magma ascent rates from mineral reaction rims...

ABSTRACT

An ongoing challenge in curriculum development formodern, process-oriented courses in mineralogy andpetrology is to design exercises that promote understand-ing of the mechanisms and rates of fundamental mineralprocesses. We have developed an exercise wherein stu-dents determine magma ascent rates for recent eruptionsof Mount St. Helens. The ascent rates are determined fromanalysis of the thickness of amphibole breakdown rimsand comparison to experimental calibration of the reac-tion kinetics. As an extension activity, the results are incor-porated into a discussion of volcanic hazards planning forthe Cascades Range, Pacific Northwest, United States.This exercise requires no special equipment and is easilyadapted to introductory courses in earth and environmen-tal sciences without compromising the overall learningobjectives.

Keywords: Education – undergraduate; education – pro-cess oriented laboratory; mineralogy; volcanic hazards

INTRODUCTION

The traditional fields of mineralogy and petrology havesteadily evolved from the description and classification ofminerals and rocks to development of an understandingof the fundamental mechanisms and rates of the processesthat form, change, and re-form minerals and rocks in thedynamic Earth system. It is critical that instruction in min-eralogy and petrology also evolve by incorporating intothe undergraduate core curriculum these exciting new in-sights into mineral processes. One of the ongoing chal-lenges in developing process-oriented mineralogy/petro-logy courses is designing student opportunities that pro-mote understanding of the kinetics (rates and mecha-nisms) of mineral and chemical processes. It is particularlyuseful if opportunities can be developed that employ geo-logical systems of inherent interest to both introductoryand advanced students as well as involve processes thatoccur on human time-scales.

We have developed a laboratory exercise wherein stu-dents are provided with the opportunity to determinemagma ascent rates for recent eruptions of Mount St. Hel-ens. The thickness of reaction rims surrounding amphi-bole phenocrysts are measured using standard thinsections, and the time required to form a rim of a giventhickness is determined from experimental calibration ofthe breakdown reaction rate (Rutherford and Hill, 1993).Magma ascent rates are then calculated based on acceptedmodels of the Mount St. Helens volcanic system, and the

experimentally determined pressure and temperatureconditions (depth) at which the amphibole began tobreakdown upon ascent to the Earth’s surface. The experi-mental calibration and application of amphibole break-down rates to magma ascent rates was first reported byRutherford and Hill (1993) for samples from the 1980-1986eruptions of Mount St. Helens, and the results of theirstudy provide the background for this laboratory exercise.

To enhance the student learning outcomes of this ex-ercise, an extension activity is included wherein the resultsof the magma ascent rate determinations are incorporatedinto a consideration of volcanic hazard planning for theCascades Range, Pacific Northwest, United States. Thisextension activity serves to reinforce the complementaryaspects of experimental and field studies in geology aswell as the critical role of geoscientists in acquiring and ef-fectively communicating information relevant to hazardplanning.

An important consideration in designing laboratoryexercises is the ease with which they can be implementedand adapted to other institutions and course settings. Thisexercise is ideally suited for easy implementation and ad-aptation; specifically, the only equipment required is astandard student microscope, the amphibole phenocrystsare identified by their distinct shape, cleavage and color,and the reaction rims are easily identified due to the pres-ence of the opaque magnetite grains, the amount of back-ground knowledge necessary to achieve meaningfulresults is minimal, yet discussions can be easily expandedto challenge the most advanced students of petrology, andthe exercise is easily adapted to other volcanic settingssuch as Mount Shasta or the recent eruptions of theSoufriere Hills volcano, Montserrat (Rutherford et al.,1998). Details of the exercise are presented below.

THE EXERCISE

Engagement - Few students can deny a fascination withvolcanic activity, especially the violent explosive behaviorassociated with andesitic volcanoes. Most introductorycourses in geology present details of the 1980 eruption ofMount St. Helens in Washington State, or the more recenteruptions of Mount Pinatuba, Philippines, or the SoufriereHills volcano, Montserrat. These lecture presentationsgenerally include a discussion of the eruptive style of dif-ferent volcanoes and the link to magma composition (es-pecially the amount of water) and tectonic setting.Instructors typically take advantage of students’ inherentinterest in volcanism to develop the principles of volcanic

140 Journal of Geoscience Education, v.49, n.2, March, 2001, p. 140-145

MAGMA ASCENT RATES FROM MINERAL REACTION RIMS AND

EXTENSION TO TEACHING ABOUT VOLCANIC HAZARDS

John R. Farver Department of Geology and Center for Materials Science, Bowling Green State University,Bowling Green, OH 43403, [email protected]

Daniel J. Brabander Department of Civil and Environmental Engineering, Massachusetts Institute of Technology,Cambridge, MA 02139, [email protected]

hazard planning and the vital role of geoscientists in theprocess.

In discussing what information would be useful in or-der to develop a volcanic hazards plan, students will typi-cally identify several of the important issues including themagnitude and frequency of eruptions based on historicalrecords, population/industry density and monitoringand early warning systems. At some point in the discus-sion, an important consideration becomes evacuationplanning models and how long after early warning sys-tems predict impending volcanic activity that an eruptionoccurs.

At this point, the instructor should be able to coach thestudents to frame the question, “How long does it takesfor a magma to travel from the magma chamber to theEarth’s surface?”, or, more importantly, “How might onedetermine the magma ascent time?” Depending upontheir background, students are likely to present a range ofpossible ways to determine magma ascent rates, and theinstructor should pursue and discuss these ideas as appro-priate to the nature of the class and the time available. Animportant goal of this discussion is to encourage studentsto “think like geologists”. To accomplish this goal, the in-structor may choose to qualify the earlier question andask, “How might a geologist determine the magma ascenttime?” In response, students should eventually arrive atthe important principle that the first place a geologistshould look to answer questions about a rock’s history isthe rock itself.

At this point, it is useful to provide students withhand-specimens of one or more of the samples to be usedin the exercise (or at least a sample of a similar rock). Stu-dents should be asked to describe the texture and mineral-ogy of their hand-specimen. They should note that it isgray in color, intermediate in composition, and is com-

posed of large phenocrysts in a very fine-grained matrix(porphyritic texture). With the aid of a hand lens, the stu-dents should be able to identify the phenocrysts of amphi-bole (by cleavage and color). The instructor should askwhat these observations imply about the magma prior toeruption, in particular, the phenocrysts of amphibole - ahydrous mineral. It is important to establish that the am-phibole was present and stable in the magma prior toeruption.

For students in mineralogy/petrology courses, it isuseful to note that a more detailed analysis indicates thatthe Mount St. Helens samples are dacites (63±1 wt% SiO2)that contain phenocrysts of plagioclase, low-Ca pyroxene,amphibole, Fe-Ti oxides, rare high-Ca pyroxene, and glass(Rutherford and Hill, 1993). Also, most of the samplesfrom eruptions after May 18, 1980, show extensive devel-opment of fine to coarse microlites (Cashman, 1992). Thesedetails, however, are not required to complete the exerciseand can be omitted without compromise to the overalllearning objectives.

This exercise was developed for Mount St. Helenssamples, however, the reaction boundary is not overlysensitive to composition, and the experimental calibrationof Rutherford and Hill (1993) is very good for a range ofandesitic volcanic settings (Rutherford et al., 1998). An al-ternative sample found in most every geology departmentin the United States comes from the Wards Scientific Co.“Collection of American Rocks”, sample #29, an andesitefrom an ancient eruption of Mount Shasta. Samples frommore recent eruptions such as the Soufriere Hills volcano,Montserrat, would also provide excellent results(Rutherford et al., 1998).

After students have had the opportunity to observeand describe the texture of the rock and establish the pres-ence of large amphibole phenocrysts, the instructorshould pose the question, “What happens to a hydrousmineral like amphibole when the magma ascends to theEarth’s surface?” Students should respond that the am-phibole will decompose in response to the decrease in wa-ter pressure in the melt during ascent (Le Chatelier’sPrinciple). The instructor should inform students that thehornblende reaction rims found in the Mount St. Helenssamples consist of a complex assemblage of minerals in-cluding clinopyroxene, orthopyroxene, plagioclase feld-spar, magnetite and ilmenite (Figure 1). Although thebreakdown rim is complex and the details of the reactionmay be better suited to advanced courses in mineral-ogy/petrology, the system can be greatly simplified with-out compromise to the general learning objectives. Forexample, the reaction need only be described as a simpledevolatilization process, and much like the introductorygeology demonstration of the rapid release of CO2 from abottle of carbonated soda when the cap is removed andthe pressure released, the amphibole releases its structur-ally bound water to the magma in response to the decreasein water pressure in the magma as it ascends to the Earth’ssurface. In addition, because this is a dehydration reaction,it provides an excellent opportunity to demonstrate whereall of the water comes from that makes these andesitic vol-canoes so violent in their eruptive style.

Farver - Magma Ascent Rates from Mineral Reaction Rims and Extension to Volcanic Hazards 141

Figure 1. Pressure-temperature diagram (Pwater = Ptotal)showing amphibole stability field for Mount St. Helensdacite. [after Figure 4, Rutherford and Hill, 1993]. Alsoshown are two possible magma ascent paths and theconditions in the 1980 magma chamber. Cpx =clinopyroxene, Opx = orthopyroxene, Plg = plagioclasefeldspar, Mt = magnetite, Ilm = ilm


Objectives - Through the instructor-moderated discus-sion of the students’ hand-specimen observations, theclass should arrive at the primary objective of this exercise,namely, to determine the magma ascent rate of volcanicrocks using the pressure-dependent breakdown of amphi-bole due to decreasing water pressure in the magma dur-ing ascent. In the process of accomplishing this objective,students will also: strengthen their skills in the use of theoptical (petrographic) microscope, develop quantitativeskills through collection, manipulation and statistical as-sessment of data, enhance their comprehension of mineralstability and reaction kinetics, demonstrate the vital linkbetween field-based and experimental, laboratory-basedgeology, and establish the important role of geoscientistsin developing well-informed societal responses to geolog-ical hazards.

Data Collection - In preparation for this exercise, studentsare instructed to read the abstract and rim thickness mea-surement technique section of Rutherford and Hill (1993).One of the greatest advantages of this exercise is howstraightforward the recognition and measurement of thereaction rim thickness is. Hornblende phenocrysts are eas-ily recognized in thin section, and, because the analysis isbest done under plane polarized light, students don’t needto be experts in petrography. Hornblende phenocrysts inthe Mount St. Helens samples are euhedral and, in thinsection, many exhibit lozenge shape with the two promi-nent cleavages at ~126° and 54° (Figure 2). Hornblende ispleochroic, going from light to dark brown, and the break-

Figure 2b. Hornblende phenocryst in an andesiteformed during an ancient eruption of Mount Shasta,CA (sample no. 29, Ward’s Collection of AmericanRocks, plane polarized light). The hornblende has an~30 mm thick breakdown reaction rim that formeddue to the decrease in water pressure during ascentof the magma. By measuring the thickness of reac-tion rims, students can determine the magma as-cent rate for recent volcanic eruptions such as the1980-1986 eruptions of Mount St. Helens, WA.

Figure 2a. Hornblende phenocryst (in plane polarized light) with ~30 mm thick reaction rim, note the lozengeshape and two prominent cleavages at ~126E and 54E. The thickness of the rim is clearly defined by the pres-ence of opaque magnetite grains, one of the products of the hornblende breakdown reaction. Sample is a dacitefrom Mount St. Helens December 1980 eruption.

down reaction produces an iron oxide (magnetite) phasethat is opaque, providing a very distinct marker for deter-mining the width of the reaction rim (Figure 2).

Following the methods employed by Rutherford andHill (1993), students determine the rim width using stan-dard thin sections of Mount St. Helens rocks. The widthmeasurements are made using a graduated ocular lensthat is either calibrated in an earlier lab or can be calibratedas part of this exercise. Students are instructed to use onlyeuhedral amphibole phenocrysts that are sufficiently large(>100 mm) and that contain substantial amphibole coresin order to avoid anomalous rim widths caused by sec-tions which are cut through margins or corners of grains.The students work in groups of two or three, and each stu-dent takes a series of at least ten measurements on eachthin section. Mean and median rim thickness values arecalculated for individuals and for the group.

Experience has shown that students have few prob-lems measuring the rim thickness, but they tend to bias thedata by reporting only the thickest rim values. The instruc-tor should emphasize that a spread in the rim thicknessvalues is anticipated, and all rim thickness values shouldbe reported. This is particularly important in applying thedata to volcanic hazards planning (outlined below) be-cause overestimating the rim thickness values results inunderestimating how quickly the magma ascends to thesurface, hence, underestimating how quickly evacuation

must proceed. The potentially disastrous consequences ofthis bias in reporting rim thickness values can serve as anexcellent illustration of the practical importance of statisti-cal analysis.

The statistical validity of the data is then discussed,and students are asked to suggest reasons for the spread intheir data. Many will note the problems associated withmeasuring the thickness of a 3D object from a 2D section.The instructor may also want to bring into this discussionquestions about nucleation and overstepping of reactionboundaries. In addition, the reaction is driven by the re-duction in the dissolved water content of the magma dur-ing ascent, and the reaction rims form only whereamphibole is in contact with the melt - advanced studentsshould be asked to explain this observation.

Data Interpretation - The average rim thickness valuesare compared to the published values of Rutherford andHill (1993), and the duration of the ascent is determinedfrom inspection of the experimental calibration (Figure 3).Knowing the ascent duration, the ascent rate can then becalculated from the ascent path length (ie, pre-eruptivedepth). Geophysical studies indicate that a 6-16 km deepmagma chamber exists under Mount St. Helens (Lees andCrosson, 1989). Consistent with the geophysical data,phenocryst and melt analyses combined with experimen-tally established phenocryst-melt equilibria indicate thatprior to eruption, the magma was at 900°±20°C and220±30 MPa, corresponding to a depth of ~8 km(Rutherford and Devine, 1988).

Although the magma equilibrated at 8 km, the break-down rim would not have begun to form until the magmacrossed out of the amphibole stability field (Figure 1)which, for the Mount St. Helens dacites that erupted be-tween 1980-1986, corresponds to ~160 MPa water pres-sure or about 6.5 km depth. The rate of magma ascent canalso be constrained by the extent of groundmass crystalli-zation and on the basis of size distribution of plagioclasecrystals in erupted products when combined with esti-mates of grain growth and growth times (Cashman, 1992).Calculated magma ascent rates for the 1980-1986 MountSt. Helens eruptions vary from 15-30 m/hr, for the postMay 18, 1980 dacites, to >66 m/hr, for the May 18, 1980eruption (Rutherford and Hill, 1993). To put these ratesinto perspective, students are asked to compare their cal-culated magma ascent rates to average tectonic plate mo-tion (2-12 cm/yr), uplift rates of active mountain chainssuch as the Alps and Himalayas (mm/yr), the rate of ba-saltic lava flowing on a shallowly sloping shield volcano(m/hr to m/day), the average walking speed of a human(4-5 km/hr), the speed of lahars (up to 50 km/hr).

EXTENSIONS

One of the strengths of this exercise is the broad range ofextension activities that it presents. At BGSU, this exercisehas been used in a sophomore-level course on Earth Mate-rials. The course typically consists of about equal numbersof geology majors and pre-service science teachers, alongwith a few environmental studies majors. To best accom-modate the range of interests and ultimate career paths


Figure 3. Experimental calibration of amphibole re-action rim thickness as a function of time for MountSt. Helens dacite [after Figure 6, Rutherford andHill, 1993]. Curve (a) is for isothermal 900°C decom-pression from 160 MPa (the breakdown reactionboundary), and curve (b) is for constant decompres-sion from 220 MPa (the Mount St. Helens source re-gion) assuming that no breakdown occurs during the220 to 160 MPa part of the ascent where amphiboleis stable.


represented by this group, the extension activity em-ployed is an application of the ascent rate measurementsto volcanic hazards planning for the Cascades Range, Pa-cific Northwest, United States. A description of the activ-ity follows.

Volcanic hazard and risk - It is important to note that, al-though hazard assessment and risk assessment are com-monly (and mistakenly) used interchangeably, they arenot synonyms (e.g., Peterson, 1988; Tilling, 1989). “Haz-ard” refers to the volcanic phenomenon that poses a po-tential threat to persons or property in a given area withina given period of time (Tilling, 1989). “Hazard assess-ment” involves developing a historical record of the loca-tions and dates of past events coupled with the severity ormagnitude and frequency of the events. The historicaldata are then used to determine what a particular eventwould be like should it occur now, in terms of the type ofeffects it would have. An important component of hazardassessment is compiling the information in a form that isaccurate and complete yet easily understood by commu-nity planners and decision makers.

“Risk” refers to the probability of a loss, such as life orproperty, within an area subject to volcanic hazards(Tillings, 1989). “Risk assessment” is a statement of theeconomic losses, injuries and deaths, and disruption of ur-ban support systems expected when a specific physicalevent strikes a given region. Risk assessment begins withestablishment of the probability of a hazardous event of aparticular magnitude occurring within a given time pe-riod. Risk also incorporates factors such as the location ofbuildings, facilities, and emergency systems and their po-tential exposure to the physical effects of the hazardousevent, as well as the community’s vulnerability for loss oflife, injury, or loss of property when subjected to the haz-ardous event. Hence, risk assessment incorporates the so-cial and economic effects along with the physical effects ofa hazardous event. In volcanic risk assessment, the rela-tionship is often expressed as risk = value x vulnerability xhazard (Fournier d’Albe, 1979).

For this extension activity, students are asked to con-sider the volcanic hazards associated with living in the Pa-cific Northwest region of the United States. The CascadesVolcano Observatory website provides an extensiveamount of information on historical volcanic events andhazard assessment maps generated by USGS personnelfor many of the volcanoes in the Pacific Northwest regionand throughout the world through the additional linksand references provided therein (Abolins, 1997).

An important component of this activity is consider-ation not only of the primary effects of an eruption (e.g.,pyroclastic ejecta, lava flows, lahars), but also of the sec-ondary and tertiary effects (e.g., mudslides, flooding, dis-ruption of municipal services, climate change, andchanges in topography/landforms). In so doing, studentsmust draw upon information obtained in other geologycourses including considerations of local topography,stream patterns, slope stability, glaciation, prevailingwind patterns, etc. The need to assimilate informationfrom different courses reinforces the interdisciplinary na-

ture of geology and illustrates the benefit of providing anintegrated geosciences instructional program.

The volcanic hazards extension activity can easily be-come a full project in its own right and, depending uponthe nature of the course and time constraints, the instruc-tor may need to limit discussion to the specific utility ofthe measured magma ascent rates. The ascent rates calcu-lated for the Mount St. Helens samples indicate ascenttimes as short as <4 days for some of the eruptions. A simi-lar analysis of amphibole breakdown rims in samplesfrom recent eruptions of the Soufriere Hills volcano,Montserrat, indicate ascent rates of 3.5 to >42 m/hr, alsocorresponding to ascent times as short as <4 days(Rutherford et al.,1998). These rapid ascent rates providevaluable constraints on how quickly evacuation of an areawould need to be accomplished should precursor eventsindicate imminent volcanic activity.

It is important to emphasize that, because so manynatural disasters are associated with geological processes,geoscientists play a vital role in furthering our under-standing of hazardous Earth processes, assessing the haz-ards and risks involved, and assisting in preventing ormitigating the events. However, all of these tasks are invain without effective communication of scientific under-standing about geological processes to community plan-ners and to the general public. Hence, an importantcomponent of this exercise is the written hazards assess-ment for the Cascades Range that each student submits aspart of their laboratory report.

ASSESSMENT

In addition to enhancing student comprehension of min-eral kinetics, the integration of data collection, manipula-tion and application within this exercise provides anexcellent opportunity to assess overall student progress inskills development and conceptual understanding. This isaccomplished by one-on-one interactions during the labo-ratory period, through follow-up group discussions in lec-ture, and through the written laboratory report.

Collaboration is strongly encouraged throughout theexercise, but individuals are required to write their ownreport. In preparing their reports, the students must syn-thesize the observations and interpretations of their col-leagues along with their own. Creating a final product inthis manner mirrors a true work environment and em-braces the NRC recommendations (National ResearchCouncil, 1996) for designing more authentic assessmenttools (Wiggins, 1989).

An important component of the written report is thestudent assessment of whether the stated objectives of theexercise were realized, and, if not, what specific changes tothe exercise they would make to improve the outcome. Inaddition, students are required to provide a self-assess-ment that includes how confident they are in their under-standing of the principles, techniques used, and outcomesof the exercise. The student assessments of the exercise asa whole and of their own learning outcomes has proven tobe of great worth not only in identifying ways for the in-structor to improve the exercise but also for the students to

reflect on how they learn. As noted above, at BGSU thisexercise is run in a course that is nearly half pre-service sci-ence teachers, and this self-inspection of learning can be asimportant for these students as any of the other stated ob-jectives of the exercise.

SUMMARY

The exercise presented draws upon students’ inherent in-terest in volcanic processes to develop a greater under-standing and appreciation of mineral stability and therates of fundamental mineral processes. In addition, theexercise demonstrates the vital link between experimentaland field geology. The extension activity illustrates the in-tegrated nature of geological investigation and serves toestablish the critical role of geoscientists in acquiring andeffectively communicating information to the public in or-der to develop effective hazard planning models.

ACKNOWLEDGEMENTS

We thank Mac Rutherford for providing the Mount St.Helens samples. We especially thank Jan Tullis and DickYund for showing by example the work and rewards of ef-fective teaching. The manuscript was improved by com-ments of the associate editor and an anonymous reviewer.

REFERENCES

Abolins, M. J., 1997, Using free digital data to introducevolcanic hazards: Journal of Geoscience Education, v.45, p. 211-215.

Cashman, K. V., 1992, Groundmass crystallization ofMount St. Helens dacite, 1980-1986: a tool forinterpreting shallow magmatic processes:Contributions to Mineralogy and Petrology, v. 109,p. 431-439.

Fournier d’Albe, E. M., 1979, Objectives of volcanicmonitoring and prediction: Journal of the GeologicalSociety of London, v. 136, p. 321-326.

Lees, J. M., and Crosson, R. S., 1989, Tomographicinversion for three-dimensional velocity structure atMount St. Helens using earthquake data: Journal ofGeophysical Research, v. 94, p. 5,716-5,728.

National Research Council, 1996, National ScienceEducation Standards: Washington, DC, NationalAcademy of Sciences Press, 262 p.

Pallister, J. L., Hoblitt, D. R., Crandell, D. R., andMullineaux, D. R., 1992, Mount St. Helens a decadeafter the 1980 eruption: magmatic models and arevised hazards assessment: Bulletin of Volcanology,v. 54, p. 126-146.

Peterson, D. W., 1988, Volcanic hazards and publicresponse: Journal of Geophysical Research, v. 93, p.4161-4170.

Rutherford, M. J., and Devine, J. D., 1988, The May 18,1980, eruption of Mount St. Helens, 3, Stability andchemistry of amphibole in the magma chamber:Journal of Geophysical Research, v. 93, p.11,949-11,959.

Rutherford, M. J., and Hill, P. M., 1993, Magma ascentrates from amphibole breakdown: an experimentalstudy applied to the 1980-1986 Mount St. Helenseruptions: Journal of Geophysical Research, v. 98, p.19,667-19,685.

Rutherford, M. J., Devine, J. D., and Barclay, J., 1998,Changing magma conditions and ascent rates duringthe Soufriere Hills eruption on Montserrat: GSAToday, v. 8, p. 2-7.

Tilling, R. I., 1989, Volcanic hazards and their mitigation:progress and problems: Reviews of Geophysics, v.27, p. 237-269.

Wiggins, G., 1989, A true test: Toward more authenticand equitable assessment: Phi Delta Kappan, v. 70, p.703-713.


Happy the man whose lost it is to knowThe secrets of the earth. He hastens notTo work his fellows’ hurt by unjust deedsBut with rapt admiration contemplatesImmortal Nature’s ageless harmonyAnd how and when her order came to be,Such spirits have no place for thoughts of shame.

Euripides

magma ascent rates from mineral reaction rims...

Documents