Petrology

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113 Terms

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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 (
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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
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})