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Define a volcano

A volcano is place in the earth's surface through which molten rock and other materials reach the surface.  There are three types of volcanoes.

1. Cinder cones- they are made mostly of cinders and rock. They cause explosive eruptions.They have narrow bases and steep sides.

2. Shield volcanoes- they are made of slow moving lava flows. The lava from a shield volcano is runny, so the lava flows over a large area. A dome shaped mountain is formed after several eruptions. 

3. Composite volcanoes- they are built up of rock particles and lava that come one after another.  A violent eruption occurs first. After that a quiet eruption produces a lava flow covering the rock particles.  After many eruptions a cone shaped mountain forms.

In an eruption, magma (lava) comes to the earth's surface, along with ash, dust, bombs, and cinders. Dust is the smallest particle. It is less than one quarter of a millimeter wide.

Lava flowing down a street

Ashes are from one quarter to five mm in diameter. Volcanic ash can affect people hundreds of miles away from the volcano. Several of the deaths from the Mount St. Helens volcano in 1980 were caused by breathing in volcanic ash.Volcanic ash can contaminate water supplies, cause electrical storms, and collapse roofs. 

Cinders are about the size of a golf ball.

Bombs can be several meters large.  However they can be about the size of a golf ball. Volcanic eruptions known as lateral blasts which are sideways blasts can shoot bombs at high speeds for several miles.  These blasts have been known to knock down entire forests!

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