Volcanoes have inspired fear, awe and wonder in societies through the history of mankind. They are among the most dramatic natural phenomena on Earth, and until last century, how and why they existed were mysteries to humanity. To understand how volcanoes are formed, it is first necessary to learn about the earth and how it is structured. Then, one can move on to the specifics of the parts of the earth that cause volcanoes and understand how volcanoes are formed.
The earth has a solid inner core and a liquid outer core. These two parts form the densest part of the earth, and incidentally also are what gives the earth its magnetic field. The next layer of the earth is called the mantle. This is also comprised of two layers, both of which form the thickest layer of the earth. The mantle is made, roughly speaking, of molten rock, which flows under the intense pressure and heat, just under the topmost layer of the earth called the crust. The crust is the cool layer of earth, like the skin which forms on a bowl of hot soup. It is the thinnest layer, and varies in depth. The oceanic crust, which is the rock underneath the oceans and seas, is only 5-10 km thick, whilst the continental crust, the part which forms the lands people live on, is as thick as 70 km. This is a broad outline of how the earth is layered, and the first part of understanding how volcanoes are formed.
The second part of understanding how volcanoes are formed is grasping plate tectonics. This is not a difficult concept, despite the strange terminology. The earth’s crust and the upper part of the mantle make up the layer of the earth known as the lithosphere. This layer is divided up into sections or plates, which shift and move all the time. The concept of plate tectonics was not developed until late in the last century, when scientists discovered that the prehistoric earth was shaped differently and was indeed, one big continent. The process of plate tectonics, or the movement of the plates in the lithosphere, caused the continent to break up, move away, and form new continents. Tectonic plates move in three different ways. They can move toward one another (convergent,) away from one another (divergent,) or alongside one another (transform boundary.) Tectonic plates are the key to understanding how volcanoes are formed because it is their very movement which causes the process of development of a volcano to begin, which is the last part of knowing how volcanoes are formed.
Volcanoes are formed in several different ways, and they form several different types of volcanoes. For the sake of clarity, though, this article will focus on the three ways that volcanoes are formed. Firstly, the most benign cause of undersea volcanic activity is divergent plate boundaries. This term implies that the plates have moved away from one another, allowing the crust to become thinner. It is in these places that the mantle melts and cools to form new crust. Convergent plate boundaries occur when tectonic plates, most commonly an oceanic plate and a continental plate, collide or push up against each other. When this happens, one plate slides up and pushes the other one down, allowing magma to escape. This magma cools very quickly in water and often is not seen at all. The last way in which a volcano is formed is possibly the most familiar. Hot spots are cases where magma forces its way to the crust in a plume, which then builds up and escapes through the crust into the atmosphere. In this case, the volcano is not caused, but averted by tectonic movement. The plate shifts away from the plume and the volcano becomes dormant.
Volcanoes are incredibly powerful demonstrations of the power and intensity of the earth. Understanding how they work can help people to better prepare for and predict their occurrence. The study of volcanoes is ongoing, since people still do not know everything about them, but knowing how they are formed is a fundamental step in this process of learning.
References
http://en.wikipedia.org/wiki/Volcano
http://en.wikipedia.org/wiki/Tectonic_plates#Key_principles
http://en.wikipedia.org/wiki/Structure_of_the_Earth
