Recognizing Rocks: Igneous, Sedimentary, and Metamorphic Types - Geology - page 76

Igneous rocks form when molten (melted) rock cools; it changes from a liquid to a solid,
or crystallizes. If the cooling process occurs underground, the melted rock (which is
called magma ) cools to form intrusive or plutonic igneous rocks. If the process occurs
above ground, such as from a volcanic eruption, the melted rock (which is called lava )
cools to form extrusive or volcanic rocks.
If you find the difference between intrusive and extrusive rocks confusing,
keep in mind that the names are telling you where the rocks are formed: Intrusive
rocks are formed below the surface, while extrusive rocks are formed above the
surface.
In the sections that follow, I show you how various igneous rocks are classified and the
process that molten rock goes through to create them. I then provide some basic inform-
ation about volcanoes — the geographic features that spout lava and create extrusive
rocks. Finally, I introduce some underground igneous rock features called plutons.
Classifying igneous rocks
Igneous rocks are classified, or named, according to their texture and mineral content.
How quickly the molten material cools into a solid (rock) determines the texture. The
elements in the magma or lava determine the mineral content (and color) of the rocks
that it creates.
Textures of igneous rocks
Texture is the appearance of the rock — and to some extent how it feels when you touch
it. It describes the size of the individual minerals in the rock. The texture of an igneous
rock provides clues to how and where the rock was formed, including how long it took
to cool from the magma (or lava), and the mineral content of the magma (or lava). As a
general rule, igneous rocks that form underground have larger minerals than ones do
that form above ground.
Following are the terms commonly used to describe the texture of igneous rocks. For
each term, I indicate whether it usually describes an intrusive (I) or extrusive (E) rock:
Phaneritic (I): An igneous rock with crystals that are large enough to see without a
microscope is called phaneritic. This texture is created when the rock forms deep
underground from a magma that cools very slowly. The slow cooling time allows
the crystals to grow, transforming liquid elements into solid mineral form (crys-