Weathering Flashcards
(27 cards)
Igneous rock
Forms from magma or lava solidification
Most resistant type of rock
Hard
No Layers
Granite: intrusive - slow magma cooling
Basalt / Obsidian: extrusive - rapid lava cooling
Sedimentary rock
Forms from sediment compaction
Crumbly
Layered
Have joints (natural weaknesses in rock) and bedding planes (horizontal lines of weakness)
Chemically weak due to calcium carbonate (especially chalk and limestone)
Sandstone: clastic - compacted broken rocks
Limestone: chemical - compacted dissolved minerals
Coal: organic - compacted biogenic matter
Metamorphic rock
Forms by transformation of other rocks
Relatively hard
May or may not have layers
Slate: foliated - has layers
Marbe: non-foliated - has no layers
Weathering
Natural process of decomposition and disintegration of rock in situ by climate and other natural factors
In situ means occuring in the place where the rock is (rock is weakened and broken down but not removed)
Mechanical / Physical weathering
Exposed rocks, limited vegetation and large diurnal temperature range leads to expansion of rocks close to surface during day and contraction at night due to cooling
Sets up stresses in the rock
Salt crystals, water and vegetation break down rocks further, especially those with joints and faults
Freeze Thaw weathering (Congelifraction)
Frozen water expands by 9% compared to the same volume of liquid water
Ice freezing places the rock under huge stress and it breaks it wide apart so more water can enter
Repeated freezing and thawing cycles requires a diurnal temperature range fluctuating around 0°C
Occurs in high latitude areas, alpine regions or winter in temperate climates
Sedimentary / highly jointed rocks are susceptible
Thermal Expansion
Rocks are poor conductors of heat so only the outer layers expand and contract because heat doesn’t penetrate deep, placing stress on the rock due to high inelasticity
Eventually, outer layers peel off in thin sheets - exfoliation
This contributes to the formation of exfoliation domes and bornhardts
E.g. Half Dome, Yosemite NP
Mainly occurs in deserts and on granite rocks
Onion Skin weathering (Exfoliation)
When the surface layers of a rock are heated to a much greater degree than the interior of the rock, and stress in these layers cause it to peel off
E.g. granite is particularly susceptible
Granular Disintegration
Different minerals within the rock have different properties and therefore expand and contract at different rates which sets up extra stresses in the rock
Common in certain types of granite where the minerals, quartz (light), mica (dark) and feldspar (medium), are of different colours
Light coloured minerals reflect heat and dark ones absorb it and so are more likely to expand
Leaves a pitted surface behind with granules accumulating on the surface beside the rock
Shattering
Homogenous rocks simply shatter as a result of the constant cycle of expansion and contraction
E.g. basalt is particularly susceptible
Block Separation
Occurs in jointed rocks with predominant bedding planes
Rocks break down in blocks along these lines
E.g. limestone and sandstone are particularly susceptible
Pressure release (Dilatation)
Overlying rocks are removed by erosion
Allows the lower layers to relax and crack
Identified by large horizontal cracks and joints resembling bedding planes (pseudo bedding planes)
This cracking then allows further erosion and weathering of the rock as it exposes more surface area
E.g. the Tors, Dartmoor
The Tors, Dartmoor
Granite which used to be buried deep underground has risen above the surface due to erosion of land around this
As rock was removed, there was a reduction of pressure on the granite as there was less above it
This caused it to crack, creating joints
Where the joints were close together weathering quickly broke down the rock
There were also areas where there were very few joints so slower weathering occurred
This continued over millions of years, leaving behind tors and valleys now seen on Dartmoor
Salt crystal weathering (Haloclasty)
Dissolved salts in surface water results in crystals forming when water evaporates
Heating causes crystals to expand so rocks are prised apart
Salt weathers rock in a process called haloclasty
If saltwater evaporates, salt crystals are left behind
As crystals grow, they put pressure on the rock, slowly breaking it apart
Crystals expand further and cause greater stress when heated
Sodium sulphate and carbonate expand by 300% by this process
Optimum temperature is 26-28°C
Not limited to coastal areas - salt upwelling and soil salinisation can cause haloclasty
Scree slopes
All physical / mechanical weathering processes produce fragments of rock which have chipped off and can accumulate to form scree and scree slopes
More pliable rocks like clay are changed to a plastic state (mostly by chemical weathering)
Chemical weathering
Chemical change to a rocks minerals
Decomposition of rock due to a chemical change
Often the result of moisture interacting with rocks
Most active in warm, humid environments
Happens in many different ways but mainly involves the action of chemical agents which are dissolved, transported and deposited by water (either from rain, fog, dew or capillary action)
Processes are carbonation, hydrolysis and hydration
Hydrolysis
Reaction between silicate minerals and acid from the slightly acidic nature of water
Clay is produced and K, Ca or Na are released
Breakdown of rock by acidic water to produce clay and soluble salts
Chemical breakdown of a substance when combined with water
The most common example is feldspar in granite rocks changing to kaolin (china clay)
This clay can be eroded more easily than resistant feldspar
Hydrolysis is one of the most widespread processes as almost all rocks contain feldspar
Hydration
Absorption of water by minerals within rocks
Certain rocks are more vulnerable, such as anhydrite
Anhydrite absorbs water which forms gypsum
Hydration expands volume by 35% and results in rock deformation and breakdown
Carbonation
Process by which CO2 and rainwater or moisture in surroundings chemically react to produce carbonic acid, a weak acid, that reacts with carbonate minerals in the rock
Simultaneously weakens rock and removes chemically weathered materials
Chalk and limestone susceptible due to calcium carbonate content
Originates from crushed sea shells and marine organic matter
Carbonation amplified in humid, tropical climates
E.g. Karst Landscape (China)
Carbonation equations
air + water = carbonic acid
CO2 + H20 = H2CO3
limestone + carbonic acid = calcium bicarbonate
CaCO3 + H2CO3 = Ca(HCO3)2
Oxidation
Reaction of rock minerals with oxygen, thus changing the mineral composition of the rock
When minerals in rock oxidise, they become less resistant to weathering
Iron becomes red or rust coloured when oxidised
E.g. hematite
Biological weathering
Both physical and chemical
Plant roots grow in the joints of sedimentary rocks and can physically open them up further (mechanical)
Burrowing animals also cause physical weathering
Decaying plant roots produce acid which can chemically weather rock
Lichens and micro-organisms produce acids which weather the rocks upon which lichens are growing
Fulvic and humic acids are the main organic agents of chemical weathering - produced by detritivores and decomposition - chelation
How climate affects weathering
Peltier diagram shows weathering rates of the world in relation to the mean annual temperature and precipitation
Mechanical weathering doesn’t operate: in extreme prolonged cold periods, prolonged hot spells, too little moisture available, too much moisture to freeze, when thick vegetation prevents insolation
Chemical weathering rates double with every 10°C (Van ‘t Hoff’s Law) and requires abundant water and high temperatures
How relief affects weathering
Affects weathering by exposing rocks to the temperature or sun and wind
Elevated areas affected more than low level areas
South facing slope recieves more sun so sees more chemical weathering
North facing slope receives less sun so is colder so sees more physical weathering
Steep scree slopes form below north face as hard rock chips off hence why north face is typically steeper
Weathering accelerates as mountains get taller as they are more exposed to elements with no protective vegetation
