field methods for geologists

Appendix A

Field Methods in Volcanic Regions

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In this book we have focused on the ways field observations of volcanic terrain can be used to locate and evaluate geothermal resources. Keeping in mind that for some geologists, these observations will be their only data sets, we included this appendix to guide the student or geologist who has not yet had a great deal of field experience. This appendix is specifically oriented toward work in volcanic areas and does not include basic instruction such as how to use a compass and clinometer; the reader should consult a handbook on field geology for this type of information (for example, Compton, 1962).

The experienced field geologist may simply ignore this appendix and use the core of this book as a reference.

Preparation for Field Work

Definition of the Problem

The first and perhaps the most painful part of a project is to define the purpose of the field work; this process will guide the planning stages. Geothermal exploration within volcanic fields has two main goals: (a) identifying and evaluating the heat source and (b) locating permeable zones and the hydrothermal system. All vents and their deposits must be mapped and the extent and thicknesses determined. These maps will also serve as a basis for stratigraphic studiesan aspect that will also require samples for petrologic analysis and age dating. The data obtained from this step will reveal the volumes, ages, genesis, and compositions of the rocks, which in turn can be used to interpret the depth and size of magma bodiesthe heat source. It is also necessary to map

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all structures, including fracture systems, flexures, and faults, and to evaluate their relationship to the volcanoes and areas of hydrothermal alteration. Most hydrothermal systems are associated with zones of fracture permeability; the careful definition of these zones is crucial in locating sites for exploration core drilling. It is wise to work with hydrogeochemists when preparing detailed maps of hydrothermal manifestations such as fumaroles, hot springs, sinter and travertine deposits, and hydrothermally altered ground.

Library Research

Prior geological, geophysical, and geochemical work will be useful in evaluating the area to be mapped and in planning the field study. Spending time in a good technical library will save a great deal of effort and money. Field geologists can gain a substantial headstart by reading all published work, summarizing the portions that might be needed later, and copying the maps. Maps copied from published material can be useful for reviewing previous work when in the field. It is prudent to keep in mind multiple interpretations for the published data; for example, a down-dropped block can be interpreted by one author as a caldera and by another as a graben.

Commercial data bases, usually accessible through libraries for a small fee, provide listings of most publications and reports unless they are truly obscure. By furnishing key words, including the subject and geographic area, one can obtain a comprehensive guide to the literature about a specific area.

If the area to be mapped has already been studied by geologists from mining or oil companies, they may share unpublished reports and data, or these same reports may be on file with the government. (However, it may be that these maps and data are proprietary and not available to you.) There also may be unpublished data available from government agencies and university geologists.

Collecting Geographic Materials

Topographic Maps

It is useful to have copies of every available topographic map and at all scales. If none are available at an appropriate working scale, a good printer can enlarge or reduce the map onto a mylar (plastic) base. If paper copies of the map are to be used in the field, it is a good idea have them waterproofed.

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Some countries have digitized topographic maps; if a mainframe computer, a geographic information system, and a large budget are available, it may be possible to obtain the maps on magnetic tape. These can be useful later for preparing final published maps, constructing three-dimensional diagrams of the area, or as a base for all of the field measurements.

Satellite Images and Aerial Photographs

Satellite images, now available for most of the Earth's surface, are essential for mapping large structural features, especially in heavily vegetated regions. (Examples of Earth Resources Technology and Landsat Thematic Mapper images are shown in Figs. A.1 and A.2.) If the region is arid, preliminary geologic maps can be prepared with the help of magnetic tapes of the satellite image and image processing programs. Processed images are sometimes for sale from commercial sources, government agencies, or university research groups. Synthetic aperture radar (SAR) imagery, either aircraft or spacecraft mounted, is useful for mapping structural features and volcanic landforms, particularly in vegetated regions or areas with continuous cloud cover (Fig. A.3).

Aerial photographs, in black-and-white or color, are frequently attainable from government agencies or companies specializing in aerial photography. However, in some countries, the use of aerial photography is restricted for security reasons.

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Fig. A.1 Earth Resources Technology Satellite (ERTS) image of the Guadalajara-Lake

Chapala-Volcn Colima region of central Mexico (Band 6). Width of the image is 115 km. The most prominent feature is the Colima graben, which is oriented north-south and is visible from Volcn Colima (bottom-center) to its intersection with the east-west

trend of volcanoes and faults. Volcanoes are visible from the area of Lake Chapala (center right), through the young caldera complex of Sierra La

Primavera (center), to Volcn Tequila (upper left). These images provide a remarkable overview of the geology of a region.

Land-Ownership Maps

These maps may be available from federal or municipal government offices. When in the field, it is usefuleven essentialto know whom to contact before crossing any private property.

Establishing the Stratigraphic Framework of a Volcanic Field

The field methods used in geothermal exploration to establish stratigraphic relationships between volcanic rocks are very similar to those used for sedimentary rocks:

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Fig. A.2 Landsat Thematic Mapper (TM) image of an area of northern Chile, on its frontier with Bolivia.

Area shown is 90 by 90 km. In the center is Lago Chungara, which was formed when a river was dammed by a debris avalanche from Volcn Parinacota (the snow-covered peak north of the lake). TM images come as color prints and

are extremely useful for mapping in arid regions. This TM image was prepared by Peter Francis at the Lunar and Planetary Institute in Houston, Texas.

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Fig. A.3 Space Shuttle Imaging Radar image of northwestern Honduras,

showing the north-south-trending Sula graben. The lake visible near the bottom of the image is Lago de Yojoa, which is 5 km wide at the narrow neck; Yojoa was formed behind a natural dam composed of basaltic cones and lavas that were erupted in the Sula Graben. North-south-trending faults, which cut these youthful lavas, are easily identified. Synthetic aperture radar images such as these are excellent for reconnaissance geologic mapping in regions with heavy vegetation or cloud cover. This image was provided by Ron Blom

at the NASA-Jet Propulsion Laboratory of Pasadena, California.

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careful geological mapping of lithologic units, measurements of many stratigraphic sections, and age-dates for those units. However, rock units within volcanic fields show much more lateral and vertical variation than do units in most sedimentary basins (Fisher and Smith, 1991). They can fill caldera depressions or deep valleys, which means that one might find younger volcanic rocks at a lower levels than older rocks, even if no folding or faulting has occurred. Pyroclastic rocks are formed quicklyinitially with abundant kinetic and thermal energyand are deposited as ashfalls that drape topography, surges that cross topographic highs, pyroclastic flows that follow the valleys, as well as wet surges of cohesive ash that defy the laws of original horizontality when plastered onto vertical surfaces.

The possibilities of facies variations within single depositional units must be considered when mapping volcanic rocks (Fig. A.4). For example, surges and pyroclastic flows can grade outward into volcanic mudflows because of cooling and condensation of steam within the flow some distance from the source. The degree of welding of pyroclasts in the flow units can vary with the unit thickness; dense rocks are found near the vent or in the center of valley fills. For detailed descriptions of facies variations in volcanic rocks, see Fisher and Schmincke (1984) and Cas and Wright (1987).

Field and laboratory observations must be adapted to fit the volcanic field of interest. For example, the approach used for a large basaltic lava plateau would differ considerably from that used to study a group of small tuff rings.

Stratigraphic analysis of volcanoes provides the necessary foundation for all other studies, including petrology, geochemistry, thermal state, and structural framework; without this foundation, sample analysis is nothing more than rock collecting. Table A.1 provides a list of further functions for which various field observations are used.



Approach

A working stratigraphy can be established by considering earlier work as well as the field geologist's own study of aerial photographs or topographic maps. Published stratigraphic studies supply useful information from nearby areas and may include radiometric dates. All of this information should be compiled in a notebook and on a map or photo base. We offer the following suggestions to be considered when entering this stage of field work.

By quickly examining the whole area, one can locate the best exposures, especially those that show contacts between depositional units. If these locations are noted on maps and/or photographs it is easy to return later to measure stratigraphic sections. As stratigraphic data are collected, the information should be entered on working copies of cross sections through the volcanic field. During a field study, geologists' ideas on the stratigraphy will evolve and it will be necessary to revisit some outcrops several times to reevaluate the interpretations.

Obviously, it is preferable to measure sections at the best exposures in unfaulted areas; however, this may not always be possible. The best way to begin is by standing back and looking at the outcrop from a distance to determine the layers or discrete rock units that stand out. They are marked on a sketch or polaroid photograph and their general characteristics are noted, including thickness, texture or structure, and color. This distant view may be useful when unraveling variations from one detailed stratigraphic section to another.

When a section is measured, the attitude (strike and dip) of the rock units is described as well as the rock types and their relation to older rocks, paleosols, and any intrusive rocks. The area traversed while measuring the section is noted on a map or aerial photograph; if neither map or photo exist, a pace-and-compass map of the traverse with distances, slope angles, and

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Fig. A.4 Schematic cross sections illustrate (a) facies changes between volcanic units and

(b) time-correlative sedimentary units. These deposits are grouped into map units that are linked to the eruption or sedimentary processes responsible for the deposit.

(Adapted from Smith, 1987.)

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Table A.1. Stratigraphy in Volcanic Fieldsa

Purpose Field Observations

Correlation of lavas, tuffs, and epiclastic sedimentary rocks; eruption types; unit volumes; location of buried or eroded volcanic vents

Individual beds; bedding sets in layered sequences; grain size; component analysis of features; fabric



Paleotopography and paleogeology; eruption history; depositional history; "basin analysis"

Facies analysis; creation of a stratigraphy; descriptions of relations at unconformities

Magma composition and volcano evolution; tectonic setting and volcanism; regional stratigraphy

Relations of rock sequences to tectonic framework in time and space; comparison of volcanic fields,

centers, and provinces

a From Fisher and Schmincke (1984).

attitudes (strikes and dips) can be useful. An altimeter is employed to determine elevations of the base, top, and important contacts, in addition to elevations chosen from the topographic map. If working as a team, field geologists use a tape measure; if by themselves, a Jacob's staff is helpful. If the stratigraphic sections are very thick, it is possible to confirm thicknesses by using an altimeter or by measuring elevations of contacts on a topographic map.

What should be measured and described? Within the field, rock-stratigraphic units are defined solely on physical differences. Fisher and Schmincke (1984) defined a formation as

"a mappable bed, bedding set or sequence of beds of any thickness set apart from rock units above and below by distinctive physical criteria such as texture, color, lithologic or mineralogic characteristic, or by weathered zones or erosional unconformities; a member is a convenient subdivision of a formation."

Fisher and Schmincke also defined the concept of an eruption unit , which is a deposit from a single eruptive pulse, eruptive phase, or an eruption. A sequence of several eruption units can be treated as a mappable unit or formation. Eruption units can refer to pyroclastic fallout deposits, pyroclast flow deposits, volcanic mudflows, lava flows, and any other deposit from a single eruptive pulse. For detailed information on defining stratigraphic units within volcanic rocks, see Fisher and Schmincke (1984), or Cas and Wright (1987).

If at all possible, a geologist should not create new stratigraphic names, but rather work within existing stratigraphic designations. The rationale for this philosophy stems from experience with such cases as the Wairakei geothermal field of New Zealand. The body of knowledge about the relationship between tuffs and lavas erupted at Wairakei during the last 200,000 years has grown as more field work, drilling, and further geological exploration revealed additional details. As volcanological concepts change and more data are available, rock sequences have been refined and redefined. Over a period of 60 years, no less than 12 different stratigraphic sequences have been described for the same rocks at Wairakei. This melange of stratigraphic names confuses the working geologist to the point of desperation.

Volcanic Rock Units

Volcanic rock units include pyroclastic and epiclastic rocks and lava flows and domes. Ideally, descriptions would go onto graph paper that is taped to a large clip boardmaking it possible to evaluate relationships

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at a glance while measuring the stratigraphic section. However, because rain and wind make it impractical to carry around a bulky board, this is not the usual means of recording field notes. The most logical method is to record field observations and sketches in a waterproof notebook and draw the section later in camp or the office (Fig. A.5). It is far better to take copious notes as well as photos and sketches while in the field than to wish for them later when one is several thousand kilometers from the field area.

Contact Relationships

Understanding the nature of the contact between rock units is critical. One should determine if there is a sharp erosional or depositional contact, a tectonic displacement, a collection of reworked clastic debris, or paleosol. In the case of a depositional contact, one should ascertain whether the deposits drape the underlying topography or are concentrated in channels and valleys. If they are deposited within a valley, it is valuable to measure the size, orientation, and slope of the valley floor.

Color

A color chart is very helpful for maintaining consistency in descriptions of rock-unit color and color variations. (Rock color charts are available from the Geological Society of America.)

Rock-Unit Classification

Volcanic rock units generally fall into the categories of pyroclastic, epiclastic, and lava. In the following sections, we discuss various field observations and measurements that are useful in writing detailed and complete rock-unit descriptions.

Characteristics of Pyroclastic and Epiclastic Rocks

Pyroclastic rocks can be classified by their textural and mineralogical characteristics (see Appendix B). Complete descriptions include important details about thickness, grain size, pyroclast types, bedding sets, grading, clast orientation, flow features, induration and welding, and thermal remanent magnetization. This information is further supplemented by sampling representative clastic rock units for laboratory analysis.

Thickness

Pyroclastic units show thickness variations that are indicative of vent location, deposit type (for instance, fallout, flow, and surge), and the effects of paleotopography (Fisher and Schmincke, 1984). Even where pyroclastic units are not fully exposed, maximum exposed thicknesses can be used in constructing isopach maps. In some cases, thicknesses are estimated from topographic constraints such as scarp heights and bedding dips.



Grain Size

Field estimates of grain size can be made using the Fisher (1961) classification, which parallels the Folk (1966) classification of clastic sediments; both of these can be done with a scale and charts. Actual measurements will be done by sample sieving or thin-section studies in the laboratory, but visual estimates are sufficient for measured sections in the field. Coarser materials, including pumice and lithic clasts, can either be sieved in the field with coarse (>4-cm) sieves or measured and described at an

outcrop within a designated area outlined on the rock surface (usually ~1 m2 ). These observations are especially useful in studies of lithic clasts within pyroclastic units. Another technique for recording the textural variations within an eruption unit is to measure the lengths of the five largest lithic clasts and those of the five largest pumices.

Pyroclasts

Most of this detailed work will be done within the laboratory, however, it is helpful while in the field to note pyroclast and lithic

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Fig. A.5 An example of notes taken during measurement of a pyroclastic rock sequence.

Field notes should be as complete as possible, including the date, location, thorough rock descriptions, thicknesses of individual units, and location and numbers

of the samples collected for later laboratory analysis.

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clast characteristics that can be used later to identify a specific formation or member: color, shape, percentage of phenocrysts, phenocryst types, and variety of lithic clasts. Lithic clasts include those of lag breccias, mesobreccias, and megabreccias (the two latter types are related to catastrophic collapses such as avalanches from a sector collapse in a volcano or wall collapse within a caldera).

Bedding

Bedform identification is helpful for interpreting the origin of a pyroclastic deposits. Fisher and Schmincke (1984) discussed various bedforms that can be related to different types of eruptions (such as Plinian, hydroclastic, Strombolian), as well as the emplacement mechanism. Where a pyroclastic deposit shows a sequence of bedforms as a coherent unit (bedding set), the



sequence can be used with other observations to identify a mappable unit in the field. For example, a specific member might consist of a fine-grained ash fallout bed overlain by a surge bed, two pyroclastic flow deposits, and a volcanic mudflow breccia. Although the thicknesses and degree of compaction and welding within the pyroclastic flow deposits might vary, if the sequence appears to be unique, it can be helpful for correlating units.

Grading

The character of grading in pyroclastic deposits is also indicative of origin. The field geologist should determine whether a bed is massive, normally graded, or reversely graded.

Clast Orientation

Within surge deposits and pyroclastic flows, there may be elongate clasts or accidental debris, such as fossil tree trunks, that can be used to determine flow directions. The orientations of the long axes of as many elongate clasts as possible should be measured and averaged for each field location.

Flow Features

Many surge deposits are characterized by dunes or antidunes. Measurements of implied current directions, descriptions of types of cross-bedding, and estimates of the magnitude of the cross-beds are all useful for evaluating eruption types and processes and for locating vent areas. In pyroclastic flow deposits, flow features should be noted, including thickening in paleovalleys and shadow areas behind paleotopographic high areas where the flow is relatively thin.

Induration and Welding

To establish whether a rock is welded, partly welded, or nonwelded, bulk sample density can be compared to that of a nonvesicular lava of similar composition; welded tuffs have densities similar to those of equivalent lavas, nonwelded tuffs have densities less than half of those for equivalent lavas, and nonwelded tuffs have intermediate densities. To determine if the rock has been indurated or cemented by post-depositional processes, one should look for vapor-phase alteration within pyroclastic flow deposits, matrix cementation by diagenesis or weathering, and secondary clays from hydrothermal activity. Other evidence of induration might be found in the form of fossil fumaroles (pipe-like zones cemented with vapor phase minerals) and compaction features such as vertical concentrations of small lithic clasts (segregation pipes).

Thermal Remanent Magnetization

Most welded tuffs have high magnetic stability and exhibit uniform thermal remanent magnetization (TRM) directions. Polarity determinations of welded ignimbrites can be made in the field with a portable magnetometer (Lipman, 1975).

Sampling

For each distinct unit (but not necessarily from all measured stratigraphic sections),

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field geologists collect a sample that is representative of that unit. If the tephra are unconsolidated and coarse grained, they are sieved, the size fractions are weighed, and chunks of the pumice are collected (in addition to a split of the 1 m wide and can be up to 30 m long in some thick plateau basalt flows. The columns can have as few as three or as

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Fig. A.6 Examples of maps and useful observations of silicic lava flows. (a) Sketch map of Little

Glass Mountain in California, made quickly from an aerial photograph. This is a young rhyolitic obsidian flow for which flow lobes and the direction of flow can be observed by mapping the ridged and furrowed flow surface; from this information and topographic profiles,

it is possible to locate the vent area. The flow lobes also can be identified through textural changes; in this example, zones of coarsely vesicular pumice can be mapped. (Adapted from Fink and Manley, 1987.) (b) Map of Little Glass Mountain that shows

zones of coarsely vesicular pumice (dark areas). (Adapted from Fink and Manley, 1987). (c) Map of the Watchman dacite flow at Crater Lake in Oregon. Flow patterns

were identified by measuring the attitudes of flow foliation. This method is particularly useful if no aerial photographs are available. (Adapted from Williams, 1942). (d) Cross section along the long axis of a silicic lava flow illustrates textural variations, including coarse rubble scattered over the flow surface, along the flow front, and at the base.

Ragged spines or slabs quite often extend out from the flow or dome.

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many as seven sides; most appear to have five or six sides (Williams and McBirney, 1979). Maps of column orientations can sometimes help determine lava flow boundaries, and this is especially useful where outcrops are poor. For example, within a valley-filling lava flow, columns in the center of the valley would be vertical; however, along the valley walls, they would be oriented at an angle and would be perpendicular to the walls, which had acted as heat sinks during cooling of the lava flow. Similar columnar jointing can also be found in dikes, plugs, and lava lakes.

If the lavas are potential reservoir rocks, maps of the size, width, and extent of cooling joints in these flows exposed at the surface may be useful for estimating their permeability.

Petrology

For field identification of lava type, geologists use the petrographic classification with which they are most familiar (such as those illustrated in Fig. 1.3 and Appendix B, for instance), but consistency is crucial. The field descriptions should be the best possible, but it is likely that these will change after thin sections have been examined petrographically, especially in the case of finely crystalline rocks.

Lava Flow Type

If possible, descriptions of the type of lava flow should include its overall texture and morphology. Most basaltic lavas can be identified by the terms pahoehoe, aa , or block lava . Details of basal breccia and lava tubes or channels should be provided if they are visible.



Thermal Effects

To ascertain whether there has been thermal alteration of rocks underlying the lava flow, field geologists look for oxidation of soil layers or older rocks, formation of pipe vesicles during heating of water in soil or bogs, and desiccation of clastic sedimentary rocks (Fig. A.7).

Thickness

In measuring thicknesses, all mappable sub-divisions (eruption unit, member, or formation) and all textural subunits are noted.

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Fig. A.7 Cross section of a generic basaltic lava flow, showing some of the basic structural features

that should be described when mapping flows. Flow surfaces, if preserved, present a variety of textures that range from smooth, ropy pahoehoe to spiny, rubbly aa lavas. Flow interiors exhibit variations in structure such as different types of columnar joints, vesicle concentrations, and lava tubes; the presence of these features often depends on flow thickness and viscosity. Pipe vesicles are formed within flows as they cross wet ground; rising steam leaves vesicle trains or small tubes that are bent by flow (a good indicator of flow direction). Lava blocks spalled or extruded from the toe of

an aa lava flow leave lava rubble beneath and in front of the flow.

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Thickness is measured from the base of a unit to the level of some significant textural change.

Sampling

Lava samples are critical for developing a sound understanding of the time-stratigraphy for a field area. In addition to providing documentation of the petrogenetic evolution of a volcanic field, carefully selected samples can provide important radiometric dates. To obtain dates and chemical analyses that are reliable, it is important to assess the evident weathering and diagenetic effects as well as phenocryst content of samples.

Correlation of Volcanic Rock Units

The ability to identify and correlate eruption units becomes much more significant if the units are large, extensive, and within a tectonically complex area. If a pyroclastic unit (either fallout deposit or ignimbrite) is to be traced to determine either its volume or its utility as a stratigraphic marker across complex terrain, then correlation criteria must be established. Geothermal exploration within calderas requires that pyroclastic deposits exposed around the margins be correlated with thick caldera-fill deposits; these tuffs are from the same eruption but may have substantially different textures.

An entire branch of volcanology, tephrochronology , has been developed to answer the need for correlating volcanic ash deposits (see, for example, Wilcox, 1965; Self and Sparks, 1981). To correlate ash beds, it is necessary to identify the mineral phases, glass compositions, and particle shapes (such as shard types and pumice characteristics) that are characteristic of each deposit. If the ash is petrographically unique, it is possible to identify it with a hand lens plus a reference sample of the known deposit. If there are several ash beds of similar composition or appearance, it may be necessary to use chemical analyses of the glass pyroclasts, including trace elements, for correlation. Ideally, radiometric age dates are employed, but they are expensive. Bulk chemical analyses are known to be a poor basis for correlation: with increasing distance from the source, the gravitational segregation of mineral phases from a glass-shard-laden eruption plume can change the bulk chemical composition. The refractive indices of glass shards (see Fig. B.1), used at one time for correlation, are not always accurate because glasses change as a result of alteration and hydration in different depositional environments.

Correlation of ignimbrites can be difficult because of facies variations, the degree of welding, postdepositional alteration, and chemical zonation of large-volume eruption units. For example, it is not easy to quickly correlate a nonwelded ignimbrite on the outer slopes of a volcano and a densely welded, hydrothermally altered ignimbrite from the same eruption within the thick caldera fill. Hildreth and Mahood (1986) have reviewed techniques for correlating ignimbrites and conclude that the following observations are the most reliable:

careful mapping of the whole unit;

stratigraphic position;

thermal remnant magnetic directions within welded tuffs and high-precision potassium-argon ages;

a distinctive suite of lithic clasts; and

petrographic characteristics within pumice clasts, pyroclast shapes, and unusual phenocrysts.

Lithology and Structure

The characterization of rock samples provides qualitative and quantitative data that are used for interpreting the origin of the rocks and their significance to the overall volcanic structure and geothermal properties. In addition, laboratory analyses of lithological character provide strong tests of field hypotheses. Appendix B outlines various rock classification methods.

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Before doing a structural analysis, it is extremely important to properly map faults, showing topographic effects that constrain their dip and strike. In addition to delineation and classification of faults and fractures, the overall volcanic structure must be evaluated. With this information, various rock associations, and the rocks' spatial and temporal variations, it it possible to constrain a probable underlying volcanic structure, as has been described in previous chapters of this book.

The following discussions cover basic aspects of the techniques employed in lithological and structural studies. More detailed information is available in popular petrographic and structural textbooks such as Williams et al . (1982) and Dennis (1972).

Lava Samples

Hand Sample Classification

Because many volcanic rocks are fine-grained, accurate hand-sample classification is often difficult. The color, texture, density, and mineral content are descriptive features that can be used to identify a sample. These macroscopic features are also invaluable for making the field identifications and correlations that are necessary for mapping.

Compositional classification is generally determined by color and phenocryst content, if any. An example of such a classification is shown in MacDonald (1972; p. 458). Mineralogical classification is greatly aided by the use of rock associations, as was described for rock families by Carmichael et al . (1974, pp. 32-37), including the basalts, andesite-rhyolite associations, trachybasalt-trachyandesites, trachyte-phonolites, lamprophyres, and nephelinites. In addition to the sample's phenocryst mineralogy, secondary mineralogy is employed to classify many volcanic rocks, especially those found in areas of geothermal activity. (This subject is discussed more thoroughly in Chapter 3.)

Textural features of samples lend a physical basis for classification to supplement the more chemical nature of mineralogical classifications. For example, textural features of lavas include vesicularity, phenocryst abundance and size, foliation and fracture, and secondary transformations such as hydration, devitrification, and weathering.

In general, a combination of the compositional and textural classifications of lava samples (for instance, aphyric rhyolite; pumiceous, hornblende-biotite dacite; flow-banded andesite) provides a satisfactory, unambiguous method of naming rocks for field and laboratory recognition.

Thin-Section Petrography

Analysis of rock samples by petrographic microscope is the most important laboratory procedure geologists use to supplement the field study; its value lies in part in the relatively simple preparation and facilities required. This work can be accomplished conveniently in the field area with a rock trim saw and lap, quick setting glues or epoxies (Hutchinson, 1974), and a polarizing, petrographic microscope. Petrographic methods, outlined in crystallography texts such as Heinrich (1965), as well as textural descriptions, well illustrated in other texts such as Williams et al . (1982), facilitate the analysis. This analysis usually includes textural classification (for example, aphyric, foliated, or vesicular) and modal analysis of the crystal content, which is quantified by point counts. One possible format for the analysis is shown in the sidebar on this page.

Petrographic analyses may also include scanning electron microscopy (SEM) of thin sections. This procedure requires sophisticated equipment that might not be readily available, but it can be extremely helpful in characterizing and interpreting phase mineralogy and textures in fine-grain samples, especially those that are pyroclastic or have been altered to secondary minerals. Etching samples with acids or by an ion beam greatly enhances poorly developed textures by

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selectively thinning the section according to mineral and glass hardnesses (Heiken et al ., 1989).

Whole-Rock and Mineral Chemistry

X-ray fluorescence spectrometry (XRF) and atomic absorption spectrophotometry (AA) are the most widely used methods for obtaining bulk chemical analyses of rock samples. (The methods and analytical problems are outlined by Hutchinson, 1974.) These data are very valuable when combined with petrographic descriptions to characterize volcanic stratigraphy and determine the nature of a magmatic source. For instance, Carmichael et al . (1974) reported that magma evolution through differentiation is revealed by the enrichment in the silica contents of erupted products with time. Samples for bulk chemical analysis must be carefully chosen to obtain a suite of samples for which analyses can be compared. Problems in discerning variable effects of secondary alterations and phenocryst contents can reduce the value of sample data for characterization and correlation.

Mineral compositions provide information to be used in detailed classification schemes that require specificity; for example, discrimination of the anorthite content of plagioclase. Mineral chemistry data also can be applied to calculation of geothermometers and geobarometers (Behen and Lindsley, 1987).

This type of information is typically obtained from thin sections by electron probe microanalysis (EPM); however, mineral separates, obtained when the sample is crushed and prepared for bulk chemical analyses (Hutchinson, 1974), can be readily analyzed by x-ray diffraction (XRD). These separates may be also useful for radiometric dating by mass spectrometry.

Alteration

Alteration mineralogy is an significant aspect of volcanic petrography in geothermal areas

1. Sample Number: 2. Date and Location:

3. Rock Name: 4. Hand Sample Description:

5. Overall Thin-Section Texture:



6. Phenocryst Description

Essential Minerals:

Varietal Minerals:

Accessory Minerals:

Secondary Minerals:

7. Phenocryst and Groundmass Textures:

8. Modes by Point Count:

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(see Chapter 3). Both the traditional XRD powder methods and the SEM are useful for identifying suites of alteration minerals such as clays and zeolites. These suites are typical of hydrothermal alteration environments and therefore can be employed to establish thermal regimes, the likely chemistry of the host rocks, and the nature of hydrothermal fluids. We recommend the review by Henley et al . (1983) for comprehensive instructions on this method.

Pyroclastic Samples

Field Classification

Several classification schemes are provided in Appendix B. Field descriptions include general grain size and sorting, bedding textures, color, and topographic effects on the pyroclastic deposit. More detailed descriptive aspects are discussed in Chapter 2.

Laboratory Analysis

Analysis of tephra samples in the laboratory involves several interdependent techniques that generally do not require elaborate analytical equipment. Figure A.8 is a flow chart that outlines laboratory treatment of pyroclastic samples, including both preparatory and analytical steps. The petrographic inspection follows procedures outlined above for lava samples and can be simply performed with a binocular scope on small sample splits or thin sections of epoxy-impregnated samples. Fundamental measurements comprise granulometry, mode and component analysis, grain shape and texture description, and mineral and glass chemical analysis.

Granulometric Analysis

Grain-size analysis of pyroclastic samples is a standard characterizing technique and, over the last 20 years, has been increasingly used to interpret the origin of samples (for instance, Sheridan, 1971; Walker, 1971; Wohletz, 1983). Granulometric characterization of samples is an especially important tool for correlation and classification in areas where many pyroclastic deposits are encountered. Interpretation is generally needed to determine the eruption and emplacement mechanisms for the deposits sampled.

Sieving is a practical approach for classifying samples in the range of ~16 to 0.064 mm, for which standard screens are readily available (see, for instance, Folk and Ward, 1957). Above this grain size, hand counts of individual fragments are useful; below this size, settling-tube measurements, based on either a pipette method (Folk, 1976) or optical methods such as fluid suspension absorbance measurements can extend the range to near 1 m. The wide range easily analyzed by screen sieves provides enough data to adequately characterize and interpret most tephra samples. Table A.2 presents class size intervals for clastic sediments and pyroclastic rocks. Because of the broad range of grain sizes represented by pyroclastic materials, it is common to use a logarithmic transformation of grain diameters called the phi (f ) scale (Wentworth, 1922):

for which dmm is the grain diameter in millimeters. Krumbein (1938) showed that on this scale transformation, plots of mass frequency vs phi size approximated a Gaussian distribution, which can be characterized by the use of log-normal statistics:



where d m/df = the mass per unit interval of f , Ks = a constant to normalize the distribution (usually Ks = 1), sd = the standard deviation in log units, d = particle diameter, and dm = the mode diameter of the distribution.

Tephra size data are useful for various types of interpretation. For example, Sparks et al .

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Fig. A.8 Flow chart for laboratory treatment of pyroclastic samples.

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(1978) discussed the importance of particle size to terminal fall velocity, which is useful in determining the amount of time required for pyroclasts to fall out of eruption plumes and clouds (Fig. A.9). Carey and Sparks (1986; Figs. 1.13 and 1.14) related maximum clast sizes to distance from source for eruptions of different magnitudes. A plot of median diameter vs distance from the source (Fig. A.10) shows the general fining of pyroclastic samples with distance for a number of different eruptions.

By using single-mode lognormal statistics, Walker (1971) characterized tephra samples of pyroclastic fall and flow origin. Wohletz (1983) described similar size data for pyroclastic surge samples. Sheridan and Wohletz (1983a) characterized size data for numerous samples of hydrovolcanic origin (see Fig. 2.20). Taken together and plotted on a sorting vs median diameter plot (Fig. A.11), these data provide a general interpretation scheme for tephra samples.

Another, more specific example illustrates the application of size data to a stratigraphic section of the Lathrop Wells scoria cone in Nevada that exhibits two main types of eruptive behavior (Wohletz, 1986): early

Table A.2. Class Size Intervals

Phi (f ) Mesh mm Clastic Sedimentsa Pyroclastic Rocksb

-10

1024.0

-9

512.0 Boulder

-8

256.0

Block, bomb

-7

128.0 Cobble

-6

64.0



-5

32.0

-4

16.0

-3 5/16 8.0 Pebble Lapillus

-2 5 4.0

-1 10 2.0 Granule

0 18 1.0 Very coarse sand

1 35 0.500 Coarse sand Coarse ash

2 60 0.250 Medium sand

3 120 0.125 Fine sand

4 230 0.063 Very fine sand

5

0.031

6

0.015 Silt

7

0.008

Fine ash

8

0.004

9

0.002

10

0.001 Clay

a Method described by Wentworth (1922).

b Method described by Schmid (1981).

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hydrovolcanic explosions and later Strombolian eruptions (Fig. A.12). Three types of bedforms were recognizable: scoria fall, fine ash layers of undetermined origin, and pyroclastic surge. Figure A.13 is a sorting vs median diameter plot that nicely differentiates between the three bedforms. Because of their relatively poor sorting, it was assumed that the fine ash layers had been emplaced by pyroclastic surge. Furthermore, a plot of median diameter and weight percent of fine ash (Fig. A.14) correlated the fine ash layers with similar size distributions from early hydrovolcanic samples in the cone stratigraphy and thus permitted their classification as hydrovolcanic. This interpretation was supported by a later study of pyroclast constituents, morphology, and surface chemistry.

We believe that size analysis can provide even more information about the history of fragmentation and dispersal of pyroclastic samples through mathematical analysis of individual size-frequency distributions. Sheridan et al . (1987) discussed the typical polymodality of tephra size-frequency distributions and possible types of interpretations. Typically, size-frequency distributions are analyzed as lognormal-type distributions, in which, for any particular sample, one or more lognormal subpopulations may overlap to form the total observed distribution. Because the single-mode lognormal statistics are not strictly applicable to tephra samples, we advocate the subpopulation discrimination technique established by Sheridan et al . (1987), in which microcomputer software can be applied to sieve data for fully characterized sized distributions. More recently, Wohletz et al . (1989) developed a new mathematical distribution, the sequential fragmentation/transport model, that relates distribution shapes to physical



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Fig. A.9 Terminal fall velocities for (a) pumice and (b) lithic fragments of varying radii in fluid of

several densities (for example, the lower two curves of each plot are for fallout in air at room temperature and steam at 1300 K). Vertical dashed lines are shear velocities of 15 to 200 m/s,

assuming a drag coefficient of 0.01; these lines define the rate of fallout of tephra from an eruption plume and velocities required to suspend the fragments in a pyroclastic flow.

(Adapted from Sparks et al., 1978.)

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Fig. A.10 Plot of median grain size vs distance from the source for various tephra deposits.

(Adapted from Fisher and Schmincke, 1984.)

processes of fragmentation and transport sorting, which allows a much more extensive analysis of size data. The distribution is given as

[Full Size]

where the normalization constant (Ks ) and the transport distance factor (x/xo ) are set to unity for frequency distributions totaling 100%, gf = a parameter analogous in part to standard deviation, and gf = gf + 2 for fragmentation processes or gf = 2 for transport processes. Because the distribution shapes for the fragmentation and transport forms of Eq. (A-3) are nearly identical and because almost all tephra samples have experienced some sorting by a transporting agent, the gf = 2 form is most appropriate. Figures A. 15 through A.17 show the results when Eq. (A-3) is applied to several tephra samples. In Table A.3, we show observed ranges and expected values of gf for volcanic fragmentation and transporting process.

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Fig. A.11 Plot of sorting (sf ) vs median diameter (Mdf ), showing ranges of values as 1 and 8%

contours for fallout (dashed lines) and pyroclastic flows (solid lines). (Adapted from Walker, 1971.)

The bold solid line encloses the range of values observed for pyroclastic surge samples from observations referenced by Fisher and Schmincke (1984); the dotted line

surrounds values of cross-bedded surge deposits. (Adapted from Fisher and Schmincke, 1984.)

Component Analysis

Tephra samples contain essential juvenile, (meaning new magma), accessory (older volcanic materials), and accidental (subvolcanic basement fragments) components. In juvenile components, fragments of glass, lava, and crystals vary in proportion in a complex fashion that is dependent on the magma composition and temperature as well as the mode of ejection and transport. Glass is often vesiculated and forms pumice or scoria. The three tephra components (glass, crystals, and lithic fragments) can be easily recognized with assistance of a hand lens or microscope. An example from Walker and Croasdale (1972) shows vertical and lateral changes of pyroclast constituents for the Fogo A tephra sampled southeast of Lake Fogo at Sao Miguel in the Azores (Fig. A.18).

An analysis of tephra components is especially important for identifying samples from deposits that have major nonjuvenile contributions. Abundant accidental and accessory lithic fragments are indicative of eruptions that have fractured and excavated rocks from around the magma conduit, as is the case for vent-opening and hydrovolcanic eruptions. A careful count of lithic-fragment abundances for the scoria cone at Lathrop Wells (Fig. A.12) showed the relative abundance of lithic fragments in pyroclastic deposits from hydrovolcanic phases (Fig. A.19). In addition, Fig. A.19 illustrates the relative increases of crystals in pyroclastic surge samples from the tuff ring and fine ashes from

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Fig. A.12 Stratigraphic section of the Lathrop Wells, Nevada, scoria cone,

showing sampled intervals. (Adapted from Wohletz, 1986.)

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[Full Size]

Fig. A.13 Plot of sorting vs median diameter for samples from the scoria cone described in Fig. A.12.



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Fig. A.14 Plot median diameter (Mdf ) and wt% ash

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Fig. A.16 Plots of the size-frequency distribution for a sample of the proximal bedded deposits

of the Mount St. Helens May 18, 1980 pyroclastic flow. The upper plot shows

a cubic spline curve fit to the data points ( ), and the lower plot shows the modeled

distribution (solid curve) made by adding four subpopulations of SFT form (dashed curves).

(Adapted from Wohletz et al ., 1989.)

Rounding: used to determine the relative degree of transport abrasion and reworking by epiclastic processes

Surface alteration: indicates weathering or hydrothermal processes; phreatomagmatic pyroclasts from wet surge deposits show abundant alteration coatings.

Grain-shape analysis can be further developed by using an SEM; methods for analysis and interpretation are discussed by Heiken (1972), Wohletz (1983; 1986), and Heiken and Wohletz (1985). The SEM micrographs in Fig. A.20 reveal prominent textural features. Figure A.21 plots the variation of grain textures that proved useful in distinguishing between Strombolian (magmatic) and hydrovolcanic samples from the scoria cone at Lathrop Wells (Fig. A.12).

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Fig. A.17 Plots of the size-frequency distribution for a sample of gray pumice fall from the AD 79

eruption of Vesuvius. The upper plot shows a cubic spline curve fit to the

data points ( ), and the lower plot shows the modeled distribution (solid curve) made

by adding three subpopulations of sequential fragmentation/transport (SFT)

form (dashed curves). (Adapted from Wohletz et al ., 1989.)



Tephra Chemistry and Alteration

Bulk chemical analyses can be obtained by x-ray fluorescence (XRF) and atomic absorption (AA) methods, as discussed for lava samples earlier; however, such analyses are not commonly performed because of the secondary alterations typical found in tephra samples. Glass silica content can easily be determined by index of refraction measurements with a petrographic microscope (see Fig. B.1). An x-ray analyzer attached to the scanning electron microscope (SEM) also provides a rapid means by which to obtain relative chemical compositions; using this technique, it is possible to analyze small alteration crystals that cover individual pyroclast surfaces (Fig. A.22). Many types of pyroclastic deposits show variations of glass surface chemistry with stratigraphic position (Fig. A.23). These variations can be interpreted with respect to the degree of secondary alteration (an essential measurement for porosity and permeability), the degree of water interaction during hydrovolcanic eruption, and changes in magma chemistry. In the Fig. A.24 plot of major-element variations for the Lathrop Wells scoria cone (Fig. A.12), the strongest variations occur in hydrovolcanic samples, as is the case for palagonitic constituents and surface alteration textures (Figs. A.19 and A.20, respectively).

Structural Analysis

Identification of geologic structures is a crucial component of field work. Frequently, these structures are most readily observed from stereo pairs of aerial photographs. In the field, definition of fault and vent structures requires careful correlation of rock units and close inspection of outcrop fabrics.

Regional Tectonic Control

Regional structures are related to past and present tectonic conditions such as crustal compression (thrusting and folding) and extension (block faulting, graben formation, and strike-slip movement). In general, regional structure exerts some control over volcanic vent locations and, to some degree, the type of volcanic complex that evolves (for instance, composite cones from compressional regions and scoria cone fields from extensional environments).

Stratigraphic Correlation and Volcano-Tectonic Models

As we discussed earlier, preparation of a detailed stratigraphy is one of the most important aspects of field work. Stratigraphic units in volcanic fields consist of old basement rocks, which are relatively large areally as a result of their sedimentary, intrusive, or metamorphic origin; rocks of petrologic consanguinty, such as older mafic rocks buried by younger intermediate extrusives; and widespread pyroclastic units.

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Table A.3. Possible Variation of gf with Fragmentation and Transport Processesa

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Correlation of these units can generally be accomplished in the field by examining out-crop textures and alteration, phenocryst contents, erosion surfaces, and overall rock type. For widespread pyroclastic units, discontinuities in surface elevation and thickness (for example, large thickness variations seen across graben- or caldera-bounding faults) are often used to identify fault locations. Composite cones can show facies such as near-vent intercalations of lava and pyroclastic units that change laterally to distal laharic deposits. Other typical stratigraphic successions for various volcanic field types are discussed in previous chapters (see also Cas and Wright, 1987).

The Map

Because planimetric maps are the most significant and tangible product of geological/volcanological investigations, we place great emphasis on detailed, accurate, and legible maps that portray as much qualitative and quantitative data as possible. The spatial relationships of observations and data collection points are not only the key to understanding the subsurface structure of an exploration property but also necessary in planning drilling operationsespecially for issues concerning topographic accessibility. The three-dimensionality of geologic investigations also dictates the need for maps with associated cross-section interpretations. The exercise of producing maps and cross sections is one way to validate spatial observationsthe spacial and temporal laws of superposition, topographic control, and cross-cutting relationships must be satisfied by data and observations before the field



geologist can produce a technically accurate map. During the map-editing stage, inconsistencies in observations and the completeness of a field study become obvious.

The stages of mapping are well described in classical texts on geological field methods.

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[Full Size]

Fig. A.18 Median size (Mdf ), sorting coefficient (sf ), and frequency distribution of pumice (light gray),

crystals (white), and lithic clasts (black) for the Fogo A tephra. (a) These variations show vertical changes that are documented within a stratigraphic section. (b) These

variations show lateral changes documented within the deposit with increasing distance from the vent.

(Adapted from Walker and Croasdale, 1972.)

For instance, Compton (1962) discusses the scale and detail, types of data and observations to be included, ways maps address specific themes or problems, note-taking and location protocol, field equipment required, sampling procedures, sample density, and traverse plans.

Generally, mapping is first approached from the reconnaissance level where previous reports and maps are compiled, accessibility is determined, and land ownerships are determined. Many of these issues were covered at the beginning of this appendix. Often maps and orientations provided by previous workers can be compiled into a working reconnaissance map. The mapping process then progresses from observations of type localities for development of the stratigraphic framework to field checking major geological contacts determined by previous work. Most useful reconnaissance traverses are along areas with outcrops, such as streams, roads, ridges, and trails that cross structural and stratigraphic contacts. At the end of the reconnaissance stage, when the area of interest has been placed in a regional context, the size of the area to be studied and the level of detail required can be determined; in addition, specific areas can be selected for detailed mapping.

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[Full Size]

Fig. A.19 Variation of pyroclast constituents for samples from the scoria cone section shown in Fig. A.12.

The curves depict variations for the size fractions indicated. (Adapted from Wohletz, 1986.)

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[Full Size]

Fig. A.20 SEM photographs illustrating four common pyroclast textures. (a) Vesicularity is well

developed for this pyroclast sampled at Surtsey. (b) Grain angularity is prominent in a hydroclastic sample from Surtsey. (c) Grain rounding indicates transport abrasion in this



poorly vesicular pyroclast from Kilbourne Hole maar in New Mexico. (d) Surface alteration coats this pyroclast from the Coliseum Diatreme maar in Arizona.

Scale and Graphic Detail

Topographic maps for most areas are available at scales of 1:250,000 to 1:25,000. The scales of satellite and aerial photo images vary, but satellite images are generally more regional. It is often satisfactory to photographically enlarge topographic base maps for more detailed investigations, but this process can make it difficult to judge absolute distances correctly and triangulation methods will be necessary.

For geothermal fields, the scale of the map may be determined by the size of the volcanic field with which it is associated. For example, if a hydrothermal system strongly interacts with regional aquifers, related hydro-thermal manifestations, geochemical survey points, and sampling locations may extend over regions of up to several hundred square kilometers. In such cases, a large-scale geological reconnaissance map helps identify the geological control of the hydrothermal manifestations and their possible relationships with primary hydrothermal prospects. These large-scale maps are valuable in establishing hydrologic recharge and hydrothermal outflow areas, which are a function of the regional hydrologic gradient. As a part of the geological and hydro-

326

[Full Size]

Fig. A.21 Variation plot for pyroclast textures determined by SEM vs scoria cone

stratigraphy (Fig. A.12). The vesicularity in magmatic (cone-forming eruptions) samples is greater than that in samples from the hydrovolcanic tuff ring, but

the hydrovolcanic phases show greater grain alteration and blocky (angular) textures. Pyroclast rounding is increased in samples abraded during surge transport.

(Adapted from Wohletz, 1986.)

geochemical survey, evaluations of the regional groundwater budget often play a major role in modeling the productivity of a hydrothermal system.

When exploration has progressed to the drilling/coring stage and production drilling plans are being considered, a detailed plane-table map showing the target area's topographic contours, geological contacts, lithology, and structures is beneficial. This process may require scales in the range of 1:1,000 to 1:10,000, which will make it easier not only to determine the well site, but also to locate geologic details and project depths accurately.

Thematic Mapping

Geological maps can be very different, depending on the theme of the mapwhether it focuses on bedrock lithology, detailed volcanologic or tectonic structure, rock facies determined by chemical or physical properties, or geothermal manifestations. The most useful type of map is

327



[Full Size]

Fig. A.22 SEM of microcrystalline alteration materials coating a vesicular pyroclast from Surtsey.

The mineralogy of these materials can constrain the alteration environment.

geovolcanological and shows aspects of bedrock lithology, volcano structure, and cognate lithologies (suites of rocks all erupted from the same volcanic edifice). A geovolcanological map adds these structural interpretations to more classical geologic base maps. Producing this type of map requires that the field geologists

recognize related rock types that can be grouped as co-genetic suites related to the evolution of particular vents;

delineate subunits or facies of rocks that reflect their genesis;

map geomorphological changes that reflect concealed vent structures; and

distinguish between regional tectonic structural fabrics and local volcanic ones.

Several map themes we have found particularly useful in specific areas are discussed in more detail below. Creating multiple maps of a geothermal area can be very useful in separating different data sets and observations so they can be judged on their internal consistency; however, multiple maps are also useful as a group when they are over-laid so as to determine areas of greatest data correspondence. (Map overlays will be discussed in the later section on 3-D models.)

328

Lithological

A geological map that emphasizes only rock data and observations illustrates the greatest number of mapped geological units and structural contacts but places little emphasis on volcanic or tectonic structure. These maps are employed to show rock sample locations and subtle variations in rock properties. However, because the detail can obscure volcanological and structural interpretations, this type of map may not be the most suitable for illustrating key rock and structural elements that control a geothermal system. The geological map of Usu volcano (Fig. 5.34) is typical of a lithological approach that shows variations of rock types according to their age, petrography, and geomorphology.

Structural

A map that emphasizes tectonic and volcanological structure may have some interpretive elements that are based on grouping of lithological units and the delineation of individual volcanic edifices by geomorphological features. Because only major rock units are shown, much greater emphasis is



[Full Size]

Fig. A.23 Variation in glass-surface chemistry for samples taken from stratigraphic sections of four

volcanoes: Crater Elegante and Cerro Colorado in Sonora, Mexico, and Panum Crater and Obsidian Dome from the Inyo-Mono volcanic field of California.

Sample types are designated as fall (F), sandwave surge (SW), massive surge (M), and planar surge (P). Fall samples are most representative of magmatic compositions

(stippled patterns), whereas the surge samples show hydrovolcanic tephra compositions that result from the rapid alteration of the magma through its interaction with water. The vertical line on plots for Crater Elegante and Cerro Colorado separates essentially

magmatic samples (left) and later-erupted hydrovolcanic samples (right). (Adapted from Wohletz, 1987.)

329

placed on structural features that affect subsurface conditions and locations of hydrothermal systems. Relative ages and the amount of recent fault movement can be depicted by variable thicknesses of contacts. Figure 5.24 provides a prototype in the detailed structural map for the Coso volcanic field.

Facies

Volcanic facies (lateral and vertical variations in single eruptive units) are manifest as gradation changes in physical or chemical properties. Some examples of mappable facies discussed in previous chapters include

downslope variations in composite cones, from lavas to lavas intercalated with pyroclastic units to dominantly epiclastic and pyroclastic textures;

variations in pyroclastic units related to median grain size, sorting, and bedding structures;

pyroclastic and lava facies of silicic domes;

contrast between caldera fill and caldera outflow rocks;

plateau forming, horizontally outcropping rocks of basin fills; and

dipping and unconformable rock strata of near-vent facies.

Other examples are described by Cas and Wright (1987).

Facies variations shown in plan view can provide information on porosity/permeability relationships that are meaningful when a potential hydrothermal reservoir must be delineated. In other cases, facies variations in some pyroclastic units can point to potential vent areas that have been eroded or concealed under younger units. Wohletz and Sheridan (1979) discussed one example of ways in which pyroclastic surge facies can indicate vent area, and another example, shown in Fig. 2.34, suggests how dry-to-wet pyroclastic rock facies might help constrain the degree of aquifer interaction for a given eruptive unit.

Geothermal Manifestations

Chapter 3 outlined aspects of geothermal manifestations, including thermal spring and fumarole locations, silica sinter and travertine deposits, hydrothermally altered rocks, and phreatic explosion craters and breccias. A map indicating locations of such manifestations is very useful for hydrogeochemical surveys; it not only points to individual sample localities, it also becomes a base map (like that shown in Fig. A.25) for plotting hydrogeochemical samples and subsequent interpretations.

Cross Sections

Construction of cross sections should begin before field work actually commences. The following approach works well to stimulate ideas about the area's geologic history and framework; it also allows the geologist to identify inconsistencies and deficiencies while still in the field.

Topographic Profile

After previous topographic maps, aerial photographs, and any other available data have been examined, it is possible to establish lines for cross sections through critical parts of the field area. If topographic maps are available, the geologist prepares topographic profiles for the cross sections at the same scale used for the working copy of the map. After the profiles are completed, several copies are made with indelible ink on sturdy paper (cross-section paper is good) or plastic mylar (Fig. A.26).



No vertical exaggeration is used, especially when sketching in lithologic units and structural features; cross sections with vertical exaggeration are often deceptive and can lead to later problems in siting wells.

Preliminary Interpretation

At this point, the geologist has examined all the older data, aerial photographs, and

330

[Full Size]

Fig. A.24 Variation of major-element chemistry for surfaces of pyroclast samples taken from the scoria

cone described in Fig. A.12. Strong variations are evident in major-element abundances for the hydrovolcanic, magmatic, and lava samples.

(Adapted from Wohletz, 1986.)

331

topographic maps and has some preliminary ideas about the structure of the area from geomorphological clues. Folds, faults, and major lithologic breaks are sketched in pencil on the cross-sections (Fig. A.26). Studying these preliminary cross sections can help in planning field traverses.

In the Field

Each evening in the field, appropriate changes are made to the working cross sections, based on the day's lithologic descriptions as well as observations of faults, attitudes, areas of alteration, etc. Sometimes it is necessary to erase an earlier interpretation or add new lines. This messy working cross section evolves, along with the geologist's ideas about the field area (Fig. A.26); the daily review exercise is stimulating and sharpens perceptions for the next day's observations.

By the end of the field season, the geologist has a fairly sophisticated set of cross-sections that are consistent with the geologic map, working hypotheses, and the logical framework within which samples were collected.

Additional Information from Drill Holes

The ultimate test of the three-dimensional view of the field area is a comparison of the map, cross-sections, and data gained through drilling. A proposed stratigraphy, based on field work, is created for the drillers; this exercise helps them prepare a drilling plan and cost estimates. In return, the drilling provides the geologist with hard data about subsurface geological features. As drilling proceeds, numerous changes to the cross-sections may be necessary. On the other hand, well-founded cross sections may be useful for interpreting core or cuttings that are difficult to classify.

The ideal exploration well is a corehole that has been sampled to its full depth. Wireline coring is a proven technology and eliminates guessing about the rock types and the degree of alteration or fracturing. Procedures for the curation and description of core samples are outlined in Appendix F. If no cores are to be collected, careful evaluation of drill cuttings can be useful; however, it is important to take into consideration the limitations of this method when cuttings from different strata are mixed



during their rise to the surface and there is a time lag involved. Onsite petrographic identification of cuttings is aided by hand lens and a binocular microscope, but ideally a geologist should set up a simple thin-section preparation system with basic equipment: quick-setting glue (super-glue ) or epoxy to fix the cuttings on a glass slide, a hot plate to set the glue, and a grinder or abrasives to grind down the cuttings mounted on the slide to the appropriate thickness. Because this method takes only a few minutes per thin section, it allows the geologist to keep up with the drilling operations.

When calibrated against core or cuttings, geophysical logs provide critical information on lithologies, temperatures, and permeability (see Appendix F). Integrating these dataperhaps with the aid of a professional well-log analystis time-consuming but well worth the effort if the geologist is to understand the third dimension within the geothermal field.

"Final" Versions of Cross-Sections

At this stage, the field geologist has confidence in the cross sections. If the cross sections are to be used in a publications or report, it is desirable to use a technical illustrator for the final drafting. However created, the final maps and cross sections should be drawn on a plastic base or good quality paper that will exhibit minimal expansion and contraction with changes in humidity. One way to avoid many problems is to prepare the cross sections and map with a computer that has computer-assisted design programs or a geographic information system

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[Full Size]

Fig. A.25 Example of a spring map from a site near Azacualpa, Honduras. Descriptions

and measurements of hot springs include details of local landmarks such as streams, large boulders, and canyon walls.

(Adapted from Eppler et al., 1987.)

333

program. Distinct patterns or conventional symbols should be used for lithologic units. Horizontal and vertical scales must be included; it is impossible to use either cross sections or maps accurately without clearly labeled scales.

The final cross sections should be laid across the map parallel to the profile lines (Fig.A.26) and several questions should be asked: do the interpretations still appear to be reasonable? Is the scale correct? Do key points on the map (for example, faults) correlate with those same features on the cross section?



The process of creating these maps is lengthy and involves many stages. The last, extremely necessary step is to proof the completed map: checking the data and spelling of place names as well as myriad other details that have been assimilated during the mapping process.

Three-Dimensional Model from Maps, Cross Sections, and Drillhole Data

The final stage of a geovolcanological field study is the compilation of all data, including maps, cross sections, stratigraphic sections, well logs, and rock chemical and physical data. At this stage of a geothermal investigation, if complementary hydrogeochemical and geophysical survey data are available and if there are any drillhole logs, a complete geothermal model might be developed. Cross sections drawn from geovolcanological maps can be greatly enhanced by drill core and cutting information (Appendix F), as discussed above, and the resulting lithological and structural sections can be compared to geophysical lines such as electrical and gravity profiles. This comparison is used in the interpretation of the geophysical data and further constrains the vertical dimension of the geological study. In addition, geochemical surveys suggest areas of recharge and outflow of thermal waters and can constrain rock chemistries of potential reservoirs (Wohletz et al ., 1984).

By developing the superposition and adjustment of the geological, geophysical, and hydrogeochemical cross sections, it is possible to formulate a three-dimensional model. This exercise generally simplifies each of the data sets but produces an internally consistent picture of the subsurface. The degree to which interpretation plays a role in developing a generalized cross sectional illustration depends largely on supporting evidence from analyses and observations that are not typically shown on thematic maps. For example, where hydrovolcanic vents have been mapped, the type and abundance of lithic constituents of pyroclastic deposits indicate the lithology of potential reservoir rocks at depth under the vents. With stratigraphic information, the projected section below the vent can be interpreted and the lithologic and structural character of the potential reservoir can be determined.

This modeling stage can be the most critical stage of a field geothermal study, even if all the desired data sets are not available. A carefully designed model portrays the dominant controlling features of a geothermal system; it is formulated to be easily tested and readily understandable. Such a model combines observed constraints on subsurface conditions with many of the more subtle aspects of field observations that can not always be easily interpreted in their raw form. A model can be a single or several

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two-or three-dimensional illustrations or a set of numerical calculations that reproduce the quantitative features of a geothermal system. We emphasize that such a model is a hypothesisone that can be tested by further detailed geological studies and specific geophysical surveys. The richness of detail portrayed by a model also indicates something about the completeness of the field study. The uncertainties shown by the model are also of great significance because they emphasize missing information and point to potential methods of obtaining that data.

Recommendations and Justification for Drilling

In the final report of a field study, data and observations must be clearly separated from interpretations and conclusions. After documentation of all the studies and their associated conclusions, a formulation of required future work and a summary should be added.

Following an extended field study (whether it has been accomplished by reconnaissance or detailed field work), the project reaches a point when one must justify future work and specify the direction it should take. Field researchers evaluate both their supporting data and their overall inclination about the potential success of a geothermal exploration project. If data and observations are sufficient to produce a three-dimensional model that can be tested by further studies such as geophysical surveys, thermal gradient boreholes, or core drilling, the justification must be succinctly presented and a strategy that will work within this framework should be suggested.

It is our experience that even if shallow thermal gradient wells are indicated, the cost of obtaining core from these boreholes is not a significant additional expense; core information greatly enhances the overall body of data that can be extracted from drilling. The location of these boreholes should be determined by (a) drilling targets specified by various field investigators; (b) ways in which the three-dimensional model can be best tested and augmented by drilling information; and (c) considerations of access and property rights.

The field geologist can also emphasize conclusions about the size of the heat source in locations where young volcanism will allow application of the methods described by Smith and Shaw (1975), which are discussed in Chapter 2. Such estimations are supported by field observations of geothermal manifestations, such as surface heat flow that can be determined from hot spring and fumarolic areas (also discussed in Chapter 3). After the temperature of a potential hydrothermal system is constrained (either through direct, surface-temperature measurements or analysis of hydrothermal-mineral assemblages) conversions of thermal resource to available heat for production and electric power generation can be generalized from graphs shown in Appendix D. Such exercises produce only crude numerical estimations of a geothermal resource, but the information could emphasize the relative potential of a geothermal prospect and help justify or discourage exploration drilling.

The final step in writing a report is a summary that compiles all aspects of the work, including the perceived regional importance; geological, hydrogeochemical, and geophysical conclusions; an overall geothermal model; the projected size and temperature of the potential resource; and recommendations for continuation or culmination of the exploration. Nontechnical language should

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Fig. A.26 Cross-section development shown in stages.

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be used wherever possible in this summary because it will be read by individuals of diverse backgrounds. Optimism about the project and its success must be carefully balanced against the data; the possibility that an exploration project does not satisfactorily justify future development is a valid recommendation. If the recommendation is to discontinue a project, it may be necessary to consider culminating work, such as releasing property rights and effecting environmental restoration where field work has infringed (such as might be required if geophysical lines caused topographic modifications or if boreholes must be capped and cemented). On the other hand, if continuation is recommended, aspects of property ownership, environmental restoration (access, governmental restrictions, and logistics), and local operational support should be discussed. These considerations can greatly facilitate promotion of future work.

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field methods for geologists

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