Most of these changes in the earth’s surface takes place so slowly that they are not immediately noticeable to the human eye.
The idea that the earth’s landmasses have broken apart, rejoined, and moved to other parts of the globe forms part of the
plate tectonic theory.
Plate Tectonic Theory A bout forty years ago, scientists exploring the seafloor found that it is full of tall mountains and deep trenches, a single seafloor mountain chain circles Earth and contains some of Earth’s tallest mountains. Along this mountain chain is a deep crack in the top layers of earth. Here the seafloor is pulling apart and the two parts are moving in opposite directions, carrying along the continents and oceans that rest on top of them. These pieces of Earth’s top layer are called tectonic plates . They are moving very slowly, but constantly. (Most plates are moving about as fast as your fingernails are growing -- not very fast!) Currently Earth’s surface layers are divided into nine very large plates and several smaller ones.
According to the theory of plate tectonics, the earth’s outer shell is not one solid piece of rock. Instead the earth’s crust is broken into a number of moving plates. The plates vary in size and thickness.
Wegner’s theory was that about 180 million years ago, Pangaea began to break up into separate continents. To back this theory up, he perserved remains and evidence from ancient animals and plants from South America, Africa, India, and Australia that were almost identical.
What happens when plates crash into each other depends on the types of plates involved.
Because continental crust is lighter than oceanic crust, continental plates ”float” higher.
Therefore, when an oceanic plate meets a continetnal plate, it slides under the lighter plate and down into the mantle. The slab of oceanic rock melts when the endges get to a depth which is hot enough. A temperature hot enough to melt si about a thousand degrees!) This process is called subduction . Molten material produced in a subduction zone can rise to the earth’s surface and cause volcanic building, mountains, and islands.
Earth’s highest mountain range, the Himalayas, was formed millions of years ago when the Indo-Australian Plate crashed into the Eurasian Plate. Even today, the Indo-Australian Plate continues to push against the Eurasian Plate at a rate of about 5 cm a year!
These areas are likely to have a rift valley, earthquake, and volcanic action.
For example: Here, the San Andreas Fault lies on the boundary between two tectonic plates, the north American Plate and the Pacific Plate. The two plates are sliding past each other at a rate of 5 to 6 centimeters each year. This fault frequently plagues California wit hearthquakes.