The Rock Cycle
The Earth is active. As you are reading this:
- Volcanoes are erupting and earthquakes
- Mountains are being pushed up and are being worn down;
- Rivers are carrying sand and mud to the sea;
- Huge slabs of the Earth's surface called tectonic plates
are slowly moving - about as fast as your fingernails grow.
Weathering and Erosion
Rocks of every sort and shape are worn away over time. Weathering is the process
which breaks rocks into smaller bits. There are three main types:
- Physical weathering is a physical action which breaks up rocks : An example
of this is called freeze-thaw weathering when water gets into tiny cracks in rocks.
When the water freezes it expands, if this is repeated the crack grows and bits
eventually break off.
- Chemical weathering is when the rock is chemically attacked: An example of
this is the breakdown of limestone by acid rain.
- Biological weathering is when rocks are weakened and broken down by animals
and plants. An example would be a tree root system slowly splitting rocks.
Erosion is a type of physical weathering which involves wearing down rocks.
Have you heard of coastal erosion? Read about the disaster at Beachy
| There is an important point to remember. ROCKS ARE WEATHERED
AT DIFFERENT RATES. Dartmoor is an upland area of 241 square
miles reaching up to 2,000 feet in height making it the largest and highest area
of moorland in the South of England. It is also the largest granite surface in
England. (see picture). Granite is made up of large interlocking crystals (igneous
rock) that give it a granular texture and make it one of the toughest rocks
on Earth. Sedimentary rocks such as sandstone tend to be much weaker
The rock cycle goes round and round, taking hundreds of millions of years.
Once the rock has been broken down into smaller bits it's got to somehow move.
Streams and rivers carry the small bits towards the sea (continually wearing down
as the they progress). Big rivers such as the Humber and the Severn carry millions
of tonnes of sediments out to sea each year.
Deposition simply means that the sand and sediments in the sea eventually settle
to the bottom.
LOOK AT THIS ANIMATION:-
Sedimentary rocks are formed in three steps:
- Layers of sediment are deposited at the bottom of seas and lakes.
- Over millions of years the layers get squashed by the layers above.
- The salts that are present in the layers of sediment start to crystallize
out as the water is squeezed out. These salts help to cement the particles together.
How can you spot a Sedimentary rock?
- Sedimentary rock will often have layers or bands across them.
- It will often contain fossils which are fragments of animals or plants preserved
within the rock. Only sedimentary rocks contain fossils....... click here
to find out why.
- The rock will tend to scrape easily and often crumble easily.
SOME COMMON SEDIMENTARY ROCKS:
|Sandstone is one of the most common sedimentary rocks. It is
made from sand grains eroded from older rocks, cemented together and then hardened
into new rock. Here we see a picture of a Jurassic sandstone from the USA, notice
the layers. Each layer is a record of an event in the past.
This is made from pebbles and smaller stones stuck together in a matrix.
Limestones are made from fragments of sea creatures that sank to the bottom
of ancient tropical seas. Many limestones from Southern England are made from
dissolved lime which builds up around sand grains to form tiny spheres called
oolites. Limestones frequently contain fossils. Here we see a stalactite from
the limestone cave system a few miles away in Ingleton.
Mudstone or Shale
These are simply just mud hardened into rock. They consist of much finer particles
than sand .They often contain fossils.
Heat and pressure make Metamorphic Rocks
Earth movements can push all types of rock deeper into the Earth. These rocks
are then subjected to massive temperatures and pressures causing the crystalline
structure and texture to change. THEY DO NOT MELT. The high pressure involved
are often associated with mountain building processes.
This is formed from mudstone or clay and is the most common kind of metamorphic
rock in Britain. Pressure causes new minerals to grow in parallel sheets - which
makes slate split easily to make roofing tiles.
Marble is limestone that has been squashed and heated .The shells of the limestone
breakdown and recrystallise into tiny crystals. Marble is chemically the same
as limestone but it is much harder and far more expensive. Some of the finest
marble comes from Italy and it is used for sculptures and as a fine building material.
Formed from mudstones subjected to great heat over long periods of time. It
looks to have layers of banded crystals (It cannot be igneous because igneous
rocks don't have layers)
Igneous rocks form when molten rock (Magma if it is below the surface or lava
if it has erupted from a volcano) solidifies. These rocks can be identified by
the following tell-tale clues:
- Igneous rocks contain a minerals randomly arranged in
(Remember CRYSTALS !!!!!!)
- If the rock has small crystals this means that it had rapidly cooled, possibly
because it was erupted into the ocean. We call it an EXTRUSIVE IGNEOUS rock.
If the rock has large crystals it means that it slowly cooled, the molten rock
solidifies deep down within the crust without ever reaching the surface via an
eruption. We call it an INTRUSIVE IGNEOUS rock.
- The rock are usually tough and hard (With the most famous exception being
This bit is worth remembering:-
||COOLED SLOWLY UNDERGROUND
||COOLED QUICKLY AFTER AN ERUPTION
COMMON IGNEOUS ROCKS
This is the most common form igneous rock which makes up most of the ocean
floors. It is smooth and velvety-black in appearance and very hard. Basalt is
formed when magma is erupted onto the sea-bed, as soon as it hits the cold sea
water it cools quickly - it's got tiny crystals.
This rock floats on water. Carbon dioxide and water dissolved in the molten
rock is released with the decrease in pressure as it reaches the surface. Lava
cools quite quickly in the air so the bubbles of gas get trapped.
If molten rock doesn't reach the surface via a volcano and cools underground
instead, it solidifies very slowly (WHAT WOULD THE CRYSTAL SIZE BE?). This is
because overlying layers of rock insulate the magma keeping it warm, this only
allows gradual cooling. Some crystals grow to a much bigger size giving granite
a speckled appearance. Granite is the most
common form of igneous rock in the UK.
Earthquakes, Folding and Faulting
Sedimentary rock are often found tilted, folded, fractured and twisted. This
indicates that the Earth has moved with enormous force (obviously over huge timescales).
Large scale movements of the Earth's crust can push up whole mountain ranges.
More information on the Himalayas can be found here.
'What goes up must come down' as the old saying goes, weathering will ensure that
the rock cycle starts all over again.
GO TO THE ROCK TEST
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