Formation
of
Earth
About 300 million years ago,
Earth looked differently …
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Moving land masses came together to form the
super continent of Pangaea
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
If they were once together, why are they now apart,
and what forces could move such large land masses?
In 1915, Alfred Wegener, a
German scientist, said that
the only possible answer
was continental drift.
He suggested that 300
million years ago all of the
earth’s land masses, which
were in constant motion,
collided to form one
supercontinent. He called it
Pangaea (“all land”).
Wegener’s
Theory
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Wegener believed that the
land masses broke into pieces
due to the weaknesses in the
earth's crust. They were made
up of less dense materials and
caused them to drift centimeter
by centimeter over millions of
years until they arrived at
where they are now. Figure
1.5 shows how the Pangaea
split up into plates and drifted
over the millions of years.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
This figure shows how the Pangaea split up into plates and
drifted over the millions of years.
What Kind of Evidence
Supported Continental
Drift?
He saw the jigsaw fit
between South
America and Africa
What Kind of Evidence
Supported Continental
Drift?
He found fossils of the
same plants and
animals on both
continents. He felt that
they could only exist in
both places if the
continents were once
joined together.
What Kind of Evidence
Supported Continental
Drift?
Wegener tried to prove that there were
mountains that were similar in age and structure
on both sides of the Atlantic Ocean - the
Appalachians in the eastern United States and
Canada, and similar mountains in the northern
British Isles and Europe. These mountains
formed about 300 million years ago when North
America collided with Europe and Northern
Africa
What Kind of Evidence
Supported Continental Drift?
• Ice sheets covered southern Africa, India,
Australia, and South America about 250 million
years ago.
• Matching glacial deposits were found
• How could this ice develop in places that are so
warm today? The only explanation seemed to be
that at one time the continents were located closer
to the South Pole.
What Kind of Evidence
Supported Continental Drift?
Past climates
EVIDENCE THAT SUPPORTED
CONTINENTAL DRIFT THEORY
1. Jig-saw puzzle fit of continents
2. Matching rocks and mountain belts
3. Similar fossils on different
continents
4. Evidence of similar past climates
5. Matching glacial deposits
Wegener’s Theory?
• Most scientists disagreed with Wegener
• he could not explain how the continents
moved
• Canadian J. Tuzo Wilson helped spark
new interest in theory.
• By 1968, a new theory, known as plate
tectonics, was developed
Plate Tectonics
What does
tectonic mean?
It relates to the
internal force
which deforms
the earth’s crust
The Earth
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
• Our planet might
seem fixed and rigid,
but a closer look
reveals that it is
constantly shifting
under our feet.
Earth's Structure
What's inside the earth?
• the early part of the 20th century,
geologists studied the vibrations
(seismic waves) generated by
earthquakes to learn more about
the structure of the earth's interior.
They discovered that it is made up
of these distinct layers: the crust,
the mantle, and the core.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Crust - hard and rigid, it’s the earth’s outermost and thinnest layer,
only a few miles (5 km) thick under the oceans and averaging 20
miles (30 km) thick under the continents
Mantle - subdivided into 2 regions, upper and lower, this dense
layer made of hot, semisolid rock is located directly below the crust
and is about 1800 miles (2,900 km) thick
Inner Core - an extremely hot, solid sphere of mostly iron and
nickel at the center of the earth. It is 3,200 to 3960 miles (1200 km)
in diameter
Outer Core - the outer core is the only liquid layer of the earth - a
sea of mostly iron and nickel. It is roughly 1800 to 3200 miles
92890 to 5150 km) below the surface and about 1400 miles (2300
km) thick
• Plate tectonics is the theory that
helps explain most geologic
processes
• It states that the earth’s outer
shell is made up of about twenty
plates
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
• Most of these plates are made up
of both a continent and an ocean.
• They are moving over a weak
layer of hot rock, several hundred
kilometres below the earth’s
surface, which flows like slowmoving plastic
• Unequal distribution of heat within
the earth causes convection
currents to move the plates
Plate Movement
• “Plates” of lithosphere are moved around
by the underlying hot mantle convection
cells
How Do The Plates Move?
• There are three ways:
Convergent: the plates move toward
each other and collide
Transform: the plates
slide past each
other
Divergent: the plates move away
from each other
Divergent Boundaries
•Spreading ridges
–As plates move apart new material is erupted to fill the gap
Convergent Boundaries
• There are three styles of convergent
plate boundaries
– Continent-continent collision
– Continent-oceanic crust collision
– Ocean-ocean collision
Continent-Continent Collision
• Forms mountains, e.g. European Alps, Himalayas
Neither side of the boundary wants to sink beneath the other side, and
as a result the two plates push against each other and the crust
buckles and cracks, pushing up (and down into the mantle) high
mountain ranges. For example, the European Alps and Himalayas
formed this way.
Continent-Oceanic Crust Collision
• Called SUBDUCTION
At a convergent boundary where continental crust pushes against oceanic crust, the oceanic crust
which is thinner and more dense than the continental crust, sinks below the continental crust.
•This is called a Subduction Zone.
•The oceanic crust descends into the mantle at a rate of centimetres per year. When the subducting
slab reaches a depth of around 100 kilometres, it dehydrates and releases water into the overlying
mantle wedge The addition of water into the mantle wedge changes the melting point of the molten
material there forming new melt which rises up into the overlying continental crust forming
volcanoes.
Ocean-Ocean Plate Collision
• When two oceanic plates collide, one runs over the
other which causes it to sink into the mantle
forming a subduction zone.
• The subducting plate is bent downward to form a
very deep depression in the ocean floor called a
trench.
• the denser oceanic crust dives beneath the less
dense continental plate.
•
• The worlds deepest parts of the ocean are found
along trenches.
– E.g. The Mariana Trench is 11 km deep!
Manned or unmanned submersible vehicles (top left photo) have
explored small parts of trenches discovering new species (like the fish
photographed here) and amazing ecosystems.
Transform Boundaries
• Where plates slide past each other
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Perhaps the most famous transform boundary in the
world is the San Andreas fault, shown in the drawing
above. The slice of California to the west of the fault is
slowly moving north relative to the rest of California.
Since motion along the fault is sideways and not vertical,
Los Angeles will not crack off and fall into the ocean as
popularly thought, but it will simply creep towards San
Francisco at about 6 centimeters per year. In about ten
million years, the two cities will be side by side!
Although transform boundaries
are not marked by spectacular
surface features, their sliding
motion causes lots of
earthquakes.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
How fast do plates move?
• About the same rate that finger
nails grow. Usually a few
centimeters per year
• (approx. 3-4 cm/year)
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.