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Plate Tectonics
Earth’s Drifting Continents
The theory of continental drift, stated by Wegner,
a German meteorologist, says that Earth once
had a single landmass that broke up into pieces,
which have since drifted apart.
Scientists call this giant landmass, Pangaea,
which means all Earth.
Evidence from Fossils
Evidence from identical fossils found in both South
America and Africa supports Wegner’s theory.
Fossils of Glossopteris, an extinct plant, found in rocks
from South Africa, Australia, India and Antarctica
provide support. The seeds of this plant were too large
to have been carried by the wind and too fragile to have
survived the ocean waves.
Antarctica
The presence of Glossopteris fossils in the
frozen wasteland of Antarctica also indicate that
the climate must have changed greatly.
Antarctica must have changed position.
Evidence from Rock
An ancient folded mountain chain formed in Africa lines
up with matching folded mountains in South America.
Coal field layers found in Brazil line up with coal field
layers in Africa.
Glacial deposits found in areas of warm climate also
provide evidence.
Salt deposits normally formed between 35 and 10
degrees North and South of the equator are found as
far north as Michigan.
Earth’s Spreading Ocean Floor
Scientists discovered a large system of
underwater mountains that have a deep crack,
called a rift valley, running through their center,
known as the midocean ridges. A great deal of
volcanic activity occurs at the midocean ridges.
Lava erupts from the rift valley. The hardened
lava forms new ocean floor.This process is
known as ocean-floor spreading.
Transform Faults
Ocean-floor spreading helps to explain how continents
drift. As a piece of the ocean floor moves, it takes its
continent (if it has one) with it.
Individual sections of midocean ridges are straight but
the ridges as a whole curve.This is because the straight
sections are offset by thin cracks known as transform
faults.The youngest rock is near the midocean ridge.
As the floor spreads, older rocks move farther away
from the ridge.
Magnetic Stripes
A permanent record of the Earth’s magnetism
remains in the rocks.
Scientist discovered that the Earth’s magnetic
poles reverse themselves from time to
time.Studies show that during the past 3.5
million years, the magnetic poles have reversed
themselves nine times. The pattern is identical
on both sides of the midocean ridge.
Subduction
As the older ocean floor moves away from the
midocean ridges, it will eventually move down
deep into the Earth along the trenches. When
the rocks are pushed deep enough, they are
melted by the heat of the Earth. Some of it will
rise up through the crust and produce volcanoes
but most of the molten rock will become part of
the mantle.
Earth’s Moving Plates
The word plate refers to the moving, irregularly shaped
slab that fit together like paving stones to form the
surface of the Earth. The word tectonics refers to the
branch of geology that deals with the movements that
shape the Earth’s crust.
The theory of plate tectonics links continental drift and
ocean-floor spreading to explain the formation,
collisions, movements and destructions of the Earth’s
crust.
Lithospheric Plates
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.Plates
usually contain both oceanic and continental
crust.
There are seven major lithospheric plates, each
of which is named after its surface features.
Seven Lithospheric Plates
The Pacific plate covers 1/5 of the Earth’s
surface.
The other major plates are the North American,
South American, Eurasian, African, IndoAustralian and Antarctic plates.
There are also smaller plates such as the
Caribbean and Arabian plates.
Plate Boundaries
Plates move at different speeds and in different
directions. Smaller landmasses move more
slowly.
In a few cases, the edges of the continents are
the boundaries of plates.Most plate boundaries
are on the ocean floor.
Types of Plate Boundaries
There are three types of plate boundaries.
Convergent
Divergent
Strike-Slip Boundaries
Divergent Boundaries
The first type occurs at midocean ridges.
Because the plates move apart, the ridges are
called divergent. These boundaries are also
called constructive boundaries.
Convergent Boundaries
The second type of plate boundary has trenches.
Because the plates come together at the trenches, the
trenches are called convergent. They are also called
destructive.
The collision of plates at convergent boundaries causes
tremendous friction and pressure. Earthquakes often
result. This explains why the Ring of Fire follows the
major ocean trenches in that area.
Strike-Slip Boundaries
The third type of plate boundary is formed by a
lateral fault. Boundaries formed by lateral
boundaries are called strike-slip boundaries. At
a strike-slip boundary, two plates grind together
and slip past each other horizontally.No new
plate material is destroyed or made.Earthquakes
often occur along strike-slip boundaries, such as
the San Andreas Fault.
Plate Motion
Scientists are not sure exactly what makes the
plates move. One hypothesis is that large
convection currents within the Earth move the
plates. A convection current is the movement of
material caused by differences in temperature.
Mantle material close to the core is very hot.
Mantle material farther from the core is cooler
and less dense. Rising and sinking of magma
occurs in a circular motion.
Collision of Oceanic/Continental
Plates
Oceanic plates are more dense than continental
plates.When an oceanic plate collides with a
continental plate, the continental plate rides over
th edge of the oceanic plate. The subduction of
the oceanic plate pushes up and folds the
continental crust on the edge of the continental
plate and forms mountain ranges such as the
Andes of South America.
Oceanic Plate Collision
When two oceanic plates collide, the older
oceanic plate is subducted under the younger.
(Plates grow denser as they cool.)The plate
being subducted melts. Molten rock then rises
up and breaks through the surface. As a result,
a string of volcanoes erupts on the ocean floor
along the trench. Sometimes they may rise
above the ocean’s surface as a string of islands,
such as Japan and the Aleutian Islands.
Collision of Continental Plates
When two continental plates collide, the edges of
the continents fold upward to form large
mountain ranges, such as the Appalachian
Mountains. They were formed when Africa
collided with North America during the formation
of Pangaea. In some cases they have same
density and the edges of the continents thicken
and are pushed upward to form ranges like the
Himalayas.
Possible Changes in Plates
Any change in one plate or boundary affects all
the other plates and boundaries. There are
many changes that can occur in plates and their
boundaries. Continental plates may fuse
together. A trench may switch direction and
begin to subduct a formerly overriding plate.
New divergent boundaries may form in the
center of continents. Plates may also be
completely subducted and disappear.