Download 5 THE EARTH`S CRUST 5.1 Text 5 Most mineral resources are

5 THE EARTH’S CRUST
5.1 Text 5
Most mineral resources are derived from the Earth's crust. The crust is
composed of minerals that are crystalline solids with specific and rather simple
composition. Minerals in the Earth's crust are concentrated into specific groups
which are called rocks. Two distinctly different types of crust are recognized:
oceanic and continental. Since it is difficult to investigate the floor of the ocean the
composition of the oceanic crust is not known completely.
Scientists say that it is relatively constant in composition. The oceanic floor
consists largely of minerals rich in calcium, magnesium, iron and silicon. It is
formed by cooling of lavas extruded on the sea floor to form a type of a rock called
basalt. It is subjected to the same forces of erosion and weathering.
The continental crust is more complicated and has a more variable thickness
and a less well defined structure. It contains less iron and magnesium than the
oceanic crust, but relatively more silicon, aluminium, sodium and potassium.
It is quite possible to say that the rock-forming processes which we can
observe today have been active for at least 3,500 million years. The rocks of the
Earth’s crust are divided into three main groups: 1) sedimentary rocks, which
consist of fragments or particles of pre-existing rocks; 2) igneous rocks, which have
been solidified from magma and metamorphic rocks; 3) metamorphic rocks which
have been derived from either igneous or sedimentary rocks.
Sedimentary rocks represent one of the three major groups that make up the
crust of the Earth. Most sedimentary rocks have originated by sedimentation. They
are layered or stratified. Thus stratification is the most important characteristic of
sediments and sedimentary rocks. It is necessary to note that the processes which
lead to the formation of sedimentary rocks are going on around us. Sediments are
formed at the very near surface of the Earth by the action of heat, water (rivers,
glaciers, seas and lakes) and organisms.
Igneous rocks have been crystallized from solidified magma. Lava flows and
hardens into massive homogenous rocks cooling on the surface in a short time.
Igneous rock is generally compact and massive in structure. Igneous rocks are rich
in minerals that are important economically or have great scientific value. Igneous
rocks and their veins are rich in iron, zinc, nickel, and other ferrous metals.
Minerals are defined as inorganic substances which occur naturally. Such
rocks have a definite chemical composition and physical properties which vary
within known limits. The major properties are colour, crystal form, hardness,
cleavage and others.
Cleavage is one of the most diagnostically useful mineralogical properties
which can be found throughout the mineral. Minerals to be useful for a man can be
grouped into two broad categories: such as aluminium, copper, gold, silver, iron,
tin, platinum, chromium, nickel, lead, and zinc and non-metallic minerals such as
diamonds, salt, limestone, cement, sulphur and asbestos.
Economic minerals are those which are of economic importance and include
both metallic and non-metallic minerals. Most minerals consist of several elements.
Such elements are oxygen, silicon, titanium, aluminium, iron, magnesium, calcium,
potassium and hydrogen. They make up more that 99 per cent by weight of all the
rock-forming minerals. Aluminium, iron and magnesium are industrial metals
among these ones. The other metals are presented in small quantities, mostly in
igneous rocks.