Friday, April 06, 2007
The way volcanoes work!
Volcanic eruptions and formations are linked processes. Basically, a volcano is a portion of earth in which subterranean materials have made their way to the earth’s surface.
Humans live at the surface level of Earth, which is called the crust. Though the earth has many small layers, scientists call the first major area beneath the crust the mantle, and the area beneath the mantle the core. Magma, the main component in volcanoes, comes from the mantle region of the earth.
Volcanism is the process through which inner materials of the earth come up to the earth’s surface. It is also the process by which a planet cools itself.
There are three major ways in which volcanic activity can occur: spreading-center volcanism, subduction-zone volcanism, and hot-zone volcanism. All three types of volcanism are caused by the movement of tectonic plates.
In spreading-center volcanism, the tectonic plates move away from one another, forming an ocean or continental ridge. As the plates separate, the mantle rock flows up into the space created between them.
Here, the mantle rock melts to form magma, which emerges and forms a new crust. This process does not create a volcano as we know it, but it does patch up holes in the earth.
Most volcanoes are formed by subduction-zone volcanism. This process occurs when two tectonic plates collide and one plate from the lithosphere (the layer made up of the upper crust and the top layer of the mantle) is pushed deeper into the mantle, which forms a trench (usually located at the ocean floor). Most scientists believe that heat and pressure eject water from the lower plate (the subducting plate) into the upper plate.
Now that the upper plate contains more water, its melting point decreases and the upper plate melts into magma. The magma flows out and up because it is less dense than the surrounding rock. If under enough pressure, it spews through the surface. Otherwise, magma forms in a chamber beneath the earth’s crust.
Another way that volcanoes form is by hot-spot formation, which is not caused by shifting plates, but rather, by hotter, deeper magma rising from the lower to the upper mantle.
The hotter material from the lower mantle can then form an area just under the earth’s crust that is called a “hot spot.” No eruptions will occur at the surface, but as the plate above the hot spot shifts, a series of volcanoes forms. This is how the tropical vacation spot we know as Hawaii formed.
In both subduction-zone and hot-spot volcanism, magma always seeks lower pressure regions, so it rises toward the surface because it is less dense than the surrounding rock.
To form the conical shape of a volcano, one of two things happens: The magma pushes crust materials up around it, or magma spews forth and cools, collecting over time to form a mountain or hill, though it is possible to have a volcano in a crater or plateau.
The reason that magma does not always reach the surface immediately is because when the surrounding rock’s pressure exceeds the pressure of the liquid magma, it can keep this magma temporarily contained just beneath the earth’s surface.
Over time, the magma pressure can rise to exceed that of the surrounding rock, causing the molten rock and magma to spew forth through the surface.
Many dissolved gases exist in magma, and as long as the surrounding rock exerts a pressure greater than the gas vapors, the magma will remain contained. However, if the gas vapor pressure ever exceeds the pressure exerted by the rock, the dissolved gases expand and form bubbles called vesicles.
When vesicles form, one of two things happens: either the pressure of the surrounding rock decreases because of the decompression caused by the lowering pressure point of the magma, or the gas pressure increases because the cooling magma has crystallized and therefore enriched the gas inside it.
Magma filled with the tiny gas bubbles has a lower density than the surrounding bubble-free magma, and so the gas bubbles push the magma up and out of the volcano chamber.
The result is lava spewing out from a chamber just beneath the earth’s crust, thus forming the best-recognized version of the volcano.