Earth Science 1121H Chemical Weathering

Chemical weathering

the mineral in rocks are attacked chemically and broken down
- involves the breaking of bonds in the minerals

composition of the atmosphere

78% nitrogen, 21% oxygen, 1% other

Origin of Atmosphere and Oceans

-atmosphere originates by volcanic outgassing of the earth
- oceans condense from the atmosphere

Role of CO2 in weathering

1. water dissolves CO2 from the atmosphere and also from the buried organic material

2. dissolved CO2 reacts with H2O to form carbonic acid

3. carbonic acid dissociates to hydrogen and bicarbonate

4. hydrogen attacks the chemical bond in silicate minerals

Weathering of Feldspar

• k-feldspar breaks down to produce residual clay and quartz

• na-feldspar: some SIO2 and Na dissolves and is removed in solution

• K, NA, MG, CA produced by weathering of minerals long with bicarbonate end up in the oceans

salinity of oceans is a result of

weathering

weathering of feldspar produces

new minerals and dissolved material

ocean water salinity

Ca is extracted from seawater by organisms and used to make shell and skeletons

chemical vs mechanical weathering

chemical: produces new minerals, solid products chemically different from parent
mechanical: rock disaggregated, same minerals, solid products chemically identical to parent

Hydrolysis

the chemical breakdown of a compound due to reaction with water.

Oxidation

A chemical change in which a substance combines with oxygen, as when iron oxidizes, forming rust

dissolution

the breaking up or dissolving of something into parts; disintegration

weathering of limestone

produces distinctive features on the surface
- and abundant cave systems

karst topography

A region in which a layer of limestone close to the surface creates deep valleys, caverns and sinkholes.

Rate of weathering depends on

1. nature of the rock

2. climate

weathering most efficient in

warm, moist climates

Natures of rocks that weather more easily

• volcanic glass

• minerals with fewer Si-O bonds

• fine0grained rocks

chemical weathering most dominant in

warm, moist regions

mechanical weathering most dominant in

cool regions

Rapid weathering

landslides and avalanches
- mass movement

differential weathering

The variation in the rate and degree of weathering caused by such factors as mineral makeup, degree of jointing, and climate.

arid climate weathering

dry climate
- most weathering during infrequent storms

spherodial weathering

A type of chemical weathering in which corners and sharp edges of rocks weather more rapidly than flat surfaces, thus yielding spherical shapes.

• onion skin weathering

• chemical attack weekends outer layer until it peels off

alfisol

• soil in arid climates

• evaporation causes groundwater to migrate upwards and precipitate calcite

oxisol

• soil in warm, moist climates

• intense weathering leads to removal of soluble materials leaving a soil composed of insoluble alumina and iron oxides

orogenesis

mountain building
-result of plate tectonics
-affects climate change

Climate change

earth's cooling began about 40-50 years ago

Increased weathering associated with

uplift of the Himalayas

• resting in the drawdown of CO2 from the atmosphere and to global cooling

• weathering used CO2, those CO2 is removed from the atmosphere to the lithosphere

Climate is affected by processes such as

convection that occur within the earth