why do we have volcanoes on earth
Volcanoes are just a natural way that the Earth and other planets have of cooling off and releasing internal heat and pressure. Volcanoes erupt because of density and pressure. The lower density of the
relative to the surrounding rocks causes it to rise (like air bubbles in syrup). It will rise to the surface or to a depth that is determined by the density of the magma and the weight of the rocks above it. As the magma rises, bubbles start to form from the gas dissolved in the magma. The gas bubbles exert tremendous pressure.
This pressure helps to bring the magma to the surface and forces it in the air, sometimes to great heights. It's sort of like the bubbles of gas in a bottle of soda. Before you open the soda you don't see many bubbles because the pressure in the bottle keeps the gas dissolved in the soda. When you open the bottle the pressure is released and the gas bubbles leave the soda. If you shake up the bottle first, the soda gets pushed out by the bubbles of gas as they rush out.
You might try this, but do it outside. Most volcanoes are found near and mid-ocean ridges. This explains why the map of volcanoes on Earth matches so well the map of tectonic plates. On the other hand, some volcanoes are located in remote places like in the middle of the Pacific plate (Hawaii), thousands of kilometres away from the nearest plate boundary. This kind of volcano is very special: it is the consequence of a hotspot. A hotspot is a plume, seemingly coming from the outer core of our planet.
It is a flow of hot and less dense material that crosses the solid mantle and breaks through the crust. This process creates intense and very localised volcanism at the surface. As the tectonic plates are continuously moving, hotspots leave a series of volcanic mounts at the surface of the Earth. A good example is the Hawaiian-Emperor seamount chain, a chain of ancient volcanoes created by the hotspot currently below Hawaii, which stretches over 5800km from Hawaii to Russia.
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