In order to understand the geography of the external landforms of the earth, it is essential that we have some idea of what lies within the earth's crust. It is not possible to know exactly how the earth was formed about 4500 million years ago, but from the evidence of volcanic eruptions, earthquake waves, deep-mine operations and crutal borings the following facts are quite
clear.
The earth is made up of several concentric layers. The outer layer is the earth's crust - the lithosphere - which comprises two distinct parts. The upper part consists of granite rocks and forms the continents. Its main mineral constituents are silica and alumina so it is collectively referred to as the sial. It has an average density of 2.7. The lower part is a continuous zone of denser basaltic rocks forming the ocean floors, comprising mainly silica, iron and magnesium. It is therefore called sima and has an average density of 3.0. The sial and sima together form the earth's crust which varies in thickness from only 5-6 km beneath the oceans to as much as 48 km under some parts of the continents. Since the sial is lighter than the sima, the continents can be said to be 'floating' on a sea of denser sima.
Immediately beneath the crust or lithosphere is the mantle or mesophere about 2900 km thick, composed mainly of very dense rocks rich in olivine. The interior layer is the core or barysphere, 3476 km in radius, and is made up mainly of iron (Fe) with some nickel and is called nife. The temperature here is estimated to be as high as 1927 degrees celcius, and the core is subject to extremely high pressure. Under such conditions, the core could be expected to be in a liquid state. But recent studies through earthquarke waves have suggested that the innermost part of the core is probably a crystalline or solid mass.
Parts of the earth's crust are immersed by oceans, seas, lakes and rivers. These form the hydrosphere. The oceans, seas, lakes and rivers cover about three-quarters of the earth's surface.
Extending skywards for over 24 kilometres, the earth is enveloped by a mass of gases which make up the atmosphere. The atmosphere is essentially a mixture of nitrogen and oxygen with smaller quantities of water vapour, carbon dioxide, and inert gases such as argon. Geographically, it is important as the medium of climate and weather phenomena such as wind, cloud, rain and snow.
Of no less importance than the physical zones is the bioshere, the sphere of life, which lies at the interfaces between the lithosphere, the hydrosphere and the atmosphere. The great equatorial forests, the temperate forests, and the tropical and temperate grass lands, with their countless swarms of animals and insects, as well as the tangles of seaweed, coral reefs and shoals of fishes are part of the sphere of life known as the biosphere. We can add to these the myraids of minute organisms such as bacteria and other microscopic plants and animals which are present in every cubic inch of air and water and soil.
There is considerable evidence to support the view that the continents are 'floating' on the more dense underlying material, and that certain parts of the continents are recovering from effects such as the removal of the ice caps following the last Ice Age. In Hong Kong there is evidence to support the view that there is a slow uplift taking place in the north-east of the Colony around the Deep Bay area, while in the Shatin Valley and other areas around the Tolo Channel the land is being slowly depressed or submerged.
The land surface of the earth is about 30 per cent of the total area, and the land-masses are separated by ocean basins. Various reasons have been put forward to attempt to explain the distribution of these land masses. There are certain areas, like most of Africa, which are made up of ancient crystalline rocks. These areas are known as shields. There is reason to believe that they had slowly drifted apart. This theory of continental drift was first put forward by Wegener in the nineteenth century and is still supported by modern research.
Most of the more recent land areas of sedimentary rocks have been formed in the basins between these ancient shield areas.
clear.
The earth is made up of several concentric layers. The outer layer is the earth's crust - the lithosphere - which comprises two distinct parts. The upper part consists of granite rocks and forms the continents. Its main mineral constituents are silica and alumina so it is collectively referred to as the sial. It has an average density of 2.7. The lower part is a continuous zone of denser basaltic rocks forming the ocean floors, comprising mainly silica, iron and magnesium. It is therefore called sima and has an average density of 3.0. The sial and sima together form the earth's crust which varies in thickness from only 5-6 km beneath the oceans to as much as 48 km under some parts of the continents. Since the sial is lighter than the sima, the continents can be said to be 'floating' on a sea of denser sima.
Immediately beneath the crust or lithosphere is the mantle or mesophere about 2900 km thick, composed mainly of very dense rocks rich in olivine. The interior layer is the core or barysphere, 3476 km in radius, and is made up mainly of iron (Fe) with some nickel and is called nife. The temperature here is estimated to be as high as 1927 degrees celcius, and the core is subject to extremely high pressure. Under such conditions, the core could be expected to be in a liquid state. But recent studies through earthquarke waves have suggested that the innermost part of the core is probably a crystalline or solid mass.
Parts of the earth's crust are immersed by oceans, seas, lakes and rivers. These form the hydrosphere. The oceans, seas, lakes and rivers cover about three-quarters of the earth's surface.
Extending skywards for over 24 kilometres, the earth is enveloped by a mass of gases which make up the atmosphere. The atmosphere is essentially a mixture of nitrogen and oxygen with smaller quantities of water vapour, carbon dioxide, and inert gases such as argon. Geographically, it is important as the medium of climate and weather phenomena such as wind, cloud, rain and snow.
Of no less importance than the physical zones is the bioshere, the sphere of life, which lies at the interfaces between the lithosphere, the hydrosphere and the atmosphere. The great equatorial forests, the temperate forests, and the tropical and temperate grass lands, with their countless swarms of animals and insects, as well as the tangles of seaweed, coral reefs and shoals of fishes are part of the sphere of life known as the biosphere. We can add to these the myraids of minute organisms such as bacteria and other microscopic plants and animals which are present in every cubic inch of air and water and soil.
There is considerable evidence to support the view that the continents are 'floating' on the more dense underlying material, and that certain parts of the continents are recovering from effects such as the removal of the ice caps following the last Ice Age. In Hong Kong there is evidence to support the view that there is a slow uplift taking place in the north-east of the Colony around the Deep Bay area, while in the Shatin Valley and other areas around the Tolo Channel the land is being slowly depressed or submerged.
The land surface of the earth is about 30 per cent of the total area, and the land-masses are separated by ocean basins. Various reasons have been put forward to attempt to explain the distribution of these land masses. There are certain areas, like most of Africa, which are made up of ancient crystalline rocks. These areas are known as shields. There is reason to believe that they had slowly drifted apart. This theory of continental drift was first put forward by Wegener in the nineteenth century and is still supported by modern research.
Most of the more recent land areas of sedimentary rocks have been formed in the basins between these ancient shield areas.
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