This is why all oceanic crust is much younger than the continental crust; it is constantly being recycled. Even though new oceanic crust is always being formed, old crust is always being destroyed, and so there is no very ancient oceanic rock around. If this didn't happen, the world would have to be constanly expanding to make way for the extra crust being formed!
As the oceanic plate gets pushed down into the mantle, a vast ocean trench is formed by the drastic lowering of the sea bed. These trenches are by far the deepest areas of the worlds' ocean and are home to some of the planet's most extraordinary wildlife. Sometimes some of the subducted oceanic plate, once melted into magma within the mantle, begins to rise and push up through the continental plate on the other side, forming volcanoes, and ultimately, a mountain range such as the Andes (caused by the Nazca plate sinking below the South American plate).
Sliding past each other.
Tectonic plates are also able to slide in opposite directions whilst lying next to one another. As crustal material is neither destroyed nor created in this procedure these are known as conservative margins. However the edges of the plates are rough and cause friction. This means that rather than sliding smoothly past each other they tend to jam and stick in one place until the pressure builds up to be so great that it has to give. At that point the plates move suddenly, causing an earthquake. For this reason the fault lines along conservative plate margins tend to often be the most dangerous earthquake zones in the world. For example, the San Andreas Fault forms a junction between the North American and Pacific Plates. Although both plates are moving northwest, the pacific plate moves faster, giving the illusion that they are moving in opposite directions at a rate of about 6cm per year. When sufficient pressure has built up, the pacific plate suddenly jerks forwards, resulting in massive earthquakes in California. If this process continues, Los Angeles will eventually end up as an island off the Canadian coast!
SUBDUCTION is the process in which one is pushed downward beneath another into the underlying when move towards each other. The that is denser will slide under the thicker, less dense . Faulting occurs in the process. The subduction zone is the place where two plates come together, one riding over the other. Most volcanoes on land occur parallel to and inland from the boundary between the two plates. The subducted plate usually moves in jerks, resulting in earthquakes. The area where the subduction occurs is the subduction zone. A long, narrow, deep depression forms in this area. It is called an oceanic trench.
There are 3 types of rock, Metamorphic, Igneous and Sedimentary.
IGNEOUS……….Granite and Basalt
Igneous rocks are formed from molten magma or lava. The word, igneous means "fire". All igneous rock starts deep in the earth as hot, molten magma. If the magma cools and hardens inside the earth it is called "intrusive" rock. These rocks cool slowly and have large crystals. When the magma comes out of the earth's crust through a volcano, it is called "extrusive".If It cools off quickly, the crystals that form are very small and if it cools slowly the crystals are larger. Molten, or hot, liquid rock is called MAGMA when it is still inside the earth, but once it comes out through a volcano it is called LAVA.
SEDIMENTARY……….Limestone and Chalk
These are layered rocks .everyppe of every day, rocks are being worn down by wind and rain. Tiny grains of dirt, sand, mud and clay are worn off and washed into streams, rivers, lakes and oceans. When these tiny bits of sand and dirt settle to the bottom of the water, they are called sediment. Minerals in the water and microscopic, or very tiny sea animals also get mixed in with the dirt and sand to form the sediment, hence sedimentary rock contains fosils. er thousands and millions of years we end up with a really deep pile of sediment. Due to great weight and PRESSURE from all the stuff on top turns the sediment on the bottom into sedimentary rock!
MEAMORPHIC……….Marble and Gniss
Metamorphic rocks are formed when other kinds of rocks are changed by great heat and pressure inside the earth. The word "metamorphic" means CHANGED. Think of metamorphic rocks as recycled rocks. When igneous, sedimentary or even metamorphic rocks get buried deep beneath the surface of the earth, over millions of years the heat and Pressure inside the earth change them into something else.They contain distorted crystals. Limestone can be changed to marble, sandstone can be changed into quartzite, and shale can be changed to slate. It's just another example of how the earth is constantly changing!
Earthquakes are the Earth's natural means of releasing stress. When the Earth's plates move against each other, stress is put on the lithosphere. When this stress is great enough, the lithosphere breaks or shifts. Imagine holding a pencil horizontally. If you were to apply a force to both ends of the pencil by pushing down on them, you would see the pencil bend. After enough force was applied, the pencil would break in the middle, releasing the stress you have put on it. The Earth's crust acts in the same way. As the plates move they put forces on themselves and each other. When the force is large enough, the crust is forced to break. When the break occurs, the stress is released as energy which moves through the Earth in the form of seismic waves which we feel and call an earthquake.
Volcanoes form when hot material from below and leaks into the crust. This hot material, called magma, comes either from a melt of subducted material, and which is light and buoyant after melting, or it may come from deeper in the Interior of a planet and is light and buoyant because it is *very* hot.
Magma, rising from lower reaches, gathers in a reservoir, in a weak portion of the overlying rock called the . Eventually, but not always, the magma onto the surface.
Volcanoes can be dome- or cone-shaped, depending on the kind, and the
composition of material ejected during eruptions. This material includes molten lava, pyroclastic material, gases, and steam.
As you can see from the map the earthquakes and volcanoes occur on or near the plate boundaries and subduction zones. As landforms, volcanoes are formed by the deposition of the magma that flows or is ejected, normally from one or several circular vents, as molten or solid material.
As You can see, the volcanoes and earthquakes generally occur along the plate boundaries , or the subduction zones, this correlation is shown on map 1.
