Weathering
Reading list:
Brantley, S., and 26 others, 2011, 12 testable hypotheses on the geobiology of
weathering, Geobiology, 9, 140-165.
Hamblin, W.K. and Dr. Eric H. Christiansen, E.H., 2004, Chapter 10 - Weathering,
Earth's Dynamic Systems, Tenth Edition, Prentice Hall.
Kump, L., Brantley, S., and Arthur, M., 2000, Chemical weathering, atmospheric CO2
and climate, Annual Reviews of Earth and Planetary Science, v. 28, pp. 611-667.
Liu, Z., Dreybrodt, W., and Liu, H., 2011, Atmospheric CO2 sink: silicate weathering or
carbonate weathering? Applied Geochemistry, v. 26, pp. S292-S294.
Riebe, C., Kirchner, J.W., and Finkel, R.C., 2004, Erosional and climatic effects on
long-term chemical weathering rates in granitic landscapes spanning diverse climate
regimes, Earth and Planetary Science Letters, v. 224, pp. 547-562.
L. Hinnov
February 17, 2012
Physical weathering is the
mechanical fragmentation of
rocks from stress acting on them
from Earth surface processes.
Chemical weathering involves chemical
reactions between Earth surface
chemistry (water) and minerals that
progressively decompose solid rock.
PHYSICAL
CHEMICAL
feedbacks
Famous experiment:
Griggs, 1936 heated and
cooled cubes of granite
140ºC to 30ºC for the
equivalent of 240 years of
daily fluctuations, but nothing
happened.
That was dry, but with
wetting, in ~2.5 years they
fell apart.
Major products of weathering:
(1) fractured rock
(2) regolith
(3) ions in solution
Brantley et al. (2011)
Hamblin & Christiansen (2004)
CONDITIONS ASSOCIATED WITH CHEMICAL V. PHYSICAL WEATHERING
COMMON MINERALS IN ROCKS
Common rock-forming minerals
Crustal abundance of minerals
Ronov & Yaroshevsky, 1969
COMMON MINERALS IN ROCKS
The higher the Si:O ratio, the more resistant a mineral is to
chemical weathering:
Goldich
Stability
Series
“Reverse Bowen Reaction Series”
High temp. minerals weather first….
QuickTime™ and a
decompressor
are needed to see this picture.
CHEMICAL WEATHERING OF ROCKS AND CO2 DRAWDOWN:
COMMON WEATHERING REACTIONS:
carbonation (breakdown by weakly acidic rainwater)
hydrolysis (breakdown by water)
hydration (breakdown by the absorption of water)
oxidation (breakdown by the oxygen in air and water)
1)
2)
Congruent-minerals totally dissolve
Incongruent-mineral is transformed into another mineral
CARBONATION (DISSOLUTION):
Calcite:
HYDROLYSIS:
Plagioclase (albite):
Crustal abundance
of minerals
Potassium Feldspar:
2KAlSi3O8 + 9H2O + 2CO2 ⇌ Al2Si2O5(OH)4 (kaolinite) + 2HCO3- + 4H4SiO4 + 2K+
3KAlSi3O8 + 14H2O + 2CO2 ⇌ KAl3Si3O10(OH)2(illite) + 2HCO3- + 6H4SiO4 + 2K+
Depending on K+ content of the solution.
CHEMICAL WEATHERING OF ROCKS AND CO2 DRAWDOWN:
Olivine:
Wollastonite:
(a pyroxene)
DISSOLUTION RATE OF PLAGIOCLASE
Brantley et al. (2011)
Laboratory rates are order of magnitude faster than natural rates
Note: BET analysis = Brunauer-Emmett-Teller (BET) Surface Area Analysis