Igneous rocks are
formed by the cooling of molten material called magma. Magma
lies deep below the surface of the earth.
It is a very hot, very,
very thick liquid made mostly of iron, magnesium, aluminum, silicon,
Rocks that Cool Deep
in the Earth
of the earth is very thick. The magma that is near
the surface cools slowly, becoming part of the crust. As it cools,
the minerals crystalize, giving the rocks a slightly rough texture
and/or speckled appearance. Later erosion removes the
older rocks above the newly cooled rock, and the now-solid minerals
appear on the surface.
Examples of igneous rocks that cool slowly
granite, a light
colored speckled rock, sometimes used for buildings
basalt, a darker colored rock
quartz, which may be white, pink (rose quartz), yellowish,
and other colors.
Igneous rocks are hard, and often used for
building because they are durable. The Sierra mountain range
is made of granite. It will take a long time for these
mountains to be eroded away!
Rocks that Cool Quickly
at the Surface
all igneous rocks cool deep in the earth.
hot rock breaks through to the surface. It spurts out and forms
cones, like the one in this picture. The cones are made of cinders
and rock, and can be recognized by their distinctive shape.
the activity that leads to the eruption of hot rock, is often
accompanied by earthquakes. It often occurs where tectonic plates
meet. The area around the Pacific Ocean, called the Ring of Fire,
is dotted with volcanoes, and is earthquake prone. Click here
for information on Volcanic
we tend to think of the surface of the planet as being unchanging,
it is actually quite active, and constantly undergoing reconstruction.
shows a place where lava once flowed like a river. You can see
a sort of little island in the middle, and smooth streams flowing
around it. On the left, near the front of the picture, you can
see a series of ledges where the hot, flowing rock paused and
cooled a little before flowing on. Volcanic landscapes
contain interesting features, showing that the molten rock flows
much as water does.
After an eruption,
most life forms have been killed by the intense heat. However,
once the rock cools, living organisms begin to reclaim the devastated
Another kind of igneous
rock that comes out of volcanoes is called obsidian. It is a
kind of volcanic glass, and usually black and shiny. Sometimes
small mountains are made of this rock. It has very sharp
edges -- it should be handled with care. Our ancestors used obsidian
to make knife blades and arrow heads.
photograph from DHD
Photos © E Viau taken at Craters of the
Moon National Monument, Idaho, USA
® 1999, 2003. Elizabeth
Anne Viau and her licensors. All
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