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Presentation on theme: "Weathering."— Presentation transcript:

1 Weathering

2 Weathering: the disintegration, or breakdown of rock material

3 Mechanical Weathering: no change in chemical composition--just disintegration into smaller pieces

4 Chemical Weathering: breakdown as a result of chemical reactions
CaCO3+CO2+H2O ---> Ca2+ + 2HCO3-

5 Mechanical Weathering
Physical breakup pressure release water: freeze - thaw cycles crystallization of salt in cracks thermal expansion and contraction plant roots friction and impact All this increases the total surface area exposed to weathering processes.

6 Mechanical Weathering
Exfoliation: Rock breaks apart in layers that are parallel to the earth's surface; as rock is uncovered, it expands (due to the lower confining pressure) resulting in exfoliation.

7 Mechanical Weathering
Many igneous rocks, particularly granite, are formed at great depth. When they come to the surface, they expand. Creates a morphology with exfoliation slabs – slabs sheet off in onionlike layers Stone Mountain, Georgia Expansion of rocks pose a risk in deep mines – explode off walls These fractures, or joints can enhance frost wedging and other erosive processes

8 Sheet Joints (Exfoliation)

9 Half Dome, Yosemite, CA Stone Mountain, GA
Summit of Half Dome, Yosemite, shows exfoliation Half Dome, Yosemite, CA Stone Mountain, GA

10 Exfoliated Domes, Yosemite

11 Stone Mountain, Georgia, showing the product of exfoliation due to unloading
Summit of Half Dome, Yosemite, shows exfoliation Stone Mountain, GA

12 Frost Wedging: rock breakdown caused by expansion of ice in cracks and joints

13 Shattered rocks are common in cold and alpine environments where repeated freeze-thaw cycles gradually pry rocks apart. Effects of freeze thaw on granite. Mt. San Jacinto,southern California, Transverse Ranges.  About 8,500 feet.

14 Thermal expansion due to the extreme range of temperatures can shatter rocks in desert environments.
Repeated swelling and shrinking of minerals with different expansion rates will also shatter rocks. Weathering Rain may enhance this process

15 Plant Roots

16 Friction and Repeated Impact

17 Burrowing of Animals

18 Role of Physical Weathering
Reduces rock material to smaller fragments that are easier to transport 2) Increases the exposed surface area of rock, making it more vulnerable to further physical and chemical weathering

19 Surface Area and Weathering

20 Rates of weathering Joints in a rock are a pathway for water – they can enhance mechanical weathering Water flowing through joints attacks the rock, rounding corners. As minerals are converted to clay, the rock expands, breaking outer layers off Can enhance mechanical weathering

21 Chemical Weathering Definition: transformation/decomposition of one mineral into another through chemical changes Agents of chemical weathering: Water Oxygen CO2 Living organisms Acid Rain Mineral breakdown carbonate dissolves primary minerals --> secondary minerals (mostly clays) Net loss of elements retained in the soil.

22 Water is the main operator:
Chemical Weathering Water is the main operator: Dissolution Many ionic and organic compounds dissolve in water Silica, K, Na, Mg, Ca, Cl, CO3, SO4 Acid Reactions CO2 dissolves in rain water and creates carbonic acid Carbonic acid easily weathers limestone and marble

23 Dissolution H2O + CO2 + CaCO3 --> Ca+2 + 2HCO3- water + carbon dioxide + calcite dissolve into calcium ion and bicarbonate ion Biological activity in soils generates substantial CO2 Bicarbonate is the dominant ion in surface runoff.

24 Chemical Weathering Oxidation Living Organisms
Oxygen dissolved in water promotes oxidation of sulfides, ferrous oxides (rust), native metals Living Organisms Plant material makes H+ ions available Lichens

25 Chemical Weathering Hydration: attachment of water molecules to crystalline structure of a rock, causing expansion and weakness Hydrolysis: combination of hydrogen and oxygen in water with rock to form new substances

26 Solution: process by which rock is dissolved in water
Chemical Weathering Solution: process by which rock is dissolved in water Is strongly influenced by pH and temperature When water becomes saturated, chemicals may precipitate out forming evaporite deposits. Calcium carbonate (calcite, limestone), sodium chloride (salt), and calcium sulfate (gypsum) are particularly vulnerable to solution weathering.

27 Resistance to Weathering
First to Crystallize Fast Weathering Bowen’s Reaction Series Goldrich Stability Series Last to Crystallize Slow Weathering

28 Olivine/pyroxene to clay
+ H2CO3 (acid)

29 Feldspars to clay + H2CO3 (acid)

30 Quartz to quartz (!) + anything

31 Calcite to ……. + anything nothing

32 Karst Topography A type of landscape in rainy regions where there is limestone near the surface, characterized by caves, sinkholes, and disappearing streams. Created by chemical weathering of limestone

33 ‘Karst’ landforms develop in areas underlain with limestone
This photo of Lime Sink was taken on 20 July 1932, over a week after the drawdown, which occurred over the night of 9-10 July. ‘Karst’ landforms develop in areas underlain with limestone When Lake Jackson’s sinks rupture and the lakewater drains out, the process is referred to as a drawdown. It has occurred many times during the last century, in 1900, 1907, 1909, 1932, 1935, 1936, 1957, 1982, and again in 1999.

34 Erosion The process by which water, ice, wind or gravity moves fragments of rock and soil.

35 Water Erosion Rivers, streams, and runoff

36 Ice Erosion Glaciers

37 Wind Erosion

38 Mass Movements Landslides, mudslides, slump and creep

39 Rates of weathering Weathering Climate
Temperature and moisture characteristics Chemical weathering Most effective in areas of warm, moist climates – decaying vegetation creates acids that enhance weathering Least effective in polar regions (water is locked up as ice) and arid regions (little water) Mechanical weathering Enhanced where there are frequent freeze-thaw cycles

40 Mechanical and Chemical Weathering
Fracturing, disintegration caused by mechanical weathering exposes more surface area. Greater surface area, means more places for chemical action to occur.


42 Alaska Seattle Altiplano Amazon

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