the meaning of Figure of the Earth & rocks and its types

Some questions and answers in geophysics

1.   Discussing the meaning of the term of figure of the earth.

The figure of the earth is the shape of equipotential surface of gravity. Particularly the one that coincides to the mean sea level, in simple terms the smooth closed surface of the earth or the precise geometric shape of the earth is refer to as the figure of the earth.

 The true shape of the earth is not uniform or regular, some parts are land and others are water, some highland and lowland. Some parts are plain while others are mountainous.

The earth was believed to be round, then sphere and now based on data available the earth shape is said to spheroid or oblate ellipsoid.

The term spheroid was first coined by Sir Isaac Newton to describe the earth, the terms means that the earth is flattened at the poles with bulges at the equator, comparing the spheroid earth’s shape to a sphere earth is said to be flattened by about 14.2km at the equator while bulged by about 7.1km at the equator.

 The cause of the polar flattening is the deforming effects of centrifugal acceleration which is at its maximum at the equator where gravitational acceleration is smallest. The polar flattening is given by the ratio of the difference between the equatorial and polar radius to the polar radius. Assuming Pr is the polar radius and Er is the equatorial radius, flattening =(Er-Pr)/Pr.

2.   Discuss rocks and its classifications

Rocks are solid mineral within the earth surface; the three types of rocks are classified by mode of their formation, they  are basically classified into igneous, sedimentary and metamorphic rocks,

i)      The igneous rocks are formed from magma that originates in the  earth crust. The hot molten magma or lava originates from the partial melting of the pre-existing rocks from earth mantle or core, this melting is as a result of either an increase of temperature, decrease in pressure or compositions. The molten magna is less dense than the surrounding rocks and by nature of gravity and density difference it will move up. If the molten liquid riches the surface of the earth before solidifying is called extrusive igneous rock but if it solidifies before reaching the surface of the earth it is called intrusive igneous rocks. Example of intrusive igneous rock is granite while an example of extrusive igneous rock is basalt. Igneous rocks are the most common types of rock within the earth surface.

Igneous rocks have the characteristics of being hard, crystalline and with luster.

ii)   sedimentary rock is formed through the process of sedimentation or compaction of sediments, this sediment can be organic, chemical or from rock minerals, sediment from rocks can be chemically formed from precipitation of chemicals from a solution or evaporation from a solution, or rock grains from weathering. This sediment will accumulate and will undergo cementation overtime to become sedimentary rocks. An example of sedimentary rock is limestone, sandstone and shale etc. They are generally stratified, rough, soft and porous.

iii)            Metamorphic rock: are rocks formed by the either igneous or sedimentary rocks, they metamorphosed due to action of heat and pressure. An example is marble.