Shield Volcanoes Basaltic Volcanoes · Basaltic Volcanoes References: Encyclopedia of Volcanoes,...

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Basaltic Volcanoes Basaltic Volcanoes References: Encyclopedia of Volcanoes, pp. 283-289 Shield Volcanoes Shield Volcanoes Low angle volcano constructed principally of basalt lavas Examples: Mauna Loa and Kilauea, Hawaii Thin lava flows Summit calderas May have lateral rift zones May have lava lakes in the crater Skaldbreidur Skaldbreidur Mauna Kea Mauna Kea Mount Etna, Italy Mount Etna, Italy Stratovolcanoes Stratovolcanoes A volcano constructed of alternating layers of lava and pyroclastic materials Steep slope angles (~35°) Examples: Fuji, Japan and Kliuchevskaya, Kamchatka May exhibit persistent activity (Etna, Sicily) Volumes of erupted lava proportional to repose time

Transcript of Shield Volcanoes Basaltic Volcanoes · Basaltic Volcanoes References: Encyclopedia of Volcanoes,...

Page 1: Shield Volcanoes Basaltic Volcanoes · Basaltic Volcanoes References: Encyclopedia of Volcanoes, pp. 283-289 Shield Volcanoes • Low angle volcano constructed principally of basalt

Basaltic VolcanoesBasaltic Volcanoes

References:Encyclopedia of Volcanoes, pp. 283-289

Shield VolcanoesShield Volcanoes

• Low angle volcano constructed principally of basalt lavas

• Examples: Mauna Loa and Kilauea, Hawaii• Thin lava flows• Summit calderas• May have lateral rift zones• May have lava lakes in the crater

SkaldbreidurSkaldbreidur Mauna KeaMauna Kea

Mount Etna, ItalyMount Etna, Italy

StratovolcanoesStratovolcanoes

• A volcano constructed of alternating layers of lava and pyroclastic materials

• Steep slope angles (~35°)• Examples: Fuji, Japan and Kliuchevskaya,

Kamchatka• May exhibit persistent activity (Etna, Sicily)• Volumes of erupted lava proportional to

repose time

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Stratovolcano structure PopocatepetlPopocatepetl

Cones in Arc SettingsCones in Arc Settings

• Tholeiitic or calc-alkaline compositions• Merge to andesites• Continuous feeding from depth into a

shallow chamber• May be associated with fissures and

monogenetic scoria cones• Cones may be truncated by edifice collapse

– Example: Augustine, Alaska

MountMount AugustineAugustine

Facies at a stratovolcano Monogenetic Volcano FieldsMonogenetic Volcano Fields

• Composed of volcanoes that erupt only once

• Common edifice is a scoria cone• Tuff rings and small shields are also

common• Example: Pinacate, Mexico• Vent systems are dikes or pipes (diatremes)

Page 3: Shield Volcanoes Basaltic Volcanoes · Basaltic Volcanoes References: Encyclopedia of Volcanoes, pp. 283-289 Shield Volcanoes • Low angle volcano constructed principally of basalt

Scoria ConesScoria Cones

Tuff RingsTuff Rings

Pillow LavasPillow Lavas

Scoria ConesScoria Cones• Simplest and commonest volcanic form

• Characterized by three parameters

– Height, width, crater width

• Standard initial slope of 30o

• Conical shape

• Occur in several environments

Sunset Crater, ArizonaSunset Crater, Arizona

Scoria Cone SectionScoria Cone Section

DiatremesDiatremes

• Breccia pipes

• Kimberlite

• Contains diamonds

• Ultramafic magmas

• Mixture of rocks

• Driven by deep CO2

Dia tre m eDia tre m e

Tuff ConesTuff Cones

• Massive deposits

• Thickly bedded

• Palagonitized

• Bedding up to 30o

• Wet surgesVulcano, ItalyVulcano, Italy

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Tuff RingsTuff Rings

• Thinly-bedded

• Poorly-indurated

• Beds less than 12o

• Sandwave beds

• Dry surges

C e rro C o lor ad o, M e x ic oC e rro C o lor ad o, M e x ic o

Basaltic Scoria Cone FieldsBasaltic Scoria Cone Fields

• 10s to 1000s of cones• General elliptical shape• Aspect ratio of 2:1 to 5:1• 10 to 70 km in length• Areas of extensional tectonics• Elongate perpendicular to σ3

• Widespread in western USA• Pinacate example

San Francisco Volcanic Field

San Francisco Volcanic Field

Small FieldsSmall Fields

• North rim of Grand Canyon

• Scoria cones aligned along fault planes

Flood Basalt FieldsFlood Basalt Fields

• Extensive sheets of lava flows• Flood the landscape• Associate with crustal dilation• May coincide with mass bioextinctions• Examples: Deccan, India and Columbia

River Plateau, USA

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Flow UnitsFlow Units

• Compound lava flows• Overlapping pahoehoe flows• Interconnected lava shields• Lava ponds in depressions• Inward directed dips of large fields• Cut by numerous dike swarms+

Columnar JoinColumnar Join

Central VolcanoesCentral Volcanoes

• A volcano that erupts magmas of various compositions– Mafic to silicic

• Many have a central caldera composed of silicic rocks and rhyolite domes

• A shallow magma chamber is part of the eruptive system

ShastaShasta

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Volcano InteriorVolcano Interior

• The eroded core of these volcanoes may expose a granite pluton

• Feeding system may be a ring complex• These may be a host for economic mineral

deposits

Alteration ZonesAlteration Zones

Albite, epidote, Chlorite

Propylitic

ChloriteChloritic

Clays, no micaArgillic

MicaSericitic

AdulariaPotassic

SilicaSilicic