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Let's review each of the main rock classes in more detail. First, let's look
at
IGNEOUS ROCKS
Igneous rocks are formed from molten rock -
called magma below the surface and lava above the surface.
Igneous rocks can be further categorized
into
Extrusive and Intrusive
| Extrusive igneous rocks are rocks that cool on the
Earth's surface from lava. The relatively cool temperature of the Earth's
surface (compared to the interior) cools the lava quickly - because of this,
crystals in the rock are not allowed to grow very big. When examining extrusive
igneous rocks, no or very small crystals can be seen. The kind of rock depends
on the chemical composition of the lava. If the lava is high in iron and
magnesium, the lava is very fluid, and a rock called basalt, a very dark or
black rock is formed. This rock is very dense, and often has holes in it (from
gas bubbles when the lava was extruded). If the lava has a higher silica
content, the lava is much less fluid and a rock called rhyolite is formed. |
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| This is basalt. Notice the dark color and lack of visible
crystals. There are also holes in this rock formed from gas bubbles in the
lava when it solidified. Basalt commonly flows great distances from a
volcano due to its fluid nature. It forms caves called lava tubes. |
| Below is a picture of a lava flow which is cooling to
form basalt. This type of basalt is called 'aa', due to it's chunky
appearance. |
Here is another lava flow that will cool to form
basalt. This lava flow is much more fluid and will form a basalt
called pahoehoe (that's pa-hoy-hoy). |
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| This is rhyolite. It is formed from thick lava high in
silica (hence the lighter color). There are some crystals visible, but the
are very small and surrounded by a tan matrix of non-crystal material. |
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| This is how pumice forms. Lava is ejected into the
air from a volcano. As the lava falls through the air, it is cooled and
incorporates trapped air leading to a rock with lots of holes and a very
low density - so low that it floats in water! Hit the refresh button to
see a movie of pumice forming from a volcano!!! |
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Intrusive rocks cool deep within the Earth, which allows larger crystals to
form. Here are some examples:
| This is diorite (the salt and pepper rock). Note the
interlocking white and black crystals. |
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| This is granite, recognizable by its pink, white and clear
crystals. Granite is a light-colored rock overall, even though there are some dark
crystals in it. Granite may also be devoid of the pink feldspar crystals,
in which case it looks like diorite (above) except that there are more
light colored crystals than dark. Granite is chemically similar to Rhyolite - the only difference
is the method of cooling. Granite cools slowly underground allowing the crystals
you see to grow. Rhyolite cools quickly. |
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Time to move on...
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