Rocks

Rocks-

A rock is a natural occurring solid cohesive aggregate of one or more mineral or mineral materials.

• Sedimentary rock makes up to 66% of the earth’s crust, with 34 % being the igneous and the metamorphics.

• The reason why sedimentary rocks accounts for most of the rocks on the earth’s surface is because they are mainly found ocean floor basins which accounts to 70% of total area of the earth. 

Monomineralic rock-

• Rocks that contain minerals of only one kind of mineral. 

Example -dunite, anorthosite and limestone. 

Polymineralic rock -

• If a rock is made of more than one mineral, it is called a polymineralic rock. 

Example- Granite. 

Basic minerals-

Minerals-

• A mineral is a naturally occurring organic and inorganic substance, having an orderly atomic structure and a definite chemical composition and physical properties.

• About 98 per cent of the total crust of the earth is composed of eight elements like oxygen, silicon, aluminium, iron, calcium, sodium, potassium and magnesium. 

Metallic Minerals-

These minerals contain metal content and can be sub-divided into three types:

(i)Precious metals -

• Gold, silver, platinum etc.

(ii) Ferrous metals -

• Iron and other metals often mixed with iron to form various kinds of steel.

(iii) Non-ferrous metals -

• Include metals like copper, lead, zinc, tin, aluminium etc.

Non-Metallic Minerals

• These minerals do not contain metal content.

• Sulphur, phosphates and nitrates are examples of non-metallic minerals. Cement is a mixture of non-metallic minerals.

Types of Rocks-

The three major rock types are:

1. Igneous rocks. 

2. Sedimentary rocks. 

3. Metarmoprphic rocks. 

1. Igneous rocks

• Rocks that have formed from an originally hot molten material through the process of cooling and crystallisation is called as Igneous rocks.

• Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of the examples of igneous rocks. 

Magma -

• The hot molten material occurring naturally below the surface of the Earth is called magma.

• As they comprise the earth’s first crust and all other rocks are derived from them, these are called the parent of all rocks or the ‘primary rocks’.

• About 95% of the volume of outermost 16 km of the earth is composed of them. 

Classification of Igneous rocks-

General classification-

A.Extrusive Igneous rocks-

• They are formed by cooling of lava on the earth’s surface.

• As lava cools very rapidly on coming out of the hot interior of the earth, the mineral crystals forming these rocks are very fine.

• They are  also called as Volcanic rocks. 

Examples- Basalt. 

B.Intrusive Igneous rocks-

• They are formed when magma solidifies below the earth’s surface.

• The rate of cooling below the earth’ s surface is very slow which gives rise to formation of large crystals in the rocks. 

Example- Gabbro and Granite. 

It is further classified as -

Plutonic and Hypabyssal-

• Deep seated intrusive rocks are termed as plutonic rocks and shallow depth intrusive rocks are termed as hypabyssal.

• Plutonic rocks occur as intrusive bodies for example Mount Abu, Ladakh batholith.

Classification on the Basis of Silica content-

1. Acid/Felsic(feldspar and silica-quartz) igneous rocks-

• These rocks have more than 66% Silica content (SiO2). 

Example- granite, rhyolite. 

2. Intermediate((magnesium and ferrum or iron) igneous rocks-

• These rocks have 52% to 66% of Silica. 

Example- syenite or diorite, trachyte, andesite.

3. Basic/Mafic igneous rocks:-

• The SiO2 content in these rocks vary between 45 to 52%.

Example- gabbro and basalt.

4. Ultrabasic/ultranafic igneous rocks-

• In these rocks SiO2 contents is less than 45%.

• They have high Mg content. 

Example- dunite, peridotite, pyroxenite.

Sedimentary Rocks-

• These rocks are formed by successive deposition of sediments.

• These sediments may be the debris eroded from any previously existing rock which may be igneous rock, metamorphic or old sedimentary rock.

• Sedimentary rocks have layered or stratified structure.

• The thickness of strata varies from few millimeters to several metres.

• So these rocks are also called stratified rocks.

• Generally, these rocks have some type of fossil between their strata.

Formation of sedimentary rocks-

Process of sedimentary rocks formation is as follows-

Weathering -

• Breaking of rocks due to physical,chemical and biological action. 

Erosion-

• It is the geological process in which earthen materials are worn away and transported by natural forces such as wind or water.

Transportation-

• It can occur by gravity, wind, water or ice. They can carry sediments. The ability of a medium to carry sediment depends on its viscosity and velocity.

Deposition-

• It is the process by which sediments (a) settles out of transporting medium due to decrease in velocity. 

Lithification and Diagenesis-

• It refers to the physical and chemical processes that affect sedimentary materials after deposition and before metamorphism and between deposition and weathering. 
• The diagenesis is achieved by ‘Welding or Cementation’.

Welding-

• It is the process of compaction of sediment accumulation in lower layer of a basin due to the pressure exerted by the load of the overlying sediment layer.

Cementation-

• Here, the loose grains of sediments gets settled and held together by a binding material. 

Lithification-

• It refers to complex physical, chemical, or biological processes whereby unconsolidated material (e.g., sand, silt, and mud) becomes converted to solid rock. 

 The difference between lithification and diagenesis

• Lithification is the compaction and cementation of sediment into rock while diagenesis is all the chemical, physical, and biological changes sediment goes through during and after lithification, not including weathering or other surface changes.

Classification of sedimentary rocks:

1.Clastic rocks-

• They are mechanically formed rocks. These are formed due to the process of weathering, erosion, transportation etc.

Based on grain size, they are further classified into

A.Rudaceous rocks-

• If the grain size of sediment or  particle are more than 2 mm in dia, they are called Rudaceous rocks.

They are further classified into-

• Gravel (2-10 mm in dia).

• Pebbles (10-50 mm).

• Cobble (50-200 mm).

• Boulder (>200 mm).

The grains may be rounded or angular / sharp which depends on the rate of transportation.

Rounded –

• Example: Conglomerate. 

Angular/sharp -

• Example: Breccia. 

Breccias are made up mostly of rough and angular fragments indicating least transport and abrasion. 

b) Arenaceous rocks-

• If the size of the particle is in between 1 and 2 mm, then such sedimentary rocks are called arenaceous rocks.

Example: Sandstone, Grit. 

c) Argillaceous rocks-

• If the size of the particle is < 1mm in dia, they are described as ‘dust’ and rocks are called argillaceous rocks.

• Argillaceous rocks contain a substantial amount of clay-grade particles. 

Example: Shale, Mudstone. 

2. Non-clastic rocks-

• They are formed either by chemical processes or organic process. 

A.Chemically formed rocks-

• They are formed by precipitation, evaporation or crystallization from natural aqueous solution.

They are further classified into

 Sileceous deposits –

• They are formed by precipitation of silica solution.

• Ex: Flint, Chert, Jasper. 

Carbonate deposits -

• They are formed by precipitation of carbonate rich water.

• Ex: Limestone, Dolomite, Magnesite. 

Ferrugeneous rocks-

• They are formed by precipitate of oxides and hydroxides of iron.

• Ex: Bog iron ores. 

 Evapories -

• They are formed by evaporation of sea or salty water.

• Ex: Rock salt, Anhydrites, Gypsum, Borates. 

B.Organically formed rocks-

• Sedimentary rocks which are formed exclusively from remains of organisms.

• Ex: Carbonaceous deposits I.e. Coal and petroleum fossiliferous limestone, geyserite etc.

Metamorphic Rocks-

• Metamorphic rocks are formed under the influence of heat or pressure on sedimentary or igneous rocks. Tremendous pressure and high temperature change the colour, hardness, structure and composition of all types of pre-existing rocks. 

Agents or factors of metamorphism:

1. Temperature-

• It is responsible in bringing the recrystallization or reconstitution of the original minerals into newer ones.

2. Pressure-

• Pressure is one of the important dominant factors in metamorphic rocks and in majority of rocks it is associated with temperature.

It is of two types

a) Load pressure -

• When pressure acts vertically. 

b) Direct pressure -

• Pressure is from different direction resulting change in volume and Rounded grains become flattened in the direction of maximum stress.

• Flaky / elongated minerals (Mica) have preferred orientation in the direction of maximum stress. 

3. Fluids-

• Fluids leads to recystallization of the rocks due to chmicals present in them. 

Metasomatism-

• Metasomatism is the process of altering the composition of a rock, either by the addition or subtraction of chemical elements. 

Types of Metamorphism-

1. Thermal metamorphism:

In this type of metamorphism temperature plays major role in formation of metamorphic rocks. 

Contact Metamorphism-

• When the thermal metamorphism occurs in the immediate contact of igneous intrusions, it is called contact metamorphism. 

Plutonic metamorphism

• When thermal metamorphism occurs on a regional scale at depth it is called Plutonic metamorphism.

• Ex: Limestone → Marble, Sandstone → Quartzite. 

2. Dynamic metamorphism:

• This type of metamorphism takes place in the rock by means of direct pressure / stress which is a dominant which leads to new structures.

• It is also called Cataclastic / kinetic metamorphism. 

• Ex. Shale → Slate. 

3. Dynamothermal metamorphism-

• It is a kind of metamorphism where temperature and pressure are the dominant factors which operates upon pre-existing rocks.

• The metamorphism may be regional / local scale and it is called Regional metamorphism.

• The minerals developed under direct pressure are usually flat, tabular, and flaky in nature.

• Ex. Granite → Gneiss. 

4.Cataclastic Metamorphism- 

• Cataclastic metamorphism occurs as a result of mechanical deformation, like when two bodies of rock slide past one another along a fault zone. 

Foliated and Non foliated metamorphic rocks-

Foliated rocks-

• All metamorphic rocks showing development of conspicuous parallelism in their mineralogical and structural constitution falling under the general term foliation are grouped together as foliated rocks.

Example- slates, phyllites, schists and gneisses etc.

Non-Foliated Rocks -

• It Included in this group are all those metamorphic rocks characterised with total or nearly total absence of foliation or parallelism of mineralogical constituents.

Example- quartzites, hornfels, marbles etc.

Rock cycle-

• Rock cycle is a continuous process through which old rocks are transformed into new ones.

• Igneous rocks are primary rocks and other rocks (sedimentary and metamorphic) form from these primary rocks.

• Igneous rocks can be changed into metamorphic rocks.

• The fragments derived out of igneous and metamorphic rocks form into sedimentary rocks.

• Sedimentary rocks themselves can turn into fragments and the fragments can be a source for formation of sedimentary rocks.

• The igneous, metamorphic and sedimentary once formed may be carried down into the mantle (interior of the earth) through subduction process  and the same melt down due to increase in temperature in the interior and turn into molten magma, the original source for igneous rocks.