14.2.1 Tectonic plates The Earth’s crust is cracked and is made up of many individual moving pieces called tectonic plates, which fit together like a jigsaw puzzle. These plates float on the semi-molten rocks (magma) of the Earth’s mantle. Enormous heat from the Earth’s core, combined with the cooler surface temperature, creates convection currents in the magma. These currents can move the plates by up to 15 centimetres per year. Plates beneath the oceans move more quickly than plates beneath the continents. Scientific evidence shows that about 225 million years ago all the continents were joined.
SkillBuilder discussion Communicating 1. Explain why tectonic plates move. 2. Identify the type of landform found where plates move away from each other. 3. Outline the landforms found where one plate subducts under another.
FIGURE2 The Earth’s core is very hot, while its surface is quite cool. This causes hot material within the Earth to rise until it reaches the surface, where it moves sideways, cools and then sinks.
Volcanic island chain
Mid-oceanic ridge (‘ridge-push’)
Island arc
Volcano
‘Slab-pull’
l e
Mantle plume
t l
D’
Core
Type of plate movement
Description
Examples
1. Convergent plates
When two continental plates of similar density collide, the pressure of the converging plates can push up land to form mountains. Oceanic and continental plates have different densities, so when they collide the thinner oceanic plate is ‘subducted’, meaning it is forced down into the mantle. Heat in the mantle melts this subducted plate and pressure forces the molten material back to the surface through cracks in the Earth’s crust. This can produce volcanoes and mountain ranges. Subduction can also occur when two oceanic plates collide. This forms a line of volcanic islands in the ocean about 70–100 kilometres past the subduction line. Convection currents can sometimes cause plates to slide, or slip, past one another, forming fault lines. In some areas, plates are moving apart, or diverging, from each other. As the divergent plates separate, magma can rise up into the opening, forming new land.
Himalayas (India and Asia), Alps (Africa and Europe)
Andes (South America)
Japan, Philippines, Mariana Trench
2. Lateral plate slippage
San Andreas Fault (California) Iceland, underwater volcanoes and islands, the Rift Valley, Africa (continued)
3. Divergent plates
TOPIC14 Geomorphological processes and hazards
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