Download Continental drift and plate tectonics

Continental drift and
plate tectonics
f you look closely at a world map, you might notice something about the
coastlines of South America and Africa. The coastlines almost seem to fit
together like two pieces of a jigsaw puzzle. In 1620, Sir Francis Bacon became the
first to write about this very observation after seeing newly drafted maps of the world.
I
ARCTIC
OCEAN
EUROPE
NORTH
A M E R I C A ATLANTIC
PACIFIC
OCEAN
OCEAN
SOUTH
AMERICA
ASIA
Modelling exercise:
evidence of continental drift
AFRICA
INDIAN
OCEAN
AUSTRALIA
During the 1800s and early 1900s,
geologists believed that, as the Earth
cooled, the crust began to shrink.
The continents were just high spots
in the wrinkling, shrinking surface.
However, in 1912, a German
meteorologist named Alfred
Wegener proposed a new theory. He
claimed that the continents were
floating on denser material below.
Wegener believed that the continents
moved around — breaking apart and
rejoining as they moved. He believed
that, at one time, all of the continents
were joined together in a single
super-continent. He called that
continent Pangaea.
Wegener’s theory of ‘continental
drift’ was first published in 1915.
Evidence to support the theory
included the:
• shape of continents
• distribution of fossils
• distribution of rocks.
At first, scientists laughed at
Wegener’s radical new theory.
However, over the past 50 years,
scientists have used the theory of
continental drift to develop the
current theory — the theory of
‘plate tectonics’.
INDIA
AFRICA
SOUTH
AMERICA
AUSTRALIA
Lystrosaurus – a land reptile
Cynognathus – a land reptile
Mesosaurus – a freshwater reptile
ANTARCTICA
Glossopteris – a fern
You will need:
enlarged copy of the map above
scissors.
• Cut out the continents from the enlarged copy of the map.
• Examine the distribution of fossils on each continent.
• Rearrange the continents into one super-continent by matching
the distribution of fossils.
1. How do you think the distribution of fossils helps to prove
Wegener’s theory of continental drift?
2. Are there any other ways that the continents can be put
together?
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Science Alive for VELS Level 5
Plate tectonics
They believe that:
• The Earth’s plates move along convection currents
that form in the mantle.
• At some plate boundaries, the plates are moving
apart (diverging). At these boundaries, mantle
material is forming new crust, which spreads and
forces the plates apart.
• At other boundaries, the plates are sliding under
each other. At these places, lithosphere material
melts into the top of the mantle. The sliding crust
pulls the rest of the plate along with it.
At first, Wegener’s theory of continental drift was
dismissed because he could not explain what force
caused the continents to move through deep
oceans.
With the help of technology like sonar, and, more
recently, satellite imaging, scientists discovered that
the Earth is made up of plates, not just separate
continents. Using seismic waves, they also discovered
that the top of the mantle is able to flow very slowly.
The current theory of plate tectonics describes how
the Earth’s plates move and why they move.
Scientists have concluded that there is more
than one reason why plates move.
Mid-ocean ridge
Through convection currents, hot mantle material rises through the
lithosphere at plate boundaries. The material cools on the ocean
floor, forming a ridge. Some peaks along mid-ocean ridges
rise more than 3 kilometres above the ocean floor.
Oceanic crust
Heavy oceanic crust is formed from
mantle material rising through plate
boundaries. The oceanic crust spreads
away from the ridge as more and more
crust forms. The spreading oceanic crust
pushes on nearby plates.
Continental crust
Subduction zone
Oceanic crust is heavier than
continental crust. In some places, it
slides under the continental crust and
melts into the top of the mantle. A
trench forms across the subduction
zone. The deepest trench is in the
Pacific Ocean. It is about 11 kilometres
deep. The sliding oceanic crust pulls
the rest of the plate along with it.
Convection currents
Scientists believe that convection currents exist in
the mantle. The movement of the convection currents
drags the Earth’s plates along.
THINK
4. What is the main difference between Wegener’s
theory of continental drift and the theory of plate
tectonics?
5. Describe three reasons why the Earth’s plates
move.
6. Explain why oceanic crust slides under continental
crust, but continental crust does not usually slide
under oceanic crust.
OBSERVE
7. Look at the map of the Earth’s plates on page 318.
Locate a plate boundary that probably involves:
(a) plates diverging
(b) one plate sliding under another.
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14. Geological processes
✓ learning
REMEMBER
1. What is Pangaea?
2. List three pieces of evidence Wegener used to help
support his theory of continental drift.
3. What is the name of the current theory that explains
how the Earth’s plates move?
I CAN:
describe the theories of continental
drift and plate tectonics
describe the evidence supporting
the existence of a super-continent
explain what the terms ‘mid-ocean
ridge’ and ‘subduction zone’ mean.
Our continent —
Gondwana
he dinosaurs that lived 225 million years ago would have been able to walk
across all of the continents. That’s because the continents were joined
together in a single super-continent called Pangaea. As the continents split, the
animals evolved into different species, adapting to their new environments.
T
Using the theory of plate tectonics, scientists have
traced the movement of the continents through
geological history. Plate tectonics is a continuous
process, so the continents are still moving today. Since
it broke away from Antarctica and drifted northwards,
Australia’s climate has changed — from very cold, to
cool and wet, and then to the hot and dry climate
typical of much of the continent today.
(a) 225 million years ago
About 225 million years ago, all of the
world’s landmasses were joined
together in the super-continent
Pangaea. Pangaea was surrounded by
a vast sea called Panthalassa.
Australia through time
About 600 million years ago, warm seas that were home
to soft-bodied creatures covered parts of Australia.
Fossils of these jellyfish-like creatures can be found in
the Flinders Ranges. Over the next 350 million years,
the sea level gradually lowered. Australia was so close
to the polar regions that it was covered in ice. Scientists
(b) 200 million years ago
Approximately 200 million years ago,
the Tethys Sea formed as Pangaea
began to split. The two continents that
formed were called Laurasia and
Gondwana. Laurasia included the
landmasses that are now North
America, Europe, Greenland and Asia.
Gondwana included South America,
Africa, Antarctica, India, Madagascar
and Australia.
(d) 65 million years ago
About 65 million years ago, Australia
separated from Antarctica.
(c) 135 million years ago
Gondwana began to break up about
135 million years ago. The South
Atlantic Ocean formed between Africa
and South America.
(e) The continents today
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Science Alive for VELS Level 5
Go to worksheet 14.2:
What’s the time?
believe that the changes in sea level and climate may
have contributed to the mass extinction of up to 96 per
cent of marine species and 70 per cent of land animals.
The extinction occurred worldwide, not just in
Australia. Within 50 million years of the mass
extinction, the sea level rose again.
Over 140 million years ago,
Australia was a warm, moist place
again. Scientists are not sure of
the exact temperatures, but
they agree that the world
temperatures were milder
than the current world
temperatures; the polar regions
were not covered in ice all through the year as
they are today. Scientists know this from fossil evidence.
The types of plant growing in polar regions millions
of years ago cannot grow there now because of the
constant icy-cold conditions.
About 100 million years ago, the world’s temperature
began to rise even further. As the world’s temperature
rose, so did the sea level. Shallow seas covered many
areas of Australia, turning it into a group of islands.
Central Australia was submerged under a cool sea
called the Eromanga Sea. As a result, many of the fossils
found in Australia are of animals that once inhabited
the sea.
About 65 million years ago, a sudden cooling
occurred across the planet. The world’s temperature
dropped so low that mass extinctions occurred again.
The dinosaurs were wiped out, but many prehistoric
mammals survived. Australia drifted away from
Antarctica. The climate started to change from humid
to dry.
The early marsupials that lived in Australia
were able to evolve separately from other mammals
around the world because Australia was so isolated.
The marsupials had no competition from placental
mammals for a long time, so they thrived. Australia’s
isolation from the rest of the world meant that
unique flora and fauna could evolve.
Even though new species have been
introduced into Australia, it is still a
continent with a large number of
marsupials and, along with Papua
New Guinea,
the only place
you can find
monotremes.
Plesiosaurs were huge sea
animals that lived in the
Eromanga Sea. They are not
dinosaurs, but prehistoric
reptiles.
REMEMBER
1. What was Panthalassa?
2. What sea formed as Pangaea began to split?
3. (a) What two continents formed from the split of
Pangaea?
(b) Of which continent was Australia a part?
4. Briefly describe how Australia’s climate has
changed since it was part of Pangaea.
THINK
5. Why are rocks like limestone and mudstone found
Dry land
in inland parts of South Australia?
Sea
6. Explain why similar species of marsupial can be
found in Australia and South America.
ICT
7. Go to www.jaconline.com.au/sciencealivevic/
Australia
Eromanga Sea
Antarctica
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14. Geological processes
✓ learning
salevel5 and click on the Plesiosaur link to answer
the following questions.
(a) What did plesiosaurs eat?
(b) In what type of environment did plesiosaurs
live?
(c) Who or what is Dave? (You may need to look
through the list of links on the web page to
find the answer to this one!)
I CAN:
describe how the movement of the
Earth’s plates led to the formation of
Pangaea, Laurasia and Gondwana
describe some of the geological
factors that led to the evolution of
Australian flora and fauna.