The causes of a volcano.
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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. 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.
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.
Deep within the Earth it is so hot that rocks slowly melt a thick flowing substance called magma. Because it is lighter than the solid rock around it, magma rises and collects in magma chambers. Eventually some of the magma pushes through vents, and fissures in the Earths surface. Magma that erupts is called lava.
Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake. When two blocks of rock or two plates are rubbing against each other, they stick a little. They don't just slide smoothly; the rocks catch on each other. The rocks are still pushing against each other, but not moving. After a while, the rocks break because of all the pressure that's built up. When the rocks break, the earthquake occurs. During the earthquake and afterward, the plates or blocks of rock start moving, and they continue to move until they get stuck again. The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus (on top of the ground) is called the epicenter of the earthquake.
Besides producing floods and destroying buildings, earthquakes that take place under the ocean can sometimes cause tsunamis, or tidal waves. Tsunamis are high and long walls of water which travel at a very rapid rate. They are notorious for destroying entire populations and cities near coastlines. In 1896 Sanriku, Japan, with a population of 20,000, suffered such a fate.
The causes of landslides are usually related to instabilities in slopes. It is usually possible to identify one or more landslide causes and one landslide trigger. The difference between these two concepts is subtle but important. The landslide causes are the reasons that a landslide occurred in that location and at that time. Landslide causes are listed in the following table, and include geological factors, morphological factors, physical factors and factors associated with human activity.
Landslide damage from the Northridge earthquake was only moderate because the area of greatest landslide activity is not yet heavily developed. However landslides did, as described below, block roads; damage and destroy homes; locally disrupt water mains, sewers, and power lines; and damage oil- and gas-production facilities.