Petrology

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Petrology

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113 Terms

1

Petrology

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

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Lithology

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

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  1. Igneous petrology

  2. Sedimentary petrology

  3. Metamorphic petrology

Three branches of petrology

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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.

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Sedimentary petrology

focuses on the composition and texture of sedimentary rocks

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Metamorphic petrology

focuses on the composition and texture of metamorphic rocks

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Texture

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

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

Structure

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  1. extrusive igneous rocks

  2. intrusive igneous rocks

The basic classification of igneous rocks

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Volcanic rocks

formed on the surface of the Earth

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Plutonic rocks

formed at considerable depths ( 7- 10 km)

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Hypabyssal rocks

formed at intermediate depths (<2km)

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coarse grain size

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

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fine grain size

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

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Mafic rocks

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

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Felsic rocks

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

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  1. Holocrystalline

  2. Holohyaline

  3. Merocrystalline

Degree of Crystallization

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  1. Coarse-grained

  2. Medium-grained

  3. Fine-grained

Granularity

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Equigranular

Types of Textures: broadly equal in size

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Inequigranular

Types of Textures: difference in their relative grain size

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Directive

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

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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

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Intergranular

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

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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

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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

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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

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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

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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

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  1. Flow structures

  2. Pillow structures

  3. Ropy and blocky lava

  4. Spherulitic structures

  5. Orbicular structures

Structures due to mobility of magma/lava

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  1. Equigranular

  2. Inequigranular

  3. Directive

  4. Intergrowth

  5. Intergranular

Types of Textures

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  1. Jointing structure

  2. Rift and grain

  3. Vesicular structure

  4. Miarolitic structure

Structures due to cooling of magma

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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

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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.

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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.

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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.

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  1. Reaction structure

  2. Xenolithic structure

Miscellaneous Structure

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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

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Xenolithic structure

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

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  1. Concordant

  2. Discordant

Forms of igneous rocks has two types:

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  1. Sills

  2. Phacoliths

  3. Lopoliths

  4. Laccoliths

Concordant Bodies

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Sills

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

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Phacoliths

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

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Lopoliths

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

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Laccoliths

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

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  1. Dykes/dikes

  2. Volcanic necks

  3. Batholiths

Discordant Bodies

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Dykes/dikes

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

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Volcanic necks

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

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Batholiths

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

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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.

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Sedimentary rocks

formed by the accumulation, compaction and consolidation of sediments

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secondary rocks

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

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Water

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

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  1. Nature of gathering ground

  2. Duration of transport

  3. Mixing up of sediments

  4. Allogenic and authigenic minerals

Factors influencing mineralogical composition:

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  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:

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Clastic and non-clastic textures

Origin of grains

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

Size of grains

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Rounded sub-rounded angular & sub-angular

Shapes of grains

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Open-packed Densely packed

Packing of grains

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Fabric of grains

Described in terms of orientation of longer axes of grains

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Crystalline granular amorphous textures

Crystallization trend

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Grain size

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

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Smaller grain sizes

generally indicate greater transport distances and duration

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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.

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Sorting

indicate selective transport of a particular grain size.

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Rounding

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

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Mechanical Structures Chemical Structures Organic Structures

Three types of structure in Sedimentary rocks

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Mechanical Structures

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

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Chemical Structures

Concretionary structures, Nodular structure, Geode structure

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Organic Structures

Fossiliferous structure, and Stromatolic structure

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Non-clastic rocks

Chemically formed rocks, Organically formed rocks

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Siliceous deposits Carbonate deposits Ferruginous deposits Phosphatic deposits Evaporites

Chemically formed rocks

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Carbonate rocks Carbonaceous rocks

Organically formed rocks

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Metamorphic rocks

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

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Metamorphism

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

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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.

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Ortho-metamorphic rocks

formed from igneous rocks

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Para-metamorphic rocks

formed from sedimentary rocks

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  1. Temperature

  2. Pressure

  3. Chemically active fluids

Metamorphic Agents

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200° C

Minerals are normally stable at temperatures below {1}

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  1. The internal heat

  2. The magmatic heat

Sources of heat for metamorphism

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300°C - 850°C

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

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  1. Uniform pressure

  2. Direct pressure

Pressure causing metamorphism is of two types:

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Directed pressure

can act in any direction

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Uniform pressure

acts vertically downwards

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  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

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  1. Thermal Metamorphism

  2. Dynamic Metamorphism

  3. Dynamo-thermal Metamorphism

  4. Metasomatism

Types of Metamorphism

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Thermal Metamorphism

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

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Dynamic Metamorphism

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

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Dynamic Metamorphism

Also known as cataclastic, mechanical or dislocation metamorphism

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Dynamo-thermal Metamorphism

Also known as Regional Metamorphism

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Dynamo-thermal Metamorphism

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

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Dynamo-thermal Metamorphism

Most prevalent of all metamorphic processes

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Metasomatism

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

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

The chemically active fluid may be provided:

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  1. Recrystallization

  2. Rock flowage

  3. Granulation

  4. Metasomatic replacement

Effects of Metamorphism

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Granites undergo dynamic metamorphism, to form crush breccia

Example of Metamorphic Changes: Igneous Rocks

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Sedimentary rocks

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

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Metamorphic Grades

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

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Metamorphic Zone

Indicate the depth wise extension of particular grades of metamorphism

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  1. 300° C

  2. 500° C

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

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