FORM FIVE SELECTIONS 2021** FORM 5 JOINING INSTRUCTIONS
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It is the outermost layer
of rocks which makes part of the solid Earth. It is distinguished from the
underlying mantle rocks by its composition, lower density, and the lower
velocity at which it conducts seismic energy. The crust is deeper under the continent than
under the ocean basins. At the continent is about 35 km thick in average; whilst
beneath the oceans has an average thickness of 5-10 km
The continental crust consists of mainly
light-coloured rocks of granitic to
granodioritic compositions. The ocean basins are dominantly underlain by
rocks of basaltic composition (dark
coloured rock). The crust is more or less floating on the mantle.
Oceanic crust
underlies two-thirds of the Earth's surface covered by the oceans.
The Earth’s crust contains about 80 stable elements.
Of these, major elements are 8 (Conte et
al., 1997; Lutgens and Tarbuck, 2000). These 8 major elements make almost
98% by weight of the Earth’s crust (Table 1.1). Given the abundance of oxygen
and silicon in the crust, it is clear that the most abundant minerals in the Earth's
crust are the silicates (Shipman et al.,
2003).
Table 1.1: Major elements in
the Earth’s crust
|
Elements
|
Approx. % by weight
|
|
Oxygen (O)
|
46.6
|
|
Silicon
(Si)
|
27.7
|
|
Aluminum (Al)
|
8.1
|
|
Iron (Fe)
|
5.0
|
|
Calcium (Ca)
|
3.6
|
|
Sodium (Na)
|
2.8
|
|
Potassium (K)
|
2.6
|
|
Magnesium (Mg)
|
2.1
|
|
Other elements
|
1.5
|
The
Earth’s mantle
It is divided into upper and lower mantle. The upper
mantle extends from about 35 to 1000 km, while the lower mantle is from 1000 –
2900 km. The Mohorovicic discontinuity separates the mantle from the overlying
crust. The mantle is composed of the molten rock material that is highly
viscous liquid known as magma. The magma is a mixture of rock silicates and
metals mainly iron and magnesium. The magma when migrates upwards into the Earth’s
crust forms the rock masses known as igneous intrusions. If the magma reaches the Earth’s
surface and flows out over it, it is called lava. The rocks formed at the Earth’s surface as a result of cooling
of this magma are called extrusive igneous rocks or volcanic rocks. Generally, the mantle contains more
iron, magnesium and calcium than the crust and is hotter and denser because the
temperature and pressure inside the Earth’s increase with depth. The mantle
forms 83% of the Earth by volume and 68% by mass (Mahapatra, 2003).
The
Earth’s core
The core is divided into inner solid core and outer
liquid core. The increase in the compresional-wave speed (seismic wave) from
the outer core to the inner core is a solid evidence for a fluid-solid
transition (Creager, 2008). The inner core is hotter than the outer core, but
it is solid because of very high pressure in comparison with the increase in
temperature (Creager, 2008). The outer core extends from 2,900 – 5,100 km,
while the inner core is from 5,100 - 6,370 km. The inner solid core is believed
to be composed of mainly iron (85%) and nickel based on the study done on the
composition of (siderites) meteorites. It is believed also to contain lighter
elements such as Si, S, C, and O (McDonough and Sun, 1995).
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