RELIEF FEATURES

RELIEF FEATURES

Various type of landform of the Earth’s crust classified under three orders of magnitude:

The first order comprises continents and Oceans which are the largest features on the Earth. The configurations of continents & ocean basins haven't remain static such as sea floor spreading and faults. Many ancient features have understandably dissolved in the mists of time. However, geoscientists have pieced together evidences which show that parts of the earth’s crust had been elevated and depressed, extensive stretched of dry lands of the present day were under water, violent episodes of volcanic activity and impact of extra-terrestrial bodies has scared the earth’s face and mountain ranges had been heaved up and worn down.

The second order comprises mountains and plains which have resulted by the action of the internal forces of the earth. Such action of forces emanating from deep within the bowels of the earth includes both orogenic and epeirogenic movements.

The third order features result by the action destructional force and give rise to residual features of peaks, erosional features including valleys and canyons and depositional features like deltas. Also, weathering, streams, waves, wind and glaciers produce relief features of the third order.

First order landforms:

The slope element of the Earth:

-deep sea platform of the ocean

- Continental slope

- Continental shelf

- Continental platform 

The earth's surface is inhomogeneous:

1. 70.8 % of the earth's surface are under water.

2. 29.2 % of the land continents

3. Land and sea are mostly antipodal arranged. Only1.5 % of the surface has land antipodal to land.

4. About two-third of land is in northern hemisphere

5. The deepest parts of oceans aren't always far out from the land and are often located clothe to mountain ranges as in Island arcs

Major topographic element:

1- Ocean ridges:

Wide oceans has brought to light traversal fractured linear ridges extending over a distance of about 64000 K m with width of 2000-4000 Km and rise from 1-3 Km from the ocean floor.

Examples:

The Atlantic & Indian ocean are irregular.
The east pacific ridge is smooth arch.
The mide Atlantic ridge is characterized by 25 – 50 Km wide axial rift valleys.

The Carlsberg ridge is a branch of Indian ocean ridge, branches off towards the north, enters the Gulf of Aden and thence the red sea.
Southerly branch runs through the rift valley system of East Africa.
The East pacific ridge isn't marked by central rift valleys; it’s bordered by faulted steps, ridges & troughs.
Ridges are tectonically unstable marked by shallow earthquakes, high heat flow & volcanic activity

2-Ocean basins:

The ocean basins flanking the mid – ocean ridges are abyssal plains which characterized by hills and sea mounts.
The abyssal plains are tectonically inactive and have gentle gradients of less than 1:1000.They are known to be up to 1000 Km in width below water columns measuring 3-6 Km in depth. They are found to increase in thickness toward the continental slope and shelf.
The sea mount in the bed are spectacular features with width 2-100 Km and rising to dizzy heights of more than 1000 m from the abyssal plain
Sea mounts have sharply pointed and flat tops and they are in the form of hills whose tops are in some cases below a water column of 200 meters.
Guyots: are steep-sided seamounts (12o- 35o) appearing to have ware leved platforms whose submergence is attributed to sea-floor subsidence and rise in water level in post –glacial time.

Continental slope and rise:

The present shoreline of ocean does not limit the extent of continental rocks. The outer edge of the continental shelf, approximately located 0.135 kilometers below the sea level delimits the continental rock.

Continental slope is the part leads into the deep sea, from its outer edge descends at slope up to 6o to depth of two kilometers. Continental sediments in the form of coalescing fans and aprons mark the base of continental slope. In seaward extension of large rivers, submarine canyons mark the continental shelf and slope.

Continental rise is a wide, gentle incline from an ocean basin to a continental slope. A continental rise consists mainly of silts, muds, and sand, and can be several hundreds of miles wide. Although it usually has a smooth surface, it is sometimes crosscut by submarine canyons.

3 - Ocean Trenches, deeps or troughs:
Oceans trenches define the deepest parts of ocean floor. They are known to be variable in length (300 – 5000 Km) and 30 – 100 Km in width, with slopes of 10o – 16o in their deeper parts. The trenches run parallel to island arcs or younger volcanic zones on their seaward side.

4- Island Arcs:

Most ocean trenches on their landward side are marked by parallel accurate festoons of islands. In certain cases, they are topographically and structurally continuous with continental belts of young folded mountains. They are tectonically active zones with profound seismicity.

5- Marginal Sea Basins:

They occur between island arcs and continents. Some of them are 500 to 1000 Km wide and have rugged bottoms with faults, undulations and small sea-mounts characterizing complex histories and different sediment sources. Both tectonically active and inactive basins are known

6- Folded Mountains

It is formed in sediments under the impact of compression. Folding, thrusting and uplift are thrown up as curvilinear mountain chains which may be associated with volcanic activity, deep igneous emplacement and metamorphism.
Broadly, folded mountains may be divided into older and younger groups. The older groups have medium scale elevations and are tectonically more stable than younger group. The younger groups include mountains of highest terrestrial elevations like ALPS and Himalayas.