Plate Tectonics
Since the 1600s, people have wondered why the Earths landmasses look like they would fit together.
Were they connected at one time? If so, how did they move apart? Finally, in 1915, someone came up
with a satisfactory—but controversial—explanation.
Earths Dividing Continents
Alfred Wegener, a German scientist, proposed a theory to explain many phenomena that occur on the
Earths crust. This theory is called the theory of continental drift. According to Wegener, the Earth once
had a single landmass, which broke up into large pieces that have since drifted apart. This giant landmass
of the distant past is called Pangaea.
There was another theory that stated that there must have been a land bridge that connected Africa and
South America at one time. There are fossils of plants and animals to support this theory.
Evidence from fossils, preserved remains of ancient organisms, supports Wegener’s theory of continental
drift. Identical types of fossils have been in Africa and South America. The only explanation is that the
places in which the fossils were found must once have been closer together.
More evidence is found in rock formations in Africa that line up with matching ones in South America.
Many glacial deposits are similar in Africa, South America, India, and Australia, indicating that they were
left by the same ice sheets.
Earths Spreading Ocean Floor
Large systems of underwater mountains that have a deep crack, called a rift valley, running through the
center are known as midocean ridges. Lave erupts from the rift valley that runs the length of a ridge. As
the ocean floor moves away on either side of the ridge, lava wells up and hardens. The hardened lava
forms new ocean floor. This process is called ocean-floor spreading. Ocean-floor spreading helps to
explain how continents drift. As a pieceo f the ocean floor moves, it takes its continents with it.
Straight ridge sections are offset by thin cracks known as transform faults. Many earthquakes occur along
the faults.
Rock samples from the ocean floor indicate that rocks next to a midocean ridge are younger than rocks
farther away. The youngest rocks are in the center of the ridge. As the ocean floor spreads, the older rocks
move farther away from the ridge.
The deepest parts of the oceans are V-shaped valleys called trenches. As new ocean floor is formed, the
older ocean floor moves down deep into the Earth along the trenches in a process known as subduction.
When the rocks are pushed deep enough they melt from heat. Then molten rock either pushes up to form
volcanoes or becomes part of the mantle. As new rocks are formed at ridges in the ocean, old rocks are
subducted into trenches and Earths crust will remain the same. So as new rocks are formed along the
midocean ridges, older rocks are subducted into the trenches.
Earths Moving Plates
The evolution of the Earth can be explained with the theory of plate tectonics. Plates are moving,
irregularly shaped slabs that fit together like paving stones to form the surface layer of the Earth. The
plates carry the continents and are edged by trenches and ridges. The word tectonics refers to the branch
of geology that deals with the movements that shape the Earths crust. The theory of plate tectonics, which
links together the ideas of continental drift and ocean-floor spreading, explains how the Earth has evolved
over time. It helps to explain the formation, movements, collisions, and destruction of the Earths crust.
According to the theory, the topmost solid part of the Earth, called the lithosphere, is made of a number of
plates. The plates contain a thin layer of crust above a thick layer of relatively cool, rigid mantle rock.
There are seven major lithospheric plates. All of them move at different speeds and in different directions
and all of the plates contain both continental and oceanic crust. The seven major lithospheric plates are:
 Pacific Plate—the largest plate that contains 1/5 of Earths surface
 North American
 South American
 Eurasian
 African
 Antarctic
 Indo-Australian
There are three types of plate boundaries.
 The first type is a midocean ridge. Because the plates move apart at midocean ridges, the ridges
are called divergent boundaries. Volcanoes erupt at the midocean ridges and when the lava
hardens new ocean floor is formed. Mid ocean ridges form the single largest mountain chain in the
world. Approximately the 20 times the length from New York to Los Angeles
 The second type of plate boundary is a trench. Because the plates come together at the trenches,
the trenches are called convergent boundaries. The collision of plates at convergent boundaries
produces earthquakes and volcanoes.
 The third type of plate boundary is formed by a transform fault. These boundaries are formed by a
transform fault. These boundaries are called strike-slip boundaries. The two plates grind together
and slip past each other horizontally. Earthquakes are common along strike-slip boundaries.
Scientists think that convection currents within the Earth may be what makes the plates move. The
convection currents are created by the circular motion of the molten material in the mantle due to
differences in its temperature. Convection currents move air in the atmosphere and water in the ocean.
Material close to the Earths core is very hot—the material farther from the core is cooler and denser. The
cooler material sinks down to the core. The hot material then pushes up to replace the cooler material. As
the cooler material nears the core, it becomes hot and rises again. This cycle repeats. This type of circular
motion carries the lithospheric plates, which causes the plates to move. When plates collide at a
convergent boundary, the denser plate is subducted and the other plate floats over it. When two oceanic
plates collide the older plate is subducted under the younger one. When two continental plated collide, a
large mountain range is formed. This motion carries the plates of the lithosphere along with it.