Volcanoes

Volcanoes

What are volcanoes?
A Volcano is a gap in the earth where molten rock and other materials come to the earth's surface. Some volcanoes are just cracks in the earth's crusts. Others are weak places in the earth's crust, which occur on places where magma bubbles up through the crust and comes to the earth's surface. Magma is molten rock that occurs by partial melting of the crust and the mantle by high temperatures deep down in the ground. Once magma comes to the earth's surface it is called lava.

Active and non-active volcanoes
There are volcanoes in different phases of activity:
Active volcanoes, which are likely to erupt at any moment, dormant volcanoes, which lie dormant for centuries, but then erupt suddenly and violently, and extinct volcanoes - ones no longer likely to erupt.

Types of volcanoes
In the surroundings of boundaries of tectonic plates the following types of volcanoes occur:

The fissure volcano:

Is a long crack in the earth's surface through which magma erupts. These cracks may form as two tectonic plates pull apart.
You'll find them mainly near mid-ocean ridges.

The ordinary volcanoes can be divided in different types, relating to their forms:

The shield volcano:	This is a broad, shallow volcanic cone, which arises because the running lava, which is fluid and hot, cools slowly.
The dome volcano:	This one has a steep, convex slope from thick, fast-cooling lava
The ash-cinder volcano:	Throws out - besides lava - much ash into the air. Through this the volcanic cone is built up from alternate layers of ash and cinder.
The composite volcano:	These are also built up from alternate layers of lava and ash but, besides its main crater, it has many little craters on its slope.
The caldera volcano:	An older volcano with a large crater which can be 62 miles(100km) wide. In this crater many little new craters are formed.

At other places, not at the margins of the tectonic plates, hot-spots can occur. Hot-spots are formed because a very hot area (focus) in the earth's mantle burns its way through the earth's crust. Examples can be found on the Hawaiian Island Chain.

Other forms of volcanism are the Geysers and the Hot Springs. You often find them in the neighborhood of volcanoes, where the earth's crust is thinner and the heat of the magma further penetrates into the earth's crust. A Geyser is a hole in the earth's crust, spouting fountains of boiling water. Hot rock heats up water in an underground chamber and when the water boils, it sends out a fountain of boiling water, up to 1,640 ft. (500 m) into the air. A Hot Spring works in the same way, but the water is not so hot that it spouts with pressure.

Full size

Types of volcano
A diagram of the structure of six types of volcano.

Image by:
Colin Salmon,
Dorling Kindersley

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