Shield Volcanoes Basaltic · PDF file 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

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)

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 surges Vulcano, ItalyVulcano, Italy

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

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

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