Figure 4.12: A close-up of a granite sample. Notice the black and white portions. Each color
represents a different mineral in the rock. You can easily see the mineral crystals that make
up this intrusive igneous rock. ( 22 )
a glass, such as obsidian (Figure4.14). Others, such as pumice, contain holes where gas
bubbles were trapped when the material was still hot and molten. The holes make pumice
so light that it actually floats in water. The most common extrusive igneous rock is basalt,
a rock that is especially common below the oceans (Figure4.15).
Figure 4.13: Extrusive igneous rocks form after lava cools above the surface. The lava spills
out from the Earth at a volcano. ( 14 )