Overview
The planet Earth is not a uniform, cold, solid ball. Scientists believe that as it condensed from the cloud surrounding the sun, it formed in many dynamic layers, some cold and solid, some hot and liquid, and some hot and pliable. As the layers interact, they give rise to cataclysmic events such as earthquakes and volcanoes. The theory of plate tectonics attempts to explain why earthquakes and volcanoes behave as they do.
Structure of the Earth
Scientists believe that the earth consists of three main layers: the core at the center, the mantle, and the crust. The core is made up of heavy iron and nickel. It is insulated by the rest of the planet surrounding it, and is mostly molten. The mantle is about 80 percent of the entire volume of the Earth. Most of it is solid, but not all. The crust is relatively brittle, and varies in thickness from 7 kilometers to about 70 kilometers. It is the most familiar part of the planet, as the surface has been the most explored.
Plate Tectonics
The cool crust and the upper part of the mantle, called the lithosphere, float on a hot, oozing lower part of the mantle, called the asthenosphere. The lithosphere is relatively brittle and consists of several major plates and many minor ones. According to the theory of plate tectonics, the plates move against one another, sometimes in the same direction, sometimes in opposite directions. The points at which they collide and move past one another are geologically active, resulting in earthquakes and volcanoes.
Earthquakes
When the lithospheric plates slip past, bump, rub, or clash, the result is an earthquake. If two or three plates separate or move apart, they form a divergent boundary. Those plates that separate may collide with others, forming a convergent boundary. If plates slide past one another, they form a transform boundary. If one plate moves downward and another moves upward at the same time, they create subduction. The movement of faults release pressure and create earthquakes.
Volcanoes
Volcanoes are also a result of geological activity, according to plate tectonics. The friction between plates as they move and collide creates heat, melting some of the rock. Other rock, such as in the flowing asthenosphere, is melted easily. This molten rock, called magma, is released through vents in mountains called volcanoes, sometimes slowly, such as the lava flows in Hawai’i , and sometimes explosively, such as in the eruption of Mt. St Helens in Washington State.
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