Download Evolution of Earth`s Atmosphere

Evolution of Earth's Atmosphere
Structure of the Earth
The internal structure of the Earth is layered in spherical shells, like an onion (Fig. 1.2).
These layers can be defined by either their chemical or their theological properties. 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 core. Scientific understanding of Earth's internal
structure is based on observations of topography and bathymetry, observations of rock in
outcrop, samples brought to the surface from greater depths by volcanic activity, analysis of
the seismic waves that pass through Earth, measurements of the gravity field of Earth, and
experiments with crystalline solids at pressures and temperatures characteristic of Earth's
deep interior.
Fig 1.2: The internal structure of Earth consists of layers of different composition and physical
properties.
The structure of Earth can be defined in two ways:
a. Mechanical properties such as theology (the study of religion and religious belief), or
chemically. Mechanically, it can be divided into lithosphere, asthenosphere, mesospheric
mantle, outer core, and the inner core. The interior of Earth is divided into 5 important layers.
b. Chemically, Earth can be divided into the crust, upper mantle, lower mantle, outer core, and
inner core (Table 1.1). The geologic component layers of Earth are at the following depths
below the surface:
Table 1.1: Depth of various layers of earth’s interior
Depth
Kilometres
0–60
0–35
35–60
35–2,890
100–200
Layer
Miles
0–37
0–22
22–37
22–1,790
62–125
Lithosphere (locally varies between 5 and 200 km)
Crust (locally varies between 5 and 70 km)
Uppermost part of mantle
Mantle
Asthenosphere
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Evolution of Earth's Atmosphere
35–660
660–2,890
2,890–5,150
5,150–6,360
22–410
410–1,790
1,790–3,160
3,160–3,954
Upper mesosphere (upper mantle)
Lower mesosphere (lower mantle)
Outer core
Inner core
a. Crust
The crust ranges from 5–70 km (~3–44 miles) in depth and is the outermost layer. The
thin parts are the oceanic crust, which underlie the ocean basins (5–10 km) and are composed
of dense (mafic) iron magnesium silicate igneous rocks, like basalt. The thicker crust is
continental crust, which is less dense and composed of (felsic) sodium potassium aluminium
silicate rocks, like granite. The rocks of the crust fall into two major categories – sial and
sima (Suess,1831–1914). It is estimated that sima starts about 11 km below the Conrad
discontinuity (a second order discontinuity). The uppermost mantle together with the crust
constitutes the lithosphere. The crust-mantle boundary occurs as two physically different
events. First, there is a discontinuity in the seismic velocity, which is known as the
Mohorovičić discontinuity or Moho. The cause of the Moho is thought to be a change in rock
composition from rocks containing plagioclase feldspar (above) to rocks that contain no
feldspars (below). Second, in oceanic crust, there is a chemical discontinuity between
ultramafic cumulates and tectonizedharzburgites, which has been observed from deep parts of
the oceanic crust that have been obducted onto the continental crust and preserved as
ophiolite sequences.
Many rocks now making up Earth's crust formed less than 100 million (1×108) years ago;
however, the oldest known mineral grains are 4.4 billion (4.4×109) years old, indicating that
Earth has had a solid crust for at least that long.
b. Mantle
Earth's mantle 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. Convection of the mantle is expressed at the
surface through the motions of tectonic plates. The melting point and viscosity of a substance
depends on the pressure it is under. As there is intense and increasing pressure as one travels
deeper into the mantle, the lower part of the mantle flows less easily than does the upper
mantle (chemical changes within the mantle may also be important). The viscosity of the
mantle ranges between 1021 and 1024Pa·s, depending on depth. In comparison, the viscosity
of water is approximately 10−3Pa·s and that of pitch is 107Pa·s( Fig. 1.3).
Lecture Delivered by: Dr. Shahid-e-Murtaza
Evolution of Earth's Atmosphere
Fig. 1.3: Internal structure of earth
c. Core
The average density of Earth is 5,515 kg/m3. Since the average density of surface
material is only around 3,000 kg/m3, we must conclude that denser materials exist within
Earth's 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 nickel. It is not necessarily a solid, but, because it is
able to deflect seismic waves, it must behave as a solid in some fashion. Experimental
evidence has at times been critical of crystal models of the core. Other experimental studies
show a discrepancy under high pressure: diamond anvil (static) studies at core pressures yield
melting temperatures that are approximately 2000K below those from shock laser (dynamic)
studies. The laser studies create plasma, and the results are suggestive that constraining inner
core conditions will depend on whether the inner core is a solid or is plasma with the density
of a solid. This is an area of active research.
Evidences for Internal Earth Structure and Composition
a). Seismic Waves
When an earthquake occurs the seismic waves (P and S waves) spread out in all directions
through the Earth's interior. Seismic stations located at increasing distances from the
earthquake epicenter will record seismic waves that have travelled through increasing depths
in the Earth.
Seismic velocities depend on the material properties such as composition, mineral phase and
packing structure, temperature, and pressure of the media through which seismic waves pass.
Seismic waves travel more quickly through denser materials and therefore generally travel
more quickly with depth. Anomalously hot areas slow down seismic waves. Seismic waves
move more slowly through a liquid than a solid. Molten areas within the Earth slow down P
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Evolution of Earth's Atmosphere
waves and stop S waves because their shearing motion cannot be transmitted through a
liquid. Partially molten areas may slow down the P waves and attenuate or weaken S waves.
When seismic waves pass between geologic layers with contrasting seismic velocities (when
any wave passes through media with distinctly differing velocities) reflections, refraction
(bending), and the production of new wave phases (e.g., an S wave produced from a P wave)
often result. Sudden jumps in seismic velocities across a boundary are known as seismic
discontinuities.
b). The Crust
Mohorovic Seismic Discontinuity: Seismic stations within about 200 km of a continental
earthquake (or other seismic disturbance such as a dynamite blast) report travel times that
increase in a regular fashion with distance from the source. But beyond 200 km the seismic
waves arrive sooner than expected, forming a break in the travel time vs. distance curve.
Mohorovic (1909) interpreted this to mean that the seismic waves recorded beyond 200 km
from the earthquake source had passed through a lower layer with significantly higher
seismic velocity.
This seismic discontinuity is now known as the Moho (much easier than "Mohorovicic
seismic discontinuity"). It is the boundary between the felsic/mafic crust with seismic
velocity around 6 km/sec and the denser ultramafic mantle with seismic velocity around 8
km/sec. The depth to the Moho beneath the continents averages around 35 km but ranges
from around 20 km to 70 km. The Moho beneath the oceans is usually about 7 km below the
seafloor (i.e., ocean crust is about 7 km thick).
c). The Mantle
Low Velocity Zone: Seismic velocities tend to gradually increase with depth in the mantle
due to the increasing pressure, and therefore density, with depth. However, seismic waves
recorded at distances corresponding to depths of around 100 km to 250 km arrive later than
expected indicating a zone of low seismic wave velocity. Furthermore, while both the P and S
waves travel more slowly, the S waves are attenuated or weakened. This is interpreted to be a
zone that is partially molten, probably one percent or less (i.e., greater than 99 percent
solid). Alternatively, it may simply represent a zone where the mantle is very close to its
melting point for that depth and pressure that it is very "soft." Then this represents a zone of
weakness in the upper mantle. This zone is called the asthenosphere or "weak sphere."
d). The Core
Gutenberg Seismic Discontinuity / Core-Mantle Boundary: Seismic waves recorded at
increasing distances from an earthquake indicate that seismic velocities gradually increase
with depth in the mantle (exceptions: see Low Velocity Zone and 670 km Discontinuity
above). However, at arc distances of between about 103° and 143° no P waves are recorded.
Furthermore, no S waves are record beyond about 103°. Gutenberg (1914) explained this as
the result of a molten core beginning at a depth of around 2900 km. Shear waves could not
penetrate this molten layer and P waves would be severely slowed and refracted (bent).
Lehman Siesmic Discontinuity / The Inner Core, between 143° and 180° from an
earthquake another refraction is recognized (Lehman, 1936) resulting from a sudden increase
in P wave velocities at a depth of 5150 km. This velocity increase is consistent with a change
from a molten outer core to a solid inner core.
Lecture Delivered by: Dr. Shahid-e-Murtaza