CONTINENTAL DRIFT
In 1912, Alfred Wegener proposed the hypothesis of Continental Drift, which
stated that the continents had moved. Wegener hypothesized that the continents
once formed part of a single landmass which he named Pangaea. According to
Wegener, about 200 million years ago, Pangaea began breaking up into smaller
continents, which drifted to their present locations. Wegener speculated that this
motion may have crumpled the crust in places, producing mountain ranges.
EVIDENCE FOR CONTINENTAL DRIFT
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Continents looked like they fit together like a Jigsaw puzzle
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Identical fossil remains of mesosaurus, a small, extinct land reptile was
found in both eastern South America and western Africa. These animals
could not have swum across the Atlantic Ocean, therefore, Wegener
concluded that South America and western Africa were once joined. Remains
of warm weather plants were found in Arctic regions where they could not
grow today.
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The age and type of rocks in the coastal regions of widely separated areas
such as western Africa and eastern Brazil, matched closely. Mountain ranges
that ended at the coastline of one continent seemed to continue on
landmasses across the ocean. For example, the Appalachian mountains in the
U.S., are similar in age and structure to mountains in Greenland and northern
Europe. If these 3 landmasses are pushed together in a model of Pangaea,
the mountains fit together in one continuous chain.
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Changes in climate patterns. Layers of debris from glaciers were found in
southern Africa and South America – areas that today have climates too
warm for glaciers. Other fossil evidence, such as the coal deposits in the
eastern U.S., Europe, and Siberia, indicated that tropical or subtropical
swamps covered much of the Northern Hemisphere. If the continents were
once joined and positioned over the South Pole, these climatic differences
would be easy to explain.
Despite this evidence, many scientists did not believe in Wegener’s hypothesis of
continental drift because they did not know what force could be causing it.
SEAFLOOR SPREADING
In 1947, a group of scientist set out to map the Mid-Atlantic Ridge, an undersea
mountain range with a steep narrow valley running down its center. The MidAtlantic Ridge is one part of an entire system of mid-ocean ridges that go around
the earth. The scientists found that the ocean floor was very young compared to
the age of continental rocks. Harry Hess suggested that the valley at the center
of the ridge was actually a break or rift in the earth’s crust and that molten rock
or magma from deep inside the earth was rising up through the rift. Hess thought
mid-ocean ridge would be newer than the rock that was farther away from the
ridge. If the seafloor was spreading through the formation of new crust, then the
continents must also be moving.
When the magma cools and solidifies, certain iron-containing minerals within the
rock become magnetized. When the rock hardens, the magnetic orientation of the
minerals becomes permanent and points north. However, scientists have
discovered that throughout the earth’s history, the magnetic field has reversed
itself several times. The ocean floor showed alternating bands of normal and
reversed magnetism. They discovered that the striped patterns of magnetism on
one side of the ridge mirrored the other side exactly. This proved that seafloor
spreading does occur.
THEORY OF PLATE TECTONICS
The hypotheses of continental drift and seafloor spreading led to the broader
theory of plate tectonics. Tectonics comes from the Greek word for construction
and is the study of the formation of features in the earth’s crust.
The earth’s crust consists of two types, oceanic crust and continental crust.
Material on the ocean floor forms oceanic crust. Land is made of continental
crust.
The oceanic and continental crust and the rigid upper mantle make up the
lithosphere. It forms the thin outer shell of the earth. Beneath the lithosphere
lies the asthenosphere, a layer of plastic-like rock that slowly flows like putty.
The lithosphere is broken into separate plates that ride on the denser
asthenosphere like rafts on water. Most lithospheric plates are composed of both
continental and oceanic crust. So far, about 30 lithospheric plates have been
identified. Some plates are moving toward each other, some are moving apart, and
some are sliding past each other.
There are three types of plate boundaries and they act in different ways. They
are divergent boundaries, convergent boundaries and transform fault
boundaries. Two plates moving away from each other form a divergent boundary.
As the plates move apart, magma, or molten rock from the asthenosphere rises and
fills the space between the plates. As the molten rock cools, it hardens onto the
edges of the separating plates and creates new oceanic crust. Most divergent
boundaries are found on the ocean floor along the mid-ocean ridges. In the center
of a mid-ocean ridge is a narrow valley formed as the plates separate. This
formation is called a rift valley. Other rift valleys may form where continents are
separated by plate movement.
As seafloor spreading pulls plates apart at one boundary, those plates collide with
other plates at other boundaries. The type of boundary where one plate collides
with another is called a convergent boundary. Three types of collisions can occur
at convergent boundaries. One type occurs when an oceanic plate collides with a
continental plate. Since the oceanic plate is denser, it is forced under the less
dense continental crust. The area where this happens is called a subduction zone.
A deep ocean trench generally formas along a subduction zone. As the oceanic
plate moves down into a subduction zone, it melts and becomes part of the mantle
material, Some of the magma formed rises to the surface through the continental
crust and produces volcanic mountains.
In the second type of convergent boundary collision, two continental plates come
together. Neither plate is subducted because the plates have the same density.
The colliding plates are crumpled and uplifted producing large mountain ranges.
The Himalayas were formed by this type of collision.
The third type of collision along convergent boundaries occurs between oceanic
crust and oceanic crust. A deep ocean trench forms when one of these plates is
subducted. Part of the subducted plate melts, and the resulting molten rock rises
to the surface along the trench to form a chain of volcanic islands.
A transform fault boundary forms where two plates are grinding past each other.
A major transform fault boundary is the San Andreas fault in California. It is the
site of many earthquakes.
CAUSES OF PLATE MOTION
Many scientists think that the movement of lithospheric plates is due to
convection, the transfer of heat through the movement of heated fluid material.
Heat from the earth’s core and mantle causes some material in the lower
asthenosphere to become hotter and therefore less dense than the material
above it. The hotter material rises. When this hot material reaches the base of
the lithosphere, it cools. When the molten material cools, it becomes more dense
and starts to sink. The cooling material is pushed to the side by new hot material
that rises. This cycle is known as a convection current. As this process takes
place, the lithospheric plate is carried along with the moving material.
VOLCANIC MOUNTAINS
Mountains that form when molten rock erupts onto the earth’s surface are called
volcanic mountains. They may develop on land or on the ocean floor. The Cascade
Range of Washington, Oregon, and northern California is composed of volcanic
mountains. Some of the largest volcanic mountains are found along divergent plate
boundaries which form the mid-ocean ridges, The mid-ocean ridges are actually
huge volcanic mountain chains that run through the centers of the Atlantic,
eastern Pacific, and Indian oceans. The peaks of the highest mountains sometimes
rise above sea level to form volcanic islands like Iceland.
Other large volcanic mountains are formed on the ocean floor over hot spots. Hot
spots are pockets of magma beneath the earth’s crust that erupt onto the surface.
The Hawaiian Islands are the tips of high volcanic mountains that were formed
over a hot spot on the seafloor.
A dome mountain forms when molten rock rises through the crust and pushes up
the rock layers above it. The result is a circular dome on the earth’s surface.