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WHAT IS GEOPHYSICS?
Geophysics is the branch of science that is concerned with the physical, chemical, geological, astronomical -and other characteristic properties of the earth.
It deals with geological phenomena such as the temperature distribution of the earth’s interior, the source, configuration and the geomagnetic field.
Interior structure of the earth
The structure of the earth is composed of three major zones arranged in a concentric manner. These are crust, mantle and core;
Internal structure of the earth
The crust
The crust is the outer solid layer of the earth. It is extremely thin (5 to 15km) compared to the radius of the earth (6371 km).
There are two types of crust, namely continental crust and oceanic crust.
Continental crust is heterogeneous and of relatively low density (2 to 2.8 tonnes per cubic meter). It is composed mainly of granites and sedimentary rock.
Oceanic crust is basaltic and has a. higher density (3.0 to 3.1 tonnes per cubic meter). Both the continental and oceanic crusts float on the denser mantle. Because of its low density, the continental crust floats on the mantle at a higher elevation, forming the land masses and mountains. The continental crust is 30 to 70km thick.
The dense oceanic crust floats at a lower elevation forming oceanic basins. It is about 8km thick.
The boundary between the crust and the mantle is called Mohorovicic discontinuity or simply Moho. It is a zone between one and several kilometers thick.
The mantle
The mantle begins from the Moho and extends to a depth of 2900km below the earth’s surface, up to its boundary with the earth’s core. This boundary is called the Gutenberg discontinuity.
The mantle contains about 70% of the earth’s mass. It is made up of rocks, both in solid and in molten states. These rocks are said to be in a plastic state. The upper part of the mantle has a temperature of about 870°C. The temperature increases downwards through the mantle to about 2200°C near the core.
Circulation of materials in the mantle is the main mechanism of heat transfer from the core of the earth to the outer regions of the earth. It is the main force that drives the movement of continents as well as volcanism and earthquakes.
The core
The core is the innermost part of the earth. It extends from the Gutenberg discontinuity to the geometric center. The core consists of two distinct regions, namely the outer core and the inner core.
The outer core is composed of a liquid of molten nickel and iron known as magma. It extends from the mantle to a depth of about 5000km below the earth’s surface. The inner core is solid; it is composed of iron-nickel alloys.
The material of the inner core is solid because of the high pressure at this depth.
Tectonic plates
The earth’s crust and part of the mantle are cracked into huge pieces called tectonic plates. These plates float on top of the semi-molten rock underneath. They move about at a very slow speed. The movements of the tectonic plates mean that some continents are moving apart and some are moving towards each other.
This process is referred to as continental drift and has been going on for hundreds of millions of years. Tectonic plate movements have split the continents as- we, know them today.
Tectonic plate
The line where two tectonic plates meet is called a boundary. There are three main types of boundaries. These are destructive boundaries, constructive boundaries and conservative boundaries.
Destructive boundary (convergent)– is one found at the edges of two plates moving towards each other.
Constructive (boundaries divergent)– are formed at the edges of two plates moving away from each other.
Conservative boundaries– are formed when two plates slide past each other without moving apart or towards each other.
Diagram constructive and destructive boundar
