Rocks from volcanoes
When a volcano erupts, rock flows down its sides and is shot out into the air. The flowing rock that forms a red river down the side of the volcano is called lava. When some volcanoes erupt the lava can flow faster than an express train but in many eruptions it moves much more slowly and can be closely studied. There are two kinds of lava flow and their names are Hawaiian words. There are many volcanoes in Hawaii and many studies have been made on lava there. Some lava has pointed lumps of rock in it. This is called aa. A second kind of lava is lump free but forms a skin on its surface like a crumpled table cloth. This lava is called pa hoe hoe. When both kinds of lava cool they turn to a black rock called basalt.

The molten rock rising up inside a volcano contains a mixture of gases. The heat makes the gases swell and push on the rock. They push on the rock so strongly that some of it breaks up and flies into the air at great speed. The larger lumps, called volcanic bombs, soon plummet towards the ground but tiny pieces of ash may form a cloud which runs down the side of the volcano. These clouds also contain gases from the volcano. Most of the gases are poisonous. The cloud of ash and poison gases can move faster down a volcano than the lava. This is the part of the eruption which is most dangerous to people living near the volcano.

Does a volcano erupt all the time?
Most volcanoes have a resting period between eruptions. At this time the volcano is said to be dormant. When a volcano is about to erupt again it swells up. Scientists who study volcanoes have equipment on the sides of volcanoes, which measure their change in size. They use their measurements to predict when a dormant volcano will erupt. In time a volcano may stop erupting. When this happens, the volcano is said to be extinct.

Are all eruptions the same size?
No. The size of the erupting depends on the thickness of the lava and the amount of gas in it. If the lava is thin and runny, gas can escape from it easily. This means that the lava only contains a small amount of gas. This gas is just strong enough to make small lava fountains. The volcanoes on Hawaii produce lava fountains. If the lava is thick, gases cannot escape easily so they are present in huge amounts as the lava comes out of the volcano. The gases swell up rapidly at this time and make the lava explode. This makes lava lumps the size of houses fly through the air and pushes a cloud of ash many kilometres into the air.

Are all volcanoes shaped like a cone?
No. A cone-shaped volcano is built up by thick lava that slowly flows down its sides. Ash and dust also settle in layers on the sides to make the volcano cone-shaped. Volcanoes which have thin lava do not build up into a cone-shape. The lava flows a long way over the ground and builds up a shape like an upside down saucer. Volcanoes with this shape are called shield volcanoes.

How does a crater form at the top of a volcano?
Under a volcano is a huge cavern called a magma chamber. This is where the molten rock collects before it comes out of the volcano in an eruption. Sometimes the top of a volcano breaks up and falls into the magma chamber. When this happens a ring-shaped structure is formed. This is called a caldera. A lake may form inside it.

Are there volcanoes in the sea?
There are hundreds of volcanoes in the sea. Many of them occur where two plates in the crust move apart. They fill the gap with lava and release huge amounts of lava over the ocean floor. When the lava flows into the water, it forms large lumps that are the size of pillows. This lava is called pillow lava.

Can a volcano make an island?
Some volcanoes in the sea grow so tall that they erupt ash and lava above the ocean surface. In time the ash and lava form an island. In 1963 an island called Surtsey formed in three weeks from an erupting volcano in the ocean near Iceland.

Why does granite look different from basalt?
Both rocks are formed from hot rocks inside the Earth so you might expect them to be the same. One reason for the difference is the way the rocks cool down. Basalt forms on the surface of the ground. It cools quickly in the air. Granite stays inside the crust as it cools. It loses heat very slowly. In fast cooling rock, like basalt, small crystals form that can only be seen well with a microscope. In slow cooling rock, like granite, large crystals form that can be seen clearly with the naked eye. A second reason why the two rocks look different is that they contain different minerals.