Types of Rocks

Igneous type of rocks
The cooling of magma is an exothermic process. During this process, a phase alteration from liquid to solid state occurs. The solid formed is crystalline in nature. This is termed as an "Igneous rock". The surface of the earth which is exposed to the coldness of the atmosphere is made of this rock. If magma reaches the surface of the earth, the rock formed is called "basalt". If magma never comes out of the magma chamber, it is called "gabbro". Both of these have the same composition, but different textures. Intermediate magmas give rise to andesite and diorite. Felsic magmas produce granite and rhyolite. Depending on composition, igneous rocks are classified into:

• felsic – these contain potash feldspar, quartz, muscovite or biotite, amphibole
• intermediate – these contain plagioclase, amphibole, muscovite or biotite, quartz
• mafic – these contain olivine, pyroxene, plagioclase
• ultramafic – these contain olivine and pyroxene

As per the texture, they are divided into:

• aphanitic or fine grain
• intermediate
• phaneritic or coarse grain
• glassy
• frothy

Sedimentary kinds of rocks
A major part of the earth's surface is made of igneous rocks. This crust is covered by a thin layer of loose sediment. The rocks which make up this debris get compacted and cemented. This rock which is caused by the accumulation of tiny pieces, that have been broken from the pre-existing rocks, is called as a "sedimentary" or "secondary" rock. This is classified into three types:

• clastic – small pieces of broken rock get compacted and cemented. They are said to be lithified. These are basic sedimentary rocks
• chemical – formed when standing water evaporates and dissolved minerals are left on the surface. Due to frequent floods and evaporation, large deposits of salt and gypsum are formed. These are a regularly found in arid lands
• organic – animals use calcium in teeth, bones and shells. These sources of calcium accumulate on the seabed and form a sedimentary rock due to organic processes eg. Limestone, coal, peat, oil

Metamorphic types of rocks
The name is made from two words – meta which means change and morph which means form. Any rock can be called as a "metamorphic rock". The necessary condition is that the rock must be surrounded by such an environment which makes the minerals in the rock unstable and in a state of non-equilibrium. Majorly, this occurs when the rocks are buried and this causes an increase in pressure and temperature. Some alterations occur in the minerals so that equilibrium is attained. Examples of such metamorphic rocks are gneiss, schist, slate and marble.

In non-foliated metamorphic rocks, litho-static pressure and heat are the major conditions. These factors are prevalent below the earth's surface. The existing rock gets recrystallized. All rocks can be said to be of this type to some extent. Some examples of these rocks are:

• quartzite – this is derived from sandstone, is hard and durable, appearance is similar to sandstone, breaks through the quartz grains and not around them
• hornfels – this is derived from shale, is hard, durable and dense
• marble – this is derived from limestone, is softer than quartzite and hornfels, made of calcium carbonate, useful for buildings, carvings and as facing stone

In foliated metamorphic rocks, directed pressure and heat are the necessary conditions. Similar elements combine in different proportions and form varying minerals under different conditions of pressure and temperature. Some examples are greenschist facies, actinolite facies and staurolite facies. The different examples of metamorphic rocks are as follows:

• slate - has microscopic grains, is hard and durable, generally used as chalkboards, roofs and pool table tops
• phyllite – mica provides the rock a shiny look, can be half-muscovite or graphite or chlorite
• schist – the foliation of micaceous minerals is well-developed and more than 50%. This is also called schistosity. The primary textures and features are altered
• gneiss – is a high grade metamorphic rock, there exists a color banding of light and dark minerals
• migmatite – due to partial melting and recrystallization of felsic minerals