The structure of the earth
- The top layer is the crust ( a hard outer shell). Underneath the crust is the mantle where temperatures are so high that rock exists in a semi- molten state (about 3800'c).Underneath the mantle is the outer core made of liquid iron and nickel. Underneath this is the solid inner core which is very hot (5500'c). The crust is broken into several large and other smaller segments called plates which float on the mantle.
- Heat from within the earth creates convection currents which cause plates to move perhaps by a few centimetres a year. Plates may move either away, towards or sideways past neighbouring plates.
- Plates meet at plate boundaries and this is where most of the worlds earthquakes and volcanic eruptions occur in long belts across the earths surface, and where high mountain ranges are located. Very little activity takes place in the rigid centre of the plates.
- Plates consist of two types of crust; oceanic and continental. It is thesedifferences in crust that account for the activity taking place at boundaries.
- Continental crust- is older (3-4 billion years old), lighter, cannot sink and is therefore permanent. 25-100 km thick. New crust isnt formed and cannot be destroyed.
- Oceanic crust- is denser, heavier, can sink into the mantle and is constantly being destroyed at destructive plate margins and replaced. 5-20 km thick. Only about 180 million years old. Forms constantly at constructive plate margins.
Distrubution of volcanoes
- The overall pattern is that they are found in long narrow belts along the edges of some of the continents. eg western coast of North and South America.
- They tend to be found along plate margins.There is ring of volcanoes surrounding the Pacific Ocean called the Pacific Ring of Fire.
Earthquakes and distribution
- It is a sudden and violent movement of the Earths crust.
- They occur at plate margins due to friction - the plates try to move. Pressure builds up and if released slowly it is hardly noticed. If released suddenly it can send out huge pulses of energy- causing the earths surface to move violently.
- They occur at all four types of plate margins.
Destructive plate margins
- Where an oceanic plate and continental plate collide.
-The oceanic plate is denser so sinks beneath the continental plate. This is known as subduction. A deep ocean trench is formed as the oceanic crust is dragged down in to the subduction zone.
- As the oceanic plate sinks down into the mantle it melts to form magma, taking down sea water with it, making it less dense than the mantle.
- It therefore rises up through cracks (fissures) in the continental crust above. This forms volcanoes when the lava erupts at the surface.
- The trapped sea water turns into steam making the volcano very explosive. Lava, ash, steam and gas come out the volcano.
- As the oceanic plate…