US Geologist - University of Sindh Geologists

IGNEOUS ROCKS

 Igneous (Latin, ignis or fire) rocks are formed by cooling and crystallization of molten rocks.

 

Magma

Hot, molten rock material, existing below the earth surface.

Acid Magma:  It is rich in Si, Na and K and poor in Ca, Mg and Fe.

Basic Magma: It is rich in Ca, Mg and Fe and poor in Si, Na and K.

 

Lava

When magma reaches the surface of the earth, it is called lava.

 

Igneous Process

Most igneous process occur underground we can not see them, geologist often rely on geophysical studies of the earths interior, computer models, that simulate subterranean temperature and pressure conditions and laboratory experiment.

 

Melting

A crystalline solid melts when the bonds between its ions break, allowing the charged particles to move freely when under ground temperatures become hight enough bonds in minerals are broken, eventually the heated rock the heated rock no longer a crystalline solid becomes magma, since the various types of  mineral melt at different temperatures, different minerals melt out of the rock as the heat gradually increases hence composition of the magma changes as each newly molten mineral enters and enriches it. Partial melting is described as melting is described as melting points of some but not all of its components minerals, factors control the melting of rocks and creating of magma.

 

1 – Heat = (a-heat produced due to radio active decay of elements, b-form the core of the earth,3-Frictional heat produced as each plate moves.

2 – Pressure = help to increase melting point of a substance.

3 – Water = Water lowers the melting point of substance.

 

Fluidity Of Magma

Its ability to flow, governed by its temperature and composition.

 

Viscosity

Fluids resistance to flow, viscosity increases with decreasing temperature, viscosity increases with silica content.

 

Crystallization

Where temperature decreases bonds start to form and tiny crystals begin to appear. As cooling progress, different minerals crystallize out of the magma at different temperatures.

 

 

 

Classification Of Igneous Rock

 

1 – Texture

2 – Mineral content

 

Texture

It means the size, shape and arrangement of mineral grains

 

Phaneritic

 

1 – State

Metamorphic rocks derived from shale or mudstone a strongly foliated rock known as state, state tends to break parallel to the basic planes into relatively thin, flat fragments, a pattern known as state cleavage.

2 – Phyllite

Low grade metamorphic rock with glossy and shinning lusture, as temperature increase to 300 C, the size of the microscopic mica, chlorite and graphite flakes in state increases, producing the foliated metamorphic rock phyllite.

3 – Schist

When the flakes become visible to the unaided eye, the metamorphic rock is classified as schist, a medium if coarse grain strongly foliated.

4 – Gnesis

It is composed of feldspars, quartz and some mafic minerals, Gneisses are distinguished by minerals that are present in comparatively large amount.

 

Nonfoliated Rocks

 

1 – Marble

When relatively pure limestone and dolostones are metamorphous, even by directed pressure, they tend to form a coarsely granular nonfoliated metamorphic rock marble.

2 – Quartz

A very durable Nonfoliated metamorphic rock come by the metamorphism of quartz sandstone.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Phaneritic

(Form the Greek Phaneros, or visible) When a magma’s minerals crystallize slowly over hundreds or thousands of years, there is ampte time for crystals to grow large enough to be seen by the unaided eye.

 

Aphanitic

(From the Greek a Phaneros, or “not visible”), when magma solidify so quickly that their crystals have little time to grow, and crystals are so small they can barely be seen by the naked eye.

 

Prophyritic

When large crystals are surrounded by small crystals.

 

Glassy

When magma cools so quickly that its ions don’t have enough time to form any crystals, then they are essentially frozen in place, randomly bonded to produce glassy texture.

 

Intrusive OR Plutonic Rocks (for Pluto, the Greek god of the underworld)

Plutonic Rocks are formed when magma cools slowly at great depth with the retention of the volatiles. The texture of such rocks are phaneric.

 

Extrusive OR Volcanic Rocks

Volcanic rocks are formed when the magma erupts at the earths surface and cools rapidly. The volatiles present in the magma escape into the atmosphere. The texture of such rocks is aphanitic or glassy.

 

Hypabyssal Rocks

Hypabyssal Rocks are formed when consolidation of magma takes place very close to the earth’s surface, fine grand with prophyritic texture.

 

Chemical Composition

The earth’s magmas consist largely of the most common elements: oxygen, silicon, aluminum, iron calcium, magnesium, sodium, potassium and sulfur. The relative proportions of these components within a body of magma, added to the prevailing temperature, pressure and its water contents, distinctly characterize magma into following types.

Ultramafic Igneous Rock

Ultramafic Igneous Rocks contain less than 45% silica, here both feldspars and quartz are essentially absent and one or more of the ferromagnesian mineral is the dominant component. These rocks are darker in colors, have a relatively higher specific gravity Gabbro, Dolerite.

 

Intermediate Igneous Rocks

These rock contain silica in between about 55% 65%. They consist of iron and magnesium silicate, along with sodium plagioclase and mica and small amount of quartz. These rocks are lighter in color although a little darker than color of acidic rocks: Example, andesite and diorite.

 

Felsic Igneous Rock

These rocks contain more then 65% silica, these rocks are rich in potassium feldspar, aluminum rich micas and quartz but poor in iron magnesium and calcium silicates. These rocks are light in color and low in specific gravity granite, rhylite.

 

 

Intrusive Rock Formations

Magma moves forcefully into the cracks in preexisting rock, actually pushing the rocks aside to create its own space. I n a similar way, rising magma may force overlying rocks to bulge upward.

 

Mode Of Occurrence

 

Sill

A sill is a sheet like body, lying parallel to layers of preexisting rocks. Sills are produced when intruding magma enters a space between layers of rocks, melting and incorporating adjacent sedimentary material.

 

Phacolith

Magma under low pressure will not exert much pressure to make space for itself, in such cases the magma may be pushed into the existing crests and troughs of a fold in the introduced rocks, and get solidified there. Such structures are in form of crescentric.

 

Laccolith

When thick, viscous magma intrudes between two parallel layers of rock and lifts the overlying rock layers and the form of a dome of arch, it is called Laccolith.

 

Dike

Dike is a vertical wall like igneous body that cuts across preexisting rocks. Dike s are generally steeply inclined or nearly vertical, suggesting that they formed from rising magma, which tend to follow the most direct route upward.

 

Batholith

Batholiths are massive discordant igneous bodies with surface areas of 100 square kilometers or more. They typically form intermediate to high viscosity magmas that, because they flow so slowly, are more likely to remain underground and crystallize here.

 

Lava Flow

Tabular shaped volcanic rock bodies, formed by lava flows.

 

Description Of Common Rocks

 

Granite: It is coarse grained rock which is composed of quartz, alkali feldspar and small amount hornblende. The granites are mostly light in color with a white or pink tint according to the color of the feldspar.

Andesite: These are light colored volcanic rocks. In nature, they are intermediate between acidic and basic rocks. They are general fine grained aphanitic rock with typical prophyritic textures.

Rhyolite: Rhyolite is similar to granite in mineral composition but different texture. The texture is often prophyritic, which contains phenocryst of quartz and feldspar, set in a finely crystalline or glassy groundmass. The color of the rock is generally white, grey or pink.

Obsidian: Obsidian is glassy rock which is acid in composition. It is generally dark colored but show brilliant lusture.

Diorite: It is light colored coarse grained rock of plutonic origin, which is mainly composed of plagioclase feldspar and hornblende. Most diorites contain little or no quartz.

Syenite: It is coarse grained rock light colored rock of plutonic origin, which is mainly composed of orthoclase, soda-plagioclase and or more mafic minerals such as biotite and hornblende.

Gabbro: It is a coarse grained plutonic rock which is dark green or black in color, it is composed of calsic plagioclase, augite and magnetite.

Dolerite: Dolerite is usually dark being almost black when fresh. Its texture is medium to fine grained. It is mainly composed of plagioclase, augite and iron oxide, it is a hypabyssal rock.

Basalt: Basalt is dense looking black volcanic rock. Its texture is fine grained to glassy. It is composed of augit, plagioclase and iron oxide. Sometimes it contains vesicles which have been filled with secondary material.

Peridotite: It is granular, massive, dark colored of plutonic origin, which is composed almost entirely of ferromagnesian minerals like olivine and augit. Quartz and felspars are mostly absent.

Dunite: It is a coarse grained rock which is composed almost entirely of living.

SEDIMENTARY ROCK

When disintegrated products of preexisting rocks are transported and deposited they get lithified forming sedimentary rocks.

 

Weathering

The process by which rocks and minerals break down at or near the earth’s surface.

 

Types Of Weathering  

Physical Weathering: In this type of weathering rocks or minerals break into smaller pieces.

 

Frost Wedging: Water expands in volume by about 9% when it freezes. In case when water enters and freezes into pores or cracks of rock it exerts a great force in surrounding areas. This causes loosening or dislodging fragments of rocks.

 

Crystal Growth: When saltwater enters a crack and evaporates, the growing salt crystals apply great pressure to the walls of the crack which finally tears them apart.

 

Thermal Expansion & Contraction: Rock surface that are exposed to high daytime and low nighttime temperatures their components minerals expands with repeated heating an contract upon cooling, which should eventually cause the rock’s outer layer to break apart.

 

Mechanical Exfoliation: When erosion of overlying rock and soil exposes a wide area of a large plutonic mass, pressure on the mass is reduced and it expands. As the rock in the structure expands, it fractures into sheets parallel to its exposed surface.

 

Chemical Weathering: In this type of weathering changes in chemical composition of minerals or rocks takes place.

 

Dissolution: Process of removal of ion or ion groups by water, layer this water eventually evaporates leaving behind the dissolved substances.   

Oxidations: In oxidation, a mineral’s ion combines with oxygen to form an oxide.

 

Hydrolysis: In hydrolysis, H positive and OH positive ions from water molecules displace other ions from a mineral’s structure, forming a different mineral.

 

Carbonation: The reaction with atmospheric carbon dioxide to form a carbonate.

 

 

 

Products Of Weathering 

Sediments (from Latin sedimentum, meaning “setting”): fragment of solid material.

 

Detrital Sediment: Composed of transported solid fragments.

 

Chemical Sediment: When dissolved minerals either precipitate from solution or are extracted from water by living organisms and converted to sells, skeletons, or other organic substance.

 

Erosion / Transportation

The process by which moving water, wind, ice or gravity carries pieces of rock and deposits them elsewhere.

 

Composition

When transporting media is unable to carry on transporting weathered product, it deposits them at suitable places.

 

Diagenesis

Heat, pressure and the ions alter the physical and chemical nature of both detrital and chemical sediments by a set of processes known collectively as diagenesis.

 

Lithification OR Consolidation:

Lithification (Greek lithos, meaning rock, and latin facere, meaning to make) process of conversion of loose sediment into solid sedimentary rock.

 

1 – Compaction & Dehydration

The process by which pressure reduces the volume of sediments by squeezing out excess water which increased the cohesion of sediments.

 

2 – Cementation

Material originally dissolved during chemical weathering precipitate from water circulating through sediment, creating a chemical cement that binds the sediment grains together.

 

Sediment Texture 

Grain Size: Different rocks produce grains of different size, shapes and resistance to weathering.

 

Grain Shape: The more vigorous collision a particle experiences and the father it moves away from its parent rock, the more rounded it becomes.

 

Sorting: The process by which a transport medium selects particles of different sizes, shapes or densities.

 

Sedimentary Structure

Physical features that reveal the conditions under which sediment deposition occurred.

 

1 – Stratification

The deposition of sediments into layers or beds is called stratification.

 

2 – Lamination

Thin bedding, less than one centimeter in thickness are called lamination.

 

3 – Grade Bedding

During deposition when particles settle at different rates, depending on their sizes, densities and shapes, they produce beds of coarse particles at bottom and fine particles at top.

 

4 – Cross Bedding

It is consist of sedimentary layers deposited at an angle to the underlying set of beds.

 

5 – Ripple Marks

Ripple marks are the symmetrical or unsymmetrical undulations which may be seen on the surface of some sedimentary deposit. Produced by waves and currents in shallow water.

 

Classification Of Sedimentary Rocks

On the basis of mode of origin.

 

1 – Detrital / Clastic Sedimentary RockRocks that consists of preexisting solid particles compacted and cemented together, depending upon the particles size they are:  

Mudstone: Rocks with grain size less than 1/256 mm to diameter lacking any fissilty in their structure.

Shale: Rock with grain size less than 1/256 mm in diameter with fissile structure, which is their ability to split into very thin parallel surfaces.

Siltstone: Detrital sedimentary rock with grain size ranging from 1/256-1/16 mm in diameter.

Sandstone: Detrital sedimentary rocks with grain size range from 1/16 mm to 2 mm in diameter.

Conglomerate: Detrital sedimentary rock with grain size larger than 2 mm in diameter, the larger grains are rounded and embedded in fine grained ground mass.

Breccia: Detrital sedimentary rock with grain size larger than 2 mm in diameter, the larger grains are angular and embedded in fine grained matrix.

 

2 – Chemical Sedimentary Rock

Rock formed when dissolved minerals either precipitate or evaporated from solution.

 

Limestone: Composed largely of calcium carbonate, forms by the process of precipitation.

Evaporite (Rock Salt, Gypsum): Formed when salty water evaporates.

Chert: Consist largely of silicate, formed by the process of precipitation.

 

Organic Sedimentary Rock

Formed by accumulation of plant remains.

Calcareous Rock

Limestone

Coal Steam

IGNEOUS ROCKS
Igneous (Latin, ignis or fire) rocks are formed by cooling and crystallization of molten rocks.

Magma
Hot, molten rock material, existing below the earth surface.

Acid Magma: It is rich in Si, Na and K and poor in Ca, Mg and Fe.
Basic Magma: It is rich in Ca, Mg and Fe and poor in Si, Na and K.

Lava
When magma reaches the surface of the earth, it is called lava.

Igneous Process
Most igneous process occur underground we can not see them, geologist often rely on geophysical studies of the earths interior, computer models, that simulate subterranean temperature and pressure conditions and laboratory experiment.

Melting
A crystalline solid melts when the bonds between its ions break, allowing the charged particles to move freely when under ground temperatures become hight enough bonds in minerals are broken, eventually the heated rock the heated rock no longer a crystalline solid becomes magma, since the various types of mineral melt at different temperatures, different minerals melt out of the rock as the heat gradually increases hence composition of the magma changes as each newly molten mineral enters and enriches it. Partial melting is described as melting is described as melting points of some but not all of its components minerals, factors control the melting of rocks and creating of magma.

1 – Heat = (a-heat produced due to radio active decay of elements, b-form the core of the earth,3-Frictional heat produced as each plate moves.
2 – Pressure = help to increase melting point of a substance.
3 – Water = Water lowers the melting point of substance.

Fluidity Of Magma
Its ability to flow, governed by its temperature and composition.

Viscosity
Fluids resistance to flow, viscosity increases with decreasing temperature, viscosity increases with silica content.

Crystallization
Where temperature decreases bonds start to form and tiny crystals begin to appear. As cooling progress, different minerals crystallize out of the magma at different temperatures.

Classification Of Igneous Rock

1 – Texture
2 – Mineral content

Texture
It means the size, shape and arrangement of mineral grains

Phaneritic

1 – State
Metamorphic rocks derived from shale or mudstone a strongly foliated rock known as state, state tends to break parallel to the basic planes into relatively thin, flat fragments, a pattern known as state cleavage.
2 – Phyllite
Low grade metamorphic rock with glossy and shinning lusture, as temperature increase to 300 C, the size of the microscopic mica, chlorite and graphite flakes in state increases, producing the foliated metamorphic rock phyllite.
3 – Schist
When the flakes become visible to the unaided eye, the metamorphic rock is classified as schist, a medium if coarse grain strongly foliated.
4 – Gnesis
It is composed of feldspars, quartz and some mafic minerals, Gneisses are distinguished by minerals that are present in comparatively large amount.

Nonfoliated Rocks

1 – Marble
When relatively pure limestone and dolostones are metamorphous, even by directed pressure, they tend to form a coarsely granular nonfoliated metamorphic rock marble.
2 – Quartz
A very durable Nonfoliated metamorphic rock come by the metamorphism of quartz sandstone.

Phaneritic
(Form the Greek Phaneros, or visible) When a magma’s minerals crystallize slowly over hundreds or thousands of years, there is ampte time for crystals to grow large enough to be seen by the unaided eye.

Aphanitic
(From the Greek a Phaneros, or “not visible”), when magma solidify so quickly that their crystals have little time to grow, and crystals are so small they can barely be seen by the naked eye.

Prophyritic
When large crystals are surrounded by small crystals.

Glassy
When magma cools so quickly that its ions don’t have enough time to form any crystals, then they are essentially frozen in place, randomly bonded to produce glassy texture.

Intrusive OR Plutonic Rocks (for Pluto, the Greek god of the underworld)
Plutonic Rocks are formed when magma cools slowly at great depth with the retention of the volatiles. The texture of such rocks are phaneric.

Extrusive OR Volcanic Rocks
Volcanic rocks are formed when the magma erupts at the earths surface and cools rapidly. The volatiles present in the magma escape into the atmosphere. The texture of such rocks is aphanitic or glassy.

Hypabyssal Rocks
Hypabyssal Rocks are formed when consolidation of magma takes place very close to the earth’s surface, fine grand with prophyritic texture.

Chemical Composition
The earth’s magmas consist largely of the most common elements: oxygen, silicon, aluminum, iron calcium, magnesium, sodium, potassium and sulfur. The relative proportions of these components within a body of magma, added to the prevailing temperature, pressure and its water contents, distinctly characterize magma into following types.
Ultramafic Igneous Rock
Ultramafic Igneous Rocks contain less than 45% silica, here both feldspars and quartz are essentially absent and one or more of the ferromagnesian mineral is the dominant component. These rocks are darker in colors, have a relatively higher specific gravity Gabbro, Dolerite.

Intermediate Igneous Rocks
These rock contain silica in between about 55% 65%. They consist of iron and magnesium silicate, along with sodium plagioclase and mica and small amount of quartz. These rocks are lighter in color although a little darker than color of acidic rocks: Example, andesite and diorite.

Felsic Igneous Rock
These rocks contain more then 65% silica, these rocks are rich in potassium feldspar, aluminum rich micas and quartz but poor in iron magnesium and calcium silicates. These rocks are light in color and low in specific gravity granite, rhylite.

Intrusive Rock Formations
Magma moves forcefully into the cracks in preexisting rock, actually pushing the rocks aside to create its own space. I n a similar way, rising magma may force overlying rocks to bulge upward.

Mode Of Occurrence

Sill
A sill is a sheet like body, lying parallel to layers of preexisting rocks. Sills are produced when intruding magma enters a space between layers of rocks, melting and incorporating adjacent sedimentary material.

Phacolith
Magma under low pressure will not exert much pressure to make space for itself, in such cases the magma may be pushed into the existing crests and troughs of a fold in the introduced rocks, and get solidified there. Such structures are in form of crescentric.

Laccolith
When thick, viscous magma intrudes between two parallel layers of rock and lifts the overlying rock layers and the form of a dome of arch, it is called Laccolith.

Dike
Dike is a vertical wall like igneous body that cuts across preexisting rocks. Dike s are generally steeply inclined or nearly vertical, suggesting that they formed from rising magma, which tend to follow the most direct route upward.

Batholith
Batholiths are massive discordant igneous bodies with surface areas of 100 square kilometers or more. They typically form intermediate to high viscosity magmas that, because they flow so slowly, are more likely to remain underground and crystallize here.

Lava Flow
Tabular shaped volcanic rock bodies, formed by lava flows.

Description Of Common Rocks

Granite: It is coarse grained rock which is composed of quartz, alkali feldspar and small amount hornblende. The granites are mostly light in color with a white or pink tint according to the color of the feldspar.
Andesite: These are light colored volcanic rocks. In nature, they are intermediate between acidic and basic rocks. They are general fine grained aphanitic rock with typical prophyritic textures.
Rhyolite: Rhyolite is similar to granite in mineral composition but different texture. The texture is often prophyritic, which contains phenocryst of quartz and feldspar, set in a finely crystalline or glassy groundmass. The color of the rock is generally white, grey or pink.
Obsidian: Obsidian is glassy rock which is acid in composition. It is generally dark colored but show brilliant lusture.
Diorite: It is light colored coarse grained rock of plutonic origin, which is mainly composed of plagioclase feldspar and hornblende. Most diorites contain little or no quartz.
Syenite: It is coarse grained rock light colored rock of plutonic origin, which is mainly composed of orthoclase, soda-plagioclase and or more mafic minerals such as biotite and hornblende.
Gabbro: It is a coarse grained plutonic rock which is dark green or black in color, it is composed of calsic plagioclase, augite and magnetite.
Dolerite: Dolerite is usually dark being almost black when fresh. Its texture is medium to fine grained. It is mainly composed of plagioclase, augite and iron oxide, it is a hypabyssal rock.
Basalt: Basalt is dense looking black volcanic rock. Its texture is fine grained to glassy. It is composed of augit, plagioclase and iron oxide. Sometimes it contains vesicles which have been filled with secondary material.
Peridotite: It is granular, massive, dark colored of plutonic origin, which is composed almost entirely of ferromagnesian minerals like olivine and augit. Quartz and felspars are mostly absent.
Dunite: It is a coarse grained rock which is composed almost entirely of living.

ROCKS:
Rock is a coherent, naturally occurring solid, consisting of an aggregate of minerals or a mass of glass. To understand this definition we lets take it apart.
Coherent: A rock holds together and it must be broken into separate pieces.
Naturally Occurring: Geologist considers only naturally occurring materials to
be rocks, so bricks and concrete do not consider as rock.
An Aggregate of Mineral OR A Mass of Glass: The vast majority of rocks
consist of an aggregate (a collection) of many crystal minerals.

CLASSIFICATION:
People have developed classification scheme for just about every group of materials on the planet.
Rocks are classified into three types which are:
(I) IGNEOUS ROCKS
(II) SEDIMENTARY ROCKS
(III) METAMORPHIC ROCKS

(I) IGNEOUS ROCKS: Igneous rocks form by the freezing and solidification of molten rock.
(II) SEDIMENTARY ROCKS: Which form either by the cementing together of fragment (grain) broken off pre-existing rocks.
(III) METAMORPHIC ROCKS: When pre-existing rock changes into a new rock under a high pressure and temperature.