320Science Alive for VELS Level 5

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

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’.

INDIAN

OCEAN

PACIFIC

OCEAN

ARCTIC OCEAN

ATLANTIC

OCEAN

N O RT H

A M E R I CA

S O U T H

A M E R I CA

E U R O P E

A F R I CA

A S I A

AUSTRALIA

SOUTH

AMERICA

AFRICA

INDIA

AUSTRALIA

ANTARCTICA

Lystrosaurus – a land reptile

Cynognathus – a land reptile

Mesosaurus – a freshwater reptile

Glossopteris – a fern

Modelling exercise: evidence of continental drift

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?

 

 

 

32114. Geological processes

Plate tectonicsAt 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.

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.

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?

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.

✓ lea

rnin

g I CAN:describe the theories of continental drift and plate tectonicsdescribe the evidence supporting the existence of a super-continentexplain what the terms ‘mid-ocean ridge’ and ‘subduction zone’ mean.

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.

Continental crust

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.

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.

Convection currents

Scientists believe that convection currents exist in

the mantle. The movement of the convection currents

drags the Earth’s plates along.

 

 

 

322

Science Alive for VELS Level 5

Our continent —

Gondwanahe 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.

Australia through timeAbout 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

(e) The continents today(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.

(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.

(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.

 

 

32314. Geological processes

✓ le

arn

ing 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.

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.

Australia

Eromanga Sea

Dry land

Sea

Antarctica

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

in inland parts of South Australia?

6. Explain why similar species of marsupial can be

found in Australia and South America.

ICT

7. Go to www.jaconline.com.au/sciencealivevic/

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!)

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.

Go to worksheet 14.2:

What’s the time?