The Restless Earth

• Very thin (about 20km).
• Divided into tectonic plates that float on the mantle
• Two types : Continental & Oceanic
• Oceanic - Thinner, More Dense
• Continental - Thicker, Less Dense

• Destructive
  Move towards each other
  Oceanic and Continental meet, oceanic forced down into mantle (subducted) and destroyed
• Constructive
  Move away from each other
  Magma rises to fill the gap and cools creating new crust
• Conservative
  Move sideways past each other
  Crust isn't created or restored

• Formed a destructive margins
  When the plates collide the sedimentary rocks that have built up between then are folded and forced upwards to form mountains.
  Where continental plate and an oceanic plate collide. Eg. Andes in South America
  Where two continental plates collide. Eg. Himalayas
  Very high mountains
  Rocky
  Steep slopes
  Often snow and glaciers in the highest bits & lakes in the valleys.

• Found at Destructive margins
  Oceanic subducts under the continental as it's more dense.
  Oceanic moves down into mantle where it's melted and destroyed.
  Magma forms.
  Magma rises through cracks in the crust called vents.
  Magma erupts onto the surface forming a volcano.
• Found at Constructive margins
  Magma rises up into the gap forming a volcano.

  Some form over parts of the mantle that are really hot (Hotspots)

• Composite eg. Mount Fuji in Japan
  Ash and lava that's erupted, cooled & hardened into layers.
  Viscous lava that hardens quickly to form a steep sided volcano.
• Shield eg. Mauna Loa on the Hawaiian Islands
  Runny lava that flows quickly over a wide area.
  Forms a low, wide, flat volcano.

• Scientists monitor
  Small Earthquakes
  Escaping Gas
  Changes in the shape ( bulges can mean lava has built up beneath)
• All mean an eruption is likely

• Magma rises up through cracks in the crust to form a large magma basin below the surface. The pressure of the magma causes a circular bulge on the surface several Km wide.
• The bulge eventually cracks, creating vents for lava to escape through. The lava erupts out of the vents causing earthquakes and sending up giant plumes of ash and rock.
• As the magma basin empties, the bulge is no longer supported so it collapses - spewing up more lava.
• When the eruption's finished there's a big crater ( Caldera ) left where the bulge collapsed. Sometimes these get filled with water to form a large lake eg. Lake Toba in Indonesia
• Characteristics
  Flat
  Cover a large area
  Have a Caldera

• Thousand of cubic kilometre of rock, ash and lava will be thrown out of the volcano.
• A thick cloud of super-heated gas and ash will flow at high speed from the volcano (Pyrocrastic flow), killing, burning and burying everything it touhes - Everything within 10 miles will be destroyed.
• Ash will shoot KMs into the air and block out almost all daylight over whole continents. This can trigger mini ice ages as less heat energy from the sun gets to Earth.
• Ash will settle over hundreds of square kilometres.

Caused by tension that builds up at all three types of plate margins.

• Destructive - Tension builds up when one plate gets stuck as it's moving don past the other into the mantle.
• Constructive - Tension builds along cracks within the plates as they move away from each other.
• Conservative - Tension builds up when plates that are grinding past each other get stuck.

The plates eventually jerk past each other, sendiing out shock waves (vibrations). These vibrations are the earthquake.

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.

The epicentre is the point on the Earth's surface straight above the focus.

Weak earthquakes happen quite often, but strong earthquakrd are rare.

The Richter Scale

• This measures the amount of energy released by an earthquake (called the magnitude).
• Magnitude is measured using a seismometer - a machine with an arm that moves with the vibrations of the earth.
• The Richter scale doesn't have an upper limit and it's logarithmic - this means that an earthquake with a magnitude of 5 is ten times more powerful than an earthquake with a magnitude of 4.

The Mercalli Scale

• This measures the effects of an earthquake.
• Effects are measured by asking eye witnesses for observations of what happened. Observations can be in the form of words or photos.
• It's a scale from 1 to 12.