are termed intrusive, or plutonic rocks. However, if the magma does not solidify before reaching the earth's
surface, the molten material that pours out onto the surface is called lava, and the rocks that crystallize from the
lava are termed extrusive or volcanic rocks. Because intrusive igneous rocks crystallize at depths where
conditions of temperature and pressure are not drastically different from those under which the original magma
formed, the cooling rate is relatively slow and coarse-grained, or phaneritic, textures develop. On the other hand,
lava cools very quickly upon exposure to atmospheric temperatures that are considerably lower than the initial
temperatures of the molten material. As a result, there is not sufficient time for extrusive igneous rocks to
develop large crystals; rather, a fine-grained, or aphanitic texture is formed. Three types of aphanitic textures
occur: stony (the rock consists of individual mineral grains that cannot be discerned with the unaided eye), glassy
(the rock cooled very rapidly, prohibiting the formation of any crystals), and frothy (the rock formed while gases
were escaping from the lava so that small cavities, or vesicles, have been preserved). The texture of an igneous
rock, coupled with its mineral content (roughly how light or dark the mineral is), is used to determine the rock
type (see figure 1-2).
Figure 1-2. Classification of igneous rocks
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