Plates are made up of two types of crust:
- Thicker (less dense) crust is called continental crust - mostly above sea level
- Thinner (more dense) crust is called oceanic crust - mostly below sea level
Plates move because of convection currents in the mantle - currents caused by temperature differences within the mantle.
Volcanoes and earthquakes most commonly occur at the boundaries where the plates meet.
There are three types of plate boundary - constructive, destructive and conservative.
Volcanoes and earthquakes at constructive boundari
A constructive plate boundary is where two plates are moving apart.
Mantle is under pressure from above. When they move apart, the pressure is released at the boundary.
Release of pressure causes the mantle to melt, producing magma.
Magma is less dense than the plate above, so it rises and can erupt to form a volcano.
The plates don't move apart in a uniform way - some parts faster than others. This causes pressure to build up. When the pressure becomes too much the plate cracks, making a fault line and causes an earthquake. Further earthquakes may also occur along the fault line once it's been created.
Volcanoes and earthquakes at destructive bounadrie
A destructive boundary occurs where two platea are moving towards each other.
Continental crust and oceanic crust are movign towards each other, the more dense oceanic crust is forced under the less dense continental crust (it's subducted).
Oceanic crust heated by friction and contact with the upper mantle, which melts into the magma.
Magma is less dense than continental crust above and will rise back to the surface to form volcanoes.
To cause an earthquake one of the plates gets stuck, the pressure bulids up and one of the plates jerks past the other, causing an earthquake.
Earthquakes at conservative boundaries
A conservative boundary occurs where two plates are moving past each other.
The two plates get locked together in places and the pressure builds up, plates jerk past each otherreleasing the energy as earthquakes.
Volcanoes away from fault lines
Volcanoes can occur away from plate boundaries eg in Hawaii.
These volcanoes are thought to be caused by magma rising from a large chamber beneath the crust. Areas like Hawaii are called volcanic hotspots.
The formation of tropical cyclones
1) Marm moist air is drawn up
2) Air cools and condenses, releasing huge amounts of latent heat powering more rising air
3) Air rises, spiraling due to the coriolis effect
4) Cold air sinks down the centre of the cyclone to form an eye and high pressure
5) Most of the cyclone has low pressure due to rising air
- Develop above sea water that's 26.5 degrees C or higher
- They lose strength when over land because the energy supply is cut off
- Occur 5-30 degrees north and south of the equator
- Cyclones spin because of the coriolis effect
- Cyclones don't occur 0-5 degrees of the equator because the coroilis effect isn't strong enough
- Corilis effect is why they move away from the equator
- They move westwards due to the east-west winds in the tropics
- Tropical cyclones are also known as hurricanes