Geography
Tectonic plates:
The Earth consists of four concentric
layers: inner core, outer core, mantle and crust. The crust is made up of tectonic
plates, which are in constant motion. Earthquakes and volcanoes are most likely
to occur at plate boundaries.
The structure of the Earth
The Earth is made up of four
distinct layers:
- The inner core
is 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. With its
immense heat energy, the inner core is like the engine room of the Earth.
- The outer core
is the layer surrounding the inner core. It is a liquid layer, also made
up of iron and nickel. It is still extremely hot, with temperatures
similar to the inner core.
- The mantle
is the widest section of the Earth. It has a thickness of approximately
2,900 km. The mantle is made up of semi-molten rock called magma. In the
upper parts of the mantle the rock is hard, but lower down the rock is
soft and beginning to melt.
- The crust
is 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.
There are two different types of
crust: continental crust, which carries land, and oceanic crust,
which carries water.
The diagram below shows the
structure of the earth. In geography, taking a slice through a structure to see
inside is called a cross section.
Cross section showing structure of
the Earth
The Earth's crust is broken up into pieces called plates. Heat rising and falling inside the mantle creates convection currents generated by radioactive decay in the core.
Divergent plate movement:
The convection currents move the plates. Where convection currents diverge near the Earth's crust, plates move apart. Where convection currents converge, plates move towards each other. The movement of the plates, and the activity inside the Earth, is called plate tectonics.
At a constructive boundary the plates are moving apart. The plates move apart due to convection currents inside the Earth.
- As the plates move apart (very slowly), magma rises from the mantle. The magma erupts to the surface of the Earth. This is also accompanied by earthquakes.
When the magma reaches the surface, it cools and solidifies to form a new crust of igneous rock. This process is repeated many times, over a long period of time.
Eventually the new rock builds up to form a volcano.
Constructive boundaries tend to be found under the sea, eg the Mid Atlantic Ridge. Here, chains of underwater volcanoes have formed along the plate boundary. One of these volcanoes may become so large that it erupts out of the sea to form a volcanic island.
Convergent plate movement:
At a destructive boundary the plates are moving towards each other. This usually involves a continental plate and an oceanic plate.
The oceanic plate is denser than the continental plate so, as they move together, the oceanic plate is forced underneath the continental plate. The point at which this happens is called the subduction zone.
Subduction process: (a. with two continental plates b. continental and oceanic plate )As the oceanic plate is forced below the continental plate it melts to form magma and earthquakes are triggered. The magma collects to form a magma chamber. This magma then rises up through cracks in the continental crust. As pressure builds up, a volcanic eruption may occur.
Tectonic plates - Test
1.Which part of the Earth is directly below the crust?
2.Which part of the Earth is the hottest?
3.What material makes up the mantle?
4.A constructive or divergent plate boundary is when:
5.A destructive or convergent boundary is when:
6.At a conservative plate boundary land is:
7.What occurrence is common along a conservative plate boundary?
8.Which of these statements about a constructive boundary is NOT true?
9.The Himalayas were formed on what kind of plate boundary?
10.The Mid-Atlantic Ridge or chain of volcanoes formed underneath the Atlantic Ocean is created by
what kind of plate boundary?
