EARTH'S LAYER
The
Crust
The crust is the outermost layer
of the earth making up 0.5-1.0 per cent of the
earth’s
volume and less than 1 per cent of Earth’s mass.
Density increases with depth,
and the average density is about 2.7 g/cm3 (average
density of the earth is 5.51
g/cm³).
The thickness of the crust
varies in the range of range of 5-15 km
in case of the
oceanic
crust and as 50-70 km in case of the
continental crust.
The continental crust can be
thicker than 70 km in the areas of major mountain
systems. It is as much as 70-100
km thick in the Himalayan region.
The temperature of the crust
increases with depth, reaching values typically in
the range from about 200 °C to 400 °C at the
boundary with the underlying
mantle.
The temperature increases by as
much as 30 °C for every kilometre in the upper
part of the crust.
The outer covering of the crust
is of sedimentary material and below that lie
crystalline, igneous and
metamorphic rocks which are acidic in nature.
The lower layer of the crust
consists of basaltic and ultra-basic rocks.
The continents are composed of
lighter silicates — silica + aluminium (also
called sial) while the oceans have the heavier silicates — silica +
magnesium
(also called sima).
The continental crust is
composed of lighter (felsic) sodium potassium
aluminium
silicate rocks, like granite.
The oceanic crust, on the other
hand, is composed of dense (mafic) iron
magnesium
silicate igneous rocks, like basalt.
In
geology, felsic refers to igneous rocks that are relatively rich in elements
that
form
feldspar and quartz.
It
is contrasted with mafic rocks, which are relatively richer in magnesium and
iron.
Felsic
refers to rocks which are enriched in the lighter elements such
as
silicon, oxygen, aluminium, sodium, and potassium.
Most Abundant Elements of the Earth’s Crust
The
Mohorovicic (Moho) discontinuity
Mohorovicic (Moho) discontinuity
forms the boundary between the crust and
the asthenosphere (upper reaches of the mantle) where there
is a discontinuity
in the seismic velocity.
It occurs at an average depth of
about 8 kilometres beneath the ocean basins and
30 kilometres beneath
continental surfaces.
Lithosphere
The lithosphere is the rigid
outer part of the earth with thickness varying between
10-200 km.
It is includes the crust and the upper part of the mantle.
The lithosphere is broken into tectonic plates (lithospheric plates), and the
movement of these tectonic
plates cause large-scale changes in the earth’s
geological structure (folding,
faulting).
The source of heat that drives
plate tectonics is the primordial heat left over
from the planet’s
formation as well as the radioactive decay of uranium,
thorium,
and potassium in Earth’s crust and mantle.
The
Mantle
It forms about 83 per cent of the earth’s volume and holds 67% of the earth’s
mass.
It extends from Moho’s
discontinuity to a depth of 2,900 km.
The density of the upper mantle
varies between 2.9 g/cm3 and 3.3 g/cm3.
The lower mantle extends beyond
the asthenosphere. It is in a solid state.
The density ranges from 3.3 g/cm3 to 5.7 g/cm3 in the lower mantle.
The mantle is composed of silicate rocks that are rich in iron and
magnesium
relative to the overlying crust.
Regarding its constituent
elements, the mantle is made up of 45%
oxygen, 21%
silicon,
and 23% magnesium (OSM).
In the mantle, temperatures
range from approximately 200 °C at the upper
boundary with the crust to
approximately 4,000 °C at the core-mantle boundary.
Because of the temperature
difference, there is a convective material
circulation
in the mantle (although solid, the high temperatures within the
mantle cause the silicate
material to be sufficiently ductile).
Convection of the mantle is
expressed at the surface through the motions of
tectonic plates.
High-pressure conditions ought
to inhibit seismicity in the mantle. However, in
subduction zones, earthquakes
are observed down to 670 km.
Asthenosphere
The upper portion of the mantle
is called as asthenosphere (astheno means weak).
It lies just below the lithosphere extending up to 80-200 km.
It is highly viscous, mechanically weak and ductile and its
density is higher
than that of the crust.
These properties of the
asthenosphere aid in plate tectonic movement
and
isostatic
adjustments (the elevated part at one part of the crust area is
counterbalanced by a depressed
part at another).
It is the main source of magma that finds its way to the
surface during volcanic
eruptions.
The
Outer Core
The outer core, surrounding the
inner core, lies between 2900 km and 5100 km
below the earth’s surface.
The outer core is composed of iron mixed with nickel (nife) and
trace amounts
of lighter elements.
The outer core is not under enough pressure to be solid, so it is
liquid even
though it has a composition
similar to the inner core.
The density of the outer core
ranges from 9.9 g/cm3 to 12.2 g/cm3.
The temperature of the outer
core ranges from 4400 °C in the outer regions to
6000 °C near the
inner core.
Dynamo theory suggests that convection in the outer core, combined with the
Coriolis
effect, gives rise to Earth’s magnetic field.
The
Inner Core
The inner core extends from the
centre of the earth to 5100 km below the earth’s
surface.
The inner core is generally
believed to be composed primarily of iron
(80%)
and
some nickel (nife).
Since this layer can transmit
shear waves (transverse seismic waves), it is solid.
(When P-waves strike the
outer core – inner core boundary, they give rise to Swaves)
Earth’s inner core rotates slightly faster relative to the rotation of the
surface.
The solid inner core is too hot
to hold a permanent magnetic field.
The density of the inner core
ranges from 12.6 g/cm3 to 13 g/cm3.
The core (inner core and the
outer core) accounts for just about 16 per
cent of
the
earth’s volume but 33% of earth’s mass.
Scientists have determined the
temperature near the Earth’s centre to be 6000 C,
1000 C hotter than previously
thought.
At 6000°C, this
iron core is as hot as the Sun’s surface, but the crushing
pressure
caused by gravity prevents it from becoming liquid.
Seismic
Discontinuities
Seismic discontinuities are the
regions in the earth where seismic waves behave
a lot different compared to the
surrounding regions due to a marked change in
physical or chemical properties.
Mohorovicic
Discontinuity (Moho): separates the crust from the mantle.
Asthenosphere:
highly viscous, mechanically weak and ductile part of
mantle.
Gutenberg
Discontinuity: lies between the mantle and the outer core.
The
information about the earth’s interior can be obtained from two sources:
(1)
Volcanoes
(2)
Earthquake
Geophysicists study the velocity
and the paths of earthquake waves to learn about the earth's
interior.
Earthquake waves transmitted
through the earth are of two types:
(1)
Primary or compressional waves, P, in which motion of solid
particles is back and forth,
parallel to the direction of
travel, and they can travel in both solid and liquid medium. Their
speed is slow in liquid.
(2)
Secondary or shear waves, S, in which particle motion is across,
transverse to the direction
of travel. They can only travel in
solid.
Here are some examples of what
we have been able to distinguish in the earth’s interior from
the study of seismic waves and
how they travel through the layers of the earth:
1. The thickness of the crust.
This is a measure of the thickness of the crust based on the
abrupt increase in speed of
seismic waves that occurs when they enter the mantle. The
boundary between the crust and
mantle, as inferred from the change in the speed of Pand
S-waves, is called the
Mohorovicic discontinuity, named after the Croatian
seismologist who first discerned
it; usually it is referred to simply as the Moho. Seismic
waves also give information
about the thickness of oceanic crust and continental crust.
2. The thickness of the
lithosphere. Whe seismic waves pass down from the lithosphere
into the asthenosphere, they
slow down. This is because of the lower rigidity and
compressibility of the rocks in
the layer below the lithosphere. The zone below the
lithosphere where seismic waves
travel more slowly is called the low velocity zone
which is called as
asthenosphere.
3. The boundary between the
upper and lower mesosphere (upper and lower mantle). This
shows up as an increase in
seismic wave speed at a depth of 660 km.
4. The boundary between the
mantle and the core. This is marked by S-waves coming to
an abrupt stop. This is because
the outer core is liquid, and a sudden large reduction in
the speed of P-waves, as they
enter the liquid core where there is no rigidity to contribute
to P-wave speed.
5. The inner core. This was
first recognized by refraction of P-waves passing through this
part of the core, due to an
abrupt increase in their speed, which was not shown by Pwaves
traveling through only the outer part of the core.
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