There is a theory that the continents were once all one continent. A young German scientist named Alfred Wegener in 1910 thought of this theory. He named this supercontinent Pangaea, which means all lands. Wegener said that this continent existed a few hundred million years ago, and has drifted apart since then. Wegener's idea continents slowly moved over earths surface became known as continental drift. Wegener got proof from different scientific fields to support the ideas of continental drift. He studied landforms, fossils and proof that showed how earth's climate changed over many of millions of years. Mountain ranges and other features on continents provided evidence for continental drift. Wegener used fossils to support the arguments of continental drift. Mountain ranges also provided evidence for Wegener's theory. He also used evidence of climate.
Wegner tried to explain to scientists how continental drift happened. He believed that when continents collided, the edges crumpled and folded. Folding continents pushed rock up into mountain ranges. Wegener couldn't provide a satisfactory argument for the force that pushes and pulls at the plates. Because of that, many scientists rejected his idea. But many years later, Geologists began to believe that the earth was slowly shrinking and cooling. This brought Wegener's theory back into consideration.
The east pacific rise is just part of the mid ocean ridge. The mid ocean ridge is the longest chain of mountains in the world. Mid ocean ridges curve like the seam of a baseball along the ocean floor. At the mid ocean ridge, molten material rises from the mantle and erupts. The molten material rises and spreads out, pushing older rock to both sides of the ridges. As molten material cools, it forms a strip of solid rock in the center of the ridge. Then the material splits apart the rock and pushes it aside. Harry Hess, a geologist called this new process that continually happened sea-floor spreading. Many different types of evidence followed this and supported Hess's theory of sea-floor spreading, evidence from molten material magnetic strips to drilled samples. Geologists found strange rocks that they knew could only come from hardened molten material. Final proof of sea-floor spreading came from drilling samples from the sea floor. The samples of rock proved their theory.
A deep ocean trench forms where the ocean crust bends down. Where there are deep ocean trenches, subduction takes place. Subduction is the process by which the ocean floor sinks beneath a deep ocean trench and back into the mantle. Subduction and sea-floor spreading can change the size and shape of the ocean over millions of years. The Pacific Ocean covers 2/3 of the planet, yet it is shrinking.
The edges of different pieces of the lithosphere meet at the lines called plate boundaries. Plate boundaries can go very deep into the lithosphere. Faults, breaks in Earth's crust where rocks slip past each other, form along the boundaries. ther are three kinds of plate boudaries; transform boundaries, convergent boundaries, and divergent boundaries. every boundary has a different type of plate movement.
Along transform boundaries, crust is neither made or demolished. A transform boundary is a place where two plates slip past each other, moving in opposite directions. Earthquakes usually happen along these boundaries.
The place where two plates move apart (diverge) is called a divergent boundary. Most divergent boundaries occur along the mid ocean ridge. Divergent boundaries can also happen on land. When a divergent boundary occurs on land, two plates pull apart. A deep valley called a rift valley forms along the divergent boundary.
Where two plates come together, or converge, it is called a convergent boundary. When two plates come together, it is called a collision. When two plates collide, the density of the plate determnes which plate will end up on top. Oceanic crust is denser than continental crust. An oceanic crust becomes cooler and denser as it moves away from the mid ocean ridge. Where two plates carrying oceanic crust come together at a trench, the denser of the two goes under the other plate and into the mantle. This is subduction. Sometimes a plate carrying oceanic crust collides with a plate carrying continental crust. The less dense continental crust is unable to sink below the dense oceanic crust. Instead, the oceanic crust sinks below the continental crust and plunges into the mantle. When two plates carrying continental crust collide, neither is able to sink. Instead, the plates crash into each other. The collision squeezes the crust into large mountain ranges.
The plates move remarkably slow, from 1-10 cm. per year. The North American and Eurasion plates are floating apart at a rate of 2.5 cm per year (that is about how fast your finernails grow). These plates have been moving for millions of years. About 260 millions of years ago, the continents were joined as one supercontinent that Wegener called Pangaea. Then, about 225 million years ago, Pangaea began to break apart. After an extremely long period of time, we have the world today, with the seven continents that were at one time Pangaea.
This picture shows some of the plates and which directions the boundaries are moving in.