It varies in thickness from 2 to 35 miles and it is this layer. Below the crust is a thick layer of rock called the mantle which is nearly 1500 miles thick. The core consists of 15 % of the Earths volume and 32% of its mass. It is very dense and is made mostly of iron. Another set of divisions of the Earths interior can be made based on mechanical differences and types of heat transfer; the lithosphere and the asthenosphere. The innermost asthenosphere is hotter and fluid-like. The lithosphere is a division made up of the crust and the uppermost part of the mantle.
It is cool and rigid and is broken up into seven major and many minor techtonic plates. These plates move in relation to one another at one of three types of plate boundaries: convergent or collision boundaries, divergent or spreading boundaries, and transform boundaries. Most of the worlds active volcanoes occur along plate boundaries. The major plates are African Plate, Antarctic Plate, Arabian Plate, Australian Plate, Caribbean Plate, Cocos Plate, Eurasian Plate, Indian Plate, Juan de Fuca Plate, Nazca Plate, North American Plate, Pacific Plate, Philippine Plate, Scotia Plate, and the South American Plate.
There are also many minor plates throughout the world. As techtonic plates move, they interact with each other and create friction, pressure and/or strain. Stress builds up in both plates and when it reaches a level that exceeds the threshold of rocks on either side of the fault, this accumulated potential energy is released. The brittle upper crust reacts by fracture, or instantaneous stress release to allow motion along the fault.
Energy released in this way is the cause of earthquakes which are commonly found along transform boundaries. The San Andreas Fault along the western coast of North America is a well known transform boundary. Here, the Pacific and North American plates meet and move against each other. Other examples of transform faults include the Alpine Fault in New Zealand and the North Anatolian Fault in Turkey. Mendocino Fracture Zone offshore northern California). Divergent boundaries consist of two plates that move apart from each other.
When this occurs, a space is created and fills with molten magma. Spreading is not usually uniform and can create massive fault zones. Divergent boundaries are common in the sea floor and are the cause of the Mid-Atlantic Ridge and the East Pacific Rise. Convergent boundary action depends on the density of the plates that are colliding. Oceanic plates tend to be more dense, with a higher percentage of heavy elements. When a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically thrust underneath.
This forms a subduction zone and is responsible for oceanic trenches and mountain ranges. An example of a continental-oceanic subduction zone is the area along the western coast of South America where the oceanic Nazca Plate is being subducted beneath the continental South American Plate. Another phenomenon that occurs as the subducting plate descends is a rise in temperature as hot water that has been encased in the porous oceanic crust is released. As the water rises into the mantle of the overriding plate, it lowers the melting temperature of the surrounding mantle, producing melts (magma).
These melts rise to the surface and are the source of some of the most explosive volcanism on Earth because of their high volumes of extremely pressurized gases. Mount St. Helens was formed in this way. As these melts rise to the surface and cool, they form long chains of volcanoes inland from the continental shelf and parallel to it. South America is dense with this type of volcanic mountain building from the subduction of the Nazca plate. In North America the Cascade mountain range, extending north from Californias Sierra Nevada, is also of this type.
The entire Pacific Ocean boundary is surrounded by long stretches of volcanoes and is known collectively as The Ring of Fire which are the most active volcanoes in the world. When two continental plates collide, they will buckle and compress or one plate goes under the other creating mountain ranges. Currently, the northern margin of the Indian plate is being pushed under the Eurasian plate, and is creating the Himalayan Mountains. When two plates with oceanic crust collide, they typically create an island of volcanoes that erupt through the overriding plate. Japan and the Aleutian Islands were formed in this way.
The world is constantly changing and the occurrence of natural disasters is a constant threat. Life in a high risk location may be uncertain, but many people have chosen to live near these potentially dangerous areas for reasons that they feel outweigh the risk. Choice is part of our rights as a human being, and we each must educate ourselves as to our environmental risks and weigh our priorities to make the decisions that are right for each of us. Our awareness of plate techtonics can allow us to assess our future risks. Not allowing people to live in high risk areas would remove their choices over their own lives.