Download - Shield Volcanoes Basaltic Volcanoes
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 surgesVulcano, 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