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Structure of the earth menu
Structure of the earth
Continental Drift and Plate tectonics
Plate boundaries


Destructive Plate Margins Constructive Plate Margins

Pacific Ring of Fire

Fold Mountains

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[image - cross section of the earth]
A cross section of the earth

Structure of Earth

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Structure of the earth

When studying plate-tectonics the best starting point is examining the structure of the earth. The earth is very similar to a peach in its structure. In the centre is a solid core. Surrounding the core is the inner core, then the mantle, which is covered in the earths 'skin' or crust.

cross section of the earth
figure 1. Cross section of the earth (source: Wikipedia)

The inner core is the centre of the earth and is the hottest part of the earth. It is a solid mass of iron and nickel. The temperature of the core is around 5500°C

The outer core is the layer around the inner core. It is also made up of iron and nickel though it is in liquid form.

The next layer is the matle.This layer is made up of semi molten rock, known as magma.

The final layer is the earth's crust. This layer is between 0-60km thick.

Continental Drift and Plate Tectonics

In 1912 Alfred Wegener published a theory to explain why the Earth looked like a huge jigsaw. He believed the continents were once joined forming a supercontinent he called Pangaea. Over 180 million years ago this supercontinent began to "break up" due to continental drift.

During the 20th Century, scientists developed the theory of Plate Tectonics. The theory suggested that the crust of the Earth is split up into seven large plates (see map below) and a few smaller ones, all of which are able to slowly move around on the Earth's surface. They float on the semi-molten mantle rocks, and are moved around by convection currents within the very hot rock. See why do plates move? for more details.

The are two types of tectonic plates - continental plates and oceanic plates. Continental plates are lighter (less dense) than oceanic plates. Oceanic crust is much younger in geologic age than continental crust. Continental crust is on average thicker than oceanic crust.
figure 2. The Earth's main plates

Why do plates move?

 The earth's tectonic plates are in constantly moving like giant 'rafts' on top of the semi-molten mantle below. However this movement is slow and rates vary from less than 2.5cm /yr to over 15cm/yr.

The movement of the earth's crustal plates is believed to be due to convection currents which occur in the semi-molten mantle. These convection currents are created by heat from within the earth - much of which is generated by radioactive decay in the core.

Convection currents diagram

So how do convection currents cause plate movements? As semi-molten rock in the mantle is heated it becomes less dense than its surroundings and rises. As it reaches the crust above, it spreads out carrying the plates above with it. As the semi-molten rock then cools, it gradually sinks back down to be re-heated. (see diagram above)

Plate Boundaries

  The point where two or more plates meet is known as a plate boundary. It is at these locations where earthquakes, volcanoes and fold mountain form. There are four main types of plate boundary. These are constructive, destructive, conservative and collision margins.

Plate Boundary

Tensional / Constructive (divergent ) plate boundaries

Constructive plate boundaries occur when two plates move away from each other. Ocean ridge and volcanic islands North American and Eurasian Plate

Compressional / Destructive (subduction zones) plate boundaries

Destructive plate boundaries occur when an oceanic plate is forced under (or subducts) a continental plate. Fold Mountains and Oceanic trenches Pacific Plate and the Eurasian Plate

Conservative (transform faults) plate boundaries

Conservative plate boundaries occur when two plates slide past each other.   North American Plate and the Pacific Plate

Collision plate boundaries

Collision plate boundaries occur when two continental plates move towards each other. Fold Mountains Indo-Australian and the Eurasian Plate

You should be aware that whilst most volcanoes / earthquakes occur along plate boundaries, there are exceptions. For example the volcanic Hawaiian islands which can be found in the middle of the Pacific Plate are formed due to a Hotspot. Hotspots are plumes of molten rock which rise underneath a plate causing localised melting and the creation of magma resulting in volcanic activity. See this animation for further explanation of hotspot activity.

Key Terms
Constructive Boundary (Divergent) - where two plates move away from each other resulting in new crust being formed.
Destructive Boundary (Convergent) - where two plates move towards each other - in the case of a plate consisting of continental crust meeting a plate consisting of oceanic crust, the oceanic crust will be subducted and destroyed as it is less dense.
Conservative Boundary - where two plates move alongside each other - although crust is neither created or destroyed here, earthquakes usually occur here.
Collision Boundary - where two plates of continental crust move towards each other creating fold mountains.
Volcano - a vent through which lava, ash etc. is erupted (often, but not always cone-shaped)
Earthquake - a sudden movement of the earth's surface

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