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Limestone is a sedimentary rock composed mainly of the mineral calcite, which is a crystalline form of Calcium Carbonate (CaCO3). Limestone often contains variable amounts of silica in the form of Jasper or Flint, as well as amounts of clay, silt, and sand as disseminations, nodules, or layers within the rock unit. The main source of this calcite in limestone is speleothems such as stalagmites and stalactites. The secondary source of calcite is the shells of sea animals and corals.

Limestone makes up about 10 percent of the total volume of all sedimentary rocks. Most Limestone starts as the floor of shallow tropical seas and can be seen in parts of the tropics that are only 30 to 40 years old. The stone has the unique property of retrograde solubility, meaning that the stone is less soluble in water as the temperature increases.

Limestone is also classified as a young marble formed from the consolidation of seashells and sediment. Shells of sea animals form grains in limestone that promotes the growth of cement crystals around themselves.

Limestone is a popular building material because of its availability and the relative ease with which it can be worked with and cut. The stone can have a very diverse chemical composition, which can result in a variety of different colours of limestone or even within a single cut of the stone.

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About 300 million years ago, in the Carboniferous period, there were huge tropical swamps filled with giant tree-like ferns. As the remains of the plants were buried and compacted in these huge, warm Swamps, they formed peat. As they sank deeper, heat and pressure changed the peat into brown coal. Further pressure changed it to black bituminous coal.

Coal swamps are the classical terrestrial (land-based) ecosystems of the Carboniferous and Permian periods. They are forests that grew during the Palaeozoic Era (encompassing the Carboniferous and Permian) in which the volume of plant biomass dying and being deposited in the ground was greater than the volume of clastic (grains of pre-existing rock) material, resulting in a build-up of peat. This was subsequently buried, and eventually turned into coal over geological time. These swamps gave rise to most of the major, industrial-grade coal reserves that are mined today. The palaeontology of these coal-forming ecosystems is well known from the Carboniferous rocks of Euramerica (modern day Europe and North America), owing to the history of coal exploitation in these regions. However, extensive swamp areas that produced thick coal reserves have also formed at other times in the Earth’s history, most notably in the Permian. During the Early Permian, the coal swamps of Euramerica continued to flourish in Cathaysia (the tectonic blocks that formed modern day China), and throughout the Permian, coal swamps dominated by seed plants called glossopterids were found on the Southern Hemisphere supercontinent Gondwana (formed from modern day India, Australia, Antarctica, Africa, Madagascar and South America). The coal swamps of the Carboniferous and Early Permian formed primarily in tropical regions, whereas the Gondwanan coal swamps of the later Permian formed in higher-latitude temperate regions. Coal forests developed primarily in lowland areas such as river deltas, but there is a bias in the plant fossil record because fossilization is most likely to occur in these waterlogged habitats, meaning that fossils of drier, upland plant communities are much less common, so little is known of the plants that grew there.

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No. Although coal is sometimes called an organic rock, it is not a proper rock as rocks are inorganic (lifeless). Coal is a fossil fuel - like oil and gas- that formed over millions of years from the remains of once-living matter.

Coal is the largest source of energy for generating electricity in the world, and the most abundant fossil fuel in the United States.

Fossil fuels are formed from the remains of ancient organisms. Because coal takes millions of years to develop and there is a limited amount of it, it is a nonrenewable resource.

The conditions that would eventually create coal began to develop about 300 million years ago, during the Carboniferous period. During this time, the Earth was covered in wide, shallow seas and dense forests. The seas occasionally flooded the forested areas, trapping plants and algae at the bottom of a swampy wetland. Over time, the plants (mostly mosses) and algae were buried and compressed under the weight of overlying mud and vegetation.

As the plant debris sifted deeper under Earth’s surface, it encountered increased temperatures and higher pressure. Mud and acidic water prevented the plant matter from coming into contact with oxygen. Due to this, the plant matter decomposed at a very slow rate and retained most of its carbon (source of energy).

These areas of buried plant matter are called peat bogs. Peat bogs store massive amounts of carbon many meters underground. Peat itself can be burned for fuel, and is a major source of heat energy in countries such as Scotland, Ireland, and Russia.

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Rocks are continually being recycled to make new rocks in a process called the rock cycle. For example, igneous rocks are gradually worn away by the weather. The weathered fragments are washed down to the sea and eventually form sedimentary rocks. Similarly, metamorphic rocks can be formed from both igneous and sedimentary rocks.

The rock components of the crust are slowly but constantly being changed from one form to another and the processes involved are summarized in the rock cycle. The rock cycle is driven by two forces: (1) Earth’s internal heat engine, which moves material around in the core and the mantle and leads to slow but significant changes within the crust, and (2) the hydrological cycle, which is the movement of water, ice, and air at the surface, and is powered by the sun.

The rock cycle is still active on Earth because our core is hot enough to keep the mantle moving, our atmosphere is relatively thick, and we have liquid water. On some other planets or their satellites, such as the Moon, the rock cycle is virtually dead because the core is no longer hot enough to drive mantle convection and there is no atmosphere or liquid water.

Magma is rock that is hot to the point of being entirely molten. This happens at between about 800° and 1300°C, depending on the composition and the pressure, onto the surface and cool quickly (within seconds to years) — forming extrusive igneous rock.

Magma can either cool slowly within the crust (over centuries to millions of years) — forming intrusive igneous rock, or erupt onto the surface and cool quickly (within seconds to years) — forming extrusive igneous rock. Intrusive igneous rock typically crystallizes at depths of hundreds of metres to tens of kilometres below the surface. To change its position in the rock cycle, intrusive igneous rock has to be uplifted and exposed by the erosion of the overlying rocks.

Through the various plate-tectonics-related processes of mountain building, all types of rocks are uplifted and exposed at the surface. Once exposed, they are weathered, both physically (by mechanical breaking of the rock) and chemically (by weathering of the minerals), and the weathering products — mostly small rock and mineral fragments — are eroded, transported, and then deposited as sediments. Transportation and deposition occur through the action of glaciers, streams, waves, wind, and other agents, and sediments are deposited in rivers, lakes, deserts, and the ocean.

Unless they are re-eroded and moved along, sediments will eventually be buried by more sediments. At depths of hundreds of metres or more, they become compressed and cemented into sedimentary rock. Again through various means, largely resulting from plate-tectonic forces, different kinds of rocks are either uplifted, to be re-eroded, or buried deeper within the crust where they are heated up, squeezed, and changed into metamorphic rock.

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There are three kinds of rocks: igneous, sedimentary and metamorphic. Igneous rocks are formed when red-hot magma flows up from Earth's hot core and cools down. Sedimentary rocks form when debris, including plant and organic matter, usually deposited on the seabed in layers, is built up, compressed and cemented into solid rock over millions of years. Metamorphic rocks are created when movements of Earth's crust or the heat of its magma transforms one kind of rock into another.

Rocks are mineral aggregates with a combination of properties of all the mineral traces. Any unique combination of chemical composition, mineralogy, grain size, texture, or other distinguishing characteristics can describe rock types. Additionally, different classification systems exist for each major type of rock. There are different types of rocks existing in nature.

Rocks which are found in nature rarely show such simple characteristics and usually exhibit some variation in the set of properties as the measurement scale changes.

Types of Rocks

There are three types of rocks:

Igneous Rock

Igneous rock is one of the three main rock types. Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.

Sedimentary Rock

The sedimentary rocks are formed by the deposition and subsequent cementation of that material within bodies of water and at the surface of the earth. The process that causes various organic materials and minerals to settle in a place is termed as sedimentation.

Metamorphic Rocks

The metamorphic rocks make up a large part of the Earth’s crust and are classified by texture and by chemical and mineral assemblage. They may be formed simply by being deep beneath the Earth’s surface, subjected to high temperatures and the great pressure of the rock layers above it.

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Rocks are the hard mass of which the ground is made. Though we usually see them exposed in places such as cliffs, mountain crags and quarries, rocks are everywhere, even deep beneath the ground. Rocks can be as old as Earth itself, and are made of tiny crystals or grains of naturally occurring chemicals called minerals.

A rock is a solid mass of geological materials. Geological materials include individual mineral crystals, inorganic non-mineral solids like glass, pieces broken from other rocks, and even fossils. The geological materials in rocks may be inorganic, but they can also include organic materials such as the partially decomposed plant matter preserved in coal. A rock can be composed of only one type of geological material or mineral, but many are composed of several types. Figure 6.2 shows a rock made of three different kinds of minerals.

Rocks are grouped into three main categories based on how they form. Igneous rocks form when melted rock cools and solidifies. Sedimentary rocks form when fragments of other rocks are buried, compressed, and cemented together; or when minerals precipitate from solution, either directly or with the help of an organism. Metamorphic rocks form when heat and pressure alter a pre-existing rock. Although temperatures can be very high, metamorphism does not involve melting of the rock.

Credit: University of Saskatchewan

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