PEROLOGY

PETROLOGY:
The word petrology is derived from word petroleum.
Petrology is the department of science which is concerned with mineralogical & chemical compositions of rocks with their classification.
Rocks are classified mainly three groups:
· Sedimentary Rocks
· Metamorphic Rocks
· Igneous Rocks
SEDIMENTARY ROCKS:
Sedimentary rocks are formed from overburden pressure as particles of sediments are deposited out of air, ice or water flows carrying the particles in suspension. As the sediment deposition builds up, the overburden pressure (Lithostatic pressure) squeezes the sediments into layered solids. This rock formation process is known as Lithification (rock formation).
These rocks are further classified in three categories. This classification is based on three main ways of formation of sedimentary rocks:
(1) Clastic / Terrigenous / Detrital / fragmental sedimentary rocks: Clastic means broken or fragmented. Clastic rocks are composed of broken up pieces of pre-existing rocks and minerals. We can further identify these rocks based on the size of the fragments that they are composed.
PARTICLE SIZE
SEDIMENT
ROCK
Over 256 mm (10")
Boulder
Conglomerate or breccia
2 to 256mm(0.08-10")
Gravel
Conglomerate or breccia
1/16 to 2mm
Sand
Sandstone
1/256-1/16mm
Silt
Siltstone*
Less than 1/256mm
Clay
Claystone*

*also known as mudstone

(2) Biological / Bioclastic: sedimentary rocks: These sedimentary rocks are formed by the deposition of result of some biogenic activities. Coals , Cherts and shell limestone are examples of biological sedimentary rocks.

(3) Chemical sedimentary rocks : These rocks are formed by deposition of chemicals, which are generally precipitated from solutions. These rocks however, form from a water solution, and not from molten rock. Various salts (rock salt and gypsum), Dolostone, limestones are few examples of chemical sedimentary rocks. Chemical sedimentation involves water and therefore the depositional environments will be most often found near or under water.
Composition of sedimentary rocks :
Sedimentary rocks are composed of mainly silica (quartz), feldspars, amphiboles and clay material.
Note:
We are fortunate that in earth’s atmosphere, we have low levels of carbon dioxide (CO2) due to limestones and dolomite sedimentary rocks. This is a fact that most of carbon (petroleum and other hydrocarbons) on earth is trapped in limestone’s and dolomite’s layers. So less free carbon is available to react with oxygen. In this way we have low percentage of co2 on earth’s atmosphere. Making existence of life on earth possible.
METAMORPHIC ROCKS:
In a simple definition we can say that metamorphic rocks are formed from transformation of one type of natural stones to another type of natural stones through mix of heat, pressure and minerals.
These changes can be either: - Crystalline formation or/and Texture change or/and colour changeBefore understanding the metamorphic rocks, it is necessary to understand METAMORPHISM.