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Rocks and the Rock Cycle

The rock cycle is an important theory in the science of geology. First proposed by Scottish chemist James Hutton (1726-1797), one of the founders of the science, the theory was later refined by Marcel-Alexandre Bertrand (1847-1907) to explain how the three basic rock types, known as igneous, sedimentary, and metamorphic, are related; that they change from one rock type to another through time; and that there must be a driving force behind the rock cycle. 
Rocks are made of minerals, such as quartz and calcite, and can be altered by changing the mineral proportions within the rock, or by altering the minerals themselves. Common methods of altering rocks include weathering and heating, both of which are fundamental to the rock cycle.

Igneous Rocks

Magma that comes to the Earth's surface is called lava, and is commonly seen pouring from volcanoes. When magma solidifies, an igneous rock is formed. Following a volcanic eruption, the lava cools quickly and the resulting rocks will often have the appearance of dark glass, such as obsidian. However, huge amounts of magma never reach the surface of the Earth, instead solidifying into igneous rocks in underground passages and chambers. The process of solidification occurs through crystallization of the magma.

When the Earth and solar system formed from dust 4.8 billion years ago, the pressure from gravity and radioactive heat melted the dust and formed the first igneous rocks. Through time, this melting has separated denser from lighter minerals, causing the light, continental igneous rocks, such as granite, to effectively float on denser, semi-molten rocks, or mantle

The ocean floor is made of similar, dense rocks, the most common of which is basalt. Granite is a common building stone and occurs naturally in the highlands of Scotland, the moors of Devon and Cornwall, and beneath the Pampas plains of Argentina. Basalt is also used in construction, especially as a roadstone, and occurs at the Giant's Causeway in Northern Ireland, throughout Hawaii, and on the Moon. The Rosetta Stone in Egypt is made of basalt. Scientists are interested in igneous rocks, and especially volcanoes, because they create fertile soil and can be tapped for geothermal energy.

Sedimentary Rocks

Rocks exposed at the Earth's surface will undergo weathering. This is because most rocks are formed at high pressures and temperatures, away from the influence of oxygen or water. When brought to the surface by volcanoes or Earth movements, such material is chemically unstable and will break down into a more stable form. The process can be compared to iron or steel formed at high temperatures in a furnace and then left in the open air, where the iron oxidizes or rusts. Rocks are weathered in two ways: chemically, as described above, and mechanically, which involves erosion by rivers, wind, waves, or glaciers

The weathered product, or sediment, will comprise fragments of rock and dissolved chemicals which can be transported into lakes, seas, oceans, or other depressions on the Earth's surface, where it may accumulate in layers. The layers may also enclose other transported objects such as dead plants or animals, which may be preserved as fossils. Some depressions, called basins, are very deep and may accumulate up to 20,000 metres (60,000 feet) of sediment. Such huge thicknesses of sediment create tremendous pressure on the underlying layers, squeezing out any water they contain and causing new minerals to form. Both chemical and mechanical weathering turn soft sediment into sedimentary rock, as well as altering the minerals in both the rocks and the fossils. 

The sediments that accumulate to form rocks come not only from transported fragments of sand and clay, but also from chemicals carried in solution and later precipitated. Rock salt and limestone are two examples of such chemical sedimentary rocks. Shale, which is formed from layers of silt and clay, is the most common sedimentary rock on the Earth's surface, followed by sandstone, which consists of compacted grains of sand. Famous sedimentary rocks include the conglomerate (made of pebbles and cobbles) that forms Uluru (Ayers) Rock, in Australia, the sandstone megalith of Avebury, near Stonehenge in England, and the yellow magnesian limestone used to build the Houses of Parliament in London.

Fossil fuels are derived from organic sedimentary rocks, which are formed from the accumulation and compaction of dead organic matter. Coal is formed from peat, which is created by the accumulation of dead organic matter in acidic waters. Over time, the peat compacts into lignite, a sedimentary rock with a woody texture, and as further layers of decayed material build up, the lignite compacts into bituminous coal. Oil shale, another organic sedimentary rock, can be heated and crushed to yield oil.

Metamorphic Rocks

Sedimentary and igneous rocks may become buried by huge layers of overlying rock, or they may be caught up in areas where movements of the Earth's crust are occurring. The pressure and heat produced by burial or movements of the crust will alter the original rock so completely that a new, or metamorphic, rock is formed. Mildly altered sedimentary rocks, such as shales, turn into slates when metamorphosed. Further pressure and heat will turn slates into schist or gneiss. Further action will completely melt the rock, creating magma and a new igneous rock when cooled. 

Burial and deformation commonly occur in areas where there are faults, such as subduction zones, and areas where mountain ranges are formed. plate tectonics is the controlling force behind the location of these features and the resulting metamorphic rocks on the Earth's surface. Slate is quarried in Gwynedd county, North Wales, and marble, which is metamorphosed from limestone, can be found at Carrara in Italy. Quartzite, another metamorphic rock consisting of sand grains cemented together by quartz, occurs in the Taunus of west central Germany and Australia's Katherine Gorge.

How do Rocks get Recycled?

The recycling of rocks can occur in a variety of ways. In the ideal development, igneous rocks exposed at the surface are weathered, eroded, transported, and deposited as sediment. Layers of sediment are buried and harden into sedimentary rocks, which are then heated and deformed to such a degree that they melt and form new igneous rocks. In this ideal cycle, shortcuts are common. For instance, igneous rocks may never be weathered, but instead are re-melted during metamorphism without experiencing a sedimentary stage. Alternatively, metamorphic rock may be uplifted away from the influence of pressure and heat, to be weathered and removed as sediment.

The rock cycle is a naturally changing process, like the weather or the growth of plants. The difference lies in the fact that the rock cycle takes much longer to complete and is driven by plate tectonics. When James Hutton put forward his rock cycle theory, he had no knowledge of the age of the Earth, or of plate tectonics. Yet the vast amount of time available for rock recycling, and the processes of rock destruction and creation inherent in tectonic theory, help to explain the driving forces behind his ideas. 

An important spin-off from the main theory, as presented by Hutton, is that all global materials are recycled by this means. When plate tectonic activity began during the early evolution of the Earth, approximately 4.8 to 3.5 billion years ago, it was accompanied by rock recycling and the release of atmospheric gases and water. Water, carbon dioxide, nitrogen, and radiation from the Earth or Sun are essential for life on Earth to survive. In the early evolution of the Earth, fresh supplies of water and carbon dioxide would not have been formed without tectonics and the rock cycle.

Uses of the Rock Cycle

The rock cycle shows how igneous rock may be eroded and transported as fragments and dissolved sediment. It also explains why fossils such as dinosaurs, ammonites, and trilobites occur most commonly in sedimentary rock, and how deposits of oil, gas, and coal occur in such a medium. The theory can also be used to demonstrate the formation of greatly deformed and altered rock, and it tells us that the Earth has its own method of recycling materials and reforming them.

The rock cycle occurs on other planets too, especially where there is, or was, plate tectonics and the agents of weathering and sediment transport (wind and water). There is good evidence of the rock cycle operating on Mars and Venus. Rock recycling also occurs at a very slow rate on the Moon and on smaller rocky bodies through the action of solar weathering and gravity. On Earth, the active tectonic system allows a faster rate of rock recycling, releasing the water and gases that allowed life to evolve on our planet. Without the rock cycle, life as we know it would not exist on Earth.