Download Unit 3 Lesson 1 Geological History

Grade 9 Geography of Canada
Unit 3 Lesson 1
Continental Drift
Question
How could
continents
move
(Continental
Drift)?
Facts
Earth’s crust
consists of about 20
plates
Plates move
(Wegener’s
Continental Drift
theory)
Plates “float” on top
of the Mantle.
Part of the Mantle is
the asthenosphere.
It is extremely hot,
liquid and moving
slowly
Wegener found
many similarities
between continents
(e.g., fossils,
landforms)
Movement of liquid
within the Mantle
occurs by
convection
Other Information
Ideas and Hypotheses
As gas and
liquid is heated,
it loses density
(e.g., hot air
rises because
molecules in
the air become
further apart
and, thus, less
dense). As the
heated
substance
rises, it cools,
becomes more
dense, and
begins to fall.
Ocean currents push
the continents
through the Earth’s
crust (Wegener’s
idea. Why did
ploughing not
change shape of
continents? This is
one reason why few
people supported
Wegener’s thesis.)
Tidal forces caused
by gravitational pull
of the moon.
Centrifugal forces as
the Earth rotates
Pressures created
by rising and
falling within
Mantle caused by
convection
movement
Grade 9 Geography of Canada
Unit 3 Lesson 1
Theory of Plate Tetonics and Continental Drift
Alfred Wegener
At the University of Marberg in 1911, Wegener read a scientific paper that listed
fossils of identical plants and animals found on opposite sides of the Atlantic.
He soon found more cases of similar organisms separated by great oceans.
Science of the period suggested land bridges, now sunken, had once connected
far-flung continents. Wegener noticed the close fit between the coastlines of
Africa and South America. He hypothesized that similarities among organisms
were due, not to land bridges, but to the continents having been joined together
at one time.
Such an insight, to be accepted, would require large amounts of supporting
evidence.
Wegener found that large-scale geological features on separated continents
often matched very closely when the continents were brought together. For
example, the Appalachian Mountains of eastern North America matched with
the Scottish Highlands, and the distinctive rock strata of the Karroo system of
South Africa were identical to those of the Santa Catarina system in Brazil.
Wegener also found that the fossils found in a certain place often indicated a
climate utterly different from the climate of today. For example, fossils of
tropical plants such as ferns and cycads are found today on the Arctic island
of Spitsbergen.
All of these facts supported Wegener's theory of Continental Drift. About 300
million years ago, the continents had formed a single mass called Pangaea (from
the Greek for "all the Earth"). Pangaea had split and its pieces had been moving
away from each other ever since.
Why was Wegener’s Theory Not Readily Accepted?
Wegener could not explain how the continents might move. He thought the
continents were moving through the earth's crust, like icebreakers plowing
through ice sheets, and that centrifugal and tidal forces were responsible for
moving the continents.
Wegener overestimated the rate of continental movement. He suggested that
North America and Europe were moving apart at over 250 cm per year or
about 100X faster than observed today.
Today, Plate Tetonics is widely accepted. Note, Wegener's theory was wrong in
one major point. Continents do not plow through the ocean floor. Instead, both
continents and ocean floor form solid plates which "float" on the asthenosphere,
the underlying rock that is under such tremendous heat and pressure that it
behaves as an extremely viscous liquid. (Incidentally, this is why the older term
"continental drift" is not quite accurate -- both continents and oceanic crust
move.)
The main features of plate tectonics are:
The Earth's surface is covered by a series of crustal plates.
The ocean floors are continually, moving, spreading from the center,
sinking at the edges, and being regenerated.
Convection currents beneath the plates move the crustal plates in different
directions.
The source of heat driving the convection currents is deep in the Earth’s
mantle.
In 1929, Arthur Holmes elaborated on one of Wegener's hypotheses: the mantle
undergoes thermal convection. As a substance is heated, its density decreases
and rises to the surface until it is cooled and sinks again. This repeated heating
and cooling results in a current which may be enough to cause continents to
move. Holmes suggested that this thermal convection was like a conveyor belt
and that the upwelling pressure could break apart a continent and then force the
broken continent in opposite directions carried by the convection currents.
In 1960, Holmes’ theory was modified and called the "Sea-floor Spreading". The
supporting evidence was seafloor features: mid-oceanic ridges, deep sea
trenches, island arcs, geomagnetic patterns and fault patterns.
Mid-Oceanic Ridges
The mid-oceanic ridges rise 3000 meters from the ocean floor and are more than
2000 kilometers wide. These huge underwater mountain ranges have a deep
trench which bisects the length of the ridges and in places is more than 2000
meters deep. Heat flow research revealed that the greatest heat flow was
centered at the crests of these mid-oceanic ridges. Seismic studies show that the
mid-oceanic ridges experience an elevated number of earthquakes. All these
observations indicate intense geological activity at the mid-oceanic ridges.
Geomagnetic Anomalies
Periodically, the Earth's magnetic field reverses. New rock formed from magma
records the orientation of Earth's magnetic field at the time the magma cools.
Study of the sea floor with
magnometers revealed "stripes" of
alternating magnetization parallel to
the mid-oceanic ridges. This is
evidence for continuous formation
of new rock at the ridges. As more
rock forms, older rock is pushed
farther away from the ridge,
producing symmetrical stripes to
either side of the ridge. In the
diagram to the right, the dark
stripes represent ocean floor
generated during "reversed" polar orientation and the lighter stripes represent the
polar orientation we have today. Notice that the patterns on either side of the line
representing the mid-oceanic ridge are mirror images of one another. The
shaded stripes also represent older and older rock as they move away from the
mid-oceanic ridge. Geologists have determined that rocks found in different parts
of the planet with similar ages have the same magnetic characteristics.
Deep Sea Trenches
The deepest waters are found in oceanic trenches, which plunge as deep as
35,000 feet below the ocean surface. These trenches are usually long and
narrow, and run parallel to and near the ocean margins. They are often
associated with and parallel to large continental mountain ranges. There is also
an observed parallel association of trenches and island arcs. Like the midoceanic ridges, the trenches are seismically active, but unlike the ridges they
have low levels of heat flow. Scientists also began to realize that the youngest
regions of the ocean floor were along the mid-oceanic ridges, and that the age of
the ocean floor increased as the distance from the ridges increased. In addition,
it has been determined that the oldest seafloor often ends in the deep-sea
trenches.
Island Arcs
Chains of islands are found throughout the oceans and especially in the
western Pacific margins: the Aleutians, Japan, Philippines, and Indonesia.
These "Island arcs" are usually situated along deep-sea trenches and are
situated on the continental side of the trench.
These observations, along with many other studies of our planet, support the
theory that underneath the Earth's crust (the lithosphere: a solid array of
plates) is a malleable layer of heated rock known as the asthenosphere which
is heated by radioactive decay of elements such as Uranium, Thorium, and
Potassium. Because the radioactive source of heat is deep within the mantle,
the fluid asthenosphere circulates as convection currents underneath the
solid lithosphere.
This heated layer is the source of lava we see in volcanoes, the source of
heat that drives hot springs and geysers, and the source of raw material
which pushes up the mid-oceanic ridges and forms new ocean floor. Magma
continuously wells upwards at the mid-oceanic ridges (arrows) producing
currents of magma flowing in opposite directions and thus generating the
forces that pull the sea floor apart at the mid-oceanic ridges. As the ocean
floor is spread apart cracks appear in the middle of the ridges allowing molten
magma to surface through the cracks to form the newest ocean floor.
As the ocean floor moves away from the mid-oceanic ridge it will eventually
come into contact with a continental plate and will be subducted underneath
the continent. Finally, the lithosphere will be driven back into the
asthenosphere where it returns to a heated state.
Grade 9 Geography of Canada - Unit 3 Lesson 1
Our Dynamic Earth
Video – National Geographic – Teacher Resource Centre – Call 551.D96 KIT
Questions
1. What does plate tectonics theory attempt to explain?
Plate tectonic theory primarily attempts to explain movements of the Earth’s crust
(lithosphere).
2. What was the name of the single supercontinent that existed 200 million years ago?
The name of the supercontinent was the Pangaea.
3. What created the Himalayas?
The HImalayas were created by the Indian Plate crashing into the Eurasian Plate.
Crashing is the common wording. Since plates only move a few centimetres per
year, it can hardly be called crashing.
4. What do the plates of the lithosphere (crust and upper mantle) float on?
The plates float on the asthenosphere within the mantle.
5. Name three major plates.
6. Describe the major forms of plate margin behaviour?
Divergent (spreading) – occurs when plates separate.
Subduction – occurs when converging plates collide and the lighter plate plunges
under the other.
Lateral or transform – occurs when plates slide past each other.
Continent-continent convergence – occurs when two continental plates collide.
7. What type of plate activity is occurring off the west coast of South America and what
is the result?
Subduction is taking place as the Nazca Plate subducts under the South American
Plate to form the Andes Mountain. Volcanoes and earthquakes also are produced
during this process.
8. What is occurring alon gthe San Andreas Fault and what can be the result?
Lateral motion is occurring as a result of the Pacific Plate sliding past the North
American Plate. As stress is released along the plate boundaries, large earthquakes
are produced.
9. Describe the appearance of lava flows along the ocean floor
They look like pillows. Hence, they are called Pillow Lavas.
Our Dynamic Earth…Page 2
10. Describe the underwater flow of lava in Hawaii.
Lava quickly changes form as it is cooled in the seawater. It also creates tubes
through which molten lava travels, re-emerges at the end of the tube, and then, flows
into the sea. When seawater comes in contact with lava, the lava is quickly chilled
and breaks down into small black particles forming black-sand beaches.
11. Describe how life can flourish at great depths where light cannot penetrate.
Seawater, which mixes with the magma beneath the seafloor, is heated and
chemically altered. It then rises to the seafloor, emerging as thermal vents and
“black smokers.” Chemosynthetic bacteria are able to metabolize sulfur chemicals in
this hot water, forming the basis of the entire food web, including crabs, tube worms
and clams.
12. What enabled people to discover the deep-sea vents?
Deep-diving submarine enabled researchers to discover deep-sea vents and study
life at extreme depths.
13. What is the promise of plate tectonics?
Knowledge of plate tectonics may allow researchers to predict future volcanic and
seismic activity, and to discover how to live in harmony with our dynamic Earth.
14. How might additional understanding of plate dynamics improve our lives?
Additional knowledge of the movement of plates and the dynamics of Earth
processes could lead to better prediction of earthquakes and volcanoes, and improve
our ability to harness the Earth’s energy.
15. What evidence is there that the Earth is a dynamic planet?
Volcanoes and earthquakes are strong indicators that the Earth is in motion. Less
obvious, but just as strong, evidence is provided by the gradual movement by the
plates.