D. TYPES OF VOLCANOES

1. General - about 550 volcanoes have erupted during historic times.

Active = erupting volcanoes (~50/yr).

Potentially active = evidence of eruption within the past 10,000 years (~2,500).

Dormant = no evidence of eruption within the past 10,000 years.

3 to 5 U.S. volcanoes are usually active at any given time:

Kilauea and Mauna Loa in Hawaii

Mount St. Helens in the Cascades

Mt. Spurr, Redoubt, and Augustine in Alaska

2. Holes

vent - general term for an eruption opening

crater - cone-shaped depression at volcano summit 76

fissure - eruption from a long crack 75 Common in Hawaii

caldera - large collapse-depression 79, 80 Like Crater Lake

3. Shape - determined by viscosity, which depends on composition, and amount of

dissolved gasses. 73

4. Shield volcano - broad, gently sloping mountain produced from fluid basalt. 74

Ex: Kilauea, Mauna Loa, Mauna Kea, Olympus Mons (Mars)

5. Cinder cones - produced by explosively erupted cinders; angle of repose = 30o; small.

Ex: many in the Mojave Desert 76

6. Composite volcano or stratovolcano - form from eruptions of cinders and flows; these volcanoes are steeper at top; usually composed of andesite. Best known volcanoes.

Ex: Shasta, Ranier, Hood, Fuji, Etna, Vesuvius, and Kilimanjaro 66, 67, 77

7. Lava domes - usually composed of viscous, felsic lava - small

Ex: Mount St. Helens (now), Lassen, Mono Domes, and Mt. Unzen

8. Submarine eruptions can form pillow lava.

Mostly located at mid-ocean ridges; mostly basalt; cover the ocean floor.

9. Plateau lavas or flood basalts - large continental areas covered by fluid basalt from extensive fissure eruptions; possibly located above a hot spot or plume.

Ex: Columbia River, Deccan (India), Karoo (South Africa), and the Moon 81

COLUMBIA RIVER BASALT GROUP

area

65,000 mi2

volume

44,000 mi3

average thickness

3,600 ft

number of flows

~311

age

16 to14.5(6)Ma

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