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Weathering and Erosion
Terms in this set (50)
General process by which rocks are broken down at Earth's surface
Set of processes that loosen and move soil and rock downhill or downwind
Why is weathering important?
Produces all clays, all soils, and the dissolved substances that are carried by rivers to the ocean
apron shaped pile of fragmented rock, accumulated at base of MT. Snowdon, Wales
What molecules can break down any rock?
Water, Oxygen, and Carbon Dioxide
Chemical Weathering Definition
Takes place when the minerals are chemically altered or dissolved.
Physical (mechanical) weathering
takes place when solid rock becomes fragmented by physical processes that do not change its chemical composition.
How do chemical and physical weathering work together?
They reinforce each other. Positive feedback.
The Faster the chemical weathering...
the weaker the pieces, more susceptible to damage, break into smaller pieces physical weathering
The smaller the pieces of rock
greater surface area available for chem weathering
Factors that control the weathering of rocks are:
1.) The properties of the parent rock
2.) the climate
3.) the presence or absence of soil
4.) the length of exposure
Slow Weathering Conditions (7 things)
Low rainfall, cold temperature, no soil layer, sparse organic activity. Short exposure. Low solubility (quartz), Massive structure
Fast Weathering Conditions (7 things)
Heavy rainfall, hot, thick soil layer, abundant organic activity, Long exposure, High solubility (calcite), fractured and thinly bedded
Minerals become unstable when exposed to certain conditions. Changes are highly specific. Must have water, an abundance of oxygen, and carbon dioxide (makes weak carbonic acid).
quartz chemical weathering
feldspar chem weather
degree of chem weathering is greatest in
warm and wet climates
imp characteristics of surface conditions that lead to chem weathering are
presence of water, abundance of oxygen, presence of CO2 producing weak carbonic acid
C02 in air, not enough to make carbonic acid, but
much more CO2 in soil, water through soil is much more acidic
water plus C02 produces
carbonic acid dissociates into
hydrogen and bicarbonate ions
Common reactions involved in chemical weathering:
Dissolution, hydrolysis, oxidation, hydration, and chemical weathering produced by organisms
Minerals dissolve completely into a solution.
(Dissolution) Calcite is
soluble in acidic solutions
calcium and bicarbonate ions combine in the ocean to form
the mineral calcite, shells of marine organisms
Calcite combines with what
The chemical breakdown due to exposure to water. K-feldspar produces kaolinite, silica, potassium ions, and bicarbonate ions. Water breaks down many other silicate minerals (amphiboles, pyroxenes, micas, and olivines).
(hydrolysis) Potassium felspar, in granite, reacts with what to produce what
acidic water (carbonic acid) to produce kaolinite (clay)
Hydrolysis reactions break down not only feldspars but also
other silicate minerals like amphiboles, pyroxenes, micas, olivines
Chemical effects of chemical weathering on silicates
1. Leaches cations and silica
3. Makes the solutions less acidic
Oxidation transforms iron-bearing minerals into mixtures of iron-oxide and iron-hydroxide. Makes hematite, geothite, and limnotite. This process is what causes rust (think red Mars). When olivine comes into contact with carbonic acid, it breaks down into dissolved iron, dissolved carbonate, and dissolved silicic acid. The dissolved iron quickly turns into hematite when oxygen is present.
Ferrous iron. Not soluble
Ferric iron. Soluble.
How to make ferric iron
Pyroxene + oxygen = hematite and dissolved silica
Hematite can also be formed from...
Biotite and amphibole
Oxidation with Sulfur
This is a special type of oxidation that takes place where the rocks have elevated levels of sulfide minerals, especially pyrite. When exposed to water and oxygen, the reaction generates iron ions, sulfuric acid, and hydrogen ions.
Acid Rock Drainage
This is the runoff from oxidation with sulfur. A lot of runoff can be produced even with small amount of pyrite. It is very prevalent near mining sites. Ex.: Mt. Washington Mine. The runoff is very acidic (<4 pH), and heavy metals become very soluble here.
The absorption of water into the crystal structure of minerals that causes some minerals, such as certain types of clay, to expand.
Chemical Weathering Produced by Organisms
Roots of plants, fungi, and lichens help dissolve minerals by extracting nutrients. Certain types of microbes obtain their life energy from the chemical bonds in minerals.
Fastest to Slowest Weathered Minerals
1. Mafic minerals (Halite, Calcite, Olivine) - Least stables
2. Feldspares (Ca-plagioclase, Pyroxene, Amphibole, Na-plagioclase, biotite, orthoclase, muscovite)
6. Hematite - most stable
Physical Weathering Processes
Natural zones of weakness, exfoliation and spheroidal weathering, root wedging, frost wedging, salt wedging, differential thermal expansion of minerals, and other forces (rivers, glaciers, waves, burrowing creatures, etc.)
natural zones of weakness
Joints, fractures, etc. Have joints when they first form.
exfoliation and spheroidal weathering
Sheets of rock that lie on top of one another and accelerate weathering. Ex. Half Dome in Yosemite.
Spheroidal has sheets that peel off like an onion.
In this type of physical weathering, the roots of a plant (typically a tree) wedge into a crack in a rock and, as the plant growns, splits the rock.
The mechanical breakup of rock caused by the expansion of freezing water in cracks and crevices. Water expands by 9% when it freezes.
In front of ocean, air loaded with salt crystals. Salt moves into cracks and build up to destroy property.
The energy of river directly affects what is being moved. Flooding has greater energy and greater force and greater weathering. Moving sand and silt acts like sandpaper on larger rocks.
Frozen river that is still moving. It moves very slowly, but can be more aggressive than just water. It picks up and move rocks that they run over, and this gravel can carve mountains down and cut valleys miles deep.
Constant crashing against rock.
Organisms in soil and burrow. Displace and break down rock.