Calc-alkaline%20Volcanic%20Rocks

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Calc-alkaline Volcanic Rocks

• Petrography
• Processes
• Field relations
• Volcanic arcs
• Petrogenesis

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Petrography

• Fabric
• Classification
• Alteration

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Fabric

• Aphanitic
• Rapid cooling
• Loss of volatiles
• Glassy (vitric)
• Chilling of viscous magma
• Clastic
• Fragmented due to rapid, violent eruption

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Compositional Classification

• Rhyolite
• Rhyodacite
• Dacite
• Andesite
• Latite
• Basalt

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Textural Classification (glassy silicic types)

• Obsidian
• Perlite
• Pumice
• Vitrophyre

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Volcaniclastic Classification

• Epiclastic
• Transport by earths hydrologic system
• Volcanic sandstone, shale, etc
• Greywacke
• Lahar

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Pyroclastic Classification

• Components
• Vitric, crystal, lithic
• Size
• Ash, lapilli, blocks bombs
• Types
• Tuff
• Welded tuff
• Breccia

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Alteration

• Deuteric alteration
• Occurs as materials cool after emplacement
• Hydrous minerals may decompose
• Due to reduction in pressure
• Fe-Ti dusty rims on reddish pseudomorphs

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Alteration

• Hydrothermal alteration
• Forms due to circulation of hot ground water
• Propylitic alteration
• Formation of hydrous minerals
• Chlorite, amphibole, epidote, phrenite
• Associated with some ore bodies

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Extrusive Processes

• Lava flows and domes
• Thicker and shorter than for basalts
• Due to higher viscosity
• Mono Domes
• Dacite lava flows

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Pyroclastic Processes

• Eruptive column
• Pyroclastic fall
• Pyroclastic flow
• Pyroclastic surge

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Ash-flow Sheets

• Flow units
• Cooling units
• Welded tuffs

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Morphology

• Controlled by topography
• Fill depressions
• Even upper surface
• Valley ponded deposits
• Veneer deposits
• Multiple lobes and fans
• Lateral levees

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Flow Unit Standard Section

• Layer 1 (ground layer or surge)
• Layer 2 (flow unit)
• Layer 2a (fine-grained basal)
• Layer 2b (main body of flow)
• Layer 3 (ash cloud)

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Welded Tuff

• Degree of welding
• Non-welded
• Partially welded
• Densely welded
• Density is a good index
• Welding (density) profiles

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Partly Welded Bishop Tuff
Partly Welded Bishop Tuff
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Welding Profiles

• Density plotted vs. elevation
• r 1.0 at base and top
• r maximum value near center
• Erosion easily removes upper part
• Welding f(temperature, Pressure)

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Vitrophyre, Armenia
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Secondary Mineralization

• Vitrophyre
• Devitrification
• Lithophysae
• Vapor-phase crystallization
• Zeolitization

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Compositional Zoning

• Initial eruptions
• Crystal-poor rhyolites
• Crystal-rich latites or dacites
• Related to zoned magma chambers
• Highly-evolved upper parts
• More primitive lower parts
• Evidence in banded pumice

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Calderas

• Cauldron subsidence
• Resurgent calderas
• Caldera complexes
• Origin of silicic calderas
• Basaltic calderas
• Active calderas

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Collapse Calderas

• Atitlan, Guatemala
• Crater Lake, Oregon
• Ksudach, Russia
• Toya Caldera, Japan
• Thera, Greece
• Taupo, New Zealand

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Caldera Characteristics

• Simple circular form
• Diameter gt 2 km
• V of collapse V of tephra
• Steep walls
• Collapse megabreccia
• Generally a lake

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Model of a collapse Caldera
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Sector Collapse Scarps

• Horseshoe shape
• Open towards debris apron
• Gravitational mechanism
• Associated with andesite cones
• Van Bemmelen model
• Mount St. Helens

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Mount St. Helens, WA
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Stages in Resurgent Calderas

• Tumescence
• Early rhyolite dome

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Stages in Resurgent Calderas

• Main events
• Plinian fall
• Ash flow
• Collapse
• Resurgent domes
• Geothermal stage

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Origin of Silicic Caldera Complexes

• Large calderas are only in continental crust
• Basaltic under plating plays a role
• Silicic magmas rise towards the surface
• Mixed magmas are evidence
• Marginal basaltic eruptions
• Zoned magma chambers
• Sequential tapping of evolving magma

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De Silva Model
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Large Composite Volcanoes

• Generally polygenetic
• Simple cones
• Composite cones
• Compound volcanoes
• Volcano complexes

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Simple Cones

• Single summit vent
• Small crater (lt200 m diameter)
• Radial symmetry
• Slopes gt 40o near summit
• Concave profiles
• Height of a volcano is limited
• May grow to 3000 m
• Mass eruption rate is a control

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Popocatepetl, Mexico
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Continental Rifts

• Afar example
• Red Sea
• Gulf of Aden
• African rift zone
• Mechanism
• Extension or thermal anomaly first?

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Convergent Zones

• Island arcs
• Oceanic/oceanic crust
• Oceanic/continental crust
• Continental margins
• Oceanic/continental crust
• Continent/continent collisions
• Himalayan Mts.

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Convergent Plate Mechanisms

• Dipping Benioff zone
• Earthquakes down to 600 km depth
• Subducted slab
• dehydrates providing rising fluids
• heats as it goes down
• Overlying mantle wedge
• partially melted by rising fluids

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Processes and Products

• Partial melting in slab and mantle wedge
• Fractional crystallization of magmas
• Assimilation of crustal material
• Formation of large magma chambers
• calderas
• batholiths
• Chemically evolved products
• Andesites and rhyolites are common

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Arc Volcanic Petrogenesis

• Magmas more felsic and diverse on continental
  crust
• Suggests involvement of continental sial
• Restricted to basalt and andesite on oceanic
  plates
• Implies mantle derived magmas

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Prominent Models

• Partial melting of the lower continental crust
• Partial melting of the subducting oceanic slab
• Melting of Peridotite in the mantle wedge