Atmosphere

Earth’s atmosphere is a layer of gases surrounding the planet earth and retained by the earth's gravity. it contains roughly (by molar content/volume) 78% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide, trace amounts of other gases, and a variable amount (average around 1%) of water vapor. this mixture of gases is commonly known as air. the atmosphere protects life on earth by absorbing ultraviolet solar radiation and reducing temperature extremes between day and night.
there is no definite boundary between the atmosphere and outer space. it slowly becomes thinner and fades into space. three quarters of the atmosphere's mass is within 11 km of the planetary surface. in the united states, people who travel above an altitude of 80.5 km (50 statute miles) are designated astronauts. an altitude of 120 km (~75 miles or 400,000 ft) marks the boundary where atmospheric effects become noticeable during re-entry. the karman line, at 100 km (62 miles or 328,000 ft), is also frequently regarded as the boundary between atmosphere and outer space.
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The tectonic plates of the lithosphere on earth.
Earth cutaway from core to exosphere.
The lithosphere from the Greek for "rocky" sphere) is the solid outermost shell of a rocky planet. On the earth, the lithosphere includes the crust and the uppermost mantle which is joined to the crust across the mohorovičić discontinuity. 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 long periods of geologic time, whereas the asthenosphere flows much more readily. as the conductively cooling surface layer of the earth's convection system, the lithosphere thickens over time. it is fragmented into tectonic plates (shown in the picture), which move independently relative to one another. This movement of lithosphere plates is described as plate tectonics.
The concept of the lithosphere as earth’s strong outer layer was developed by barrell, who wrote a series of papers introducing the concept (barrell 1914a-c). the concept was based on the presence of significant gravity anomalies over continental crust, from which he inferred that there must exist a strong upper layer (which he called the lithosphere) above a weaker layer which could flow (which he called the asthenosphere). These ideas were enlarged by Daly (1940), and have been broadly accepted by geologists and geophysicists. Although these ideas about lithosphere and asthenosphere were developed long before plate tectonic theory was articulated in the 1960's, the concepts that strong lithosphere exists and that this rests on weak asthenosphere are essential to that theory.
The division of earth's outer layers into lithosphere and asthenosphere should not be confused with the chemical subdivision of the outer earth into mantle, and crust. All crust is in the lithosphere, but lithosphere generally contains more mantle than crust.
There are two types of lithosphere:
Oceanic lithosphere, which is associated with oceanic crust
Continental lithosphere, which is associated with continental crust
Oceanic lithosphere is typically about 50-100 km thick (but beneath the mid-ocean ridges is no thicker than the crust), while continental lithosphere has a range in thickness from about 40 km to perhaps 200 km; the upper ~30 to ~50 km of typical continental lithosphere is crust. The mantle part of the lithosphere consists largely of predictive. the crust is distinguished from the upper mantle by the change in chemical composition that takes place at the moho discontinuity.
Oceanic lithosphere consists mainly of mafic crust and ultramafic mantle (peridotite) and is denser than continental lithosphere, for which the mantle is associated with crust made of felsic rocks. Oceanic lithosphere thickens as it ages and moves away from the mid-ocean ridge. This thickening occurs by conductive cooling, which converts hot asthenosphere into lithospheric mantle, and causes the oceanic lithosphere to become increasingly dense with age. Oceanic lithosphere is less dense than asthenosphere for a few tens of millions of years, but after this becomes increasingly denser than asthenosphere. The gravitational instability of mature oceanic lithosphere has the effect that at subduction zones the oceanic lithosphere invariably sinks underneath the overriding lithosphere, which can be oceanic or continental. New oceanic lithosphere is constantly being produced at mid-ocean ridges and is recycled back to the mantle at subduction zones. As a result, oceanic lithosphere is much younger than continental lithosphere: the oldest oceanic lithosphere is about 170 million years old, while parts of the continental lithosphere are billions of years old. The oldest parts of continental lithosphere underlie cartons, and the mantle lithosphere there is thicker and less dense than typical; the relatively low density of such mantle "roots of cratons" helps to stabilize these regions (e.g., Jordan, 1978)
Another distinguishing characteristic of the lithosphere is its flow properties. under the influence of the low-intensity, long-term stresses that drive plate tectonic motions, the lithosphere responds essentially as a rigid shell and thus deforms primarily through brittle failure, whereas the asthenosphere (the layer of the mantle below the lithosphere) is heat-softened and accommodates strain through plastic deformation.
Geoscientists can directly study the nature of the sub continental mantle by examining mantle xenoliths brought up in kimberlite and other volcanic pipes. the histories of these xenoliths have been investigated by many methods, including analyses of abundances of isotopes of osmium and rhenium. Such studies have confirmed that mantle lithospheres below some cratons have persisted for periods in excess of 3 billion years, despite the mantle flow that accompanies plate tectonics
Earth’s hydrosphere
the earth's hydrosphere consists of water in all forms: the ocean (which is the bulk of the hydrosphere), other surface waters including inland seas,lakes, and rivers; rain; underground water; ice (as in glaciers and snow); and atmospheric water vapor (as in clouds). the average depth of the oceans is 3,794 m (12,447 ft), more than five times the average height of the continents. the mass of the oceans is approximately 1.35 × 1018 tonnes, or about 1/4400 of the total mass of the earth (ranges reported: 1.347 × 1021 to 1.4 × 1021 kg.)
The abundance of water on earth is a unique feature that distinguishes our "blue planet" from others in the solar system. Approximately 70.8 percent (97% of it being sea water and 3% fresh water of the earth is covered by water and only 29.2 percent is landmass. earth's solar orbit, volcanism, gravity, greenhouse effect, magnetic field and oxygen-rich atmosphere seem to combine to make earth a water planet.
Earth is actually beyond the outer edge of the orbits which would be warm enough to form liquid water. Without some form of a greenhouse effect, earth's water would freeze. Paleontological evidence indicates that at one point after blue-green bacteria (cyanobacteria) had colonized the oceans, the greenhouse effect failed, and earth's oceans may have completely frozen over for 10 to 100 million years in what is called a snowball earth event.
on other planets, such as venus, gaseous water is destroyed (cracked) by solar ultraviolet radiation, and the hydrogen is ionized and blown away by the solar wind. This effect is slow, but inexorable. This is one hypothesis explaining why Venus has no water. Without hydrogen, the oxygen interacts with the surface and is bound up in solid minerals.
in the earth's atmosphere, a tenuous layer of ozone within the stratosphere absorbs most of this energetic ultraviolet radiation high in the atmosphere, reducing the cracking effect. the ozone, too, can only be produced in an atmosphere with a large amount of free diatomic oxygen, and so also is dependent on the biosphere (plants). The magnetosphere also shields the ionosphere from direct scouring by the solar wind.
Finally, volcanism continuously emits water vapor from the interior. earth's plate tectonics recycle carbon and water as limestone rocks are subducted into the mantle and volcanically released as gaseous carbon dioxide and steam. it is estimated that the minerals in the mantle may contain as much as 10 times the water as in all of the current oceans, though most of this trapped water will never be released.
the water cycle describes the methods of transport for water in the hydrosphere. this cycle includes water beneath the earth's surface and in rocks (lithosphere), the water in plants and animals (biosphere), the water covering the surface of the planet in liquid and solid forms, and the water in the atmosphere in the form of water vapor, clouds, and precipitation. Movement of water within the hydrosphere is described by the hydrologic cycle. it is easy to see this motion in rivers and streams, but it is harder to tell that there is this motion in lakes and ponds.
The water in the oceans moves as it is of different temperature and salinity on different locations. Surface waters are also moved by winds, giving rise to surface ocean currents. Warm water is lighter or less dense than cold water which is more dense or heavier and salty water is also more dense than fresh water. The combination of the water's temperature and salinity determines whether it rises to the surface, sinks to the bottom, or stays at some intermediate depth.