Weathering and Erosion - Class 7 Geography

Weathering - Weathering is the breaking up of rocks on or near the Earth’s surface.

Types of Weathering

Physical weathering -It is the breaking or fracturing of rocks due to temperature change, frost action, salt formation in drier areas. It is helped by the growth of plants in rocks and the burrowing of animals. The rock simply breaks and there is no decay.

Chemical weathering -It happens when there is a chemical change in the rock structure and new minerals are formed. This sort of weathering is more common in warm, moist climates. Chemical weathering causes rock decay

Erosion -Erosion is the process of wearing away of the land by natural agents such as running water, ice, wave action and wind.

Denudation -Erosion removes the soil and rock debris and ceases them away leading to a reduction in elevation and in relief of landforms and of landscapes.

Difference between weathering and erosion

Weathering	Erosion
It is the breaking of rocks	It is the displacement of rock debris, soil
It is caused by physical and chemical factors, plants growing on rocks, burrowing done by animals	It is caused by running water, glaciers, waves, wind
The weathered materials are not displaced	The eroded materials are displaced

Difference between erosion and denudation

Erosion	Denudation
Erosion is the transport of soil and rocks from one location to another	Denudation is the sum of processes that result in the lowering of the Earth's surface, including erosion

Types of Erosion

Aggradation -Aggradation is the process by which a landform is built by the deposition and accumulation of material. Example – An alluvial plain.

Degradation -Degradation is the process of lowering a surface by erosion and the removal of rock water. Example - Pedestal rock.

Types of Erosional Agents

Running Water - Includes rivers or streams.

Three steps in the life of a river

Youth - In its youth, the river flows very fast through narrow channels and erodes the sides forming V-shaped valleys, gorges or canyons. It is generally seen in mountainous areas where the slope of the land is steep. There is no depositionat this stage.

Middle stage-In the middle stage the slope of the land becomes gentler and the river also slows down. Here, the river starts depositing the heavy load that it carries. Its valleys now become broader and wider also the river starts winding to go around the material it has deposited. Here both erosionand depositionoccurs.

Meanders-The bends of the river are called meanders.

Floodplains-The depositional plain formed in between the meanders are called floodplains.

Last stage-In the last stage the river becomes even slower with the decreasing slope of land. The river valley is very broad at this stage. It is here that river forms a delta at the mouth of the seas. The river deposits all the remaining debris it contains here and then splits into small streams, avoiding the deposition to reach the sea. This stage is marked by depositiononly.

Delta-Deltas are wetlands that form as rivers empty their water and sediment into another body of water.The Ganga-Brahmaputra Delta is the biggest delta in the world.

Estuary-The part of river where it joins the sea and where fresh water of river and saline water of sea is mixed is called Estuary.

Distributary-Distributary channel is a stream that carries water away from the main river channel and distributes it to other area.

Tributary- Tributaries are small streams of water that join the mainstream river to increase its water supply.

Stream-A small narrow river.

Wind - Wind is another erosional agent, wind picks up loose material in its path and rolls them along the ground.

Sand Dune-The transportation and deposition of soil by wind in desert causes soil to pile and create a hill of soil, this is known as sand dune

Barchan-The sand dune keeps changing shape when wind blows, when sand dune takes the shape of crescent it is known as barchan

Loess-In mid-latitude areas, the wind has transported silt(rock debris) to form large surfaces with it. This is called loess

Glaciers - Glaciers are also a cause of erosion

Glaciers-Large blocks of ice that move forward under their own weight are called glaciers.These can be found where snowfall is high, such as high mountains like the Himalayas, or in the polar regions like the Antarctica.99% of glaciers are found in Antarctica and Greenland.

Moraine-Mountain glaciers carry debris of rocks in their lower parts, and the accumulation of such debris is called moraine.

Glacial Troughs-The glaciers form steep-sided wide valleys which seem to be U-shaped. These valleys are called Glacial troughs

Cirques-There are smaller troughs, with steep walls which are called cirques.

Tarns-In many glaical troughs some lakes are formed, these lakes are known as tarns

Fjords-Glaciers also form narrow deep valleys with steep sides called fjords

Icebergs-When glaciers break and huge blocks of ice drift into the sea then those huge blocks of ice are known as icebergs.

Waves - Waves are another factor that cause erosion

Waves - Waves are large bodies of water that form a ridge and move forward on the water surface by the action of wind. The waves shape the landform on the coasts. Waves carry out deposition and erosion

Arch - When waves cut through a cliff then archs are formed

Stack - When the arch breaks down, the rock column that remains is called the stack

Marine cliffs-When the wave breaks on the coastline, many tons of water fall on it. If the rock is weak, the coasline will be eroded rapidly and these leads to the formation of marine cliffs

Sea caves-The points of weakness in the cliffs get cut back by the waves and deep gaps or sea caves are formed in those cliffs

Beach-When waves bring along with them sand and when lots of sand deposits to form a thick surface then such thick surfaces of sand are known as beaches