plate margins

• the inner core


   in the centre and is the hottest part of the Earth. It is solid and made up of iron and nickel with temperatures of up to 5,500°C.
• outer core


  the layer surrounding the inner core. It is a liquid layer, also made up of iron and nickel.
• the mantle


  the thickest section of the Earth at approximately 2,900 km. The mantle is made up of semi-molten rock called magma.
• the crust


   the outer layer of the Earth. It is a thin layer between 0 - 60 km thick. The crust is the solid rock layer upon which we live. It is either continental or oceanic.
• The Earth's crust is broken into tectonic plates. It was once believed that convection currents in the mantle caused the plates to move. However, it is now recognised to be more complicated than this. Mechanisms called slab pull and ridge push are believed to move the tectonic plates. Ridge push is where the new crust formed at divergent plate margins is less dense than the surrounding crust and so it rises to form oceanic ridges. The older seafloor either side of the ridge slides away and this moves the seafloor apart – moving the tectonic plates.
• Therefore, instead of tectonic plates moving because of the convection currents, evidence suggests it is the plates that drive the convection. Slab pull occurs where older, denser tectonic plates sink into the mantle at subduction zones. As these older sections of plates sink, newer and less dense sections of plate are pulled along behind. Sinking in one place leads to plates moving apart in other places.
• oceanic crust


   found underneath the oceans. It is denser than continental crust and can be subducted.
• continental crust


   found under land masses or continents. It is generally older than oceanic crust and is less often destroyed.
• A destructive plate margin usually involves an oceanic plate and a continental plate. The plates move towards one another and this movement can cause earthquakes.
• As the plates collide, the oceanic plate is forced beneath the continental plate. This is known as subduction. This happens because the oceanic plate is denser (heavier) than the continental plate.
• When the plate sinks into the mantle it melts to form magma. The pressure of the magma builds up beneath the Earth's surface. The magma escapes through weaknesses in the rock and rises up through a composite volcano. The volcanic eruptions are often violent, with lots of steam, gas and ash.
• If two continental plates collide, neither can sink and so the land buckles upwards to form fold mountains. This is called a collision margin. Earthquakes can occur at collision margins.
• At a constructive plate margin the plates move apart from one another. When this happens the magma from the mantle rises up to make (or construct) new land in the form of a shield volcano. The movement of the plates over the mantle can cause earthquakes.
• At a conservative plate margin, the plates move past each other or are side by side moving at different speeds. As the plates move, friction occurs and plates become stuck. Pressure builds up because the plates are still trying to move. When the pressure is released, it sends out huge amounts of energy, causing an earthquake. The earthquakes at a conservative plate boundary can be very destructive as they occur close to the Earth's surface. There are no volcanoes at a conservative plate margin.