Orders of Relief

The topographical features that appears on the surface of the earth are actually the products of endogenic and exogenous processes.

What is Relief?

Relief is known as the difference between two elevation points on the surface of the earth. If a surface is relatively flat then it is considered to be having low relief, while mountainous regions are said to be having high relief.

Classification or Orders of Relief

Relief can be classified as follows:

• First order relief
• Second order relief
• Third order relief

First order relief: Entire lithosphere which consists of continental crust as well as the oceanic crust comes under first order relief.

Continental crust have lower density than the oceanic crust and is made up of granitic rock consisting of Silica and Aluminium predominantly. While oceanic crust is made up of basaltic rocks and majorly consists of Silica and Magnesium.

First order relief basically reflects the original cooling and solidification of Earth's crust at the time of its formation. Continental shields and contracted Panthalassa (largest ocean known as Pacific) are considered to be included in first order relief.

Second order relief: This type of relief basically consists of all endogenic forces which originates from within earth's crust/surface. Endogenic forces are responsible for developing variations on the surface of the earth, hence they are also known as variability developers.

Endogenic processes are classified as follows:

• Diastrophism aka mountain building forces is responsible for the development of block as well as fold mountains.
• Volcanism/Earth quakes.

Mountains are the best example of product of endogenic processes on continental crust. While submarine ridges (formed due to solidification of magma) and trenches forms the best example in oceanic crust.

Third order relief: This type of relief basically consists of exogenic forces. Exogenic forces are those forces which originates on the surface of the earth.

All exogenic forces are also known as levellers because they are responsible for levelling (bringing variations) the surface of the earth. The levelling process includes erosion, transportation and deposition and as a result valleys (due to erosion) and deltas (due to deposition) are formed. Following are the agents which carry out the whole levelling process:

• Running water.
• Wind.
• Underground water.
• Glacier.
• Sea waves.

Important note: All the above mentioned levelling agents don't work after the shoreline. This means that third order relief is only confined to the continental crust.

However continental margin (consisting of continental rise, continental slope and continental shelf) may experience third order relief features because of change in mean sea level due to change in climatic conditions or region specific endogenic modifications.

Image credit: http://en.wikipedia.org/wiki/File:Continental_shelf.png