VOLCANOES AND VOLCANISM

SUBAQUEOUS ERUPTIONSSubmarine eruptions

Quiet and effusive.May produce submarine shield volcanoes.If deep enough, water pressure prevents gas

escape.Produces pillow structures.

Shallow eruptions can be explosive.Contact of water with hot magma produces lots of steam

and consequently tephra.

VOLCANOES AND VOLCANISM

SUBGLACIAL ERUPTIONS

Mafic lava erupts under glaciers.Causes rapid melting of snow and ice.

Large amounts of meltwater is freed.

Bardabunga and Grimvötn volcanoes erupted underVatnajökull on Iceland in 1996.

Outburst flood occurred.Termed JOKULHLAUP.

VOLCANOES AND VOLCANISM

JOKULHLAUP

VOLCANOES AND VOLCANISM

PYROCLASTIC ERUPTIONS

Involve viscous, gas-rich magmas.Produce large amounts of tephra.Area receives large amounts of hot ash, nuée ardente,

and hot lahars.

VOLCANOES AND VOLCANISM

Mt. Vesuvius & Herculaneum, Italy, 79 A.D.

VOLCANOES AND VOLCANISM

With little gas or water felsic magmas will produce a VOLCANIC DOME in the crater.

If pressure is not released, massive eruptions occur.Produces CALDERA.

Mt. Mazama, ORKrakatau, IndonesiaMt. Desert Island, MEMt. St. Helens, WA

VOLCANOES AND VOLCANISM

Mt. St. Helens, WA

Crater Lake and Wizard Island (Mt. Mazama), OR

VOLCANOES AND VOLCANISM

PYROCLASTIC VOLCANIC CONES

Structure and shape depends on magma type.Intermediate and felsic volcanoes produces large

amounts of tephra.Tephra generally accumulates close to vent.Forms steep-sided cinder piles.The cinders are covered with the next lava flow event.Process repeats.Produces COMPOSITE or STRATOVOLCANO.Possesses alternating layers of lava and pyroclastics.

Mt. Hood, OR , Mt. Ranier, WA, Mt. Shasta, CA

VOLCANOES AND VOLCANISM

PYROCLASTIC VOLCANIC CONES

COMPOSITE or STRATOVOLCANO

Mt. Hood, OR

VOLCANOES AND VOLCANISM

PYROCLASTIC CONES or CINDER CONES

Composed entirely of volcanic cinders.Generally small volcanoes <1000 m (~3000’) in height.Have steep sides.No intervening lava between cinders.Easily eroded.Can form from any type of magma as long as enough

gas exists to keep blowing particles up into theatmosphere.

Sunset Crater, AZ and Paricutin, Mexico

VOLCANOES AND VOLCANISM

PYROCLASTIC CONES or CINDER CONES

Sunset Crater, AZ

VOLCANOES AND VOLCANISM

PYROCLASTIC CONES or CINDER CONES

Paricutin, Mexico

Volcano Explosivity Index (VEI)