|
|
|
| Sunday, 19 June 2005 |
 |
|
 |
Junior Observer |  |
| News Business Features Editorial
|
How the Earth is formed Our planet was most probably created about five billion years ago. It is believed that it was just a ball of molten rock and gases at first, with little order to its structure. The massive amount of heat energy released then is still being radiated, even today, as the planet slowly cools down.
* The core which is approximately 7000 kilometres in diameter (3500 kilometres in radius) and is located at the Earth's centre. * The mantle which surrounds the core and has a thickness of 2900 kilometres. * The crust floats on top of the mantle. It is composed of basalt rich oceanic crust and continental crust rich in granite. The core is a layer rich in iron and nickel that is composed of two layers: the inner and outer cores. The inner core is theorised to be solid with a density (mass per unit of volume) of about 13 grams per cubic centimetre and a radius of about 1220 kilometres. The outer core is liquid and has a density of about 11 grams per cubic centimetre. It surrounds the inner core and has an average thickness of about 2250 kilometres.
Scientific studies suggest that this layer has physical properties that are different from the rest of the upper mantle. The rocks in this upper portion of the mantle are more rigid and stiff because of cooler temperatures and lower pressures. Below the upper mantle is the lower mantle that extends from 670 to 2900 kilometres below the Earth's surface. This layer is hot. The higher pressure in this layer causes the formation of minerals that are different from those of the upper mantle. The lithosphere is a layer that includes the crust and the uppermost portion of the asthenosphere. This layer is about 100 kilometres thick and has the ability to glide over the rest of the upper mantle. Because of increasing temperature and pressure, deeper portions of the lithosphere are capable of plastic flow over geologic time. The lithosphere is also the zone of earthquakes, mountain building, volcanoes, and continental drift. The topmost part of the lithosphere consists of the crust. This material is cool, rigid, and brittle. Two types of crust can be identified: oceanic crust and continental crust. Both these types of crust are less dense than the rock found in the underlying upper mantle layer. Ocean crust is thin; it has a thickness between 5 and 10 kilometres. It is also composed of basalt and has a density of about 3 grams per cubic centimetre. The continental crust is 20 to 70 kilometres thick and composed mainly of lighter granite. The density of continental crust is about 2.7 grams per cubic centimetre. It is thinnest in areas like the Rift Valley of East Africa and in an area known as the Basin and Range Province in the Western United States (centred in Nevada, this area is about 1500 kilometres wide and runs about 4000 kilometres North/South). Continental crust is thickest beneath mountain ranges and extends into the mantle. Both these crust types are composed of numerous tectonic plates that float on top of the mantle. Convection currents within the mantle cause these plates to move slowly across the asthenosphere. One interesting property of the continental and oceanic crust is that these tectonic plates have the ability to rise and sink. This phenomenon, known as isostasy, occurs because the crust floats on top of the mantle like ice cubes in water. When the Earth's crust gains weight due to mountain building or glaciation, it deforms and sinks deeper into the mantle. If the weight is removed, the crust becomes more buoyant and floats higher in the mantle. This process explains recent changes in the height of sea-level in the coastal areas of eastern and northern Canada and Scandinavia. Some locations in these regions of the world have seen sea-levels rise by as much as one metre over the last one hundred years. This rise is caused by isostatic rebound. Both these areas were covered by massive glacial ice sheets about 10,000 years ago. The weight of the ice sheets pushed the crust deeper into the mantle. Now that the ice is gone, these areas are slowly increasing in height to some new steady level.The addition of glacial ice on the Earth's surface causes the crust to deform and sink. When the ice melts, isostatic rebound occurs and the crust rises to its former position before glaciation. A similar process occurs with mountain building and mountain erosion. |
|
||||||||
| News | Business | Features
| Editorial | Security
| Politics | Produced by Lake House |
The mantle is almost 2900 kilometres thick and comprises about 83 per
cent of the Earth's volume. It is composed of several different layers.
The upper mantle exists from the base of the crust downward to a depth of
about 670 kilometres. This region of the Earth's interior is thought to be
composed of peridotite, a rock made up of the minerals olivine and
pyroxene. The top layer of the upper mantle, 100 to 200 kilometres below
surface, is called the asthenosphere. 
