ES 104: Laboratory # 7
IGNEOUS ROCKS
Introduction
Igneous rocks form from the cooling and crystallization of molten
rock material. This can occur below the surface of the earth forming
intrusive rocks (also called plutonic rocks) or on or above the surface
as extrusive rocks (also called volcanic rocks). As a generalization,
extrusive igneous rocks cool rapidly when compared with intrusive
igneous rocks.
The rate of cooling has a profound influence the size of the crystal
grains formed in the igneous rock, so the textures of igneous rocks
tell us much about the rate of cooling of the rock and thus whether
their origin is plutonic or volcanic. If cooling is slow, atoms have
plenty of time to migrate to the growing nuclei of growing crystals,
thereby enlarging these mineral grains. So, a longer cooling time
results in larger crystal grains. Intrusive igneous rocks have coarse-
grained textures, including pegmatitic (crystals larger than 10 mm).
Because of their rapid cooling, extrusive igneous rocks generally
have fine-grained textures, including porphyritic, glassy, and frothy.
In addition to texture, the mineral composition also determines
the appropriate classification of an igneous rock. Determining
composition of an igneous rock is not always easy, particularly
because as the texture gets finer, the individual mineral grains
become too small to be seen easily. In these cases, the color of a
rock becomes helpful. As a general rule, dark rocks are typically
mafic and light colored rocks are typically felsic. NOTE: be aware
that there are a few important exceptions to this general rule.
Be sure to define the terms in italics in your notes.
Objectives
• Learn to recognize the major types of igneous rocks
• Understand the significance of texture and composition in the
formation of igneous rocks
