*information presented comes from Earth Science published by Glencoe McGraw-Hill

Volcano Note


I. Magma

A. All volcanoes are fueled by magma


1. Magma forms deep within the Earth when temperatures are high enough (800C to 1200C) to melt rock

a. Besides temperature, pressure and presence of water contribute to the melting of rocks

b. a wet rock will melt at a lower temperature than the same rock when dry, regardless of pressure


2. Types of magma

CompositionSource Material
Basaltic Magma
Upper mantle
Andesitic Magma
Oceanic crust and oceanic sediments Intermediate Intermediate
Rhyolitic Magma
Continental crust
High Greatest

3. Viscosity: the internal resistance to flow. The higher the viscosity, the slower the lava flow.

II. Volcanoes

A. Anatomy of a volcano


1. Lava: magma once it reaches the earth's surface

2. Vent: opening in the crust where lava erupts

3. Crater: a bowl-shaped depression at the top of a volcano around the vent

4. Calderas: A depression much larger than a crater; forms when the top or side of a volcano collapses into the magma chamber

B. Types of Volcanoes


Volcano type
Shape Lava Type
Shield volcano
broad, gently sloping sides, circular base
layers of basaltic lava
Hawaiian Islands
Cinder-cone volcano
steep sides, generally small
basaltic and/or andesitic
greater than shield volcanoes
Izalco volcano in El Salvador
Compositemuch larger than cinder-cone volcanoes
violently explosive, greatest danger to humans
Mount St. Helens, WA

C. Locations

1. Most volcanoes form at plate boundaries

a. 80% at convergent boundaries

b. 15% at divergent boundaries

c. 5% occur far from plate boundaries


2. Hot spots: unusually hot regions of Earth's mantle where high temperature plumes of mantle material rise toward the surface

a. volcanoes far from plate boundaries are a result of hot spots

b. Hawaiian islands are a result of hot spots

Do NOT follow this link or you will be banned from the site!

Non-profit Tax ID # 203478467