Types of Rocks

The solid masses occurring naturally can be described as Rocks. The rocks contain properties of mineral traces. The rocks usually contain a unique combination of chemicals, minerals, textures, size, shape, grains etc. 

The crust, that is the Earth’s outermost solid layer, is made of rocks, a major part of earth’s interior subtracting the liquid outer core and pockets of magma present in the asthenosphere. 

The origin of rocks and the character of these rocks’s scientific study is known as petrology. It is considered as an important discipline in geology. 

These distinguishing characteristics play a very important role in further categorizing rocks. 

Classification of Rocks

Rocks are mainly made up of grains of minerals, they are Crystalline solids which are formed from atoms chemical bonded in a structure of order. Mineraloids are found in some rocks that are mineral-like substances which are rigid in nature, like volcanic glass, Crystalline solutions lack in it. 

The type of the rock and it’s abundance of minerals is described by the way it is formed. Silicate mineral, a compound that is found in many rocks, in their Crystal lattice includes silica tetrahedra and it is accountable for one third of all the mineral species that are known and 95% of the crust of the earth. 

The major factor in determining the name and properties of rocks and minerals is the proportion of silica in it. The characteristics in which rocks are classified are their mineral and chemical composition, permeability , it’s texture of particles and the size of the particles. The processes in which the rocks are formed, are the results of these physical properties. Within the period of time, one type of rock can be transformed into another, which is described in the rock cycle, a geological model. 

Following are the different types of rocks:

• Igneous Rock

• Sedimentary Rock

• Metamorphic Rock

Igneous Rock

Rocks that are formed by the solidification and cooling of magma or lava are called Igneous Rock. This rock can be formed with or without the crystallization. They can be found on the surface that are extrusive rocks or below the surface as intrusive or plutonic Rocks. The magma melts due to increase in temperature, change in composition or decrease in pressure. Magma is also a form of molten rock made by the partial melting of rocks under high temperature or high-pressure conditions. Lava is the form of magma that reaches the surface of the earth. Igneous rocks are smooth and shiny as well. An example of Igneous Rock includes Basalt, Granite etc. The igneous rock gets cooled over the period of time and has large structures like crystals. The earth crust has many igneous rocks. 

Sedimentary Rock

The rocks formed by the sediment or debris of the liquid care called Sedimentary Rocks. The particles that form sedimentary rocks are termed as sediments. The sediments are transported with the water, ice or wind. They cement together to form a rock-like structure. The sedimentary rocks are not primary rocks, they are secondary rocks. They are formed by the deposition of sediments or debris on the preexisting rock. The preexisting rock is also called the Parent Rock. The sedimentary rocks are also formed due to the weathering, erosion or the mass movement. The rock is formed after the constant deposition and settling of the sediments on the rock. The process of sedimentation forms this rock. It takes many years for a sedimentary rock to form. The porosity of the rock rescues due to the compaction and piling of layers on the rock. Some of the examples of sedimentary rock include Sandstone, Limestone, Mud Stone etc.

The sedimentary rocks are further classified based on the type or the source of the sediments. The three types of sedimentary rocks are:

• Clastic Sedimentary Rock: The rock formed from classics or small pieces of fragmented rock due to mechanical weathering is called Clastic Sedimentary Rock. These rocks are formed due to the compaction and cementation effect. Examples of Clastic Sedimentary Rock are Sandstone, Siltstone etc.

• Chemical Sedimentary Rock: The rock formed due to the deposition of chemicals on the parent rock is called Chemical Sedimentary Rock. These rocks are formed due to the mineral elements in the water and the evaporation, which leaves the minerals on the rock. One finds chemical sedimentary rock often in the areas with arid lands having good deposits of gypsum and salts. The example of Chemical Sedimentary Rock includes Rock Salt, some LimeStone, Iron ore, Flint etc.

• Organic Sedimentary Rock: The rock that is formed due to the deposition of plant and animal debris such as corals, shells or bone are called Organic Sedimentary Rock. This rock is made up of sand, pebbles, shells and various other materials. The plant and animal debris have a high content of calcium minerals and they get to pile up over the time to form the Organic Sedimentary Rock. One can also find fossils in this type of rocks. The example of Organic Sedimentary rock is the Dolomites and some Lime Stones. 

Metamorphic Rock

Some of the rocks on the earth’s crust are metamorphic rocks. These rocks are classified based on their chemical, minerals, and texture. The Metamorphic Rocks are also formed beneath the earth's surface due to high temperature and a good amount of pressure. The temperature, pressure and various other conditions can physically or chemically change the properties of Metamorphic Rock. This process is called Metamorphism. The Metamorphic rock is found beneath the surface of the earth and has ribbon-like layers. These layers are caused due to heat. Many metamorphic rocks have shiny crystals as well. The exposure to extreme conditions can also change or alter the mineralogy, texture and chemical composition of these rocks. 

The Two Types of Metamorphic Rocks Are:

•  Foliated Metamorphic Rock: This kind of rock is formed in the earth’s interior when the pressure is high in one particular direction. The minerals in the original rock are reorient with flat and long minerals. The Foliated Metamorphic rock is formed where the pressure is discreet. The foliation leads to the development of new minerals. The foliated metamorphic rocks are classified on the basis of their texture. Slate is formed at low temperatures and pressures. This kind of rock breaks in the perfect parallel line. Phyllite is the crystalized rocks having a shiny appearance. Schist is larger crystals with larger grains. They have parallel and sub parallel lines. Genesis is formed by high pressure and temperature and has a coarse grained texture.

• Non Foliated Metamorphic Rock: This kind of metamorphic rock is found due to igneous intrusions where the pressure is relatively low but the temperature is high. The minerals in the rock crystalize and are packed tightly together. The Non foliated metamorphic rock is distinguished based on their composition. 

Difference Between Foliated and NonFoliated Metamorphic Rocks

The major difference between Foliated metamorphic Rock and Non Metamorphic rock is their texture and appearance as well. They are also different as the pressure of recrystallization varies. The pressure applied during the recrystallization process decides the appearance of the rock. The geological changes have a great impact on the metamorphic rock and decide the appearance of the rock as well as minerals. The moment of the tectonic plates under the earth's surface can cause friction and distortion. It can also lead to the formation of magma. 

The type of pressure on the rock plays a prominent role in the formation of a rock. If the pressure is unequal then the rock formed during the process will be foliated and if the pressure is equal then the rock formed will be non-foliated. The content of minerals also varies for foliated and non foliated rocks. The foliated rocks usually have mineral flakes and they look layered as well. One can find a thin fragment of rock when a foliated rock breaks. The minerals appear randomly in case of Non-Foliated Rock. The appearance of this rock is granular as there are no layers. The foliated Rocks contain minerals that are very hard to see with naked eyes. Whereas, Non Foliated rocks contain coarsely grained minerals and have a random shape. The example of Non Foliated Rock includes Marble, Anthracite Coal and Quartzite.

What are Rocks Made of?

The rocks consist of several minerals and are formed in specific conditions. The characteristics of each rock is different. Based on their appearance, texture, and minerals the use of rock is determined. The rocks are used for different purposes in our day to day life such as for household tiles, Coal, sand making for construction, Tool Making, Infrastructures, Decorations etc. The study of the type of rock helps one to understand the appearance, texture, minerals and different characteristics of rocks.