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Sedimentary rocks
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Igneous rocks
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Metamorphic rocks
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Sedimentary rocks
These rocks are formed at the surface of the earth by accumulation of particles of solid matter and by the precipitation of substances dissolved in water. These particle can either be the material derived from the physical and chemical breakdown of pre-existing rocks or from diverse organic process (animals and plants contribute more material than might be imagined). Water, ice and air – the rivers, seas, glaciers and wind – are responsible for transporting pebbles, sand and silt over the surface and finally for sweeping them into hollows on the land or into the depths of the sea. The formation of sedimentary rocks is thus bound up with happenings familiar to everyone – a heavy shower washing soil into a stream sets off a train of events which may end with the settling out of mud in a pool downstream and the formation of a new rock.
The composition of a sedimentary rock can be any other rock (physical and chemical breakdown of pre-existing rocks), any combination of other rocks, and/or any product of organic processes. Color of the final rock can be used to guess the conditions in which the sediment was formed. For instance red and reddish brown are attributed to hematite, which is most frequently formed in sediments that are intermittently oxygenated and dark green are due to presence of ferrous minerals.
As previously mentioned, sediments are deposited from water, ice or air on the sea floor. They are made layer by layer meaning each new deposit or bed will sit on top of the previously laid down layers. This layered structure of sediments is considered as a potential way to distinguish these rocks from other types of rock. Another way to recognize sediment is the existence of a fossil on the rock. The shells and skeletons of animals and seeds, stems and leaves of plants are often mixed with sand and mud, and become incorporated in the sedimentary pile as fossils.
The composition of a sedimentary rock can be any other rock (physical and chemical breakdown of pre-existing rocks), any combination of other rocks, and/or any product of organic processes. Color of the final rock can be used to guess the conditions in which the sediment was formed. For instance red and reddish brown are attributed to hematite, which is most frequently formed in sediments that are intermittently oxygenated and dark green are due to presence of ferrous minerals.
As previously mentioned, sediments are deposited from water, ice or air on the sea floor. They are made layer by layer meaning each new deposit or bed will sit on top of the previously laid down layers. This layered structure of sediments is considered as a potential way to distinguish these rocks from other types of rock. Another way to recognize sediment is the existence of a fossil on the rock. The shells and skeletons of animals and seeds, stems and leaves of plants are often mixed with sand and mud, and become incorporated in the sedimentary pile as fossils.
Finding these fossils has two main advantages:
First, the nature of the fossils in a sediment throws light on the environment of deposition. For instance, the remains of marine such as sea urchins show that the sediments containing them were laid down in the sea. Second is the fact that they can be used as indicators of geological age. Since life began, organisms have undergone progressive changes in structure adapting them to their conditions of life. As a result of this process of organic evolution, fossils of different ages record different stages of evolution.
First, the nature of the fossils in a sediment throws light on the environment of deposition. For instance, the remains of marine such as sea urchins show that the sediments containing them were laid down in the sea. Second is the fact that they can be used as indicators of geological age. Since life began, organisms have undergone progressive changes in structure adapting them to their conditions of life. As a result of this process of organic evolution, fossils of different ages record different stages of evolution.
Most of the sediments that accumulate in the seas are soft, aunconsolidated and full of water. As materials pile up on top of each other, they are compressed and compacted and lose much of their water. The remaining spaces between grains are often filled by participation of a cement of cacium carbonate, iron oxides or silica. These processes convert the soft, unconsolidated sediment to a hard rock.
Sedimentary rocks are mainly divided into three man categories: 1) detrital sedimentary rocks which are formed by the accumulation of solid particles derived from the breakdown of pre-existing rocks during breakdown and erosion 2) chemical and biochemical sedimentary rocks, which are made up of precipitated materials, both those that have remained where they originally deposited and those that have later been transported and deposited as intrabasinal clastics. 3) diagenetic rocks, which form as the result of wholesale prelithification recrystallization, replacement, or other chemical or biochemical modification of the original sediments.
Example of sedimentary rock include, dolomite,flint, marcasite, chert.
Sedimentary rocks are mainly divided into three man categories: 1) detrital sedimentary rocks which are formed by the accumulation of solid particles derived from the breakdown of pre-existing rocks during breakdown and erosion 2) chemical and biochemical sedimentary rocks, which are made up of precipitated materials, both those that have remained where they originally deposited and those that have later been transported and deposited as intrabasinal clastics. 3) diagenetic rocks, which form as the result of wholesale prelithification recrystallization, replacement, or other chemical or biochemical modification of the original sediments.
Example of sedimentary rock include, dolomite,flint, marcasite, chert.
sedimentary rocks gallery
Dolomite crystallizes in hexagonal system. dolomite is also used to describe the sedimentary carbonate rock. Its color is white, gray, pink to pale brown.
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Igneous rocks
Igneous rocks are those that are formed by consolidation of molten rock or magma. Usually it is called lava when it flows on the earth’s surface, however, magma is molten rock material plus dissolved gases. The temperatures at which most rocks melt are at least eight times higher than the boiling point of water. Therefore, large volumes of magma can be formed at depths of many kilometres from the earth’s surface. It should be noted that the melting process is very complicated. In addition of involvement of high pressure and temperature, the presence of gases such as water and carbon dioxide has a great impact on the melting process.
Some magmas cool and consolidate essentially where they are formed. Most however , move, generally upward because liquids tends to be less dense than solid, to consolidate elsewhere. Generally speaking there are two types of igneous rocks, some magma solidifies within the Earth’s crust to form intrusive igneous rocks, for example, diorite, gabbro, granite, pegmatite, and peridotite; the reminder erupts from volcanoes at the surface to form extrustive igneous rocks. Andesite, basalt, obsidian, pumice, rhyolite, scoria, and tuff belong to this family.
Some magmas cool and consolidate essentially where they are formed. Most however , move, generally upward because liquids tends to be less dense than solid, to consolidate elsewhere. Generally speaking there are two types of igneous rocks, some magma solidifies within the Earth’s crust to form intrusive igneous rocks, for example, diorite, gabbro, granite, pegmatite, and peridotite; the reminder erupts from volcanoes at the surface to form extrustive igneous rocks. Andesite, basalt, obsidian, pumice, rhyolite, scoria, and tuff belong to this family.
Igneous rocks gallery
Metamorphic rocks
Metamorphic rocks are formed by transformation of preexisting rocks while they remain in the solid state. These are rocks formed from sedimentary or igneous parents which have been modified by high temperatures and pressures inside the crust. This pressure involved in metamorphism is produced partly by the weight of overlying rocks. The process of metamorphism (the term was applied to rocks in 1820, is derived from two Greek words, meta meaning after or change and morphe meaning form) does not lead to a complete melting, therefore, metamorphic rocks usually retain some of the features of their sedimentary or igneous parents. It should be noted that not only igneous and sedimentary rocks, but also previously metamorphosed rocks may be metamorphosed.
The transformation may be textural, mineralogical, or more commonly both and may or may not involve changes in the overall chemical composition of the rock.
The environments in which metamorphism takes place most likely are, naturally, those in which exceptionally high temperatures or directed stresses are generated. Environments of metamorphism include zones of dislocation, contact zones, and relatively deeply buried zones. The causative processes are generally referred to as dislocation metamorphism (or cataclasis), contact metamorphism and reginal metamorphism, respectively. Rocks that are formed within deferent environments are very distinctive: cataclastic rocks exhibit shearing, commonly granulation; contact metamorphic rocks tend to have nearly random arrangement of their constituent minerals; and rocks formed in regional tracts are foliated.
The transformation may be textural, mineralogical, or more commonly both and may or may not involve changes in the overall chemical composition of the rock.
The environments in which metamorphism takes place most likely are, naturally, those in which exceptionally high temperatures or directed stresses are generated. Environments of metamorphism include zones of dislocation, contact zones, and relatively deeply buried zones. The causative processes are generally referred to as dislocation metamorphism (or cataclasis), contact metamorphism and reginal metamorphism, respectively. Rocks that are formed within deferent environments are very distinctive: cataclastic rocks exhibit shearing, commonly granulation; contact metamorphic rocks tend to have nearly random arrangement of their constituent minerals; and rocks formed in regional tracts are foliated.
Metamorphic rocks gallery
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