Volcanoes 2

Volcanoes occur at?

Plate boundaries, hotspots

Volcanoes occur at what types of plate boundaries?

divergent or convergent

Mid ocean ridge

• most volcanism on earth

• not exposed at earths surface except iceland b/c presence of hot spot

Subduction volcanoes (continental)

• felsic and intermediate magmas/lavas

“local” continental volcanic rock

• subduction of juan de fuca under north american plate

• has active plates

Oceanic Volcanic arcs

• mafic magma

• volcanic island arc

• ex. western aleutian arc

Why are volcanes typically explosive?

they erupt gas-rich magmas

Hot spot definition

• mafic magma from a plume of hot mantle

• plumes are stationary and pulsatory

Cinder cones: (felsic/mafic)-(exp/non.exp)

mafic, explosive

Shield cones: (felsic/mafic)-(exp/non.exp)

mafic, non explosive

Stratovolcano: (felsic/mafic)-(exp/non.exp)

intermediate, felsic, mixed

Calderas: (felsic/mafic)-(exp/non.exp)

felsic, explosive

Cinder Cone info

• layers of pyroclastic eject from fire fountaining

• mafic/ basalt

• angle of repose 30-40•

Cinder cone activity

• small volcanoes that never grow up

• usually erupt for a few years then never again

• 2km by 500m

Shield Volcano info

• lava erupts from fissure, runs down gentle slopes, cooling

• mafic lava flow

• Flows: Aa and Pahoehoe

• usually not explosive

• 100km by 4-10km

Shield volcano activity

erupts often

Stratovolcano info

• interbedded lava flows, pyroclastic flows, lahars

• usually intermediate or felsic

• 10 km by less than 5km

Stratovolcano activity

• frequently explosive

• viscous magma

• may stay activity for 100,000 years

Calderas info

• created from large explosive eruptions of pyroclastic material

• when roof of magma chamber collapses

• different from a crater

Frequency of eruptions

1) cinder cone —> frequent

2) shield volcanoes —> frequent continuous

3) stratovolcanoes —> frequent

4) calderas —> rare

Cinder cone examples

• paracutin, mexico

• elfin lakes and opal cone, garibaldi park

• hawaii

Shield volcano examples

• mauna loa, hawaii

Stratovolcano examples

• mt. st helens

• mt.garibaldi

• mt. baker

Calderas examples

• crater lake, OR

• Yellowstone, WY