SIMA / SIAL???

• SIMA – Silicon and Magnesium• SIAL – Silicon and Aluminium

Last week we studied the inside of the earth.

Today – we are mainly staying on the surface.

True or False?

• The Earth is 4.6 billion years old.• The earliest human remains are dated at less

than 2 million years old.• Systematic study of the Earth only began about

300 years ago.• No major understanding of its structure or

evolution was achieved until the past century.• The country of Iceland did not exist 20 million

years ago.

Plate Tectonics Theory

Key questions:- What did the Earth used to look like?- Why does it look different today?- How has the theory of Plate Tectonics

developed?

Supercontinents!

• The Earth did not always look as it does today. • It is believed that ~ 250 million years ago all

the continents were joined together to form one “supercontinent” called Pangaea.

• Pangaea was a roughly “C” shaped landmass that spread across the equator.

Alfred Wegener put forward this theory in 1912.

Task...• Cut out the continents from the map of how

we see them today, also separate India from continental Asia.

• Try matching up the coastlines and fitting the continents together to form the supercontinent Pangaea.

Compare...

Did you have trouble fitting all of the continents together? Why?

• There are differences in modern and ancient coastal morphologies.

• Variations in sea level.

Evidence for Pangaea:

1: Continental fit What else?Use your text book to find out about the other

evidence that supports the existence of Pangaea.

Geological evidence

• The Appalachian Mountains of eastern North America are thought to link to the Caledonides of Ireland, Britain, Greenland, and Scandinavia and the Anti-Atlas Range in Morroco.

Climatological evidence

• Coal deposits that would have been formed in tropical climate conditions are found in places that do not have a tropical climate. Therefore, they must have drifted. E.g. Antarctica

Biological evidence

• Mesosaurus remains were found in southern Africa and eastern South America, two far away places. Mesosaurus was a freshwater animal, and could not have crossed the Atlantic Ocean, this indicates that the two continents used to be joined together.

• Marsupials are only found in Australia because it drifted away from the main supercontinent before the predators that wiped them out elsewhere had migrated there

So far:

Wegener had convincing evidence for continental drift.

However, sceptics were quick to point out that there was no explanation of the mechanism by which continents could move over a solid earth.

It was not until the second half of the 20th century that major discoveries began to suggest how this might be possible.

• 1948 – A survey of the floor of the Atlantic Ocean revealed a continuous ridge running north to south.

• 1000km wide.• Heights of 2.5km• Composed of volcanic

rocks.• Similar found in Pacific

Ocean.

• Iron particles in lava erupted on the ocean floor are aligned with the Earth’s magnetic field.

• As the lavas solidify, these particles provide a permanent record of the Earth’s polarity at the time of the eruption - called palaeomagnetism.

Geomagnetic Polarity Reversals

However, the Earth’s polarity reverses at regular intervals [approx every 400,000 years].

The result is a series of magnetic ‘stripes’ with rocks aligned alternately towards the north and south poles.

The striped pattern (which is mirrored exactly on either side of a mid-oceanic ridge) suggests that the ocean crust is slowly spreading away from this boundary and new rocks are being added equally on either side.

This process is known as sea floor spreading.

More evidence – The age of the ocean floor.

• Surveys recorded very young ages for places on or near the ridges.

• E.g Iceland – less than 1 million years old.

• Much older ages were recorded for ocean floor rocks nearer the continental masses (200 million years).

• Older crust is continuously being pushed aside by new crust.

Young

Old

Old

BUT!• There is no evidence for the planet growing in

size as all of this ocean crust accumulates.• So, what happens to all of this oceanic crust?• This realisation led to the discovery of huge

trenches where large areas of the ocean floor were being subducted.

• Oceanic crust is being created in some areas and destroyed in others.

• The less dense, more buoyant continental crust is not consumed.

One more very important thing...• Convection currents:1. Higher temperatures at the

Earth’s core and heat from the mantle help to create convection currents.

2. These are zones of hotter, more liquid magma.

3. Thought to have a continuous circulatory motion in the asthenosphere that causes the plates to move.

The Driving Mechanism for Plate Movement

Pull crust apart at spreading ridges.

Pull slabs of oceanic crust back down into the mantle at suduction zones.

Summary...

1912 – Alfred Wegener put forward a range of evidence for the existence of Pangaea – the supercontinent.

Later 20thC – major discoveries:1.Mid-Altantic Ridge – palaeomagnetism, sea

floor spreading.2.Subduction and ocean trenches.3.Convection currents – the cause of plate

movement.

Task:

• Add notes to the blank areas of your sheet.• Use the text book to help.• Finish for homework.

Next lesson:

• The Earth’s surface is divided up into over 50 plates moving at different rates and in different directions.

• Towards each other, away from each other, alongside each other.

• We will look at what processes happen at the edges of these plates as these movements occur.

Silly game:

• http://kids.earth.nasa.gov/archive/pangaea/Pangaea_game.html