GL 107 Earth Systems Science

K. Hannula

Discussion questions for 9/12/07

1.  Explain the difference between "volcanic" and "plutonic" igneous rocks.

Volcanic igneous rocks form when molten rock cools at the earth's surface.

Plutonic igneous rocks form when molten rock cools underground.

2.  How are the textures in volcanic rocks different from the textures in plutonic rocks?

Volcanic rocks have smaller crystals or no crystals; plutonic rocks have visible crystals.

3.  What is the difference between "felsic" and "mafic" igneous rocks?

Felsic rocks contain more Si, K, and Na; mafic rocks contain less Si and more Fe, Mg, and Ca.  Also, mafic rocks are typically darker in color than felsic rocks.

4.  Define:

• granite: a felsic plutonic igneous rock; typical minerals are quartz, potassium feldspar, some plagioclase, and possibly biotite, muscovite, or hornblende.

• rhyolite: a felsic volcanic igneous rock

• basalt: a mafic volcanic igneous rock

• andesite: an intermediate composition volcanic igneous rock