1) What are intrusive and extrusive rock?

2) What are igneous rock structures and how are they made?

3) What are the types of volcanoes and how are they similar and different?

Dear Kip,

1) Intrusive igneous rocks form by the solidification of magma below the Earth's surface. Extrusive igneous rocks form by the solidification of magma on the Earth's surface.

2) Probably the most common igneous rock structures are plutons, batholiths, stock, dikes, and sills. A pluton is any body of intrusive igneous rock. Plutons are masses of solidified magma that are exposed after erosion removes the overlying rocks. A batholith is a pluton that is more than 40 square miles in size. The Sierra Nevada batholith in California is an example. A stock is a pluton that is less than 40 square miles in size. A dike is a tabular body of igneous rock that cuts across layered rocks. A sill is a tabular body of igneous rock that intrudes parallel to layered rocks.

3) Of the 1,511 known volcanoes that have erupted in the last 10,000 years: 699 (46%) were stratovolcanoes, 164 (11%) were shield volcanoes, 105 (7%) were submarine volcanoes, 92 (6%) were volcanic fields, 87 (6%) were cinder cones, 83 (6%) were calderas.

The other 281 (18%) are complex volcanoes, lava domes, pyroclastic cones, compound volcanoes, cones, crater rows, explosion craters, fissure vents, fumarole fields, hyrothermal fields, lava cones, maars, pumice cones, pyroclastic shields, scoria cones, somma volcanoes, subglacial volcanoes, tuff cones, tuff rings, or a volcanic complex.

All of these volcanoes are made of lava, pyroclasts or a combination of lava and pyroclasts. They differ in shape, size, how they erupt, and how long they erupt.

Steve Mattox, University of North Dakota

Sources of Information:
Simkin, T., and Siebert, L., 1994, Volcanoes of the World: Geoscience Press, Tucson, Arizona, 349 p.

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