Intrusive Igneous Rock

INTRUSIVE IGNEOUS ROCK

Intrusive rocks involve magma below the surface. Magma isn't as dense as rock so it will work its way toward the earth's surface, if possible. It will fill in any existing cracks and melt the rocks with cracks in it. This is called country rock. Country rock might fall into magma, which is called stopping of the rock. These melted fragments are called xenoliths. Look for odd-shaped black mafic rock within harder rock like granite to find xenoliths. Figure 30 shows one of these:

Figure 30.

A pluton is a piece of magma that worked its way to the surface but did not make a volcano. Some will be imbedded in country rock. A big pluton with an irregular shape is a batholith or a stock. A batholith has a large, exposed area of more than 100 kilometers squared, while a stock has a smaller surface area exposed – less than 100 kilometers squared. A large area of batholiths is the Coast Range Plutonic Complex – starting in Vancouver and ending in the southeastern region of Alaska.