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 of years.

     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 few months.

    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 feet!

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.

     The 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 it.

     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 called pumice.

    Here 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 own.

Most calderas do not have lakes in them.

• States of Matter
• Making a Relief Map
• From Contour Map to Landscape Profile
• The Structure of the Earth
• How Rocks Are Formed
  • Igneous Rock
  • Sedimentary Rocks
  • Metamorphic Rocks