A. The thoery of plate motion
The plates of lithosphere float on top of the asthenosphere. Convection currentsrise in the asthenosphere and spread out beneath the lithosphere. Most geologists think that the flow of these Currents causes the movement of earth's plates. No plate can budge without affecting the other plates surrounding it. As the plates move, they collide,pull apart , or grind past eachother, producing spetacular changes of earth's surface these include volcanos,mountain ranges and deep-sea-trenches.
The edges of different peices of lithosphere-earth's rigid shell called plate boundaries. Plate boundaries extend deep into the lithosphere. Faults-brake in earth's crust where rocks have slipped past eachother form along these boundaries. There are three kinds of plate boundaries:Transform boundaries, Divergent boundaries,and convergent boundaries. For each type of boundary , there is a different type of plate movement.
The plates move at amazingly slow rates:from about one-ten centimeters per year. The North American and Eurasain plates are floating apart at a rate of 2.5 cm per year - that's about as fast as your fingernail grows. These plates have been moving for tens of millions of years. About 260 million years ago,the continents were joined together in the super continent that Wegener called Pangaea. Then 225 million years ago, Pangaea began to break apart.
Along transform boundries, crust is neither created nor destroyed. A transform boundry is a place were two plates slip past eachother, moving in opposite directions. Earthquakes occur frequantly along these boundries.
The place were two plates move apart, or diverge, is called a diverging boundry. Most ciivergent boundries occur at the mid-ocean ridge. Divergent boundries occur on land. When a divergent Boundry develops on land, to of earth's plates pull Apart. A deep valey called rift valley forms along the divergent boundary. The rift may someday split the eastern part of Africa away from the rest of the continent. As a rift valley widens, it's floor drops eventually, the floor may drop enough for the sea to fill the widining gap.
The place where two plates come together,or converge, is called a convergent boundary. When two plates converge, The result is called a collision. Collisions may bring together oceanic crust, continential crust. When two plates collide, the density of the plates determines which one comes out on top. Oceanic crust, which is made of mostly of basalt, is more dense than continental crust, which is mostly made of granite, and ocenic crust becomes cooler and denser as it spreads away from mid-ocean ridge. Where two plates carrying oceanic crust meet at a trench, the plate that is more dense dives under the other plate and returns to the mantle. Sometimes a plate carrying ocenic crust collides with a plate carrying continental crust. When two plates carrying continental crust collides subtraction does not take place. Both continental plates are mostly low-density grnite rock. Therefore, neither plate is dense enough to sink into the mantle.