Litho: Weathering Flashcards
What are the types of weathering?
Physical
- pressure release*, freeze thaw weathering, insolation weathering, salt weathering
chemical
- solution, carbonation, hydrolysis, hydration, oxidation
biological
- biomechanical, biochemical
Define physical weathering
Involves an application of a physical/mechanical force to the rock, breaking the rock apart. It is only superficial and produces little and shallow regolith since effects are confined to rocks near the surface.
Describe pressure release on a macro scale (eg. Granite)
Confined rock decompresses as regolith above it removes by erosion. The expansion generates stresses and fractures due to upward and outward expansion of the rock.
- less serious: sheet joints
- more serious: exfoliation (detachment of surface layers) forming exfoliation domes, followed by block disintegration
Describe pressure release on a micro scale
Release of strain energy results in stress between minerals in metamorphic rocks and cements between grains in sedimentary rocks. Breakage along mineral boundaries result in granular disintegration.
Describe freeze-thaw weathering on a macro scale
Leads to block disintegration when temperature fluctuates around freezing point. Water penetrates joints or beddings.
It expands about 9% as it freezes, exerting pressure on rock. A closed system is produced, sealing in water. When maximum pressure exceeds tensile strength, and repeated stress is exerted with each cycle, the rock is hammered apart over time. Widening of fissures cause rocks to be prised away over time, leading to block disintegration.
Describe freeze thaw weathering on a micro scale
Occurs when water penetrates pore spaces in between minerals, especially rocks with high permeability and porosity (eg. Chalk). Minerals break along their boundaries over many cycles.
Describe insolation weathering on a macro scale (2 parts answer)
Refers to the disintegration of rocks by repeated expansion and contraction through solar heating and cooling. Particularly effective in areas with large diurnal temperature range, such as in deserts.
Exfoliation: as rock is a poor conductor of heat, the effects of diurnal heating are confined to the surface layers. A sharp thermal gradient develops and the surface of the rock expands more, forming stresses leading to exfoliation.
Describe insolation weathering on a micro scale
Granular disintegration occurs as dark coloured minerals absorb more heat, causing them to expand and contract more than light coloured minerals. Unequal rates of expansion and contraction leads to development of stress between minerals. (eg. Dark crystals of mica vs paler Quartz and feldspar)
Describe salt weathering (general)
Salt such as sodium chloride in water that enters the rock crystallises, exerting pressure on the rock. Salt crystals grow, splitting the rocks when stress exceeds tensile strength of rock.
Salt weathering can also produce carvenernous weathering (eg. Honey combing of rock surfaces)occurs in coastal regions.
Describe salt weathering in arid regions
arid areas (deserts) where ground water is drawn to the surface by capillary action due to high PET. When water evaporates, salts are left behind as surface coatings or in rock joints, leading to surface scaling and granular disintegration.
In salt weathering, how do salts set up stresses? (3 ways)
1) Thermal expansion of salt crystals within pores
2) hydration of salts during high humidity
3) repetitive growth over many wetting cycles
Define chemical weathering
The breakdown of rocks by alteration of the chemical composition of rock minerals by agents such as water, oxygen and mild acids. It can occur at great depths (vs physical which is superficial) regolith produced is finer grained.
Define weathering
The in situ breakdown of rock by natural agents
Describe solution
Affects both original rock minerals and already weathered rock. Effectiveness determined by pH of water. Highly alkaline and acidic water increases solubility of silica and alumina
Eg. Quartz + water -> silica in solution
Describe carbonation
Occurs in limestone landscapes where very weak carbonic acid provided by rainfall as it dissolves co2, carbonates limestone minerals.
Calcium carbonate (calcite) + weak carbonic acid -> calcium + calcium bicarbonate
Important in cold regions (low temp) as co2 is more soluble at 0 degrees than 20degrees
Describe hydration
Affects rock minerals which have the capacity to take up water. Can cause expansion of a mineral, weakening the rock, causing cracking and granular disintegration.
Describe hydrolysis (granite)
Only water required.
Feldspar + water -> KOH (carbonated and removed in solution) + alumino silicic acid (left behind as kaolinite clay, silicic acid removed in solution)
More effective with higher temperature (Van Hoff Theory - increase 10degrees, 5 times increase in weathering)
Describe oxidation
Involves reaction of a mineral with oxygen to form oxides or hydroxides.
‘Rusting’ of iron: iron + oxygen -> iron oxide
Define biological weathering.
Refers to any sort of weathering carried out by a living organism, or a by-product of a living organism
Describe biomechanical weathering
Involves penetration of rock joints by tree roots. When they grow, they prise the rock apart by setting up physical stresses, causing block disintegration.
Tree roots also open up joints where waters can flow, increasing vulnerability to other forms of weathering
Describe biochemical weathering
A form of chemical weathering carries out by plants. When organic matter decay, organic acids produced dissolve into water and is an effective agent of chemical weathering. This is part the process of chelation.
What factors affect weathering?
- Climate
- Vegetation and soil cover
- Topography of local area
- Rock characteristics - structure, texture, composition
- Man’s activities
Describe weathering in the humid tropics
Chemical weathering is dominant and rapid by 4 times.
1) high annual precipitation (2500mm/yr) - abundant soil moisture for chemical reaction: solution, hydration, hydrolysis, carbonation
2) high temperature - rise of 10degrees, chemical reaction increase by 2.5 times
3) dense vegetation including dead vegetation matter increases rate of biological weathering
Limited physical weathering due to masking effect of thick regolith and lack of exposure of solid rock since physical weathering is a surface phenomenon. Also, uniformly high temps don’t favour such weathering.
Describe weathering in the seasonally humid tropics
Chemical weathering is dominant and rapid but rate and regolith formation less rapid than in the humid tropics. More regolith removal due to less dense vegetation and more effective surface wash erosion.
Describe weathering in the hot arid (deserts) region
Physical weathering is dominant due to high diurnal temperature range, favouring insolation and salt weathering. Shallow mantles of broken rocks vs deep weathered regolith in the humid tropics.
Chemical weathering experienced to small extent due to presence of sources of moisture such as infrequent rains, heavy dews and fog. Especially for chemical weathering that does not require much water such as hydration and solution.
Describe weathering in the temperate region
Moderate temperatures between 10-20degrees and moderate rainfall. Freeze thawed weathering occurs during winter and at high altitudes but rarely severe. Chemical weathering far less active except for carbonation since co2 is 2 times more soluble at 0degrees.
Describe weathering in the glacial regions
Abundant snowfall and generally low temperatures. Freeze thaw weathering is dominant. Carbonation may occur since water from melted snow contain high levels of dissolved co2, providing weak carbonic acid to arrack rock minerals.
How does rock strength affect weathering?
Stronger and harder rocks are more resistant to physical weathering. Strength is based on:
1) hardness of mineral constituents and strength of cementation between minerals (eg. Granite is hard as its hard minerals - Quartz and feldspar are tightly interlocked during crystallisation)
2) differences in rock age - older rocks get harder due to more time for cementation. Compression from young rocks above hardens the rock too
How does chemical composition affect weathering
Compare limestone and granite less chemicals stable minerals are selectively weathered, weakening the rock over time
How does rock texture affect weathering?
Coarse grained rocks where there are large voids between large grains, the high primary permeability increases the possibility of trapping water for chemical and freeze thaw weathering. Boundaries along tiny minerals of fine grained rocks also provide lines of weakness for weathering
How does rock structure affect weathering? (Presence of joints and beddings)
Joints allow surface area of rocks that can be weathered by water, air and acids to increase, leading to high secondary permeability.
How does topography affect weathering?
Affects climate on a more micros ale by influencing altitude and aspect.
Higher altitude -> freeze thaw weathering but too high will decrease the number of diurnal cycles that limits pressure exerted on rocks
Aspect -> no direct sunlight means temps are permanently below freezing point so no freeze thaw. Areas with regular insolation as well as temps are above freezing point.
Imagine/ draw peltier’s diagrams
Physical, chemical X axes (mean annual precipitation): 600,1200,1800mm Y axes (mean annual temp): 0, 10, 20, 30
Physical: 3 lines (inverse S, face own direction) rapid, fairly rapid, moderate, slow
Chemical: 4 lines (all C shape) very slow, slow, moderate, fairly rapid, rapid
Define BSW
The boundary that divides weathered from unweathered rock
What landforms are associated with weathering? (3)
1) Scree slopes made of angular fragments of rocks that have fallen and accumulated at the bottom of slopes (associated with freeze thaw weathering)
2) exfoliation domes (eg. Granite) due to pressure release and sheeting
3) limestone pavements (eg. Clints and grykes)
