WEATHERING PROCESSES
Physical and Chemical
Weathering
Weathering:
in-situ breakdown of material
Weathering:
in-situ breakdown of material
• physical (or mechanical) weathering
• chemical weathering
• resistance to weathering function of:
– internal resistance of material
– magnitude of external forces
Weathering Process
• weaken resisting forces
• produce unique landforms
• produce regolith (weathering mantle),
which may evolve to soil
This week’s agenda:
explore physical and chemical
weathering processes and landforms
Frost action (physical weathering)
(Talus cones, Banff National Park, Alberta. Photo: Marli Miller,
University of Oregon. http://marlimillerphoto.com/talus.html)
Frost action
• 9% volumetric expansion upon freezing
– effective only in closed voids that are almost
entirely saturated
– effective when temperatures oscillate above and
below freezing point
Frost action
•
ice segregation
– frozen water in voids generates a suction force pulling liquid
water toward the ice
– migrating water generates pressure forces sufficient to enlarge
cracks
– primary cause of frost heave in soils
– only more recently accepted as mechanism working in solid rock
– most effective in temperature range -3 to -8C
Frost action
• Landforms:
– talus, talus cones, scree slopes
Salt weathering (physical weathering)
• salt crystallization: occurs as saline solutions
evaporate
• salt crystal expansion: occurs when salt
crystals get wet
Honeycomb weathering in graywacke
sandstone, Golden Gate National Recreation
Area. Photo: National Park Service,
http://www.nps.gov/goga/forteachers/
graywacke-sandstone-faq.htm
graywacke: a coarse usually dark gray
sandstone or fine-grained conglomerate
composed of firmly cemented fragments
(as of quartz or feldspar)
also called dirty sandstone
Salt weathering
• occurrence:
• hot and cold arid and semi-arid environments
capillary rise brings saline water toward surface
limited liquid water (either due to supply or phase)
incapable of washing salts away
hot arid regions: large diurnal changes in temperature
and relative humidity promote repeated wetting and drying
cold regions: cold temperatures encourage salt precipitation
from solutions
• rocky coastal areas
Salt weathering
• rock susceptibility to salt weathering
… is a function of:
• proportion of micro pores
• water absorption capacity
• surface texture
• presence of clay minerals
Photo: K. Segerstrom, USGS
Photographic Library image
sk000626. http://libraryphoto.cr.usgs.gov/
Salt weathering
• landforms
•
tafoni
•Cavernous weathering of
granite ("tafoni") near
Caldera. Chile. No date.
• honeycomb weathering
• granular disintegration
• spalling
Photo: K. Segerstrom, USGS
Photographic Library image
Tafoni
sk000626. http://libraryphoto.cr.usgs.gov/
Salt weathering
• landforms
• tafoni
•
honeycomb
weathering
granular disintegration
• spalling
Honeycomb weathering in block of Wingate
sandstone Silver Falls canyon. Garfield
County, Utah. 1921. Plate 9-B in U.S.
Geological Survey. Professional paper 164. 1931.
ID. Moore, R.C. 98 mrc00098
Salt weathering
• landforms
• tafoni
• honeycomb weathering
•
granular
disintegration
• spalling
Image file: /htmllib/batch68/batch68j/batch68z/mfe01153.jpg
Sequoia National Park, California. Northern edge of
the Siberian Outpost. The frost-split blocks of granite
might readily be mistaken for glacial boulders, but,
having the same composition as the underlying bedrock,
they have obviously been formed in place. The granite
here is sparsely jointed and breaks up into large blocks.
These, in turn, are subject to granular disintegration
which at this lower altitude is probably effected not only
by frost action, but also by the solvent action of carbon
dioxide. The latter, derived from decaying lichens and
pine needles, is carried by water into the interstices
between individual granules. Eventually the blocks lose
their sharp angles and become rounded. 1935.
Photo: Marli Miller, University of Oregion. Earth Science World Image Bank,
photo hhrhuz, http://www.earthscienceworld.org/
Salt weathering
• landforms
• tafoni
• honeycomb weathering
• granular disintegration
• spalling
J.R. Stacey, USGS Photographic Library, Image
hcb00980. http://libraryphoto.cr.usgs.gov/
Wetting & drying (physical weathering)
• susceptible soils & rocks:
– soils with 2:1 layered clays (e.g.
montmorillonite)
– shale, clayey siltstones and
sandstones, granite
• result: spalling, granular disintegration
Thermal expansion and contraction
(physical weathering)
• different minerals have different
coefficients of thermal expansion
– e.g. quartz is about 3 times that of feldspar
• effectiveness of insolation debated
• result: spalling
Pressure release
(physical weathering)
• sheet joints and
exfoliation
• rock bursts in deep
mines
Unloading (sheeting) joints in granodiorite.
Yosemite National Park, California. SrF-52.jpg
Photo showing sheet jointing: F.E. Matthes, USGS
Photographic Library, Image mfe00007.
http://libraryphoto.cr.usgs.gov/
Unloading joints in granitic rock, Yosemite National Park, CA. Note how
joints on either side of the creek dip towards the creek. SrF-49.jpg M Miller
CHEMICAL WEATHERING
• progression from less stable minerals to
more stable minerals
– mineral stability
CHEMICAL WEATHERING
• progression from less stable minerals to more
stable minerals
– primary minerals - secondary minerals - new secondary
minerals
CHEMICAL WEATHERING
•
progression from less stable minerals to more stable minerals
• water is critical
• geochemical weathering:
– driven by inorganic processes;
– produces "rotten" rocks or saprolites
• pedochemical weathering:
– controlled by biologic processes;
– leads to formation of soil from saprolites
CHEMICAL WEATHERING TYPES
• Solution
– virtually all chemical weathering involves some
solution
– the most common minerals are soluble to some
degree in normal waters
CHEMICAL WEATHERING TYPES
• Solution
– virtually all chemical weathering involves some
solution
– the most common minerals are soluble to some
degree in normal except:
• silica when contained in quartz
• aluminum oxides - virtually insoluble under normal
conditions
• ferric iron - requires very acidic fluids
– solution of halite (NaCl) and calcite (CaCO3)
CHEMICAL WEATHERING
• result:
– granular disintegration,
– spheroidal weathering,
– weathering pits, karst
Photo right: N.K. Huber, USGS Photographic Library, Example of
spheroidal weathering found in Isle Royale National Park, Michigan.
Zone of spheroidal weathering in a basalt lava flow. This zone of
partly decomposed rock escaped removal by glacial erosion because
of its protected location. Circa 1971. Figure 7, U.S. Geological
Survey Professional paper 754-A. Image hnkb0004.
http://libraryphoto.cr.usgs.gov/
Photo left: Characteristic outcrop of gabbro, one-half mile north of Toecane, looking
northeast. Spheroidal weathering is characteristic of the gabbro in this region, and
rounded boulders strew its surface. The rock is not metaphosed and presents a
strong contrast to the Roan gneiss, which is similar composition. Mitchell County,
North Carolina. 1897. Figure 15 in U.S. Geological Survey. Folio 151. 1907. Image
file: /htmllib/btch325/btch325j/btch325z/kei00117.jpg
Hydrolosis (Chemical
Weathering)
• water dissociates into H+
(hydrogen cation) and OH(hydroxyl anion)
• H+ displaces other cations in
mineral structure
– K+, Na+, Ca2+, Mg2+
– may combine with hydroxly anion
or be carried away in solution
Hydrolosis (Chemical
Weathering)
• hydrolysis promoted by:
– decreasing pH (increasing H+)
– decomposition of organic matter (releases
H+)
– increased water temperatures (promotes
dissociation)
• important mechanism for breaking
apart primary minerals
– example: albite weathers to kaolinite plus
some residual silica
– albite + water = kaolinite + silica + sodium
ion + hydroxyl ion
– 4NaAlSi3O8 + 6H2O = Al4Si4O10(OH)8 +
8SiO2 + 4Na+ + 4OH
Hydrolosis (Chemical
Weathering)
• Result of hydrolosis:
–
–
–
–
–
spalling,
weathering pits,
spheroidal weathering,
weathering rinds,
production of clay
mineral
Weathering rind in granitic rock.
M.B. Miller WE-29.jpg
fine grained mafic igneous rock with orange-brown, iron rich
weathering rind (Photo: USGS
http://pubs.usgs.gov/of/2002/of02-437/gallery.htm)
Oxidation/reduction
• oxidation
– element in a mineral
structure loses electrons
increasing their charge
– reaction between ions and
oxygen results in formation
of oxides and hydroxide
– occurs above water table
– examples:
(Chemical Weathering)
Oxidation/reduction
(Chemical Weathering)
• Oxidation examples:
– ferrous iron (Fe+2) oxidizes to ferric
iron (Fe+3)
• 4Fe+2 + 3O2 = 2Fe2O3
• iron + oxygen = iron oxide (hematite)
– olivine weathers through
combination of hydrolysis and
oxidation to form hematite
• olivine + water + oxygen = hematite +
silicic acid
• 2Fe2SiO4 + 4H2O + O2 = 2Fe2O3 +
2H4SiO4
• Most elements at earth's surface
exist in an oxidized state
Oxidation/reduction (Chemical Weathering)
• reduction: opposite reaction
– occurs below water table
– reduced form of elements
are more mobile than
oxidized because they're
more soluble
• result: weathering rinds
Original porosity is still retained in this sandstone sample from the
Nanushuk Group, but some reduction in porosity probably took place
during compaction. Quartzose grains were dissolved at grain contacts
and welded. At some later stage, minor secondary porosity was
created by dissolution of quartzose grains (arrow). Growth of chlorite
in pore spaces completes diagenetic cycle. Sample 78ACh32, Kurupa
anticline; magnification, 6.3 x 10, photomicrograph. Central North
Slope, Alaska. Published as Figure 45 in U.S. Geological Survey.
Bulletin 1614, 1985.
Image file: /htmllib/btch434/btch434j/btch434z/bws00009.jpg
Granite bowlder undergoing reduction by the separation of large flakes; spur between forks of Little Kern River,
California. The flaking is believed to be due to heat caused by forest fires. Tulare County. 1903. Image file:
/htmllib/btch163/btch163j/btch163z/ggk02031.jpg …. NOTE: different meaning of “reduction” in the literature.
Ion Exchange
(Chemical Weathering)
• Exchange of ions in minerals
(usually cations) with ions in
solution
• most common in clay minerals
– cations held to clay surface by
adsorption; not held too tightly
– H+ and Ca+2 are most readily
adsorbed; Na+2 is most readily
released
• cation exchange capacity:
propensity for adsorbing cations
Ion Exchange
(Chemical Weathering)
• result:
– production of new, more stable
secondary (clay) minerals
Cation-Exchange Capacity (CEC)
Cation-exchange capacity is defined as the degree to which a soil can adsorb
and exchange cations.
Cation-a positively charged ion (NH4+, K+, Ca2+, Fe2+, etc...)
Anion-a negatively charged ion (NO3-, PO42-, SO42-, etc...)
Soil particles and organic matter have negative charges on their surfaces. Mineral
cations can adsorb to the negative surface charges or the inorganic and organic soil
particles. Once adsorbed, these minerals are not easily lost when the soil is leached
by water and they also provide a nutrient reserve available to plant roots.
These minerals can then be replaced or exchanged by other cations (i.e., cation
exchange)