3.2. Weathering

Question 1

Weathering

Answer 1

The process of breaking down rocks ‘in situ’ (without movement)

Question 2

Physical Weathering (mechanical)

Answer 2

Rocks are broken down into smaller pieces without changing their composition

Question 3

Chemical Weathering

Answer 3

Rocks break down as their minerals change in composition

Question 4

Types of Physical Weathering

Answer 4

1) Freeze Thaw weathering
2) Heating / cooling (exfoliation or onion skin weathering)
3) Pressure release (dilatation)
4) Salt crystal Growth
5) Biological weathering (vegetation root action + animals)

Question 5

Freeze Thaw weathering

Answer 5

PHYSICAL WEATHERING
Most effective in environments where moisture is plentiful and there are frequent fluctuations above and below freezing point
- Occurs when water enters joints in rocks and freezes, resulting in 9% expansion, exerting outward pressure in the crack and causing the crack to expand.
- Over time, stresses are created within the rock, causing the rock to weaken and break up

Question 6

Heating / cooling (exfoliation or onion skin weathering)

Answer 6

PHYSICAL WEATHERING
Occurs in areas with a wide range in diurnal temperatures. Presence of moisture will speed up the expansion.
- Different minerals expand and contract at different temperatures - can cause granular disintegration in rocks composed of different materials
- As rocks are poor conductors of heat, stresses (expanding and contracting) occur only in the outer layers. These stresses are released as the outer layers peel off in thin sheets - exfoliation occurs

Question 7

Pressure release (dilatation)

Answer 7

PHYSICAL WEATHERING

• Process whereby overlying rocks are removed by erosion
• This causes underlying rocks to expand and fracture parallel to the surface
• Over time, sheets of rock break away from the exposed rock along the fractures by sheeting.
• This leads to formation of granite domes.

Question 8

Salt Crystal Growth

Answer 8

PHYSICAL WEATHERING

• In areas where temperatures fluctuate around 26-28 degrees celsius, where sodium sulfate and sodium carbonate expand by about 300% creating pressure on joins, forcing them to crack
• When saline water evaporates, salt crystals may be left behind. As temperature rises, the salts expand and exert pressure on rock
• An example of this can be seen in the honeycombed stones in sea walls

Question 9

Biological weathering (vegetation root action + animals)

Answer 9

PHYSICAL WEATHERING

• Roots of plants can widen cracks
• Burrowing animals can mix soil / sediment particles and allow water to penetrate more quickly and deeply, speeding other weathering processes.

Question 10

Types of Chemical Weathering

Answer 10

1) Carbonation
2) Hydration
3) Hydrolysis
4) Oxidation
5) Biological (Chelation)

Question 11

Carbonation

Answer 11

CHEMICAL WEATHERING

• Occurs on rocks that contain calcium carbonate such as limestone and chalk.
• Rain combines with carbon dioxide forming carbonic acid (ACID RAIN).
• This carbonic acid (acid rain) dissolves the calcium carbonate (limestone) and forms calcium bicarbonate
• Calcium bicarbonate forms stalactites and stalagmites in caves where carbonation occurs.
• “Gnarled rock” is a formation of limestone chemically weathered by rain.

Question 12

Hydration

Answer 12

CHEMICAL WEATHERING

• Process whereby certain minerals absorb water into their structure, expand and change
• As the rock minerals take up water, the increased volume separates chemical parts within the rock
• For example, when anhydrite absorbs water to become gypsum - expands 0.5%

Question 13

Hydrolysis

Answer 13

CHEMICAL WEATHERING
Occurs mostly in areas with plenty of water and high temperatures. Common in hot, humid tropics.
- During hydrolysis, water gets added into the chemical structure of a mineral, which turns the mineral into a new one
- Hydrolysis adds a water molecule, breaking bonds
- Important for weathering of silicate materials
- Hydrolysis changes crystals into clay minerals. etc. Feldspar (like in granite) –> Kaolinite

Question 14

Oxidation

Answer 14

CHEMICAL WEATHERING
Occurs in permeable soil areas
- Occurs when iron compounds react with oxygen to produce a reddish-brown coating
- In this way, dissolved oxygen in the soil or the atmosphere affects iron minerals
- FeO is oxidised to Fe2O3 - very acidic and gives rocks and soils a red colour

Question 15

Biological Weathering (chelation)

Answer 15

CHEMICAL WEATHERING

• Biological process where organisms produce organic substances, known as chelates, that have the ability to decompose minerals and rocks by the removal of metallic cations
• Organic acids (from living plants) and Humic acids (from decomposing plants) can wear down the rock

Question 16

Factors affecting rate of weathering

Answer 16

1) Climate
2) Geology
3) Vegetation
4) Relief

Question 17

Climate (Factors affecting rate of weathering)

Answer 17

Peltier’s diagram shows how weathering is related to moisture availability and average annual temperature

Question 18

Geology (Factors affecting rate of weathering)

Answer 18

Rock type and rock structure influence the rate and type of weathering in many ways depending on:

• chemical composition
• the nature of cements in sedimentary rock
• joints and bedding planes - join patterns act as lines of weakness creating differential resistance within same rock type

Question 19

Vegetation (Factors affecting rate of weathering)

Answer 19

• The presence of vegetation can increase weathering systems through organic acids and through the growth of root systems

Question 20

Relief (Factors affecting rate of weathering)

Answer 20

• On very steep slopes weathered material may be removed quickly but scree slopes can develop, which protect the rock-face from further weathering
• On very flat slopes weathered material will not be removed and fresh rock faces will not be exposed
• On intermediate slopes, some weathered material will be removed, exposing fresh rock faces