FORM FOUR PHYSICS STUDY NOTES TOPIC 7: GEOPHYSICS

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TOPIC 7: GEOPHYSICS

Geophysics
is a branch of science that deals 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.
The Structure of the Earth
Describe the structure of the earth
The structure of the earth is composed of three major zones arranged in concentric manner. These are crust, mantle and core.
The crust
Is
the outer solid layer o the earth.It is extremely thin(5 to 15km)
compared to the radius of the earth (6,371km).There are two types of
crust, namely:
  1. Continental crust:
    This is heterogeneous and of relatively low density(2 to 2.8 tonnes per
    cubic meter).-It is composed mainly of granites and sedimentary rocks.
  2. Oceanic crust: This
    is basaltic and more denser (3.0 to 3.1 tonnes per cubic meter). Both
    the continental and the oceanic crusts floats 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 70 km thick. The denser 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 Mohorocivic discontinuity or simply Moho. It is a zone between one and several kilometers thick.
The mantle
It
begins from the Moho and extends to a depth of 2,900km 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 composed of
rocks,both in solid and mountain states.The upper surface of the mantle
has the temperature of about 870°C,and this temperature increases
downwards through the mantle to about 2,200°C near the core.
The core
Is
the innermost part of the earth.It extends from the Gutenberg
discontinuity to the earth’s geometric centre. The core consists of two
distinct regions namely:
  1. The inner core: It is composed of solid material because the high pressure at this depth.-It is composed of iron-nickel alloys.
  2. The
    outer core: Is composed of liquid of molten nickel and iron known as
    magma. It extends from the mantle to a depth of about 5,000km below the
    earth’s surface.
Activity 1
In groups of five, discuss why the outer core is liquid while the inner core is solid.
The Composition of the Layers of the Earth
Describe the composition of the layers of the earth
Continental
crust is made of granite and sedimentary rocks forming the lands and
the mountains while the oceanic crust forms oceanic basins. Mantle is
made of solids and molten rocks. The outer core is made of molten nickel
and iron called magma while the inner core is solid because of the high
pressure. The crust and the mantle are separated by the mohorovicic
discontinuity.
The Importance of the Layers of the Earth
Explain the importance of the layers of the earth
Continental
crust forms the land and mountains of the earth on which all human
activities are carried out e.g farming, housing etc. Oceanic crust forms
the base of the oceans and seas on which oceanic water rests and all
aquatic organisms like fishes live. The mantle provides the heat
transfer from the core to the outer layers a process which causes the
volcanic actions and earthquakes.

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