Rocks and weathering: Weathering

Question 1

Freeze thaw & when most effective?

Answer 1

Occurs when water in joints and cracks freezes at 0. Expands by about 10%. Most effective in environments where moisture is plentiful and there are frequent fluctuations above an below freezing point.

Question 2

Salt crystallisation - two types & where most frequent?

Answer 2

1. Where temperatures fluctuate around 26-28, sodium sulphate and sodium carbonate expand by about 300% - creates pressure on cracks. 2. When water evaporates, salt crystals may be left behind. As the temperature rises, the salts expand and exert pressure on rock. Most frequent in hot desert regions where low rainfall and high temperatures cause salts to accumulate just below the surface (may also occur in polar regions - salt in snowflakes)

Question 3

Heating and cooling

Answer 3

May cause disintegration in hot-desert areas where there is a large diurnal temperature range - results in exfoliation

Question 4

Pressure release/dilation

Answer 4

The process whereby overlying rocks are removed by erosion. This causes underlying rocks to expand and fracture parallel to the surface e.g. glaciers. Pseudo-bedding planes form.

Question 5

Vegetation roots

Answer 5

May physically break down rocks.

Question 6

Carbonation-solution & effectiveness

Answer 6

Occurs on rocks with calcium carbonate, such as chalk and limestone. Rainfall combines with dissolved carbon dioxide or organic acid to form a weak carbonic acid. This reacts with the calcium carbonate to form calcium bicarbonate. Effectiveness related to the pH of the water.

Question 7

Hydrolysis

Answer 7

Occurs on rocks with orthoclase feldspar, notably granite. Feldspar reacts wit acid water and forms kaolin, silicic acid and potassium hydroxyl.

Question 8

Hydration

Answer 8

Process whereby certain minerals absorb water, expand and change - for example, anhydrite is changed to gypsum (expands by 0.5%). NB. shales and mudstones can increase in volume by up to 1600% when clay minerals absorb water.

Question 9

Van’t Hoff’s Law

Answer 9

The rate of chemical weathering increases 2-3 times for every increase in temperature of 10 degrees.

Question 10

What is the efficiency of physical weathering affected by? (3)

Answer 10

Critical temperature changes, frequency of cycles, diurnal and seasonal variations in temperature.

Question 11

What factors affect the rate of chemical weathering? (2)

Answer 11

Moisture and temperature

Question 12

Rock structure influence rate and type of weathering due to: (3)

Answer 12

Their.. chemical composition, the nature of cements in sedimentary rock, joints and bedding planes.

Question 13

Controls of weathering (5)

Answer 13

Climate, rock type, rock structure, vegetation, relief.