Geology
Geology is the study of the origin and evolution of the earth and its inhabitants, as disclosed by the study of the rocks and fossils. The word “geology” is derived from the Greek “ge” meaning “earth”, and “logos” meaning “study” and was first used in 1661.
The great mass of detail which constitutes geology is classified under the headings:
Physical geology is concerned with the physical processes that have given the rocks of the earth’s crust their composition and structure, and the forces that have shaped the landscapes.
Mineralogy is the science of minerals. Petrology is the science of rocks.
Structural geology is the study of the architecture of the earth that determined by earth movements. Geomorphology deals with the origin of landscapes and with the changes occurring in them. Historical geology is the science that traces the evolution and development of the earth and its animal and plant inhabitants with time. Paleontology is the science that deals with the study of animals and plants of the geologic past. Stratigraphy is concerned with the order and sequence of the rocks that make up the earth’s crust. Economic geology is the application of the science of geology to the uses of man.
The earth is divided into a number of spheres:
Atmosphere is a gaseous envelope which covers both the lithosphere and hydrosphere.
Hydrosphere is water which covers only a portion of the lithosphere.
Lithosphere is the solid portion of the earth.
Minerals
A mineral is a natural inorganic substance having a characteristic range of chemical composition, usually definite crystal form and exhibiting other physical properties.
Minerals may be classified according to their properties.
A. Crystal form. A crystal is a solid having a definite atomic or molecular structure and bounded by surfaces called crystal planes. All crystal forms are classified in six systems, each of which is determined by the relation of axes.
1. Isometric system.
2. Tetragonal system.
3. Hexagonal system
4. Orthorhombic system.
5. Monoclinic system.
6. Triclinic system.
B. Cleavage. Property of minerals to split more easily in certain directions. It depends upon atomic arrangement of crystals.
C. Fracture. Property of minerals to break with curved or uneven surface.
D. Hardness. Resistance to abrasion or scratching.
E. Color. Some minerals may include a number of color varieties.
F. Luster. Appearance of a mineral in reflected light.
The common rock minerals may be divided into three general groups:
1) Native minerals. Composed of uncombined elements.
2) Mineral sulphides, chlorides, and oxides. Composed of metals or silicon, with either sulphur, chlorine, or oxygen.
3) Mineral silicate, carbonates, or sulphates. Composed of compounds of basic oxides with oxides of silicon, carbon or sulphur. The silicates and carbonates are the most abundant and common rock-forming minerals.
ROCKS
Rocks are aggregates of minerals. They form the solid crust of the earth. Different kinds of rocks contain different minerals, have different physical properties.
Geologists group the rocks into three types according to their mode of origin: igneous, sedimentary and metamorphic.
Deep in the earth’s crust, temperatures are high enough to melt rock into magma. Magma erupts to the surface as lava, or it may force its way into other solid rock underground. When magma cools, it solidifies, forming igneous rocks, such as granite and basalt.
Two subdivisions of the igneous rocks are recognized. The lavas and solid fragments erupted from volcanoes are called extrusive rocks that composed of microscopic mineral crystals and glass. Intrusive rocks are coarse-grained igneous rocks that were not thrown up to the surface like lavas, but solidified deep underground.
The sedimentary rocks have been formed at the surface of the earth, either by accumulation and cementation of fragments of rocks, minerals, and organisms, or as precipitates from sea water.
Most sedimentary rocks form distinct layers or strata. Most often the youngest stratum is at the top and the oldest at the base. Two general subdivisions may be recognized: clastic sedimentary rocks and organic and chemical sedimentary rocks.
It should be noted that sedimentary rocks are of great interest to petroleum geologists because most oil and gas accumulations occur in these rocks due to their most important physical characteristics: porosity and permeability. Limestone, sandstone, and clay are considered to be typical sedimentary rocks.
Metamorphic rocks are rocks that have been buried deep in the earth where they are subjected to high temperatures and pressures. When undergoing the metamorphic process, physical and chemical properties of the original rock are found to change, thus altering rock composition and appearance. To form a metamorphic rock metamorphic agents (heat, pressure, various gases and vapors) interact with the parent rock. So, for example, limestone can be metamorphosed into marble, and sandstone into quartzite.