Geography - Tectonic Activity

Earthquakes and volcanoes occur in many differnt parts of the world. Their distrubution is not random.

Both tend to occur mainly in narrow bands, although some do fall out of this general pattern. In many cases the two occur in the same places, although this is not always the case.

Earthquakes and volcanoes tend to occur together in bands that are found: -

• In the middle of oceans, such as the Atlantic
• Along the edges of continents, such as the west coast of South America

There is a particularly strong pattern around the edges of the Pacific Ocean, known as the 'Ring of Fire'

Earthquakes occur without volcanoes through central Asia. Volcanoes are also found in more isolated clusters, such as the Hawaiian Islands.

Tectonic Plate Theory.

• Earthquakes and volcanoes occur due to the movement of the tectonic plates.
• The Earth's surface is divided into a number of large and small slabs of rock that together make up the Earth's crust.
• These plates 'float' on the hot molten rock - magma - that is in the mantle below.
• These plates move in relation to each other because of convection currents in the mantle's magma.
• Depending on the direction of the convection currents, the plates can move towards each other, away from each other or slide past each other.

Two types of crust - Continental (between 25 and 100 km thick, made up of relatively low density material) and Oceanic (between 5 and 10km thick, made up of relatively high density material.)

CONVERGENT PLATE BOUNDARIES.

Collision Boundary:

• Convergence of two continental plates.
• Low density rock so both plates are pushed upwards by the collision.
• Earthquakes. (Pakistan 2005.)
• No volcanoes, general formation of fold mountain ranges. (Himalayas.)

Subduction:

• Convergence of an oceanic plate and a continental plate.
• The ocenanic plate is subducted beneath the less dence continental plate, forming a deep ocean trench on the seabed.
• The edge of the oceanic plate melts in the high temperature mantle and the convergence forces the molten rock up through weaknesses in the overlying continental plate to form volcanoes that are often very violent in their eruptions.

Divergent Plate Boundarys:

• The oceanic plates are moving away from each other
• The pressues exerted by the divergence of the plates lead to fractures being formed in the cursal rocks.
• Rising magma is then able to enter the crust, filling the gaps and creating new sea floor.
• If a lot of magma escapes to the surface, submarine volcanoes occur, which may build up over time to form volcanic islands. (Island.)

Conservative Plate Boundary:

• Two plates slide past each other, not smoothly.
• No magma rises. No subduction takes places. Volcanoes do not occur.
• Extreme stresses build up in the crustal rocks, when the pressure is released, the result is an earthquake. The best example is the San Andreas Fault.
• Friction between the two plates cause them to stick and only when enough pressure has been built up will it allow them to move again.
• If the pressure is released frequently, the earthquakes will be small. If the pressure has built up over time, the occasional large earthquake will result. (San Francisco 1989.)

HAWAIIAN ISLANDS IN THE PACIFIC OCEAN.

• High temperature magma rises from the mantle, even though there is no plate boundary
• If the crust is thin or weak, the magma can escape through it to form volcanoes
• These locations are called HOT SPOTS
• In Hawaii, the volcanoes originally formed on the sea floor, but over time the lava has built up high eniugh to stick above sea level to form volcanic islands.