Geography

The ultimate source of atmospheric energy is in fact heat and light received through space from the Sun. This energy is known as solar insolation. The Earth receives only a tiny fraction of the total amount of Sun’s radiations. Only two billionths or two units of energy out of 1,00,00,00,000 units of energy radiated by the sun reaches the earth’s surface due to its small size and great distance from the Sun. The unit of measurements of this energy is Langley (Ly). On an average the earth receives 1.94 calories per sq. cm per minute (2 Langley) at the top of its atmosphere.

Incoming solar radiation through short waves is termed as insolation. The amount of insolation received on the earth’s surface is far less than that is radiated from the sun because of the small size of the earth and its distance from the sun. Moreover water vapour, dust particles, ozone and other gases present in the atmosphere absorb a small amount of insolation.

The amount of insolation received on the earth’s surface is not uniform everywhere. It varies from place to place and from time to time. The tropical zone receive the maximum annual insolation. It gradually decreases towards the poles. Insolation is more in summers and less in winters.
The following factors influence the amount of insolation received.
(i) The angle of incidence:-The angle formed by the sun’s ray with the tangent of the earth’s circle at a point is called
angle of incidence. It influences the insolation in two ways. First, when the sun is almost overhead, the rays of the sun are vertical. The angle of incidence is large hence, they are concentrated in a smaller area, giving more amount of insolation at that place. If the sun’s rays are oblique, angle of incidence is small and sun’s rays have to heat up a greater area, resulting in less amount of insolation received there. Secondly, the sun’s rays with small angle, traverse more of the atmosphere, than rays striking at a large angle. Longer the path of sun’s rays, greater is the amount of reflection and absorption of heat by atmosphere. As a result the intensity of insolation at a place is less.
(ii) Duration of the day. (daily sunlight period) :-The duration of day is controlled partly by latitude and partly by the season of the year. The amount of insolation has close relationship with the length of the day. It is because insolation is received only during the day. Other conditions remaining the same, the longer the days the greater is the amount of insolation. In summers, the days being longer the amount of insolation received is also more. As against this in winter the days are shorter the insolation received is also less. On account of the inclination of the earth on its axis at an angle of 23 ½ , rotation and revolution, the duration of the day is not same everywhere on the earth. At the equator there is 12 hours day and night each throughout the year. As one moves towards poles duration of the days keeps on increasing or decreasing. It is why the maximum insolation is received in equatorial areas.

(iii) Transparency of the atmosphere.Transparency of the atmosphere: Transparency of the atmosphere also determines the amount of insolation reaching the earth’s surface. The transparency depends upon cloud cover, its thickness, dust particles and water vapour, as they reflect, absorb or transmit insolation. Thick clouds hinder the insolation to reach the earth while clear sky helps it to reach the surface. Water vapour absorb insolation, resulting in less amount of insolation reaching the surface.

Heat Budget

Energy emitted by the Earth’s climate system tends to maintain a balance with solar energy coming into the system. This balance, known as the radiation budget, allows the Earth to maintain the moderate temperature range essential for life as we know it.
There is positive radiation balance between 35°S and 40°N, which drives the weather systems. Ocean currents even out the difference
When incoming short-wave solar radiation (Figure 3), known as insolation, enters the Earth’s climate system, a portion of it is absorbed at the Earth’s surface, causing the surface to heat up. Some of the absorbed energy is then radiated outward in the form of long-wave infrared radiation. Cloud layers trap some of the radiation from the Earth’s surface, and then emit long-wave radiation, both outward and back to the surface. The temperature of the Earth’s surface is about 33°C higher due to long-wave radiation contribution from the atmosphere .
The amount of radiation emitted by the Earth’s surface that makes it back to space is the result of many interrelated influences, such as the amount of cloud cover, cloud heights, characteristics of cloud droplets, amount and distribution of water vapor and other greenhouse gases, land features, surface temperature, and the transparency of the atmosphere. In the warm tropical areas, low values of outgoing longwave radiation (OLR) correspond to large amounts of high, cold clouds while high values of OLR correspond to relatively clear areas or cloudy areas with low, warm clouds. In the extra-tropics OLR values typically decrease with decreasing temperature.

Let us suppose that the total heat (incoming solar radiation) received at the top of the atmosphere is 100 units (see fig. 10.2) Roughly 35 units of it are reflected back into space even before reaching the surface of the earth. Out of these 35 units, 6 units are reflected back to space from the top of the atmosphere, 27 units reflected by clouds and 2 units from the snow and ice covered surfaces.
Out of the remaining 65 units (100-35), only 51 units reach the earth’s surface and 14 units are absorbed by the various gases, dust particles and water vapour of the atmosphere.
The earth in turn radiates back 51 units in the form of terrestrial radiation. Out of these 51 units of terrestrial radiation, 34 units are absorbed by the atmosphere and the remaining 17 units directly go to space. The atmosphere also radiates 48 units (14 units of incoming radiation and 34 units of outgoing radiation absorbed by it) back to space. Thus 65 units of solar radiation entering the atmosphere are reflected back into the space. This account of incoming and outgoing radiation always maintains the balance of heat on the surface of the earth.

Beginning of the Universe started about 13.6 billion years ago,when the Big Bang created the universe from a point source.
During this process, light elements, like H, He, Li, B, and Be formed. From this point in time, the universe began to expand and has been expanding ever since.
Concentrations of gas and dust within the universe eventually became galaxies consisting of millions of stars.
Within the larger stars, nuclear fusion processes eventually created heavier elements, like C, Si, Ca, Mg, K, and Fe.
Stars eventually collapse and explode during an event called a supernova. During a supernova, heavier elements, from Fe to U, are formed. (See figure 1.9 in your text).
Throughout galaxies clusters of gas attracted by gravity start to rotate and accrete to form stars and solar systems. For our solar system this occurred about 4.6 billion years ago.
The ball at the center grows dense and hot, eventually nuclear fusion reactions start and a star is born (in our case, the sun).
Rings of gas and dust orbiting around the sun eventually condenses into small particles. These particles are attracted to one another and larger bodies called planetismals begin to form.
Planetesimals accumulate into a larger mass. An irregularly-shaped proto-Earth develops.
The interior heats and becomes soft. Gravity shapes the Earth into a sphere. The interior differentiates into a nickel-iron core, and a stony (silicate) mantle.
Soon, a small planetoid collides with Earth. Debris forms a ring around the Earth.The debris coalesces and forms the Moon.
The atmosphere develops from volcanic gases. When the Earth becomes cool enough, moisture condenses and accumulates, and the oceans are born.

The uppermost outer solid and rigid layer of the earth is called crust. Its thickness varies considerably. It is as little as 5 km thick beneath the oceans at some places but under some mountain ranges it extends upto a depth of 700km. Below the crust denser rocks are found, known as mantle crust. This upper part of mantle upto an average depth of 100 km from the surface is solid. This solid mantle plus upper crust form a comparatively rigid block termed as lithosphere. Mantle is partially molten between 100 to 250 km depth. This zone is said to be asthenosphere, also known as Mohr discontinuity, a simplification of Mohorovicic, the name of the seismologist who discovered it.
The lithosphere is broken into several blocks. These blocks are known as plates, which are moving over asthenosphere. There are seven major plates.

While the continents do indeed appear to drift, they do so only because they are part of larger plates that float and move horizontally on the upper mantle asthenosphere. The plates behave as rigid bodies with some ability to flex, but deformation occurs mainly along the boundaries between plates.

The plate boundaries can be identified because they are zones along which earthquakes occur.Plate interiors have much fewer earthquakes.

There are three types of plate boundaries:

1. Divergent Plate boundaries, where plates move away from each other.
2. Convergent Plate Boundaries, where plates move toward each other.
3. Transform Plate Boundaries, where plates slide past one another.

Divergent Plate Boundaries

These are oceanic ridges where new oceanic lithosphere is created by upwelling mantle that melts, resulting in basaltic magmas which intrude and erupt at the oceanic ridge to create new oceanic lithosphere and crust. As new oceanic lithosphere is created, it is pushed aside in opposite directions. Thus, the age of the oceanic crust becomes progressively older in both directions away from the ridge.

Because oceanic lithosphere may get subducted, the age of the ocean basins is relatively young. The oldest oceanic crust occurs farthest away from a ridge. In the Atlantic Ocean, the oldest oceanic crust occurs next to the North American and African continents and is about 160 million years old (Jurassic)

. In the Pacific Ocean, the oldest crust is also Jurassic in age, and occurs off the coast of Japan.

Because the oceanic ridges are areas of young crust, there is very little sediment accumulation on the ridges. Sediment thickness increases in both directions away of the ridge, and is thickest where the oceanic crust is the oldest. Knowing the age of the crust and the distance from the ridge, the relative velocity of the plates can be determined.

Relative plate velocities vary both for individual plates and for different plates.

Sea floor topography is controlled by the age of the oceanic lithosphere and the rate of spreading.

If the spreading rate (relative velocity) is high, magma must be rising rapidly and the lithosphere is relatively hot beneath the ridge. Thus for fast spreading centers the ridge stands at higher elevations than for slow spreading centers. The rift valley at fast spreading centers is narrower than at slow spreading centers. As oceanic lithosphere moves away from the ridge, it cools and sinks deeper into the asthenosphere. Thus, the depth to the sea floor increases with increasing age away from the ridge.

Convergent Plate Boundaries

When a plate of dense oceanic lithosphere moving in one direction collides with a plate moving in the opposite direction, one of the plates subducts beneath the other. Where this occurs an oceanic trench forms on the sea floor and the sinking plate becomes a subduction zone. The Wadati-Benioff Zone, a zone of earthquakes located along the subduction zone, identifies a subduction zone. The earthquakes may extend down to depths of 700 km before the subducting plate heats up and loses its ability to deform in a brittle fashion.

As the oceanic plate subducts, it begins to heat up causing the release water of water into the overlying mantle asthenosphere. The water reduces the melting temperature and results in the production of magmas. These magmas rise to the surface and create a volcanic arc parallel to the trench. If the subduction occurs beneath oceanic lithosphere, an island arc is produced at the surface (such as the Japanese islands, the Aleutian Islands, the Philippine islands, or the Caribbean islands

Transform Plate Boundaries

Where lithospheric plates slide past one another in a horizontal manner, a transform fault is created. Earthquakes along such transform faults are shallow focus earthquakes.

Most transform faults occur where oceanic ridges are offset on the sea floor. Such offset occurs because spreading takes place on the spherical surface of the Earth, and some parts of a plate must be moving at a higher relative velocity than other parts One of the largest such transform boundaries occurs along the boundary of the North American and Pacific plates and is known as the San Andreas Fault. Here the transform fault cuts through continental lithosphere

Triple Junctions occur at points where thee plates meet.

Hot Spots

Areas where rising plumes of hot mantle reach the surface, usually at locations far removed from plate boundaries are called hot spots. Because plates move relative to the underlying mantle, hot spots beneath oceanic lithosphere produce a chain of volcanoes. A volcano is active while it is over the vicinity of the hot spot, but eventually plate motion results in the volcano moving away from the plume and the volcano becomes extinct and begins to erode.

Because the Pacific Plate is one of the faster moving plates, this type of volcanism produces linear chains of islands and seamounts, such as the

• Hawaiian – Emperor chain, the Line
• Islands, the Marshall-Ellice Islands,
• and the Austral seamount chain