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To understand what causes volcanoes, you need to understand how the earth is made up. The earth has three main layers: the crust, the mantle and the core. The crust is made up of solid rock and varies in thickness[1]. It is more than 60km thick under mountain chains like the Alps and Himalayas, but just 5km under the oceans. The mantle is a thick layer of molten rock (called magma), and the core is made up of an outer liquid layer and a solid centre.

What Goes On Inside That Makes The Eruption

Temperatures inside the earth are very high - over 5000'C in the core. This means that the planet on which we live is like a huge fiery ball of hot molten rock, surrounded by a few kilometers of relatively cool, hard rock - the crust. Because heat rises, the magma in the earth's mantle has to find a way to rise upwards though the crust above it, rather like the way that hot air rises[2].

A volcano erupts when magma escapes from inside the earth. As the magma is escaping from a confined space, a lot of energy is released with it, as happens with any other explosion. This is why many eruptions also produce huge quantities of gases and dust. Magma sometimes rises under enormous pressure, so it not only finds cracks in the earth's crust, it can also create them. When magma reaches the earth's surface it is called lava[3].

The earth's crust is its thinnest layer. It is broken up into large pieces, called tectonic plates. These plates lie above the hot, liquid mantle. Each plate contains some continental crust (land) and some oceanic crust (sea-bed). Huge currents of molten rock circulate deep in the mantle, causing the plates to move about very slowly on the earth's surface.

Thesis Statement

A volcano erupts when magma escapes from inside the earth.

Volcanic Sites Around The World

If you look at the location of volcanoes in relation to these plates, you will notice some similarities. Many of the world's volcanoes occur along the edges of boundaries of the plates. This is no coincidence. Plate boundaries are among the most geologically active places on earth. Here, new rock is being both created and destroyed, so this is where most of the world's volcanic eruptions and earthquakes occur[4].

After 9,000 years of dormancy, the Chaiten volcano in southern Chile awoke in 2008 and began a series of eruptions that spewed ash miles into the sky, as shown in this image (see below).

Italy's Mount Vesuvius is most famous for the A.D. 79 eruption that buried the cities of Pompeii and Herculaneum. Despite the dark history, millions of people today live near the volcano.

Mexico City, a metropolis of 18 million people, sits 40 miles to the east of Popocatepetl, the second tallest volcano in North America. The volcano has been relatively quiet since a bout of activity between 1920 and 1922, though it rumbled back to life in 2000, as shown in ...
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