Rock

INTRODUCTION

Rock is a natural substance, a solid aggregate of one or more minerals. For example, granite, a common rock, is a combination of the minerals quartz, feldspar and biotite. The Earth's outer solid layer, the lithosphere, is made of rock. Botin, J.A. (2009).

Rocks are composed of grains of minerals, which are homogeneous solids formed from a chemical compound arranged in an orderly manner. The aggregate minerals forming the rock are held together by chemical bonds. The types and abundance of minerals in a rock are determined by the manner in which it was formed.

Many rocks contain silica (SiO₂); a compound of silicon and oxygen that forms 74.3% of the Earth's crust. This material forms crystals with other compounds in the rock. The proportion of silica in rocks and minerals is a major factor in determining their names and properties.

Rocks are classified according to characteristics such as mineral and chemical composition, permeability, texture of the constituent particles, and particle size. These physical properties are the result of the processes that formed the rocks.

Therefore there are various types of rocks these includes igneous rocks, sedimentary rocks and metamorphic rocks.

 Petroleum is a naturally occurring, yellowish-black liquid found in geological formations beneath the Earth's surface. It is commonly refined into various types of fuels. Components of petroleum are separated using a technique called fractional distillation, i.e. separation of a liquid mixture into fractions differing in boiling point by means of distillation, typically using a fractionating column. It consists of hydrocarbons of various molecular weights and other organic compounds. Ollivier,( 2005).

A fossil fuel, petroleum is formed when large quantities of dead organisms, mostly zooplankton and algae, are buried underneath sedimentary rock and subjected to both intense heat and pressure.

The types of rocks that rich in petroleum are all sedimentary rocks, rocks formed when grains and mineral particles deposited by running water fuse together. Because these rocks are cemented together from such small components, they are porous, full of spaces in which energy-rich carbon compounds can settle, later to be liberated in the form of either oil or gas

The following are the characteristics of Tanzanians rock rich in petroleum

Contain fossils of various organisms. Fossil remains of the organisms are mainly of the 'hard' body parts such as bones, teeth, etc. Fossils found in sedimentary rocks have many a time led to interesting discoveries and have helped in establishing many links in the evolution of life. Sedimentary rocks are formed of sediments derived from the older rocks, plant and animal remains and thus these rocks contain fossils of plants and animals. The age of the formation of a given sedimen­tary rock may be determined on the basis of the analysis of the fossils to be found in that rock.

 Most of the rocks are perme­able and porous but a few of them are also non-porous and impermeable. The porosity of the rocks depends upon the ratio between the voids and the volume of a given rocks mass. Like other rocks this rocks are not found in massive forms such as batholiths, laccoliths, dykes etc.

 Layers of rocks are seldom found in original and horizontal manner. Sedimentary layers are generally deformed due to lateral compressive and tensile forces. The beds are folded and found in anticlinal and synclinal forms. Tensile and compressive forces also create faults due to dislocation of beds. When sedimentation continues, an older rock layer becomes buried deeper as a result. The lithostatic pressure in the rock increases due to the weight of the overlying sediment. This causes compaction, a process in which grains mechanically reorganize. Compaction is, for example, an important diagenetic process in clay, which can initially consist of 60% water.

The deposition of sediments of rocks takes place in certain sequence and system. The size of sediments decreases from the littoral margins to the centre of the water bodies or sedimentation basins. Different sediments are consolidated and compacted by differ­ent types of cementing elements e.g., silica, iron com­pounds, calcite, clay etc.

The rocks are saturated with seawater or groundwater, in which minerals can dissolve, or from which minerals can precipitate. Precipitating minerals reduce the pore space in a rock, a process called cementation. Due to the decrease in pore space, the original connate fluids are expelled. The precipitated minerals form cement and make the rock more compact and competent. In this way, loose clasts in a sedimentary rock can become "glued" together.

 During compaction, this interstitial water is pressed out of pore spaces. Compaction can also be the result of dissolution of grains by pressure solution. The dissolved material precipitates again in open pore spaces, which means there is a net flow of material into the pores. However, in some cases, a certain mineral dissolves and does not precipitate again. This process, called leaching, increases pore space in the rock.

 The color of a rock is often mostly determined by iron, an element with two major oxides: iron (II) oxide and iron (III) oxide. Iron (II) oxide (FeO) only forms under low oxygen (anoxic) circumstances and gives the rock a grey or greenish color. Iron (III) oxide (Fe₂O₃) in a richer oxygen environment is often found in the form of the mineral hematite and gives the rock reddish to brownish color. In arid continental climates rocks are in direct contact with the atmosphere, and oxidation is an important process, giving the rock a red or orange color. Thick sequences of red sedimentary rocks formed in arid climates are called red beds. However, a red color does not necessarily mean the rock formed in a continental environment or arid climate.^[8]

The rock consists of carbonate minerals such as calcite, aragonite or dolomite. Both the cement and the clasts (including fossils and ooids) of a carbonate sedimentary rock can consist of carbonate minerals. The mineralogy of a clastic rock is determined by the material supplied by the source area, the manner of its transport to the place of deposition and the stability of that particular mineral. The resistance of rock-forming minerals to weathering is expressed by Bowen's reaction series. In this series, quartz is the most stable, followed by feldspar, micas, and finally other less stable minerals that are only present when little weathering has occurred.^[18] The amount of weathering depends mainly on the distance to the source area, the local climate and the time it took for the sediment to be transported to the point where it is deposited. In most sedimentary rocks, mica, feldspar and less stable minerals have been reduced to clay minerals like kaolinite, illite or smectite

REFERENCES

Botin, J.A., ed. (2009). Sustainable Management of Mining Operations. Denver, CO: Society for Mining,

Gajul, Shekhar (2018). "Underground Mining Equipment Market 2017 Global Key Players, Share, Challenges, Industry Size, Growth Opportunities & Forecast To 2021". Journalist Book.

Ollivier, Bernard & Magot, Michel (2005). Petroleum Microbiology. Washington, DC: American Society of Microbiology.

Terrascope (2014). "Environmental Risks of Mining". The Future of strategic Natural Resources. Cambridge, Massachusetts, USA: Massachusetts Institute of Technology.