weathering and denudation

What is weathering?

The gradual disintegration of rock in situ due to temperature change chemical reactions or other causes

Name the 3 types of weathering

mechanical, chemical, biological

Mechanical weathering.

The physical breakdown of a rock into smaller pieces without any accompanying change in the original constituents of the rock. There is no change in mineral composition

What is another term for frost action?

Freeze thaw

Where is frost action common?

Where there are freeze thaw cycles eg. High altitudes and latitudes.

What types of rocks are affected by frost action or freeze thaw?

Jointed rocks such as granite and limestone

Explain freeze thaw weathering

When rain falls or ice thaws water enters the cracks within the rocks as temperatures drop it freezes and expands by 9% its volume. As a result pressure is exerted sideways and downwards - thus the crack gets bigger and bigger with successive freeze thaw cycles. The rock eventually crumbles into large blocks. This is known as block disintegration. The boulders then roll down the base to form scree or a talus pile.

In what kind of region does exfoliation occur?

Arid and semi avid regions where there is a general lack of land cover.

What occurs during the day in order for exfoliation to take place?

Temperatures can reach over 45°C. Since rocks are poor conductors of heat the outer layers tend to become very hot and expand along the bedding plane.

What happens during the night in order for exfoliation to take place?

Temperatures sometimes drop to lower than 10°CAs a result, night time frost can occur. The outer layer of the rock then cool and contra as. This continuous expansion and contraction crumbles to form scree and a new layer is exposed once again.

Why is exfoliation given the name the 'onion peeling effect"?

The rock disintegrates layer by layer hence the name

Denudation

Stripping away of the outer layer of the land scape by erosion, weathering and mass wasting.

Erosion

The active breaking down and removal of rock and debris by the agents of erosion

What are the agents?

Running water, ice, wind and wave action.

Wetting and drying

When the sun comes out the surface layer dries out and cracks resulting in the formation of flakes. They tend to he light and easily blown away by the wind. This affects rocks such as clay.

What makes up carbonic acid? H2CO3

CO2 and water

What makes up calcium bicarbonate? Ca(HCO3)2

Carbonic acid and limestone calcium carbonate CaCO3

Describe the process of carbonation.

When rain falls Carbon dioxide dissolves in the rain water to form the weak acid carbonic acid. This reacts with limestone calcium carbonate to form calcium carbonate which is soluble in water.

What type of rock is limestone?

Highly jointed rock

What is meant by the term highly jointed?

Assimilated water circulates deep with the rock resulting in many voids and cavities where carbonation takes place.

What are two characteristics of limestone that make it vulnerable to carbonation?

Highly jointed rocks increase the surface area along assimulated water to circulate deep within the rock.

It is made up of calcium carbonate which reacts with acids thereby making it vulnerable to carbonation

Hydrolysis

Where water acts as a chemical agent altering minerals within rocks. During the process a water molecule splits into hydrogen and hydro-oxides. These ions react with minerals chemical bonds disrupting them forming new and weaker minerals

Give an example of hydrolysis

Granite consists of several minerals like quartz. Feldspar is vulnerable to hydrolysis and is converted to a clay. The quartz is resistant and is left behind in grains. Elements like potassium, sodium or calcium are released from the mineral structure and dissolved into the surrounding water.

Oxidation

Minerals react with oxygen in the presence of water. This results in the addition of oxygen atoms into the molecule. With the absorption of oxygen the molecules get larger.

Give an example of oxidation

Iron changes from a grey ferrous state to a ferric state in reddish brown. Rocks containing iron can become weakened and fall apart.

What two things play an important role in chemical weathering?

Heat and water

Note!

Water acts as the main reagent and heats up as a catalyst to speed up the reaction.

Give an example of where chemical weathering is fast

Climates that are hot and wet example the Amazon + Congo basins in the equatorial regions.

Physical weathering is common where

Temperatures and rainfall are low.

Physical weathering on plants

When a seed gets into a crack, the war month and moisture from the rock will encourage germination- as the roots begin to grow within the crack they expand and pry open the rock- causing it to fall apart..

Physical weathering on animals

Burrowing animals like bunnies burrow into the rock, creating voids that weaken the rocks

Chemical weathering by plants and animals

When plants and animals die they decompose to produce humic acid which can react with rocks like limestone.

Chemical weathering

Break down of rock due to chemical changes in minerals

Biological weathering

This refers to the action of plants and animals breaking up the rocks.