Types of Rocks- Igneous, Sedimentary and Metamorphic with examples

Before we begin to understand what is Rock, it is imperative to understand the term Mineral.

Mineral: It is a naturally occurring, inorganic substance having the definite chemical composition and atomic structure, formed by the mixture of various elements present in the
earth’s crust.

Rock: It is formed by the aggregation of various naturally occurring minerals and study of rocks (branch of geology)  is known as Petrology which deals with its origin, structure, composition and occurrence. Rocks are present on the earth surface in various forms like boulder, pebbles, cobbles or even as a bedrock which we see in our everyday life.

Depending upon their origin, rocks are classified into three types- 

Types of rocks:

   1. Igneous Rocks –  are formed due to the cooling further solidification of high-temperature molten material ie., MAGMA which is  present beneath the earth surface. These rocks occupy 95% of the lithosphere according to geologists.  

     Silicates are the majority of minerals present in these rocks due to the abundance of silica in the earth crust. Most common silicate minerals are quartz, feldspar, olivines, pyroxenes and mica.

      Cooling, crystallization and solidification of magma can occur above or below the earth surface which further classified igneous rocks into two types:   

•    Intrusive Igneous Rocks (plutonic rocks): Gradual  (very slow) cooling of magma inside the solid earth's crust results in the formation of large mineral crystals with a coarse-grained texture. Examples: diorite, gabbro, granite, pegmatite, and peridotite. 

  

  
  

  Diorite 
  Image source - Google | Image by -

  
  

  https://commons.wikimedia.org/wiki/File:Diorite_closeup.jpg

  
  

Gabbro

Image source - Google | Image by -

James St. John

•  Extrusive Igneous Rocks (Volcanic rocks): Magma which burst out from the earth’s surface and pours out in the form of lava. Rapid cooling of such lava results in the formation of small size mineral crystals of very fine-grained texture. Examples: basalt, Rhyolite, obsidian, scoria, and pumice.

  

  Basalt  volcanic rock
  

   Image by Vilve Roosioks from Pixabay 

Sometimes rapid cooling of lava also results in the formation of small-sized mineral crystals, jet black in colour and glassy lustre named as obsidian ( volcanic glass).

Obsidian 
Image by Hans Braxmeier from Pixabay 

        

   Enclosed gases cause frothing of magma which expands and form a spongelike rock known as Scoria or Pumice.

Image by :

James St. John

[Tephra:- It is the collective term for all sizes of solid particles which blow out from the volcanic vent. ]

     2. Sedimentary rocks- also known as pre-existing and secondary rocks and there is a reason behind it. These are formed due to weathering (breakdown)  process it can be a physical, chemical or biological process of pre-existing rocks which can be igneous, metamorphic or even previously formed sedimentary rocks.

    The physical process includes geological agents like wind, water and glacier other factors include abrasion, exfoliation, thermal expansion and frost wedging which promotes the weathering process. 
   The chemical process follows desired temperature and pressure along with Hydrolysis, Oxidation, Carbonation and acid rain whereas, the biological weathering occurs due to the activities of plants, animals certain microorganisms and bacteria ( most common fungi and lichens show a symbiotic relationship in which fungi releases certain chemicals which cause the breakdown of minerals in the rock).

lichens on the rock 

    Based on their origin sedimentary rocks are classified into two types-

       Clastic rocks- a rock which is formed by the weathering, erosion and deposition of pre-existing rocks to form a clast (broken) which comprises of silicate minerals like quartz, feldspar and clay minerals.  

       Clastic rocks can be further categorised on the basis of grain size and shape.

     The various process like weathering, erosion, deposition,   transportation and lithification cause mineral grains to attain different size and shapes. 

     classification on the basis of size is given in the table below:                                                               

Classification of clast sediments

     Different shapes can be angular, roughly rounded and smoothly rounded.

     Sediments from glaciers have generally angular shape, sediments formed from the wind have roughly rounded edges whereas sediments carried by water have a smooth spherical form.

         Non-Clastic rocks- are formed due to two processes: chemical and organic. In the chemical process, water vapours evaporate leaving dry minerals behind mainly found in arid regions whereas organic rocks are formed due to the accumulation of animal or plant remains like bones and shell which contain calcium as a main component which piles near the earth surface or seafloor and turns into an organic sedimentary rock.

     Examples of non-clastic rocks- limestone, marl, chert.diatomite, coal etc.

   
    3. Metamorphic Rocks- as the name suggests a change in the form ie., pre-existing rocks which are buried deep below the earth's surface when subject to high temperature and pressure results in changes in the mineralogy, texture and chemical composition. It takes millions of years to form such rocks.

     These rocks are categorised into two types depending upon their texture ie., presence of alternating light or dark coloured bands.

     Foliated metamorphic rocks having layered or banded appearance on a rock surface. Examples: mica, slate, gneiss, schist, etc.

Image source - Google | image by-

James St. John

 

      Non- Foliated metamorphic rocks do not have any layers or band on the surface of the rock. Examples: marble, quartzite, novaculite, etc.

Image source - Google | image by-

James St. John