Most volcanoes occur where two tectonic plates meet. When two tectonic plates seperate, they create a gap, or fissure, and hot molten rock rises up through the fissure. This type of volcano is mainly found on the ocean floor and is mostly invisible. If a big enough fissure is formed, a lot of lava may ooze out of it, sometimes even enough to rise to the surface of the ocean and form an island.
If two tectonic plates coincide, it causes one plate to be forced beneath the other. The resulting friction generates a lot of heat, causing the magma to liquefy, and rise up in the form of lava. Only a few volcanoes on earth were formed like this, but their eruptions are the most violent and dangerous ones. This is on account of the fact that the friction greatly increases temperature, causing the gases to expand. This in turn generates a huge amount of pressure, literally throwing the lava out with great force, and to great heights.
Finally, the last type of volcanoes form in the middle of a tectonic plate. In the course of normal tectonic movement, magma is pushed up little by little, till it enters crevices in the rock of the lithosphere. A volcano does not form every time this happens. However, if the overlying rocks are brittle, they give away to allow the magma to flow out onto the Earth's surface in the form of lava. This process is called a diapir, and such volcanoes are commonly called 'hotspots'. These are places that are connected by channels to the hot mantle of the earth.
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