“Rocks are constantly being changed by internal and external forces”. Discuss this statement with reference to the rock cycle diagram.
Rocks are constantly being changed or altered by extrusive forces such as weathering and erosion and intrusive forces such as heat, pressure, compaction and compression. Rocks may be changed frequently and quickly depending on their type and location, or, they might stay in their current form for a long period of time.
External Forces: Weathering and Erosion Exposed 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.
It is important to remember that rocks are weathered at different rates. 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
Transportation 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 they progress). Big rivers such as the Humber and the Severn (in the UK) carry millions of tonnes of sediments out to sea each year.
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.
Limestone Limestone is made from fragments of sea creatures that sank to the bottom of ancient tropical seas. Limestone frequently contains fossils.
Mudstone or Shale These are simply just mud hardened into rock. They consist of much finerparticles than sand .They often contain fossils.
Internal forces: Heat and pressure make Metamorphic Rock 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 is often associated with mountain building processes.
Slate 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 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.
Schist 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 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 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 rocks are usually tough and hard (With the most famous exception being pumice stone).
COMMON IGNEOUS ROCKS
Basalt 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.
Pumice 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.
Granite If molten rock doesn't reach the surface via a volcano and cools underground instead, it solidifies very slowly. 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.
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.
