Petrology

Petrology

the branch of geology that studies the origin, composition, distribution and structure of rocks.

Lithology

focuses on macroscopic hand-sample or outcrop-scale description of rocks, while petrography is the speciality that deals with microscopic details.

1. Igneous petrology
2. Sedimentary petrology
3. Metamorphic petrology

Three branches of petrology

Igneous petrology

focuses on the composition and texture of igneous rocks (rocks such as granite or basalt which have crystallized from molten rock or magma). Igneous rocks include volcanic and plutonic rocks.

Sedimentary petrology

focuses on the composition and texture of sedimentary rocks

Metamorphic petrology

focuses on the composition and texture of metamorphic rocks

Texture

refers to the mutual relationship of the different mineralogical constituents in a rock

refers to the large scale features or field characteristics of the rocks

Structure

1. extrusive igneous rocks
2. intrusive igneous rocks

The basic classification of igneous rocks

Volcanic rocks

formed on the surface of the Earth

Plutonic rocks

formed at considerable depths ( 7- 10 km)

Hypabyssal rocks

formed at intermediate depths (

coarse grain size

(> 1 mm) is associated with plutonic, or intrusive rocks. Slow cooling usually causes this texture.

fine grain size

(< 1 mm) is associated with volcanic, or extrusive rocks. Rapid cooling usually causes this texture.

Mafic rocks

richer in Mg, Fe, and Ca. They are also darker in color and denser

Felsic rocks

richer in K, Na, Al and Si, and, compared to mafic rocks, are lighter in color as well as density.

1. Holocrystalline
2. Holohyaline
3. Merocrystalline

Degree of Crystallization

1. Coarse-grained
2. Medium-grained
3. Fine-grained

Granularity

Equigranular

Types of Textures: broadly equal in size

Inequigranular

Types of Textures: difference in their relative grain size

Directive

Types of Textures: exhibit perfect or semi perfect parallelism of crystals or crystallites in the direction of the flow of magma

Intergrowth

Types of Textures: two or more minerals may crystallize out simultaneously in a limited space so that the resulting crystals are mixed up or intergrown

Intergranular

Types of Textures: specifically termed intersertal if the material filling the spaces is glassy in nature

Flow structures

Structures of Igneous rocks due to mobility of magma/lava:
development of parallel or nearly parallel layers or bands or streaks in the body of an igneous roc

Pillow structures

Structures of Igneous rocks due to mobility of magma/lava:
development of bulbous, overlapping, pillow like surfaces in the body of igneous mass

Ropy and blocky lava

Structures of Igneous rocks due to mobility of magma/lava:
surfaces show broken and fragmented appearance, these are called the blocky lava

Spherulitic structures

Structures of Igneous rocks due to mobility of magma/lava:
distinguished by the presence of thin mineral fibers of various sizes arranged in perfect or semi perfect radial manner about a common centre

Orbicular structures

Structures of Igneous rocks due to mobility of magma/lava:
rare type of structure of igneous rocks, rock mass appears as if composed of ball like aggregations

1. Flow structures
2. Pillow structures
3. Ropy and blocky lava
4. Spherulitic structures
5. Orbicular structures

Structures due to mobility of magma/lava

1. Equigranular
2. Inequigranular
3. Directive
4. Intergrowth
5. Intergranular

Types of Textures

1. Jointing structure
2. Rift and grain
3. Vesicular structure
4. Miarolitic structure

Structures due to cooling of magma

Jointing structure

Structures due to cooling of magma:
development of cracks or joints in the rocks formed from these sources, these joints sometimes follow definite patterns

Rift and grain

Structures due to cooling of magma:
indicate two separate directions, often used by quarry men, in which the igneous rocks like granite can be broken from the main rock body with a comparative ease. The equally spaced joints are producing cubical blocks.

Vesicular structure

Structures due to cooling of magma:
escape of gases while cooling is going on leads commonly to the formation of cavities of various sizes and shapes in the cooled mass.

Miarolitic structure

Structures due to cooling of magma:
sometimes small and distinct cavities are formed during the crystallization of magma, these cavities often containing projecting crystals are called miarolitic cavities.

1. Reaction structure
2. Xenolithic structure

Miscellaneous Structure

Reaction structure

Miscellaneous Structure:
characterized by the presence in the rock of some incompletely altered minerals conspicuously surrounded on their borders by their alteration products, often happens that some earlier formed minerals react with the magma during the subsequent stages of crystallization

Xenolithic structure

Miscellaneous Structure:
imposed on the igneous rocks because of incorporation of foreign material, the foreign fragments are termed xenoliths

1. Concordant
2. Discordant

Forms of igneous rocks has two types:

1. Sills
2. Phacoliths
3. Lopoliths
4. Laccoliths

Concordant Bodies

Sills

igneous intrusions that have been injected along or between the bedding planes or sedimentary sequence are known

Phacoliths

small sized intrusives that occupy positions in the troughs and crests of bends called folds

Lopoliths

igneous intrusions, which are associated with structural basins, that are sedimentary beds inclined towards a common centre

Laccoliths

concordant intrusions due to which the invaded strata have been arched up or deformed into a dome

1. Dykes/dikes
2. Volcanic necks
3. Batholiths

Discordant Bodies

Dykes/dikes

defined as columnar bodies of igneous rocks that cut across the bedding plane or unconformities or cleavage planes and similar structures

Volcanic necks

in some cases vents of quiet volcanoes have become sealed with the intrusions, such congealed intrusions

Batholiths

these are huge bodies of igneous masses that show both concordant and discordant relations with the country rock

Sedimentary petrology

the classification and study of sedimentary deposits/rocks. This study is the basis for understanding sediment transport and deposition processes, as well as shedding light on the environmental setting where the sediments were formed.

Sedimentary rocks

formed by the accumulation, compaction and consolidation of sediments

secondary rocks

They are {1}, derived from the sediments produced by the weathering of pre-existing rocks

Water

The accumulation and compaction of these sediments usually take place in the presence of {1}

1. Nature of gathering ground
2. Duration of transport
3. Mixing up of sediments
4. Allogenic and authigenic minerals

Factors influencing mineralogical composition:

1. Origin of grains
2. Size of grains
3. Shapes of grains
4. Packing of grains
5. Fabric of grains
6. Crystallization trend

Textures of sedimentary rocks are determined by:

Clastic and non-clastic textures

Origin of grains

Coarse-grained - avg grain size >5mm
Medium-grained - avg grain size b/w 5 & 1mm
Fine-grained - avg grain size

Size of grains

Rounded
sub-rounded
angular & sub-angular

Shapes of grains

Open-packed
Densely packed

Packing of grains

Fabric of grains

Described in terms of orientation of longer axes of grains

Crystalline granular
amorphous textures

Crystallization trend

Grain size

It is a good indicator of the energy or force required to move a grain of a given size

Smaller grain sizes

generally indicate greater transport distances and duration

Sorting

It will generally improve with the constant or persistent moving of particles, and thus can indicate if particles were transported over a long distance or for a long time period.

Sorting

indicate selective transport of a particular grain size.

Rounding

a good indicator for the amount of abrasion experienced by sediments.

Mechanical Structures
Chemical Structures
Organic Structures

Three types of structure in Sedimentary rocks

Mechanical Structures

stratification, Lamination, Cross bedding, Graded bedding, Mud cracks, Rain prints, Ripple marks

Chemical Structures

Concretionary structures, Nodular structure, Geode structure

Organic Structures

Fossiliferous structure, and Stromatolic structure

Non-clastic rocks

Chemically formed rocks, Organically formed rocks

Siliceous deposits
Carbonate deposits
Ferruginous deposits
Phosphatic deposits
Evaporites

Chemically formed rocks

Carbonate rocks
Carbonaceous rocks

Organically formed rocks

Metamorphic rocks

form by alteration or modification of any kind of preexisting rock.

Metamorphism

may be caused by pressure, heat, or by water or other fluids or gases that infiltrate a protolith.

process of metamorphism

can involve changes in the minerals present, changes in rock texture, or changes in rock composition, or any combination of the three.

Ortho-metamorphic rocks

formed from igneous rocks

Para-metamorphic rocks

formed from sedimentary rocks

1. Temperature
2. Pressure
3. Chemically active fluids

Metamorphic Agents

200° C

Minerals are normally stable at temperatures below {1}

1. The internal heat
2. The magmatic heat

Sources of heat for metamorphism

300°C - 850°C

Metamorphic changes take place between {1} - {2}

1. Uniform pressure
2. Direct pressure

Pressure causing metamorphism is of two types:

Directed pressure

can act in any direction

Uniform pressure

acts vertically downwards

1. These fluids act as carriers of chemical components that drive the chemical reactions with the minerals
2. The pore fluids undergo expansion, with rise in temperature
3. Fluids present around rocks may react with the minerals within them, at elevated temperatures

Chemically active fluids

1. Thermal Metamorphism
2. Dynamic Metamorphism
3. Dynamo-thermal Metamorphism
4. Metasomatism

Types of Metamorphism

Thermal Metamorphism

Refers to all metamorphic processes in which heat plays a predominant role.

Dynamic Metamorphism

Pressure causes movement of and interaction between rocks, resulting in their mechanical breakdown - cataclasis.

Dynamic Metamorphism

Also known as cataclastic, mechanical or dislocation metamorphism

Dynamo-thermal Metamorphism

Also known as Regional Metamorphism

Dynamo-thermal Metamorphism

It refers to metamorphism under the combined action of all the three agents

Dynamo-thermal Metamorphism

Most prevalent of all metamorphic processes

Metasomatism

Refers to the formation of new minerals by the chemical replacement of the existing ones, under the influence of chemically active fluids

rom within the rock (mineral metasomatism)
from outside the rock (rock metasomatism)

The chemically active fluid may be provided:

1. Recrystallization
2. Rock flowage
3. Granulation
4. Metasomatic replacement

Effects of Metamorphism

Granites undergo dynamic metamorphism, to form crush breccia

Example of Metamorphic Changes: Igneous Rocks

Sedimentary rocks

Example of Metamorphic Changes: Pure limestone, re-crystallizes under conditions of contact metamorphism, to marble

Metamorphic Grades

Represents the extent to which an original rock has been changed by metamorphism

Metamorphic Zone

Indicate the depth wise extension of particular grades of metamorphism

1. 300° C
2. 500° C

The Epizone (temperature < {1})
The Mesozone (temperature b/w {1} - {2})
The Ketazone ( above{2})