3.2. Weathering and rocks

Question 1

Define weathering

Answer 1

Decomposition and disintegration of rocks in situ

Question 2

Define mechanical weathering

Answer 2

The action of the physical environment which breaks down rock into smaller pieces e.g. Temperature. Mainly associated with block disintegration

Question 3

Define chemical weathering

Answer 3

The action of a chemical found in nature that breaks down rock or changes its substance e.g. carbonic acid. Mainly associated with granular disintegration

Question 4

Define biological weathering

Answer 4

Happens when a bio-agent is present and it aids the weathering process. e.g tree roots penetrate the ground and weakens the rocks or when animal excretion contains minerals which help accelerating the weathering process

Question 5

Types of mechanical weathering

Answer 5

• Freeze thaw - Exfoliation - Salt crystal growth - Pressure release

Question 6

Freeze thaw

Answer 6

• Occurs when water in joints and cracks freezes at 0 degree and expands 10% when it turns to ice
• It exerts a pressure of 2100kg/cm2 when rocks can only withstand 500kg/cm2
• Most effective in environments where moisture is plentiful and there are frequent fluctuations of temperature

Question 7

Exfoliation

Answer 7

• Found in hot desert areas where there is a large diurnal temperature range
• Rocks heat up and expand by day and contract by night
• Rocks is a poor conductor of heat so stresses occur only on the outer layers and cause peeling to occur

Question 8

Salt crystal growth

Answer 8

• Occurs in areas where temperatures fluctuate around 26-28 so sodium sulphate and sodium carbonate expands by 300%
• Or occurs when water evaporates, salt crystals may be left behind, heated up and exerts pressure on rocks
• The more porous the rocks, the more susceptible it is to salt crystallisation

Question 9

Pressure release

Answer 9

Process whereby overlying rocks are removed by erosion thereby causing underlying ones to expand and fracture parallel to the surface

Question 10

Types of chemical weathering

Answer 10

• Carbonation-solution
• Hydrolysis
• Hydration
• Oxidation

Question 11

Carbonation solution

Answer 11

• Occurs on rocks containing calcium carbonate
• Rainfall and dissolved carbon dioxide forms a weak carbonic acid which reacts with the rocks to form calcium bicarbonate which is soluble and removed by percolating water

Question 12

Hydration

Answer 12

The process whereby certain minerals absorb water, expand and change. e.g. clay minerals in shales and mudstones absorb water => larger volume => heavier weight and cause cracking and breakdown of rocks

Question 13

Hydrolysis

Answer 13

• Occurs on rocks containing feldspar. e.g. granite
• Feldspar reacts with acid water and form kaolinite, silicic acid and potassium hydroxyl. The acid and the hydroxyl are removed by the water solution leaving clay kaolin as the end product

Question 14

Oxidation

Answer 14

• Occurs when iron compounds react with oxygen to produce a reddish brown coating known as rusting
• Oxygen comes from the air or the soil

Question 15

Factors influencing weathering

Answer 15

• climate
• rock type and structure
• vegetation

Question 16

How does climate affect weathering?

Answer 16

Question 17

How does geology affect weathering?

Answer 17

They affect weathering due to

• Chemical composition
  
  • Some are susceptible to carbonation solution (calcium)
  • Some are susceptible to hydrolysis (granite
• Joints and bedding planes
  
  • Space for water or salt to enter => freeze thaw or salt crystallisation

Question 18

Difference between porosity and permeability

Answer 18

• Porosity is a measure of how much of a rock’s volume is open space
• Permeability is a measure of the ease with which water can move through a porous rock

Question 19

What is limestone?

Answer 19

• Sedimentary rock that consists mainly of calcium carbonate
• It is formed by the remains of organic matter, notably plants and shells

Question 20

Properties of limestone

Answer 20

• Permeability
• Solubility in rain and groundwater

Question 21

Why are karst features best developed on carboniferous limestome?

Answer 21

• Due to its strength, lower porosity and permeability
• Carboniferous limestone has a distinctive bedding plane and joint pattern => allowing water to percolate and dissolve it

Question 22

2 main types of weathering affecting limestone

Answer 22

• Mechanical: freeze thaw
• Chemical: carbonation

Question 23

How to increase rate of eroding limestone?

Answer 23

• Alluvial erosion - intense solution takes place by water which passes through alluvium and gravels
• Corrosion by mixture - occurs when waters of different hardness mix
• Limestone denudation increases as annual rainfall and runoff increases

Question 24

What are the products of limestone scenery?

Answer 24

• Limestone pavement
• Clints
• Karren or lapies
• Grikes
• Swallow holes (or sinks)
• Stalactites
• Stalagmites
• Resurgent streams
• Caves and tunnels
• Dry vallyes

Question 25

What is limestone pavement?

Answer 25

Large areas of bare exposed rock

Question 26

What are grikes?

Answer 26

Deep grroves formed by acid rainwater running over surface limestone

Question 27

What are clints?

Answer 27

Elevated rocks between grikes

Question 28

What are karren or lapies?

Answer 28

Small-scale solution grooves, only a few centimetre deep, caused by run-off and solution on limestone

Question 29

What are stalactites and stalagmites?

Answer 29

Are both deposits of calcium carbonate. The former hang down from a cave ceiling, the latter are formed at the base of a cave. Rates of deposition are slow. The speed of with which water drips from a cave ceiling have influence - fast drip => stalagmites or slow drip => stalactites

Question 30

What are swallow holes or sinks?

Answer 30

• Depressions in the landscape caused by limestone solution
• They can be formed by:
  
  • the enlargement of grike
  • carbonation
  • fluvial activity
  • the collapse of a cave

Question 31

What are resurgent streams?

Answer 31

Arise when the limestone is underlain by an impermeable rock

Question 32

What are caves and tunnels?

Answer 32

Underground features formed by carbonation solution and erosion by rivers

Question 33

What is a dry valley?

Answer 33

• A river valley without a river, a common feature on chalk limestone
• Formed by:
  
  • Rapid freeze thaw, helped by exposed joints and bedding planes
  • Carbonation was increased owing to increased solubility of CO2 and low temperatures

Question 34

Define tor

Answer 34

an isolated mass of bare rock. Some of the boulders of the mass are connected to the bed rock, some rest on top

Question 35

What is granite?

Answer 35

Igneous rocks that are formed from magma

Question 36

Properties of granite and why does it have those properties (formation)?

Answer 36

• Batholiths formed underground a long time ago. These are very large magma intrusions into the earth’s crust (igneous intrusions)
• Once the magma had cooled it has created a mass of granite rock in amongst the layers of rock which were already there
• The other rock around the granite is eventually eroded and weathered away until only the rock of the batholiths is left standing
• The particular way in which the batholith then weathers creates the jointed (cracked) structure of a tor. Granite is impermeable, so it only really will weather along the joints.

Question 37

What are the 2 theories on how batholith was weathered?

Answer 37

• Linton – 1955– Chemical weathering theory
• Palmer and Nielson- 1962– Mechanical weathering Theory

Question 38

Linton’s chemical weathering theory

Answer 38

• Linton believed chemical weathering (Hydrolysis) created Tors
• He argued that deep sub-surface chemical weathering of the batholith occurred with acidic ground water travelling through the joints of the granite rock
• He believes this occurred during humid conditions in the warm, wet periods in the Tertiary era (before the ice age)
• Decomposition was most rapid along joint planes where the water could trickle
• This led to the formation of core stones (large rounded boulders of rock) under the ground
• Denudation ( removal of the weathered material) then occurred during the ice age in glacial conditions taking the growan (granite broken into pieces) away.
• This left a Tor standing

Question 39

Palmer and Nielson’s mechanical weathering theory

Answer 39

• x=, , ,sss sss s saThey believed mechanical (freeze thaw) weathering was the dominant process in creating tors)
• They believed freeze-thaw activity broke up the rock during the ice age
• Solifluction ( the gradual movement of material down a slope) then removes the debris left from frost shattering.
• Clitter (a collection of rocks, pebbles lying on the ground) is sometimes left at the base of the hill below the tor.
• Their evidence to suggest that Linton is wrong, is that little Kaolin has been found in the joint, which should be left behind after hydrolysis has occurred.