RHYOLITE. Rhyolite is the extrusive equivalent of granite and, like granite, is composed
essentially of the light-colored silicates (see Figure 3.11). This fact accounts for its color,
which is usually buff to pink or occasionally very light gray. Rhyolite is fine-grained and
frequently contains glass fragments and voids, indicating rapid cooling in a surface
environment. When rhyolite contains phenocrysts they are small and composed of either
quartz or potassium feldspar. In contrast to granite, which is widely distributed as large
plutonic masses, rhyolite deposits are less common and generally less voluminous.
Yellowstone Park is one well-known exception. Here, rhyolite lava flows and thick ash
deposits of similar composition are extensive.
OBSIDIAN. Obsidian is a dark-colored glassy rock that usually forms when silica-rich lava
is quenched quickly (FIGURE 3.13). In contrast to the orderly arrangement of ions characteristic of minerals, the ions in glass are unordered. Consequently, glassy rocks such as
obsidian are not composed of minerals in the same sense as most other rocks.
Although usually black or reddish-brown in color, obsidian has a composition that is
more akin to light-colored igneous rocks such as granite, rather than to dark rocks such as
D I D Y O U K N O W ?
Although Earth’s upper mantle is often
depicted in diagrams as a reddishcolored layer to show its high
temperature, its color is actually a dark
green, but how do we know it is green?
On occasion, unmelted fragments of
mantle rocks are brought to Earth’s
surface by rapidly rising magma and are
ejected from volcanoes. These mantle
rocks, called peridotite, are composed
mainly of olivine, which, as the name
suggests, is olive green, and pyroxene,
which is blackish green.
FIGURE 3.12 Rocks contain information about the processes that produce them.
This mass of granitic rock in Yosemite National Park, California, was once a
molten mass deep within Earth’s crust. (Photo by Enrique R. Aguirre/agefotostock)
73
Naming Igneous Rocks
essentially of the light-colored silicates (see Figure 3.11). This fact accounts for its color,
which is usually buff to pink or occasionally very light gray. Rhyolite is fine-grained and
frequently contains glass fragments and voids, indicating rapid cooling in a surface
environment. When rhyolite contains phenocrysts they are small and composed of either
quartz or potassium feldspar. In contrast to granite, which is widely distributed as large
plutonic masses, rhyolite deposits are less common and generally less voluminous.
Yellowstone Park is one well-known exception. Here, rhyolite lava flows and thick ash
deposits of similar composition are extensive.
OBSIDIAN. Obsidian is a dark-colored glassy rock that usually forms when silica-rich lava
is quenched quickly (FIGURE 3.13). In contrast to the orderly arrangement of ions characteristic of minerals, the ions in glass are unordered. Consequently, glassy rocks such as
obsidian are not composed of minerals in the same sense as most other rocks.
Although usually black or reddish-brown in color, obsidian has a composition that is
more akin to light-colored igneous rocks such as granite, rather than to dark rocks such as
D I D Y O U K N O W ?
Although Earth’s upper mantle is often
depicted in diagrams as a reddishcolored layer to show its high
temperature, its color is actually a dark
green, but how do we know it is green?
On occasion, unmelted fragments of
mantle rocks are brought to Earth’s
surface by rapidly rising magma and are
ejected from volcanoes. These mantle
rocks, called peridotite, are composed
mainly of olivine, which, as the name
suggests, is olive green, and pyroxene,
which is blackish green.
FIGURE 3.12 Rocks contain information about the processes that produce them.
This mass of granitic rock in Yosemite National Park, California, was once a
molten mass deep within Earth’s crust. (Photo by Enrique R. Aguirre/agefotostock)
73
Naming Igneous Rocks