Weathering and Erosion
Midterm II: 1- Igneous Rocks 2- Volcanism 3- Weathering and erosion 4- Soils 5-Materials covered in recitations 3,4, and 5.
Terms in this set (44)
What is weathering?
Weathering is the general process by which rocks are broken down at Earth's surface.
What is erosion?
Erosion is a set of processes that loosen and move soil and rock downhill or downwind.
What is the difference between weathering and erosion?
Erosion has to do with moving soil/rock whereas weathering is just the breaking down of rock.
Why is weathering important?
It produces dissolved substances (that are carried to oceans by rivers), and all the clays and soils.
What is a talus?
A talus is an apron-shaped pile of fragmental rock.
What are the types of weathering?
when minerals in a rock are chemically altered or dissolved as a result of contact with water solutions or air.
when solid rock becomes fragmented by physical processes that do not change its chemical composition
How do chemical and physical weathering interact?
Chemical and physical weathering help & reinforce each other.
Faster chemical decay = weaker pieces, more breakable.
Smaller pieces = greater surface area for chemical weathering.
Why do some rocks weather more rapidly than others?
1. Properties of parent rock
3. Presence or absence of soil
4. Length of exposure
Low weathering rate caused by...
Parent rock has low mineral solubility and large rock structure (ex. quartz)
Climate has low rainfall and cold temperature.
Sparse organic activity.
Short length of exposure.
High weathering rate caused by...
Parent rock with high mineral solubility (ex. calcite) and fractured/thinly bedded rock structure.
Climate has heavy rainfall and hot temperature.
Long length of exposure.
What is the effect of mineral solubility on weathering?
Low (ex. quartz) = slow weathering.
High (ex. calcite) = fast weathering.
Effect of Rock Structure on Weathering
Massive rock structure = slow weathering.
Very fractured or thinly bedded = fast weathering.
Effect of Climate on Weathering.
Low rainfall & low temp = slow weathering.
Heavy rainfall & high temp = fast weathering.
What is the effect of soil & vegetation on weathering?
No soil & sparse veg = slow weathering.
Thick soil & abundant veg = fast weathering.
What is the best location for chemical weathering?
Tropics (warm & wet)
What is saprolite?
Saprolite is a layer of rotten rock.
Can be over 100 m thick.
Produced by chemical weathering in warm, wet climates.
Reactions involved in chemical weathering
Chemical weathering produced by organisms
Carbon Dioxide (CO2) + Water (H2O) = Carbonic Acid (H2CO3)
Calcite (CaCO3) + Carbonic Acid = Calcium ion (Ca2+) + Bicarbonate Ion (2HCO3-)
Example: weathered limestone in Segesta, Italy & Ireland
Water chemically reacts with minerals and breaks them down.
Works faster in acidic water.
Example of Hydrolysis
Carbon dioxide gas molecules in air dissolve into rain droplets to form carbonic acid molecules. Some of those ionize to form hydrogen ions and bicarbonate ions, making water acidic.
Potassium feldspar (common mineral in granite) reacts with acidic water to produce kaolinite (type of clay) and other dissolved ions.
Hydrolysis reactions break down:
Feldspars & other silicate minerals: amphiboles, pyroxenes, micas, olivines
Effects of Chemical Weathering of Silicates
1. Leaches (dissolves away) cations and silica
2. Hydrates (adds water to) minerals
3. Makes solutions less acidic
What causes dissolution and hydrolysis?
Element loses electrons by combining with oxygen.
What kind of rocks does oxidation affect?
Rocks with iron-bearing minerals (biotite, pyrite) transformed into rusty-brown mixtures of various iron-oxide and iron-hydroxide minerals (hematite and goethite)
Why is the surface of Mars red?
Oxidation reactions in rocks with iron-bearing minerals
Example of Oxidation
Iron pyroxene (4FeSiO3) + Oxygen (O2) --> Hematite (2Fe2O3) + Dissolved Silica (4SiO2)
Absorption of water into crystal structure of minerals, causing those minerals to expand (example: clay)
Chemical Weathering Produced by Organisms
Roots of plants, fungi, lichens have secret organic acids that help dissolve minerals in rocks and extract nutrients from minerals.
Microbes (Bacteria) eat minerals. (can live up to a few km below earth's surface).
Chemical Weathering of Silicates (Quartz)
Quartz is very stable and weathers slowly
Chemical Weathering of Silicates (Feldspars)
Feldspars are moderately stable and weather moderately fast. The weathering forms clay minerals.
Chemical Weathering of Silicates (Mafic Minerals)
Mafic minerals weather by oxidation and decompose to oxides.
Which are the least stable minerals at Earth's surface?
Minerals that form early in Bowen's reaction series (Mafic) are among least stable minerals at Earth's surface. (Olivine, pyroxene)
Which are the most stable minerals at Earth's surface?
Minerals that are the product of weathering reactions (ex. Hematite) are among the most stable minerals.
How do felsic minerals weather?
Felsic minerals, formed late in Bowen's reaction series, weather by hydrolysis.
How do carbonate minerals and salts weather?
Carbonate minerals and salts weather by dissolution.
How do oxide minerals weather?
Oxide minerals don't weather at all.
Which minerals don't weather at all?
Oxide minerals don't weather at all.
Types of Physical/Mechanical Weathering
1. At natural zones of weakness (joints/fractures, etc.)
2. Exfoliation and spheroidal weathering
3. Root wedging
4. Frost wedging
5. Salt wedging
6. Differential thermal expansion of minerals
7. Water (river, glacier, waves)
8. Burrowing creatures (earthworms, gophers, humans)
How much does water expand when it freezes?
Water expands by 9% when it freezes.
River energy affects what is being moved.
Rivers in flood stage have greater energy, more mechanical weathering.
Moving sand & silt acts like sandpaper on larger rocks in riverbed.
Rock collisions break rocks.
Glacier = frozen river still moving.
Erodes rock beneath faster & more aggressively than if it were water.
Picks up & moves rocks - these carve mountains down and cut vallleys.