3.2. Weathering and rocks Flashcards
1
Q
Define weathering
A
Decomposition and disintegration of rocks in situ
2
Q
Define mechanical weathering
A
The action of the physical environment which breaks down rock into smaller pieces e.g. Temperature. Mainly associated with block disintegration
3
Q
Define chemical weathering
A
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
4
Q
Define biological weathering
A
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
5
Q
Types of mechanical weathering
A
- Freeze thaw - Exfoliation - Salt crystal growth - Pressure release
6
Q
Freeze thaw
A
- 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
7
Q
Exfoliation
A
- 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
8
Q
Salt crystal growth
A
- 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
9
Q
Pressure release
A
Process whereby overlying rocks are removed by erosion thereby causing underlying ones to expand and fracture parallel to the surface
10
Q
Types of chemical weathering
A
- Carbonation-solution
- Hydrolysis
- Hydration
- Oxidation
11
Q
Carbonation solution
A
- 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
12
Q
Hydration
A
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
13
Q
Hydrolysis
A
- 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
14
Q
Oxidation
A
- Occurs when iron compounds react with oxygen to produce a reddish brown coating known as rusting
- Oxygen comes from the air or the soil
15
Q
Factors influencing weathering
A
- climate
- rock type and structure
- vegetation
16
Q
How does climate affect weathering?
A
17
Q
How does geology affect weathering?
A
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
18
Q
Difference between porosity and permeability
A
- 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
19
Q
What is limestone?
A
- Sedimentary rock that consists mainly of calcium carbonate
- It is formed by the remains of organic matter, notably plants and shells
20
Q
Properties of limestone
A
- Permeability
- Solubility in rain and groundwater
21
Q
Why are karst features best developed on carboniferous limestome?
A
- 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
22
Q
2 main types of weathering affecting limestone
A
- Mechanical: freeze thaw
- Chemical: carbonation
23
Q
How to increase rate of eroding limestone?
A
- 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
24
Q
What are the products of limestone scenery?
A
- Limestone pavement
- Clints
- Karren or lapies
- Grikes
- Swallow holes (or sinks)
- Stalactites
- Stalagmites
- Resurgent streams
- Caves and tunnels
- Dry vallyes
25
What is limestone pavement?
Large areas of bare exposed rock
26
What are grikes?
Deep grroves formed by acid rainwater running over surface limestone
27
What are clints?
Elevated rocks between grikes
28
What are karren or lapies?
Small-scale solution grooves, only a few centimetre deep, caused by run-off and solution on limestone
29
What are stalactites and stalagmites?
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
30
What are swallow holes or sinks?
* 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
31
What are resurgent streams?
Arise when the limestone is underlain by an impermeable rock
32
What are caves and tunnels?
Underground features formed by carbonation solution and erosion by rivers
33
What is a dry valley?
* 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
34
Define tor
an isolated mass of bare rock. Some of the boulders of the mass are connected to the bed rock, some rest on top
35
What is granite?
Igneous rocks that are formed from magma
36
Properties of granite and why does it have those properties (formation)?
* 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.
37
What are the 2 theories on how batholith was weathered?
* Linton – 1955– Chemical weathering theory
* Palmer and Nielson- 1962– Mechanical weathering Theory
38
Linton's chemical weathering theory
* 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
39
Palmer and Nielson's mechanical weathering theory
* 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.
