The Earth’s Structure And Tectonic Processes

The Earth’s Structure And Tectonic Processes

IB SL

Cross section of the Earth

Breakdown• Crust• Thin, outer-layer of the earth. • It is as thin as the skin of an apple is to its

flesh. • Mantle• Composed mainly of silicate rocks, which

contain a lot of magnesium and iron. • It extends to a depth of 2900km. • Temperatures can reach 5000 degrees

Celsius, which generate convection currents.

• Core• Contains iron and nickel. • Situated approximately 6371km below the

surface. • Temperature is around 5500 degrees

Celsius.

Evidence for plate tectonics1. Study of fossils - similar fossils are found on differentcontinents. This is evidence that these regions were once very close or joined together.

2. Pattern of rocks - Similar pattern of rock layers on different continents is evidence that the rocks were once close together or joined.

3. Shape of continents fit together like a jigsaw

Africa

SouthAmerica

N N N N NS S S SS

N N N N NS S S SS

Constructivemargin

Pattern same both sides

The reversal of the Earth’s magnetic field is recorded in the rocks that solidify at constructive margins. The symmetry around the margin is evidence that the rocks are moving apart.

4. Magnetic field pattern in iron containing rocks

Convention Currents

Mantle

Crust

Plate Tectonic Theory

• The location of continents today of far removed from what it was millions of years ago, when it is believed that all continents were joined to one land mass. Alfred Wegener put forward a theory regards their movement in 1912 with his theory of Continental Drift. His theory was based on observations such as:

• Biological: Coal is found in UK but needs warm, wet, humid conditions to form.

• Observational: The shapes of countries appear to 'fit' one another, for example, S. America and Africa.

Plate Movements

• In the 1940's-1960's the theory was revised and Plate Tectonic theory emerged.

• It is based on the premise that the lithosphere (crust and rigid upper mantle) is divided into plates, which are moved by convection currents coming from the earth's core.

2 Types Of Crust

Oceanic Crust (Sima)

Young (under 200 million years), thin, dense and heavy, will sink. Always being created and destroyed. For example, Basalt.

Continental Crust (Sial)

Old, light, thick (up to 150km under mountains) permanent, does not sink. For example, Granite.

Plate Margins

• Movement of plates is in one of 3 ways:

• Towards each other: Convergent (destructive or collision).

• Away from each other: Divergent.• Alongside each other: Transform or transcurrent.

• There is a further subdivision for Convergent plates, which is dependent on the properties of the plates moving towards each other (oceanic / continental crust).

Constructive Plate Boundary

At a constructive plate boundary, two plates move apart. As the two plates move apart, magma rises up to fill the gap. This causes volcanoes at this type of boundary. However, since the magma can escape easily at the surface the volcano does not erupt with much force. Earthquakes are also found at constructive boundaries.

An example of a constructive boundary is the Mid-Atlantic Ridge.

Constructive Plate Boundary

Sea Floor Spreading!

Did you know that the ocean floor in the Atlantic is growing by 3cm per year?

Which of the following pairs of continents are moving further away from each other?

1) Europe and Africa 3) South America and North America

2) Europe and North America

Mid Atlantic Ridge

6 metres 36 metres 928 metres200 metres

The plates move at different rates. The Nazca and Pacific plates are moving apart at a rate of 18cm per year while the Eurasian and North American plates are moving apart at a rate of 3cm per year.

To the nearest metre, how far will the Nazca and Pacific plates have moved over the next 200 years?

How fast do plates move?

Convergent Plate Boundaries (Together)

• The two types of margins that occur here are Destructive Margins or collision Zones...

Destructive margins

Oceanic crust moves towards continental crust. Oceanic crust sinks, and slowly destroyed. Results in deep ocean trenches, island arcs and fold mountains. Can be referred to as subduction zones.

Collision Zones

Two continental crusts meet, neither sinks, fold mountains result. For example, the Alps.

Divergent Plate Boundaries (Away)

• Also known as a Constructive margin.

• Plates move away from each other, for example, N. American and Eurasian plates, cresting mid-ocean ridges such as the Mid Atlantic Ridge.

• New material appears at the ocean ridge.

Transcurrent Boundaries (Side-By-Side)

• Here plates simply move alongside each other, land is neither created nor destroyed.

• Plates rub against each other causing friction resulting in earthquakes only at these boundaries.

Examples

Destructive Plate Margins• Peru - Chile Trench as oceanic crust is subducted.• Earthquakes, due to the heat created as the

Nazca plate is destroyed.• Fold Mountains such as the Andes. • Volcanoes if heat from the subduction zone has

the opportunity to reach the surface, for example, Cotopaxi.

• Island arcs and volcanic islands, for example, Japan.

Examples

Collision Zones • Indian Plate and Eurasian Plate created the

Himalayas. Constructive Margins

• North American plate moving away from the Eurasian plate resulting in the mid Atlantic ridge. Transform Faults

• Earthquakes such as those linked with the San Andreas Fault.

Hot Spots

• There are examples where volcanic activity is not linked to plate margins as in the case of the Hawaiian Islands.

• This is believed to be due to the presence of 'hot spots' - places of localised heat under the earth's crust that then find their way to the surface.

Name this plate boundary.

Continental crust MantleThe oceanic crust melts and risesExplosive volcanoes

Oceanic plate

The oceanic crust sinks under the less dense continental crust Earthquakes occur due to friction

Match the labels to the letters

A B

C D

E

F

G

Constructive plate boundaries

Ocean

Oceanic Crust

Mantle

MID OCEAN RIDGE

Where would you find older rocks – at A? or at B?

AB

Activity 1

Using Essential AS Geography P31, answer the following…

1.What are the 3 major relief features associated with constructive margins?2.Create a diagram to show/describe the processes operating at constructive margins.3.What is “Palaeomagnatism” and how does it explain the idea of sea floor spreading?

Activity 2

Using Essential AS Geography P33-35, answer the following…

1.What is a destructive margin?2.How are Earthquakes, Volcanoes, Ocean Trenches, and Island Arcs associated with these boundaries?3.Describe the formation of the Himalayas and explain why there are few earthquakes and no volcanoes there.