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Plate Tectonics
Chapter 10-1 Continental Drift
Objectives:
Describe the hypothesis of continental drift.
Identify evidence supporting continental drift.
The Theory of Continental Drift
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.
Wegener’s Evidence
Wegener’s evidence for continental drift came
form four areas:
Fit of the coastlines
Fossil
Rocks
Climate
Fit of the Coastlines
Many of the landmasses,
such as South America
and Africa, appear to fit
together as if they are
pieces of a puzzle.
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.
Evidence from Climate
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.
Break Up of Panagaea
How did the continents move?
Wegener could not explain how the continents
moved. He though they plowed through the
ocean floor. It did not seem possible that the
continents could plow through the harder
basaltic rock of the ocean floor.
Most scientists did not believe his theory until
later it was explained how the continents moved.
Chapter 10-2
Sea floor Spreading
Objectives:
Explain seafloor spreading.
Recognize how age and magnetic clues support
seafloor spreading.
Mapping the Ocean Floor
During World War I German
scientists introduced the
idea of using sound waves
to detect submarines.
In the 1940s scientists
began to use sound waves
to map the ocean floor.
This is sometimes called
echo sounding.
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 mid-ocean ridges.
A great deal of volcanic activity
occurs at the mid-ocean ridges.
Lava erupts from the rift valley.
The hardened lava forms new
ocean floor. This process is known
as sea-floor spreading.
Faults along the Mid-Ocean Ridge
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 mid-ocean
ridges are straight but the ridge
curves. It curves because the
straight sections are offset by thin
cracks known as 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 mid-ocean
ridge.
Ocean Floor
The ocean floor is divided
into three big areas: margin,
basin and ridge.
The ridge is an underwater
mountain chain formed by
cooling magma that comes
out the division between
plates. As they cool, they
smoke and are called black
smokers.
Mid Ocean Ridge
The mid ocean ridge is
the longest topographic
feature on Earth.
There is always a rift
valley on the top of an
ocean ridge system.
The Continental Margin
The margin is made up of
the shelf, the slope and the
rise.
The shelf is the submerged
part of the continent.
The slope is the area that
drops off into the ocean floor.
The rise is made up of the
sand that is dropped at the
bottom of the slope.
Subduction
As the older ocean floor moves
away from the mid-ocean ridges, it
will eventually move down deep
into the Earth along the trenches,
(a long narrow valley on the
ocean floor). 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
along the trench but most of the
molten rock will become part of the
mantle.
Chapter 10-3
The Theory of Plate Tectonics
Objectives:
Compare and contrast different types of plate
boundaries.
Explain how the heat inside the Earth causes
plate tectonics.
Recognize features caused by plate tectonics.
Plate Tectonics
In the 1960s scientists
developed a new theory that
combined continental drift
and seafloor spreading.
This new theory was called
plate tectonics.
According to this theory the
earth is broken into
irregularly shaped sections
that move on a plastic like
layer called the mantle.
Composition of Lithospheric Plates
Plates are made of crust
and a part of the upper
mantle. These two parts
combined are the
lithosphere. It is rigid
and about 100 km thick.
The plastic like layer
below the lithosphere is
known as the
asthenosphere.
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,
Indo-Australian 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
quickly.
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.
Divergent Boundaries
Convergent Boundaries
Transform Boundaries
Divergent Boundaries
The first type occurs at
mid-ocean ridges.
Because the plates move
apart, the ridges are
called divergent. These
boundaries are also
called constructive
boundaries. The average
rate of seafloor
spreading is 5 cm/yr.
Ocean Floor
None of the ocean floor
is older than 180 million
years.
The East Pacific Rise is
spreading more rapidly
than the Mid Atlantic
Ridge.
Continental Rifting
Sometimes there will be
a divergent boundary
under a land mass.
This is happening on the
African plate today.
This is known as
continental rifting.
Red Sea
The Red Sea was a
continental rift valley in
the past.
As Africa and Saudi
Arabia moved apart,
water moved into the rift
valley created and
formed a sea.
Convergent Plate Boundaries
There are three types of convergent plate boundaries:
Ocean - Ocean
Ocean - Continent
This type of plate boundary has trenches. Because the
plates come together at the trenches, these boundaries
are called convergent. They are also called destructive.
Continent – Continent
This does not produce a trench but some land is
destroyed.
Convergent Boundaries
Ring of Fire
The collision of plates at
convergent boundaries
causes tremendous
friction and pressure.
Earthquakes and
volcanoes often result.
This explains why the
Ring of Fire follows the
major ocean trenches in
that area.
Ocean-Continent Plate Boundary
Oceanic plates are more dense than
continental plates. When an oceanic
plate collides with a continental plate,
the continental plate rides over the
edge of the oceanic plate and pushes
the ocean plate underneath. This is
known as subduction.
The subduction of the oceanic plate
causes the plate to start to melt back
into magma. Some of the magma
bubbles up through the land and forms
continental volcanic mountain ranges
(arcs) such as the Andes of South
America and the Cascades of North
America.
Ocean- Ocean Plate Boundary
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,
the Tonga Islands, the Philippines
and the Aleutian Islands.
Continent-Continent Plate Boundary
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.
The Urals formed when Siberian
plate collided with the Baltic plate
and North America.
The Himalayan Mountains formed
when India collided with Eurasia
after the break up of Pangaea.
Transform Boundaries
The third type of plate
boundary is formed when
plates move side by side.
No new plate material is
destroyed or made.
Earthquakes often occur
along strike-slip
boundaries, such as the
San Andreas Fault.
Pacific-North American Plate
Boundary
The Pacific Plate is
moving in a northwest
direction. The North
American plate is moving
to the southwest.
Eventually the part of
California on the Pacific
Plate will move up
toward Alaska and
collide with the Aleutian
Islands.
Causes of Plate Tectonics
Today most scientist think plates
move as convection currents in the
mantle circulate.
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 more dense.
Rising and sinking of magma
occurs in a circular motion.
Faults
When rocks break and
move along surfaces, a
fault occurs.
Faults interrupt rock
layers by moving them
out of place.
Entire mountains can
form this way and are
called fault-block
mountains.
Evidence of Plate Movement
Magnetic stripes on ocean floor
Volcanoes and earthquakes
Lasers and satellites
Hawaiian Islands
The Hawaiian Islands show
direction of plate movement
as well as the rate of
movement in the past.
The Hawaiian Islands formed
over a hot spot in the Pacific
Plate.
As the plate moves over the
hot spot new islands are
created with the youngest
island being over the hot
spot.
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.
Prediction of our Future World