Rocks are classified according to how they were formed: igneous (fire), metamorphic (changed form), or sedimentary (deposition of sediments). Within each class of rocks, different rocks are identified according to their mineral and chemical composition. Depending on the composition, their overall texture and by the processes that formed them, they are defined by three major groups of rocks: Igneous rocks, Sedimentary rocks and Metamorphic rocks.

Igneous rocks

Igneous rocks are called “fire rocks” and can be formed either underground or overground.  Magma is formed by the melting of the Earth’s mantle and sometime’s the lower crust. These are formed when molten rock, magma, cools and crystallises to the solid state. When this magma slowly cools down, it becomes igneous rocks.

It is composed of some gases (in particular water vapour), solids (mineral) and the majority are elements which are the main parts of silicate minerals.  As magma is less dense than the surrounding rocks it will rise to the surface. Igneous Rocks can either be “intrusive” (plutonic) or “extrusive” (volcanic).  These first ones occur when magma cools down and solidifies inside the earths crust, whereas the extrusive types are formed on the surface of earths crust. We can also mention a third type of igneous rocks, which are formed between both the intrusive and extrusive types, in other words, just below the earth’s surface. This type is less common and is classified as “hypabyssal” igneous rocks. When it cools at the surface and solidifies, the igneous rocks that are formed are termed “extrusive” or “volcanic”.   However most magma will solidify or crystallise underground.  The igneous rocks formed there are termed “intrusive” or “plutonic”.   These rocks are only exposed to the surface by tectonic forces raising the crust in combination with erosive forces on the surface.   

The rate at which magma cools will determine the crystal size of the mineral. Basically the slower the magma cools the larger the crystal.  Therefore if you can see the crystals with the naked eye you know that the magma cooled slowly and if the crystals need a microscope to be seen, they cool quickly.

Texture

Texture refers to the general appearance based n size, shape and arrangement of the minerals appearance. The texture of an igneous rock reflects the way that the magma cooled:

  • A course-grained texture of crystals is created when the magmas cools deep within the crust, and therefore, very slowly, eg: granite;
  • A fine-grained texture results when the magma is extruded on the earth’s surface, cools and crystallises;
  • A glassy texture forms when the magma is cooled very quickly, as when it flows into the sea, eg: obsidian;
  • A frothy texture (like foam with little cavities) results when the magma is filled with gasses, and cools quickly;
  • Pyroclastic or fragmental textures result from explosive volcanic activity.

Examples
Igneous rocks includes granite, basalt, obsidian, pumice and rhyolite. Igneous rocks can be identified by their composition and texture. Their mineral composition reflects the chemical composition of the original magma. Course-grained granite and the fine-grained rhyolite are formed from the same kind of magma, but under different conditions, so they both contain about 10-20% quartz. Basalt contains no quartz, but much more metallic content than granite. Obsidian and pumice may or may not contain quartz, and are less metallic than basalt.

 

Sedimentary rocks

These are the most common and widely found of the three rock classes. They are formed from sediments that have been compressed and sometimes chemically affected to form rock.  Erosion plays a big part of the formation of sedimentary rocks as there has been a slow settlement of tiny particles of the earth at the bottom of rivers, lakes and oceans. Throughout the time, these sediments have been forming layers, one over the other, which are continuously being pressed down, until the bottom layers are slowly turned into rocks. Because of the method of their formation, they often contain features, such as fossils, that tell us about the origin and history.
There are two main kinds of sedimentary rocks:

Detrital rocks – which are formed from sediment that has been transported by wind or water and deposited in layers or beds, where they are consolidated over time into rock. Detrital rocks are made up largely of clay and sand. Clay is formed into rocks mainly by compression, as overlaying layers squeeze them together. Sand is generally formed into rock by silica, calcium carbonate, iron oxides or clay. Some common detrital rocks are quartz sandstone, shale, conglomerate, and mudstone;

Chemical and biochemical rocks – These are produced by chemical reaction with minerals in a body of water as they compress. Chemical rocks contain no previously organic material (they are not made of pieces of sediment) whereas biochemical rocks contain organic material. Coal is a well-known example. Others are rock salt, gypsum and limestone.

Texture

The texture of detrital rocks can range from coarse (over 2mm) gravel particles in Conglomerate to the very fine (less than 1/256 mm) mud particles in Shale.  Sandstone particles are considered medium (1/16 to 2 mm).  

Chemical sedimentary rocks also vary but within a much more limited range. For example, Crystalline Limestone will have fine to coarse crystalline, whereas the texture of Chalk consists of microscopic shells and clay.  Note that the while particle size is the main criteria for naming detrital rocks, for chemical rocks it is the chemical composition that is used.

Features

Because they are formed by the gradual build-up of minerals and sediments under water or in watery areas, the material of sedimentary rocks often appears in wavy or straight lines, or obvious layers.  These layers are called strata, or beds are the single most important characteristic of sedimentary rocks.  These beds can range in size from microscopically thin to meters thick.   In between the strata are bedding planes.  The sedimentary rocks will usually break along these planes.

Because of the deposition over time that leads the formation of sedimentary rocks, it is possible to find evidence for past environmental conditions. The size of the sediment particles in the rock can indicate a high or low energy environment.  For example Coal will indicate a swamp while a conglomerate rock will indicate fast moving water as only large grains will settle.

Metamorphic rocks

Metamorphic rocks are really recycled rocks. Its name comes from the word “morph”, which means “form”. These were once igneous or sedimentary, but, as parts of the earth’s crust are subducted in plate tectonics activity, they have been formed (metamorphosed) into another type of rock by a build up of heat due to these being under an enormous amount of pressure. Metamorphic rocks are formed at depths in the earth’s crust, most commonly in mountain regions.

Most metamorphic rocks form when the parent rocks become compressed at convergence zones. Though the rocks remain solid, the great heat and pressure causes the minerals in them to become unstable and form into higher density minerals. Intense folding and other processes cause many of them to become foliated, that is, for the minerals to reform in sheets or streaks. Some metamorphic rocks form by recrystallising into a more compact texture, without changing the nature of the minerals or foliating. Pure limestone can be metamorphosed into marble in this way. Other foliated rocks are slate and schist.