The planet Earth is the densest planet in the solar system,
the largest of the solar system’s four terrestrial planets(other three
terrestrial planets- Mercury, Venus and Mars) , and the only celestial body
known to accommodate life. It is home to untold number of species, including
billions of humans who depend on its biosphere and minerals. You live and die
on it. You walk on it. You drive your car over it. As it moves around the sun
it gives you a ride. Have you wondered how this vehicle on which you float in
space has been structured? This issue fired me to find out how our home has
been structured. What I found I am sharing with you in this post.
The structure
of Earth can be defined in two ways: by mechanical properties such as rheology, or chemically. Mechanically, it is
divided into 5 important layers.
The layering of
Earth has been inferred indirectly using the time of travel of refracted and
reflected seismic waves created by earthquakes. The core does not allow shear
waves to pass through it, while the speed of travel (seismic
velocity) is different
in other layers. The changes in seismic velocity between different layers
causes refraction owing to Snell's law, like light
bending as it passes through a prism. Likewise, reflections are caused by a
large increase in seismic velocity and are similar to light reflecting from a
mirror.
The five layers
are:
v LITHOSPHERE
v ASTHENOSPHERE
v MESOPHERE
v OUTER CORE
v INNER CORE
LITHOSPHERE
Earth's lithosphere includes the crust and the uppermost mantle, which
constitute the hard and rigid outer layer of the Earth. The lithosphere is
broken into tectonic
plates. The uppermost
part of the lithosphere that chemically reacts to the atmosphere, hydrosphere and biosphere through the soil forming process is called the pedosphere. The lithosphere is underlain by the asthenosphere, the weaker, hotter, and deeper part of the upper
mantle. The boundary between the lithosphere and the underlying asthenosphere
is defined by a difference in response to stress: the lithosphere remains rigid
for very long periods of geologic time in which it deforms elastically and
through brittle failure, while the asthenosphere deforms viscously and
accommodates strain through plastic deformation. The study of past and current
formations of landscapes is called geomorphology
There are two
types of lithosphere:
- Oceanic lithosphere,
which is associated with oceanic
crust and
exists in the ocean basins (mean density of about 2.9 grams per cubic
centimeter)
- Continental
lithosphere, which is associated with continental crust (mean density of about 2.7 grams
per cubic centimeter)
ASTHENOSPHERE
The asthenosphere (from Greek asthenēs (ἀσθενὲς) 'weak' + sphere) is the
highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth. It lies below the lithosphere, at depths between ~80 and ~200 km (~ 50 and
124 miles) below the surface. The lithosphere-asthenosphere boundary is usually
referred to as LAB. Asthenosphere is generally solid although some of its
regions could be melted (e.g. below mid-ocean ridge). The lower boundary of the asthenosphere is not
well defined. The thickness of the asthenosphere depends mainly on the
temperature. For some regions asthenosphere could extend as deep as 700 km
(430 mi). It is considered the source region of mid-ocean ridge basalt (MORB).
The asthenosphere is a part of the upper mantle just below the lithosphere that is involved in plate tectonic movement and isostatic adjustments. The lithosphere-asthenosphere
boundary is conventionally taken at the 1300°C isotherm, above which the mantle behaves in a
rigid fashion and below which it behaves in a ductile fashion. Seismic waves pass relatively slowly through the asthenosphere compared to the
overlying lithospheric mantle, thus it has been called the low-velocity zone (LVZ), although the two are not exactly
the same.
MESOSPHERE
The mesosphere refers to the mantle in the region under the lithosphere and the asthenosphere, but above the outer core. The upper boundary is defined as the sharp
increase in seismic wave velocities and density at a depth of 660 kilometers (410 mi).At a depth of 660 km, ringwoodite (gamma-(Mg,Fe)2SiO4) decomposes into Mg-Si
perovskite and magnesiowustite. This reaction marks the boundary between upper
mantle and lower mantle. This measurement is estimated from seismic data and
high-pressure laboratory experiments.
The base of the mesosphere includes the D'' zone which lies just above the mantle-core
boundary at approximately
2,700 to 2,890 km (1,678 to 1,796 mi). The base of the lower mantle
is at about 2700 km.
"Mesosphere" (not to be confused with mesosphere, a layer of the atmosphere) is derived from “mesospheric shell”, coined by Reginald
Aldworth Daly, a Harvard
University geology professor.
According to Daly, the base of the solid Earth mesosphere could extend to
the base of the mantle (and, thus, to the top of the core).
OUTER CORE
The Earth's outer core is a liquid layer about 2,300 km
(1,400 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 the Earth's surface. The temperature of
the outer core ranges from 4,300 K (4,030 °C; 7,280 °F) in the
outer regions to 6,000 K (5,730 °C; 10,340 °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 convects turbulently. Eddy currents in the nickel iron fluid of the outer core are believed to influence the Earth's
magnetic field. Without
the outer core, life on Earth would be very different. Convection of liquid
metals in the outer core creates the Earth's
magnetic field.
INNER CORE
The Earth's inner core is the Earth's innermost part and is a
primarily solid ball with a radius of about 1,220 km (760 mi), according to seismological studies.[1][2] (This is about 70% of the Moon's radius.) It is believed to consist primarily of an iron–nickel alloy and to be approximately the same temperature as the surface of the Sun: approximately 5700 K (5400 °C).
The Earth was discovered to have a solid inner core distinct from its
liquid outer core in 1936, by the seismologist Inge Lehmann,[4] who deduced its presence from
observations of earthquake-generated seismic waves that reflect off the boundary of the inner core
and can be detected by sensitive seismographs on the Earth's surface. This boundary is known as
the Bullen discontinuity, or sometimes as the
Lehmann discontinuity. A few years later, in 1940, it was hypothesized that
this inner core was made of solid iron; its rigidity was confirmed in 1971.
The Earth is the largest of the four terrestrial planets of the solar
system. It is the only celestial body known to accommodate life. The structure
of the Earth can be determined by its mechanical properties or chemically.
Mechanically, the Earth has been structured into 5 layers, namely, lithosphere,
asthenosphere, mesosphere, outer core and inner core. So in a nutshell this is
the structure that keeps you aloft in space.
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