Unit 4
Chapter 10
Plate Tectonics
Section 1 Continental Drift
Wegener’s Hypothesis
In 1912 German scientist Alfred Wegener
proposed the theory of Continental Drift.
He stated that the continents seemed to fit
together and have since moved away from
each other. According to Wegener, the
super continent started breaking up about
200 million years ago.
Evidence
Fossil Evidence
Wegener found similarities in the coastlines of the continents, and he found other evidence as
well. He found identical fossils of the mesosaurus that were located on the coasts of
Eastern South America and Western Africa. This animal did not swim across the ocean.
Evidence from Rock Formations
Along with fossil evidence, rock evidence pointed to the lands being joined at one time. The
Appalachian Mountains extend Northward along Eastern Coast of North America are the
same age and structure of the mountains found in Greenland, Scotland and Northern
Europe.
Climatic Evidence
Ancient glacial debris was found in Southern Africa and South America.
Missing mechanisms
Wegener could not come up with the “how” part of the hypothesis. He died in 1930 before
anyone would discover the actual answer.
Mid Ocean Ridges
In 1947 Scientists set out to map the Mid-Atlantic Ridge, which was part of the underground
mountain ranges. They discovered two surprising trends.
Sediments were thinner near the center and thicker farther away from the center.
The ocean floor was younger than the continents. Youngest rocks were by the mid-ocean
ridge.
Sea-Floor Spreading
In the late 1950’s geologist Harry Hess came up with a
new hypothesis. He proposed that the center valley
was actually a ridge where molten rock would come
to the surface and fill the cracks and push the land
away.
Robert Dietz renamed the process as “Sea Floor
Spreading”. Hess’s hypothesizes were proven with
more evidence discovered in the mid 1960’s. The
evidence was discovered through paleomagnetism
which is the study of the magnetic properties of
rocks.
Paleomagnetism
Paleomagnetism- when magma hardens the iron-rich mineral
aligns with Earth’s magnetic field.
Magnetic Reversals
It has been discovered that Earth’s magnetic properties have changed orientation. Since the
rocks magnetism goes along with the magnetic field, Scientists have discovered a pattern
of normal (points north) and reversal (points south) magnetic orientations in rocks along
the mid ocean ridge. The pattern it created is called the geomagnetic reversal time scale.
Magnetic Symmetry
Scientists discovered that our form the center of the mid ocean ridge to the edges of the
continents a pattern of normal and reversed polarity formed. These patterns were mirror
images on either side of the ridge. They also discovered the youngest rocks were on either
side of the ridge with the oldest rocks by the continents.
The symmetry of both the magnetic patterns and the rock ages supports Hess’s theory.
Wegener Redeemed
Another group of scientists discovered the reversal pattern on land was similar to the one in
the ocean. This added to the support of the continental drift theory. They also suggested
that the mechanics involved was similar to a conveyor belt moving on both sides of the mid
ocean ridge.
Section 2 The Theory of Plate Tectonics
In the 1960s, the evidence supporting the continental drift theory and sea floor
spreading lead to a new theory called Plate Tectonics.
Plate Tectonics Is the study of the formation and movement of those plates
The plates are attached to the lithosphere and move that way.
How continents Move
Lithosphere - solid layer of crust & uppermost part of the mantle
-Similar composition (100 kms)
Two types of crust
Continental – made up of rocks rich in silica (granites), less dense 2.7
Oceanic – made up of rocks rich in iron and magnesium (basalts), more
dense 3.0
Asthenosphere -lower part of the lithosphere similar composition but much
hotter almost melted - the rock flows in CONVECTION CURRENTS which
allows the rigid upper crust to move.
Tectonic Plates
There are approximately 15 major plates that fit together on the earth’s
surface like a jigsaw puzzle.
http://www.p12.nysed.gov/osa/reftable/earthscience-rt/esrt2010engw.pdf
Earthquakes
Sudden shifts or movements along plate boundaries
Volcanoes
They are formed when the movement of plates generates a magma
buildup that erupts.
Types of Boundaries
Boundaries are areas on Earth’s surface where volcanic and earthquake
activity occurs.
Diverging Boundaries
When two plates moving apart, deep areas the length of the
ridge
Mid ocean ridges (between America & Africa)
Convergent Boundaries - 2 plates moving together (converging)
Convergent Boundaries - 2 plates moving together (converging)
Collision Boundary -two continents colliding together to make a larger continent - India moving
into Asia- Himalayan Mountains are the result.
Transform Boundaries -plates sliding past each other - California/Pacific plate sliding past the
American Plate
Fault -crack in the earth's crust where movement has occurred
San Andrea's Fault moves 5cm/year
Thin Skin Thrusting -pushing of thin rock layers at continental margins over long distance
(Appalachian Mountains)
Causes of Plate Motion
Mantle convection is the driving force of the plates. Convection is the
movement of heated material.
Mantle Convection
This is the heat transfer through the mantle where less dense, heated material
rises. The cooler denser material flows away from the hot material and sinks
back into the mantle to replace the rising material.
Moho – Real name Mohorovicic discontinuity
It is the boundary between the crust and the mantle where dense rock of the
mantle meets less dense rock of the crust. It is an average of 32 km under the
continents, and 8 km under the sea. This is where most of the action occurs.
Ridge Push
This happens when the cooling lithosphere slides down the slope formed
by the elevation of the mid-ocean ridge. The force pushed the rest of the
plate away.
Slab Pull
This happens when the leading edge of the subducting plate moves under
the upper plate. It pulls the plate back into the asthenosphere.
Scientists believe that all three mechanisms together create the
convecting system that makes the Earth’s tectonic plates move
constantly.
Section 3 The Changing Continents
The continents did not always look they way they do
now and through time, they will once again change
shape and positions. Remember, we are Dynamic.
Reshaping Earth’s Crust
Craton – large areas of rock more than 540 million years
old
Shields – areas of craton exposed to earth’s elements
Rifting and Continental Breakup
Rifting
Is the process by which Earth’s crust breaks apart
anywhere, either on a continent or in the ocean. It is
the pressure build up that causes the crust to break
and move.
Terranes and Continental Growth
Terrane
It is a piece of lithosphere that has a unique geologic history
Contains rocks & fossils that differ from the surrounding area
Has major faults at the boundaries
Has magnetic properties that do not match an area
As oceanic crust subducts, a terrane is scraped off and becomes part of
the continental crust (accretion). It can also be continental crusts
colliding. The Himalaya Mountains formed this way 500 million
years ago.
Effects of Continental Change
When the continent moved, it changed the climate. The nearness of the
continent to the equator or the poles brought about a different climate.
The location by oceans and other continents plays an important role
too. When continents move, they also change the flow of air round
the globe.
Change in Climate
Geologic evidence shows ice once covered most of Earth’s surfaces. Even
the Sahara had ice. This occurred about 450 million years ago when
all the land was located near the South Pole.
Change in Life
The separating of the continents caused species to have to evolve into
different ones. Isolation of species also caused life to adapt to its new
environments.
The Supercontinent Cycle
When the continents are arranged as to create a large landmass
called a Supercontinent. These supercontinents break apart
and can form new and different ones. This is called the
Supercontinent cycle.
Why Super Continents Form
Supercontinents form when convergent boundaries between two
continents collide, neither plate subducts, the boundaries
become inactive and a new supercontinent is formed. When
the pressure builds up, the continent breaks apart.
Formation of Pangaea
Pangaea formed 300 million years ago. It also formed the
Appalachian Mountains and the Ural Mountains of Russia.
The Tethys Sea cut into Pangaea and the ocean that
surrounded it was later called Panthalassa.
Break up of Pangaea
200 million years ago during the Mesozoic Era, Pangaea broke up into 2 continents,
Laurasia and Gondwanaland.
Laurasia drifted northward and rotated and then split into North America and Eurasia
with the North Atlantic Ocean between them. It also shrank the Tethys Sea to
become the Mediterranean Ocean.
Gondwanaland also changed, South America and Africa were formed. Then about 150
million years ago a split began to happen between South America and Africa opening
up the South Atlantic Ocean. The other chunk of Gondwanaland broke to form India,
Australia and the Antarctic. India started to move northward and at about 50 million
years ago, it collided with Eurasia to form the Himalayan Mountains.
The Modern Continents
As the continents continued to shift and collide,
other features were formed. The Rocky
Mountains, the Alps and the Andes Mountains
were all formed at this time.
Geology of the Future
Some Scientists believe in approximately 150 million
years, Africa may collide with Eurasia and the
Mediterranean will close. New subductive zones
will form by the East Coast of North and South
America and they will collide with Eurasia. The
Atlantic Ocean will disappear. Mexico’s Baja
Peninsula will become part of Alaska. In about
250 million years from now we will have another
supercontinent.