THE EARTH'S STRUCTURE

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THE EARTH

The Earth has a layered structure made up of the core, mantle and crust. The lithosphere - the crust and upper part of the mantle - is broken into large pieces called tectonic plates. These move slowly over the mantle.

Volcanoes occur when molten rock pushes up through weaknesses in the crust. The molten rock cools and solidifies to form igneous rocks, such as basalt, gabbro, rhyolite and granite.

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PARTS OF TH EARTH

The Earth is almost a sphere. These are its main layers, starting with the outermost:

  1. crust - relatively thin and rocky
  2. mantle - has the properties of a solid, but can flow very slowly
  3. outer core - made from liquid nickel and iron
  4. inner core - made from solid nickel and iron

The lithosphere consists of the crust and outer part of the mantle. It is the relatively cold outer part of the Earth’s structure.

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PLATE TECTONICS AND VOLCANOES

The Earth’s lithosphere is broken into large pieces called tectonic plates:

  • oceanic plates occur under the oceans
  • continental plates form the continents - land

Tectonic plates are less dense than the mantle, so they are found on top of it. The plates are moving constantly, at a rate of a few centimetres each year.

Volcanic activity


Where tectonic plates meet, the Earth’s crust becomes unstable as the plates push against each other, or ride under or over one another. Earthquakes and volcanic eruptions happen at the boundaries between plates.

Magma - molten rock - is less dense than the crust. It can rise to the surface through weaknesses in the crust, forming a volcano.

Geologists study volcanoes to try to predict future eruptions. Volcanoes can be very destructive, but some people choose to live near them because volcanic soil is very fertile. It contains minerals needed by plants for healthy growth.

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IGNEOUS ROCKS

Lava is the molten rock that erupts from a volcano. Some eruptions produce runny lava, while others produce thick lava that escapes violently. Igneous rocks are made when molten rock cools down and solidifies.

Crystal size

if it cooled quickly, the crystals are smallIgneous rocks contain interlocking crystals. The size of the crystals depends on how quickly the molten rock cooled:

  • if it cooled slowly, the crystals are large

The slower the molten rock cools, the larger the crystals in the igneous rock.

formed from runny lavaformed from thick lava small crystals - lava cooled quickly basalt rhyolite large crystals - lava cooled slowly gabbro granite

Basalt is rich in iron, and rhyolite is rich in silica.

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THE THEORY OF PLATE TECTONICS

The mantle is the zone between the Earth’s crust and its core. It is relatively cold and rigid just below the crust. At greater depths, the mantle is able to flow because it is hot and non-rigid.

The mantle is the zone between the Earth’s crust and its core. It is relatively cold and rigid just below the crust. At greater depths, the mantle is able to flow because it is hot and non-rigid.

Subduction

Oceanic plates are denser than continental plates. Where they meet, the oceanic plate goes under the continental plate. This is called subduction. The oceanic plate is partially re-melted. Check your understanding of this using the animation below.

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