Major Rock Categories

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Abstract

Rocks are divided into three main categories according to how they were formed: igneous, sedimentary or metamorphic processes. Rocks differ in their texture, mineral and chemical composition, and bedding characteristics depending on which of these three processes that formed them.

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The Earth's crust is made up of rocks that are heterogeneous aggregates of minerals. They may crop out at the surface or be covered by a layer of unconsolidated materials (like sand, gravel, silt and clay).

Most rocks are mixtures of two or more minerals, but some, such as pure sandstones (made up entirely of quartz) and pure limestones (consisting of calcite), contain only one. Other rocks, such as coal, are made up of organic materials and contain no minerals.

Igneous Rocks

Igneous rocks, which are also referred to as magmatic rocks, are formed from materials that come from the depths of the Earth. There are two categories of igneous rocks: intrusive (or plutonic) rocks and volcanic rocks.

Plutonic rocks are consolidated at depth. Because they crystallize slowly, they have a coarse-grained texture and their crystals can be distinguished with the naked eye. Granite is a good example of a plutonic rock.

Volcanic rocks are born of volcanic eruptions and consolidated at the surface. The molten materials cool more rapidly at the surface than they would in the depths of the Earth, which gives volcanic rocks their fine-grained crystals and glassy appearance. Rhyolite, basalt and obsidian are examples of igneous rocks that have formed at the surface of the Earth.

Volcanic and plutonic rocks are similar in composition since they form from the same materials. They differ mainly in terms of crystal size, which is a function of the environments in which they formed.

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Columnar joints in rhyolite flow, MacDougalls Island, New Brunswick

Sedimentary Volcanic Rocks

Some rocks are partly volcanic and partly sedimentary in origin. They are produced by volcanic eruption when materials that have already crystallized in the chimney of a volcano are broken into large angular blocks and small fragments that are transformed into ash and projected into the air. As this material is deposited, it lands in successive layers on the ground or the sea floor and is later lithified. Rock that contains larger fragments is called volcanic breccia or agglomerate; finer-grained rock is called tuff. These rocks are classified according to their composition.

Sedimentary Rocks

Sedimentary rocks are the product of the consolidation of loose sediment that has accumulated in beds. Such sediment may be produced by the disintegration of previously existing rock or the precipitation of dissolved minerals, or it may consist of plant and animal remains. Regardless of origin, these deposits settle gradually under the weight of overlying beds and are transformed into solid sedimentary rock by cementation.

Sedimentary rocks are classified according to grain size and composition. The largest grain sizes are seen in conglomerates, tillites and sedimentary breccias. Sandstones are formed by the consolidation of sand grains. If the grains are very small, the rock is referred to as a siltstone. Still finer sediments, such as clays, produce shales.

Both fresh and salt water often contain chemical compounds, such as calcium carbonate, in solution. Under the right chemical conditions, these compounds can precipitate, forming deposits that harden into rock. The most common sedimentary rocks in this category are limestone, which consists of calcium carbonate, and dolomite, which contains significant amounts of magnesium carbonate.

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Calcite crystals. Calcite is the principal constituent of limestone.

Metamorphic Rocks

A metamorphic rock is derived from a pre-existing igneous or sedimentary rock that is transformed, to a greater or lesser degree, by the action of physical factors such as temperature and pressure, or chemical factors that include the addition of new minerals. Such transformations take place at depth in the Earth’s crust.

Metamorphism reorganizes the minerals in the rock, changes mineral grains sizes to produce a somewhat coarser-grained texture and creates new structures, primarily by aligning minerals in foliated layers or parallel bands. If rocks are sufficiently metamorphosed, it can be difficult, if not impossible, to identify the original rock.

Examples of metamorphic rocks include: marble, serpentinite, quartzite, argillite, slate and gneiss.

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Tourmaline is widely distributed in acid igneous rock, and metamorphic rock such as gneisses.