SECTION ‘A’
UNIT 1 & FOR PRACTICAL RECORD
I. How to obtain information about the internal structure
The Interior of the Earth information obtained from
the study of the propagation of the earthquake waves (P-, S- & L- waves)
through the earth. The P- and S- wave velocities change with depth and each
change can be related to a change in materials. Each region of changing shock-
wave velocity demarcates a zone of discontinuity. On the basis of seismic
investigations, the earth’s interior has been broadly divided into three major
parts- Crust, Mantle & Core.
[*P-wave can travel in solid, liquid
and gaseous media; and have short wave length and high frequency. S-waves
travel only in solid medium and have short wavelength and high frequency.
L-waves are confined to the outer skin of the earth crust; they have short
wavelength and high frequency].
II. Core
1)
It is the innermost part of the Earth. It
constitutes around 17% of the volume and 34% of the mass of the earth.
2)
It 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.
3)
The average magnetic field strength in the Earth's outer core is
estimated to be 25 Gauss.
4)
Earth's inner core rotates slightly
faster than the rest of the planet, and rotates approximately 0.3 to 0.5 degrees per year relative
to the rotation of the surface.
5)
The density of the core ranges from 10.6x103 kgm-3
to 12x103 kgm-3.
6)
The core is largely composed of iron
(~80%) and metallic nickel. The liquid outer core surrounds the inner core and is believed to
be composed of iron mixed with nickel and trace amounts of lighter elements.
The innermost part of the core is enriched in Gold, Platinum and other
siderophile elements.
III. Mantle
1)
It extends to a depth of 2,890 km, making it the thickest
layer of the Earth. It forms about 83%
of the earth by volume and 68% by mass.
2)
It is a highly viscous (1021 and 1024 Pa.s)
layer between the crust and the outer core.
3)
The temperatures range between 500 to 900oC at the
upper boundary with the crust, to over 4000oC at the boundary with
the core.
4)
Two main zones are distinguished in the upper mantle: the inner
asthenosphere composed of flowing rock in the state of plasticity, about
200 km thick and the lowermost part of the lithosphere, composed of rigid
rock, about 50 to 120 km thick. A thin crust, the upper part of the
lithosphere, surrounds the mantle and is about 5 to 75 km thick.
5)
Convection of the mantle is expressed at the surface through the
motions of tectonic plates.
6)
The mantle is composed of silicate rocks that are higher magnesium to iron ratio, and a
smaller proportion of silicon and aluminium than the crust.
IV. Crust
1)
The crust ranges from 5–70 km in depth and is the outermost
layer.
2)
The thin parts are the oceanic crust, which underlie the ocean
basins (5–10 km) and are composed of dense (mafic) iron magnesium silicate
rocks, like basalt.
3)
The thicker crust is continental crust, which is less dense and
composed of (felsic) sodium potassium aluminium silicate rocks, like granite.
4)
The main mineral constituents of the continental mass are silica
and alumina, it is thus called sial (si-silica, 65–75% and al-alumina).
The oceanic crust mainly consists of silica and magnesium. it is therefore
called sima (si-silica and ma-magnesium).
5)
It has been observed that the L- waves while passing through ocean
floors acquire more velocity in comparison to its propagation through the land
masses. This indicates that the sialic layer (granitic material) with which the
landmasses are usually composed of, is practically absent on the ocean floors.
6)
The ocean floor is of basaltic composition, which is poorer in
potassium and richer in aluminium than the basalts of the land surface and are
called ‘oceanic tholeiites’.
V. Discontinuities
Conrad
discontinuity: The boundary between the sial and the sima, located at a depth of
11 kms in the Crust layer.
Mohorovicic
discontinuity: Boundary between the Crust and the Mantle at 33 kms.
Guttenberg-Weichert
discontinuity: Between the Mantle and the Core at 2900 kms.
Lehmann
discontinuity: The boundary between outer core and inner core.
No comments:
Post a Comment