The Asthenosphere and Lithosphere We can all agree that the Earth is rotating on its axis, making one rotation about every twenty-four hours. We can also agree that the Earth is revolving around the sun in its happy 364-day elliptical orbit. Correct? But why? We have proof to support these ideas. We can observe night and day and the changing positions of the sun as the Earth rotates and we can see the change in seasons as it revolves (1). Would you believe that the Earth makes its own movements within? The Earth is composed of four layers: the inner core, outer core, mantle, and crust. The crust (or lithosphere) is divided into sections called plates. The mantle, also referred to as the asthenosphere, is a hot liquid layer, made up of magma, or molton rock. The magma closest to the core is the hottest, and thereby less dense than the magma near the crust. What happens next? The less dense magma cannot support the denser magma so the hot, less dense magma shifts upward while the denser molton rock near the core. This is all repeated again, continuously as the cooler magma is heated. This process is called convection currents (2). The plates in the lithosphere glide over the top of this magma, causing them to reposition themselves over the globe. How can we accept this theory? If we go back to the motions of the Earth through space, we can make daily observations that will support the idea. With this plate techtonic theory, proposed by Alfred Wegener, we cannot go outside and see the ground in motion, nor can we obtain a satillite picture and see a change in the continents from what they looked like yesterday, or even last decade. So we would naturally be forced to think this is false, which is what scientists did when Wegener conceived this hypothesis. After initially being dismissed, evidence came forth that facilitated the proving of plate techtonics. It was found that the continents have a "puzzle-like" fit, telling us that they could have one day been together and broken apart. Also the same fossils of organisms only thought to have been in Africa were found as well in the Americas. These creatures could not have swam across the ocean, so there must have been land connectiong them. The same rock layer have even been found in different continents, indicating that they were once joined. Given this evidence, we can now support this theory, as scientists today do. So, what happens to these continents? They obviously do not all travel in the same direction. This is where we get into the effects of plate techtonics. The Earth's season are caused by its 23.5 degree tilt on its axis, making either the northern or southern hemisphere closer to the sun. The seasons change as the Earth rotates around the sun. With the Earth's axis always pointing in the same direction (Polaris, or the North Star), different hemispheres will be closer to the sun with each few months. 2. For a good demonstration of convection currents, fill a large, clear container with cold water and two small cups with boiling water. Place the two cups side by side, about five or six inches apart (depending on the size of the container). Place the large container of cold water on top of the two cups. Put a few drops of food coloring (blue, red, or green) in the water and you will see how the water closer to the heat source rises and the cooler water falls, etc. The Effects of Plate Tectonics As we have stated earlier, the plates do not move in the same directions, making way for other plates. For example, The plate that contains North America is colliding with the plate that contains the Pacific Ocean, and at the same time, it is moving away from the Atlantic plate. There are different interations between plates. Collisions occur when two plates of the same density (Oceanic plates are more dense than continental plates) collide, resulting in a mountain range. Diversions occur when any two plates move away from eachother, resulting in a rift, ridge, or rise. Subductions, a kind of collision, occur when two plates of different densities collide. The denser plate, the oceanic plate, collides with the continental plate and begins to sink under it. These plate collisions sometimes cause volcanic eruptions. An example of a subduction that we already mentioned is North America's collision with the Pacific plate. The oceanic plate is sinking below the continent, causing great deals of earthquakes and volcanoes, like Mt. St. Helens on the west coast of North America. Another way that plates may interact is when they move against one another, side to side, creating faults. An example of this is the San Andreas fault in California. The moving of these plates do not occur over night. It has taken millions of years since the continent of Pangea broke apart (3). Pangea was a super continent that was thought to have existed in the Mesozoic Era in the geologic time era. Given this, we will not see major changes in the continents in our lifetimes. So, the next time someone ask you why volcanoes and earthquakes always occur in one area, you will tell him/her: That area is located on a plate boundary, a place where much stress in built up in the collision/ diversion motion of the plates all caused by plate tectonics. 3. A long time ago, more than 65 million years, a super continent existed. This was the time that all of today's continents were joined into one, creating one continent, Pangea, and one ocean Panthalassa. Later, the continent was split into two, Gondwanaland and Laurasia. From there, caused by plate techtonics, we got our modern-day continent positions.