 The molten, or
fluid, rock erupting from the volcano at the left is lava. When the
volcano stops erupting, there will still be plenty of molten rock inside and just below
the volcano, and that "un-erupted lava" is known as magma. Both
lava and magma can be composed of very different mixtures of minerals. When lava and magma
cool, they solidify into igneous rock, and the nature of that rock depends largely on what
minerals the lava or magma were made of.
When magma cools very slowly inside a volcano, crystals have plenty of time to develop and grow, and you get a kind of rocky mush. When the rocky mush solidifies, the resulting rock is often called porphyry.
Sometimes plutonic rocks are much darker than granite, though the texture is the same, and these are usually known as diorite. |
The kind of rock that develops from lava and magma also depends on how fast
the lava or magma cools. The faster it cools, the smaller will be the crystal structures
composing the resulting rocks. If lava falls into water, for instance, it will cool and
harden very fast, relatively speaking, forming the glasslike rock called obsidian.
Lava that cools a little slower, so that you get a blackish rock that's not glassy but
rather fine-grained, like glued-together fine sand, is mostly basalt. If
lava hardens into a light-colored, fine-grained rock, then that's usually called felsite.
There's another group of igneous rock, those formed from magma
that gathers deep within the earth, often over large areas, and of course that magma cools
so slowly that its crystals become quite large. These are called plutonic rocks
and the most famous of this group is granite, a close-up shot of which is
shown at the right. Notice the rock's lumpiness. The larger white masses are crystals of
quartz and orthoclase feldspar, and the dark spots are dark minerals.