When hot magma flows up to the
surface of the earth, it cools quickly. The magma bursts out
of the ground as hot lava: this is called a volcanic eruption.
Sometimes there is just a crack in the ground, and lava begins
to bubble out. Sometimes an already existing volcano will become
active and erupt after a quiet period of hundreds or thousands
Great clouds of steaming water
vapor, carbon dioxide, and other gases come out of the volcano.
This process is called "out-gasing". There are also
tiny bits of rock in the eruption which cool in the air and fall
as volcanic ash.
Once it is in the air, the lava
cools quickly. If the lava is fairly thick, a cone of rock builds
up around the vent of the volcano. Gobs of lava fly out and fall,
beginning to cool. Volcanoes can be built up in just a
If the lava is very
hot and liquid, it may just flow out over the
ground, building a low, flat shield volcano. Very hot lava is
a thin fluid, rather like hot syrup. It will flow along in valleys
like a river made of water. If it comes out onto level ground
it will form a large, flat plain. Parts of Oregon are made of
ancient lava flows nearly a mile thick.
In this picture you can
see layers formed by successive flows of lava. Some of the lava
is still red-hot. As you would expect, the outside of the lava
flow cools and solidifies first.
The lava that
cools on the surface of
a lava flow may be a jumble of rough rocks. The Hawaiian people,
who live on volcanic islands, call this kind of cooled lava a-a-lava
because this is what people say when they walk on it with bare
Another kind of lava is smooth with soft curving folds in
it. It looks like icing when you are pouring it out of
a bowl. This is called pahoi-pahoi lava.
Here we see a picture of what is going
on under the ground when magma pushes up to the surface. The
magma is being pushed up through cracks in the overlying bedrock.
The heavy bedrock presses down on the magma, which is like very
thick icing. It is forced upwards towards the surface.
eruption that began in the picture on the left is progressing
in the picture on the right. A definite cone is being built on
the lower eruption site, and hot lava is slowly flowing out above
You can see pieces of rock in
the air. These are becoming flying "bombs". The lava
comes up under pressure because it has a lot of rock lying on
top of it and pressing it down. This leads to some of the rock
spattering out when it reaches the surface.
The rock that comes out of volcanoes
is often full of bubbles perhaps a quarter of an inch across.
This rock is usually dark
colored. Another kind of rock that
comes out is light colored and full of tiny bubbles. This rock
has so many bubbles in it that it will float on water! It is
you see that the eruption has resulted in a steep, conical mountain.
Lava is still coming up from the magma below, but a lot of the
rock is already cooling.
After a while the pressure on
the lava will lessen and the lava will come up more slowly. It
will continue to cool, and will eventually plug up the channel
through which it has been flowing. As the pressure on the lava
lessens the lava sinks down again. The top of the mountain may
then collapse inward, forming a bowl-shaped depression called
a caldera. The volcano will then become dormant, and may
not erupt again for hundreds or even thousands of years. If the
volcano never erupts again, we say that it is an extinct volcano.
Mount St Helens is an example of a dormant volcano
that erupted in 1980 after being dormant for 123 years. Check
the Helpful Web Sites
to find out more about Mount St Helens.
This picture shows the caldera at
Crater Lake in Oregon. In this case the caldera is filled with
water. You can see the steep sides of this deep and
beautiful lake. The lake is roughly circular.
Notice the island in the lake.
It is a typical volcanic cone and has a small caldera of its
Most calderas do not have lakes in them.
photograph from DHD
Other photographs from a Corel CD-ROM
: for viewing only, not for downloading. More
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