Calc-alkaline Volcanic RocksCalc-alkaline Volcanic Rocks

PetrographyPetrography

ProcessesProcesses

Field relationsField relations

Volcanic arcsVolcanic arcs

PetrogenesisPetrogenesis

PetrographyPetrography

FabricFabric

ClassificationClassification

AlterationAlteration

FabricFabric

AphaniticAphanitic• Rapid coolingRapid cooling• Loss of volatilesLoss of volatiles

Glassy (vitric)Glassy (vitric)• Chilling of viscous magmaChilling of viscous magma

ClasticClastic• Fragmented due to rapid, violent eruptionFragmented due to rapid, violent eruption

CompositionalCompositional Classification Classification

RhyoliteRhyolite

RhyodaciteRhyodacite

DaciteDacite

AndesiteAndesite

LatiteLatite

BasaltBasalt

TexturalTextural Classification Classification (glassy silicic types) (glassy silicic types)

ObsidianObsidian

PerlitePerlite

PumicePumice

VitrophyreVitrophyre

VolcaniclasticVolcaniclastic Classification Classification

EpiclasticEpiclastic

• Transport by earth’s hydrologic systemTransport by earth’s hydrologic system

• Volcanic sandstone, shale, etcVolcanic sandstone, shale, etc

• GreywackeGreywacke

• LaharLahar

PyroclasticPyroclastic ClassificationClassification

ComponentsComponents• Vitric, crystal, lithicVitric, crystal, lithic

SizeSize• Ash, lapilli, blocks & bombsAsh, lapilli, blocks & bombs

TypesTypes

• TuffTuff

• Welded tuffWelded tuff

• BrecciaBreccia

AlterationAlteration

Deuteric alterationDeuteric alteration• Occurs as materials cool after emplacementOccurs as materials cool after emplacement

Hydrous minerals may decompose Hydrous minerals may decompose • Due to reduction in pressureDue to reduction in pressure• Fe-Ti dusty rims on reddish pseudomorphsFe-Ti dusty rims on reddish pseudomorphs

AlterationAlteration

Hydrothermal alterationHydrothermal alteration• Forms due to circulation of hot ground waterForms due to circulation of hot ground water

Propylitic alterationPropylitic alteration• Formation of hydrous mineralsFormation of hydrous minerals• Chlorite, amphibole, epidote, phreniteChlorite, amphibole, epidote, phrenite• Associated with some ore bodiesAssociated with some ore bodies

Extrusive ProcessesExtrusive Processes

Lava flows and domesLava flows and domes

• Thicker and shorter than for basaltsThicker and shorter than for basalts

• Due to higher viscosityDue to higher viscosity

Mono DomesMono Domes

Dacite lava flowsDacite lava flows

Pyroclastic ProcessesPyroclastic Processes

Eruptive columnEruptive column

Pyroclastic fallPyroclastic fall

Pyroclastic flowPyroclastic flow

Pyroclastic surgePyroclastic surge

Ash-flow SheetsAsh-flow Sheets

Flow units Flow units

Cooling unitsCooling units

Welded tuffsWelded tuffs

MorphologyMorphology

Controlled by topographyControlled by topography

Fill depressionsFill depressions

Even upper surfaceEven upper surface

Valley ponded depositsValley ponded deposits

Veneer depositsVeneer deposits

Multiple lobes and fansMultiple lobes and fans

Lateral leveesLateral levees

Flow Unit Standard SectionFlow Unit Standard Section

Layer 1 (ground layer or surge)Layer 1 (ground layer or surge)

Layer 2 (flow unit)Layer 2 (flow unit)

Layer 2a (fine-grained basal)Layer 2a (fine-grained basal)

Layer 2b (main body of flow)Layer 2b (main body of flow)

Layer 3 (ash cloud)Layer 3 (ash cloud)

Welded TuffWelded Tuff

Degree of weldingDegree of welding

• Non-weldedNon-welded

• Partially weldedPartially welded

• Densely weldedDensely welded

Density is a good indexDensity is a good index

Welding (density) profilesWelding (density) profiles

Partly Welded Bishop TuffPartly Welded Bishop Tuff

Partly Welded Bishop TuffPartly Welded Bishop Tuff

Welding ProfilesWelding Profiles

Density plotted vs. elevationDensity plotted vs. elevation

at base and topat base and top

maximum value near centermaximum value near center

Erosion easily removes upper partErosion easily removes upper part

Welding = f(temperature, Pressure)Welding = f(temperature, Pressure)

Vitrophyre, ArmeniaVitrophyre, Armenia

Secondary MineralizationSecondary Mineralization

VitrophyreVitrophyre

DevitrificationDevitrification

LithophysaeLithophysae

Vapor-phase crystallizationVapor-phase crystallization

ZeolitizationZeolitization

Compositional ZoningCompositional Zoning

Initial eruptionsInitial eruptions• Crystal-poor rhyolitesCrystal-poor rhyolites• Crystal-rich latites or dacitesCrystal-rich latites or dacites

Related to zoned magma chambersRelated to zoned magma chambers• Highly-evolved upper partsHighly-evolved upper parts• More primitive lower partsMore primitive lower parts

Evidence in banded pumice Evidence in banded pumice

CalderasCalderas

Cauldron subsidenceCauldron subsidence

Resurgent calderasResurgent calderas

Caldera complexesCaldera complexes

Origin of silicic calderasOrigin of silicic calderas

Basaltic calderasBasaltic calderas

Active calderasActive calderas

Collapse CalderasCollapse Calderas

Atitlan, GuatemalaAtitlan, Guatemala

Crater Lake, OregonCrater Lake, Oregon

Ksudach, RussiaKsudach, Russia

Toya Caldera, JapanToya Caldera, Japan

Thera, GreeceThera, Greece

Taupo, New ZealandTaupo, New Zealand

Caldera CharacteristicsCaldera Characteristics

Simple circular formSimple circular form

Diameter > 2 kmDiameter > 2 km

V of collapse = V of tephraV of collapse = V of tephra

Steep wallsSteep walls

Collapse megabrecciaCollapse megabreccia

Generally a lakeGenerally a lake

Model of a collapse CalderaModel of a collapse Caldera

Sector Collapse ScarpsSector Collapse Scarps

Horseshoe shapeHorseshoe shape

Open towards debris apronOpen towards debris apron

Gravitational mechanismGravitational mechanism

Associated with andesite conesAssociated with andesite cones

Van Bemmelen modelVan Bemmelen model

Mount St. HelensMount St. Helens

Mount St. Helens, WAMount St. Helens, WA

Stages in Resurgent CalderasStages in Resurgent Calderas

TumescenceTumescence

Early rhyolite dome Early rhyolite dome

Stages in Resurgent CalderasStages in Resurgent Calderas

Main eventsMain events• Plinian fallPlinian fall

• Ash flow Ash flow

• CollapseCollapse

Resurgent domes Resurgent domes Geothermal stage Geothermal stage

Origin of Silicic Caldera Origin of Silicic Caldera ComplexesComplexes

Large calderas are only in continental crustLarge calderas are only in continental crust

Basaltic under plating plays a roleBasaltic under plating plays a role

Silicic magmas rise towards the surfaceSilicic magmas rise towards the surface

Mixed magmas are evidenceMixed magmas are evidence

Marginal basaltic eruptionsMarginal basaltic eruptions

Zoned magma chambersZoned magma chambers

Sequential tapping of evolving magmaSequential tapping of evolving magma

De Silva ModelDe Silva Model

Large Composite VolcanoesLarge Composite Volcanoes

Generally polygeneticGenerally polygenetic

Simple conesSimple cones

Composite conesComposite cones

Compound volcanoesCompound volcanoes

Volcano complexesVolcano complexes

Simple ConesSimple Cones

Single summit ventSingle summit vent Small crater (<200 m diameter)Small crater (<200 m diameter) Radial symmetryRadial symmetry Slopes > 40Slopes > 40oo near summit near summit Concave profilesConcave profiles Height of a volcano is limited Height of a volcano is limited May grow to 3000 mMay grow to 3000 m Mass eruption rate is a control Mass eruption rate is a control

Popocatepetl, MexicoPopocatepetl, Mexico

Continental RiftsContinental Rifts

Afar exampleAfar example

• Red SeaRed Sea

• Gulf of AdenGulf of Aden

• African rift zoneAfrican rift zone

MechanismMechanism

Extension or thermal anomaly first?Extension or thermal anomaly first?

Convergent ZonesConvergent Zones

Island arcsIsland arcs

• Oceanic/oceanic crustOceanic/oceanic crust

• Oceanic/continental crustOceanic/continental crust

Continental marginsContinental margins

• Oceanic/continental crustOceanic/continental crust

Continent/continent collisionsContinent/continent collisions

• Himalayan Mts.Himalayan Mts.

Convergent Plate MechanismsConvergent Plate Mechanisms

Dipping Benioff zoneDipping Benioff zone

Earthquakes down to 600 km depthEarthquakes down to 600 km depth

Subducted slabSubducted slab

• dehydrates providing rising fluidsdehydrates providing rising fluids

• heats as it goes downheats as it goes down

Overlying mantle wedgeOverlying mantle wedge

• partially melted by rising fluidspartially melted by rising fluids

Processes and ProductsProcesses and Products

Partial melting in slab and mantle wedgePartial melting in slab and mantle wedge

Fractional crystallization of magmasFractional crystallization of magmas

Assimilation of crustal materialAssimilation of crustal material

Formation of large magma chambersFormation of large magma chambers

• calderascalderas

• batholithsbatholiths

Chemically evolved productsChemically evolved products

Andesites and rhyolites are commonAndesites and rhyolites are common

Arc Volcanic PetrogenesisArc Volcanic Petrogenesis

Magmas more felsic and diverse on continental Magmas more felsic and diverse on continental

crustcrust

• Suggests involvement of continental sialSuggests involvement of continental sial

Restricted to basalt and andesite on oceanic platesRestricted to basalt and andesite on oceanic plates

• Implies mantle derived magmasImplies mantle derived magmas

Prominent ModelsProminent Models

Partial melting of the lower continental Partial melting of the lower continental

crustcrust

Partial melting of the subducting oceanic Partial melting of the subducting oceanic

slabslab

Melting of Peridotite in the mantle wedgeMelting of Peridotite in the mantle wedge