ROCKS OF THE EARTH CRUST
Rocks of the earth crust
In generally use, rocks are large hard consolidated compact part of the earth crust. In geology, rocks are any naturally formed aggregate of mineral particles weather it is hard, relatively soft, unconsolidated or incompact which constitute of integral part of the lithosphere (dust include mud, clay, sand etc.)
A mineral is a natural occurring inorganic substance possessing a definite chemical composition and characteristics atomic structure.
The earth crust is made up of several types of rocks, differing from one another in texture, structure, color, permeability mode of occurrence and degree of resistance to denudation. Generally, all rocks can be classified into three major groups: Igneous, sedimentary and metamorphic.
Igneous rocks:
The name igneous is derived from Latin “Ignis” meaning “fire”. Igneous rocks originated from cooling and solidification of magma from the heated lower layers from the earth crust. Crystals are formed uncooling hence called crystalline rocks. Igneous rocks do not occur in layers nor do they contain fossils which are the chemically change remains of ancient plants and animals embedded rocks. In terms of origin, there are two main classes of igneous rocks; Plutonic and Volcanic
Plutonic or Intrusive rocks:
This are igneous rocks formed when molten rocks called magma cools and solidifies very slowly at great debts to the earth crust. The gradual cooling ends in the formation of large and even recognize crystals. Such rocks are known as intrusive rocks for they have been intruded or forced into the surface rocks from magma reservoirs below. Usually to they occur in large masses. They are seen on the surface only after been exposed by prolonged erosion: examples of plutonic rocks are gabbro, granite, deceit, diorite.
Volcanic or extrusive rocks
These are igneous rock formed when molten rocks called lava is poured on the earth surface where is quickly cool and solidifies. They usually have very fine crystals and glassy in appearance. They are also called exclusive rocks example of volcanic rocks include. Basalts, phyllite and trachyte
Most igneous rocks are extremely hard and resistance for these reasons they are quarried(broken) for road making and poliste as monument and grave: stones. In Cameroon, volcanic igneous rocks are common on mount Cameroon region, the Kupe and manenguba mountains and the western highlands.
Sedimentary rocks
These are rocks formed by break down of pre-existing rocks into sediments or smaller particles and deposited by running water or wind or ice or from chemical or biological activity on the surface. They have been formed by deposition either upon land surface or under water.
The sediments are laid down in starter or layers. One on top of the other and in time, this layers turn into rocks and they become harden by compression.
Sedimentary rocks are differentiated from the other rock types by their characteristics layer formation and are termed stratified rocks. The layers may vary in thickness from a few centimeters to many centimeters. This layers are called bed. Each layer is separated by a bedding plane. The rocks may be wares a fine-grained soft or hard.
Strata and sedimentary planes of a sedimentary rocks.
Sedimentary rocks are still distinguishing from other rocks in that they are non-crystalline. Another distraction of sedimentary rocks from other rocks is that they contain fossils like remains of plants and animals.
Sedimentary rocks can be classified into three types according to their origin.
Mechanically formed sedimentary rocks.
These are rocks divided or formed from small particles or fragments or rocks wasted e.g. pebbles, sand and fine clays carried by running water, glacial or wind and redeposited in layers of different thickness. The sediments are derived from process of weathering and erosion of rocks. The sediments are compressed by the covering layers and by earth movement until, eventually they form rocks. The rocks of most lowlands are like the Congo basin, china plain, amazon basins, the Benue basins, Manyu depression and many other depressions are of this origin.
Chemically formed sedimentary rocks
These are rocks derived from they evaporation or salt solution on the earth surface. Rainwater that enters the ground usually dissolves salt from rocks. When the water reaches the surface again, it carries the dissolved salt from rocks. When the water reaches the surface again it carries the dissolved salt in solution. Where the water evaporates it leaves behind the salt, which gather as chemically curved sedimentary rocks. Examples are gypsum, rock faults, borax, potash and nature.
Organically formed sedimentary rocks.
they are rocks formed from the remains of once living organism whose hard part has accumulated over a long period of time. Those formed from animal remains e.g. chalk, coal, limestone are called calcareous rocks, meanwhile those formed from plants remains e.g. peat, lignite, coal are called carbonaceous rocks.
Metamorphic rocks:
These are rocks that were organically igneous or sedimentary but have been changed so that they now have quite different characteristics from the igneous metamorphic rocks are formed hen igneous and sedimentary rocks undergo physically or chemically changes. The change may be brought about by great heat and pressure.
|
Original form |
Metamorphic form |
| Limestone | Marble |
| Clay | Slate |
| Sandstone | Quartzite |
| Granite | Gneiss |
| Coal | Graphite |
| shale | schist |
Importance or significance of rocks to man
- Rocks are important sources of raw materials for the construction of roads, houses, factories etc. granite, sandstone and basalts.
- Extrusive igneous rocks like basalts break or decompose into fertile volcanic soils which are good farming
- Organically formed sedimentary rocks like coal and petroleum and are as important sources of thermal electricity
- Coal and petroleum are also used as an important source of raw materials for the manufacture of perfumes, disinfectants and plastics.
- Limestones and chalk are used in manufacturing cement
- Marble is used for the building and decoration of monuments and houses.
