Tectonics

The Earths Surface is Separated into Tectonic Plates

1) At the centre of the Centre is a ball of solid iron and nickel called the core.

2) Around the core is the mantle, which is smei-molten rock that moves very slow.

3) The outer layer of the Earth is the crust. Its very thin (about 20km)

4) The crust is divided into lots of slabs called tectonic plates (they float on the middle).

Plates are made of two types of crust - continental and oceanic:

• Continental crust is thicker and less dense.
• Oceanic crust is thinner and more dense.

5) The paltes are moving because the rock in the mantle underneath them is moving.

6) The places where plates meet are called boundaries or plate margins.  

Volcanoes are Found at Destructive and Constructive Plate Margins

1) At destructive plate margins the oceanic plate goes under the continental plate because its more dense.

(This also creates an ocean trench):

• The oceanic plate moves down into the mantle, where its melted and destroyed.
• A pool of magma forms.
• The magma rises through cracks in the crust called vents.
• The magma erupts onto the surface (where its called lava) forming a volcano

2) At constructive margins the magm rises up into the gap created by the plates moving apart, forming a volcano.

3) Some volcanoes also form over parts of the mantle that are really hot (called hotspots), e.g.in Hawaii.

Earthquakes Occur at All Three Types of Plate Margin

1) Earthquakes are caused by the tension that builds up at all three types of plate margin:

• Destructive margins- tension builds up when one plate gets stuck as its moving down past the other into the mantle.
• Constructive margins- tension builds along cracks within the plates as they move away from each other.
• Conservative margins- tension builds up when plates that are grinding past each other get stuck.

2) The plates eventually jerk past each other, sending out shock waves (vibrations). Thes vibrations are the earthquake.

3) The shock waves spread out from the focus - the point in the Earth where the earthquake starts. Near the focus the waves are stronger and cause more damage.

4) The epicentre is the point on the Earths surface straight above the focus.

5) Weak earthquakes happen quite often, but strong earthquakes are rare.