mining engineering UET Peshawar
EARTH STRUCTURE 1
The study of the passage of
seismic (earth quack)
waves through the earth
have helped greatly in
knowing the interior of the
The interior structure of
the Earth is layered in
spherical shells, like
an onion. Earth has an
outer silicate solid crust, a
highly viscous mantle, a
liquid outer core that is
much less viscous than the
mantle, and a solid inner
EARTH STRUCTURE 2
The structure of Earth can
be defined in two ways: by
mechanical properties such
as rheology ( the study of
the flow of matter, primarily
in the liquid state) , or
Mechanically, it can be
into lithosphere, asthenosp
mantle, outer core, and
the inner core.
The interior of Earth is
divided into 5 important
layers. Chemically, Earth can
be divided into the crust,
upper mantle, lower mantle,
outer core, and inner core.
EARTH STRUCTURE 3
At depth of 30 to 40 km, the velocity
of both P and S waves increase
abruptly. The velocity of p waves
increases from about 6.5 km per
second to about 8 km per second.
This discontinuity surface is called
the “mohorvicic discontinuity”.
Ta depth of 2900 km, from the
Mohorvicic discontinuity to a depth
of 2900 km, the velocity of both P
and S wave increases gradually. But
at 2900 km the P waves are sharply
reflected and refracted, and their
velocity drops abruptly from about
13 km per second to about 8 km per
The S waves are lost entirely. This
discontinuity surface separates the
“mantle” of the earth from the “core”.
At depth of 5000 km below the
earth’s surface, there is an abrupt
increase in the velocity of P waves.
This discontinuity surface suggests
the existence of an “inner core”.
EARTH STRUCTURE 4
The core is situated from 2900 km depth up the
centre of the earth (6370 km ).
Since the S waves do not pass through the
core. Seismic measurements show that the core is
divided into two parts, a "solid" inner core with
a radius of ~1,220 km and a liquid outer
core extending beyond it to a radius of ~3,400 km.
The densities are between 9,900 and
12,200 kg/m3 in the outer core and 12,600–
13,000 kg/m3 in the inner core. The inner core was
discovered in 1936 by Inge Lehmann and is generally
believed to be composed primarily of iron and some
EARTH STRUCTURE 5
The inner core is believed to consist primarily of a nickel-iron
alloy known as NiFe: 'Ni' for nickel, and 'Fe' for ferrum
or iron. Because the inner core is denser (12.8 ~ 13.1)g⁄cm³ than
pure iron or nickel, even under heavy pressures.
Earth's outer core is a liquid layer about 2,266 km (1,408 mi)
thick composed of iron and nickel that lies above Earth's
solid inner core and below its mantle. Its outer boundary lies
2,890 km (1,800 mi) beneath Earth's surface. The transition
between the inner core and outer core is located approximately
5,150 km (3,200 mi) beneath Earth's surface.
The temperature of the outer core ranges from 4400 °C (8000 °F)
in the outer regions to 6100 °C (11000 °F) near the inner core.
Because of its high temperature, modeling work has shown that
the outer core is a low viscosity fluid (about ten times the
viscosity of liquid metals at the surface) that
EARTH STRUCTURE 6
The mantle lies beneath the earth’s crust and
it extends to a depth of 2,890 km, making it the
thickest layer of Earth. The pressure at the bottom of
the mantle is ~140 GPa (1.4 Matm). The mantle is
composed of silicate rocks that are rich in iron and
magnesium relative to the overlying crust. Although
solid, the high temperatures within the mantle cause
the silicate material to be sufficiently ductile that it
can flow on very long timescales. It has been divided
into the upper mantle and lower mantle. The upper
mantle lies between 35 km and 1000 km depth, and
is made up of ultra basic rocks like peridotite ( sp. Gr.
3.3) the lower mantle is between 1000 km and 2890
km, and contains pallasite (sp. Gr. 5.7) which is a
mixture of basic rocks and metallic iron.
EARTH STRUCTURE 7
The outer most layer of solid rock,
which lies above the Mohorovicic
discontinuity and has an average
thickness of 35 km, is known as
the crust of the earth. Under the
continents it is possible to
distinguish two layers in the earth
crust. The upper layer which is less
dense (sp. Gr. 2.65) and granitic in
character is known as the “sial” ;
while the lower layer which is
basaltic in character (sp. Gr. 3.0) is
known as “sima”. The term sial
represents rock rich in silica and
alumina, and term sima represents
rock containing silica and
magnesia. Under oceans only sima
layer is found and the sial layer
appears to be absent.
EARTH STRUCTURE 8