Rocks, Resources and Scenery

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Igneous

All formed when molten rock (magma) from the mantle cools down and hardens. There are two types depending on where the magma has cooled down:

Intrusive eg. Granite

  • Form when magma cools down below the Earth's surface.
  • It cools very slowly forming large crystals that give the rocka a coarse texture.
  • Large domes of cooled magma form domes of igneous rock called batholiths.
  • Where the magma has flowed into gaps in the surrounding rock it forms dykes (in vertical gaps) and sills (in horizontal gaps).

Extrusive eg. Basalt

  • Form when lava cools down after it's erupted from a volcano onto the earths surface.
  • It cools down very quickly formiing small crystals that give the rocks a fine texture.
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Sedimentary

Formed when layers of sediment are compacted together until they become solid rock. The process of compaction is called lithification.

  • Carboniferous limestone and chalk are formed from calcium carbonate. Layers of tiny shells and skeletons of dead sea creatures are deposited on the sea bed and compacted together over time.
  • Clays and shales are made from mud and clay minerals. The particles have been eroded from older rocks, deposited in layers on lake or sea beds then compacted together.

* Sedimentary rocks often contain fossils.

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Metamorphic

Formed when other rocks (igneous, sedimentary or older metamorphic rocks) are changed by heat and pressure.

  • Rocks deep in the Earth are changed by the pressure from the weight of the material above them.
  • When tectonic plates collide, rocks are changed by the massive heat and pressure that builds up.
  • Magma from the mantle heats the rocks in the crust causing them to change.

The new rocks are harder and more compact eg. Limestone becomes Marble. Clay becomes Slate.

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Mechanical weathering

Freeze thaw

  • When it rains water gets into the cracks in the rocks
  • At night the water freezes and expands, which puts pressure on the rock
  • The water thaws the next day, releasing the pressure, then re freezes the next night
  • Repeated freezing and thawing widens the cracks and causes the rock to break up

Exfoliation

  • Each day the surface layers of rock heat up and expand faster than the inner layers
  • At night the surface layers cool down and contract faster than the inner layers
  • This creates pressure within the rock and causes thin surfaces to peel off
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Chemical weathering

Solution weathering

  • Some minerals that make up rock dissolve in water
  • The minerals dissolve in rainwater, breaking the rock down

Carbonation weathering

  • Rainwater has carbon dioxide dissolved in it, which makes it a weak carbonic acid
  • Carbonic acid reacts with rocks that contain calcium carbonate, eg Carboniferous limestone, so the rocks are dissolved by the rainwater
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Biological weathering

  • Plant roots break down rocks by growing into cracks on their surfaces and pushing them apart
  • Burrowing animals may loosen small amounts of rock material
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Granite landscapes

Tors

  • Granite has lots of joints which aren't evenly spread ( they're closer together in some parts )
  • Freeze-thaw and chemical weathering wear down the parts of the rock with lots of joints faster because there are more cracks for the water to get into.
  • Sections of granite hat have fewer joints are weathered more slowly than the surrounding rock and stick out at the surface forming tors

Granite is also impermeable - it doesn't let water through
This creates moorlands - large areas of waterlogged and acidic soil, with low growing vegetation

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Chalk and clay landscapes

Escarpments

  • Horizontal layers of chalk and clay are sometimes tilted diagonally by earth movements
  • The clay is less resistant than the chalk so is eroded faster
  • The chalk is left sticking out forming escarpments/cuestas ( hills ). Where they clay has been eroded it forms vales - wide areas of flat land
  • Escarpments have steep slopes ( scarp slope ) on one end and gentle slopes (dip slope ) on the other

Chalk is an aquifer - a permeable rock that stores water

Water flows throughout the chalk and emerges where the chalk meets impermeable rock
Where the water emerges is called a spring line

Areas of chalk can also have dry valleys - valleys that don't have a river or stream flowing in the, because the water is flowing underground

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Carboniferous limestone features

  • Limestone pavements are flat areas of limestone with blocks separated by weathered-down joints
  • Swallow holes are weathered holes in the surface
  • Caverns form beneath swallow holes where the limestone has been deeply weathered
  • Limestone gorges are steep sided gorges formed when caverns collapse
  • Limestone is permeable so limestone areas also have dry valleys and resurgent rivers ( rivers that pop out at the surface when limestone is on top of impermeable rock )
  • Water seeping through limestone contains dissolved minerals. When the water drips into a cavern the minerals solidify and build up over time to produce stalactites ( on the ceiling ) and stalagmites ( on the ground )
  • When stalagmites and stalactites meet in the middle they form a pillar. When water flows in as a sheet a curtain builds up
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