Weathering & Soils
Laterization
occurs in tropical climate zones and produce highly oxidized soils and intense leaching of silica and accumulation of iron and aluminum oxides w/ no distinct horizons and a zone of nutrients in a thin, fragile O-horizon. Produces Laterites.
Most duricrusts form in the _____ horrizon of a ______ weathering profile an are composed of _______,_______, or ________.
upper tropical silica (silcrete) iron (ferricrete) aluminum (alcrete)
Carbonate rocks (limestone & dolostone & marble) are
very soluble in humid climate and hence weather away very quickly whereas in semi-arid to arid climate, (little to no moisture) limestone, dolostone and marble stand for a long time and often form the top of weathering resistant ridges.
Ultramafic and Mafic Igneous rocks are
very susceptible to chemical weathering b/c they are composed of high temp minerals from Bowen's reaction series
The primary substances necessary for chemical weathering are
water, oxygen, and hydrogen ions. The Hydrogen ions are supplied primarily by dissociation of carbonic acid or water.
different types of duricrusts yield important info about the past climate conditions in which they formed. EX:
*Calcrete duricrusts & Gypcrete Duricrusts form only under semi-arid to arid climate *Sicrete, ferricrete and alcrete form under sub-tropical to tropical climate conditions.
Soil Texture
*Depends on the proportion of sand, silt, & clay sizes *Specific combinations of sand,silt, and clay define the 12 Soil Texture Classes *Loam or loamy soil refers to a soil w/ aprox equal proportions of sand, silt and clay. (good for agriculture)
Climate related Major Soil Processes & Types
*Podzolization *Calcification *Laterization
5 mechanical weathering processes
*frost wedging *stress release *Salt Crystal Growth *Thermal Expansion *Contraction *Organic Activity
Resultant products of weathering
*responsible for distinctive landforms *first step in soil formation *reduces the strength and increases the permeability of earth materials so that erosion processes are more effective
Landforms produced by weathering
*tors & castle koppies *turrets *Pedestal Rocks *Tepee Butte/Rock *Tafoni or Cavernous Weathering *Weathering Pits
Type: Oxidation
02 removed (oxygen) Needed: Oxygen
Surface area to volume ratio
Chemical weathering is dependent on available surface for reaction temp and presence of chemical active fluids. Smaller particle sizes weather by chemical means faster due to an increase of surface area.
The second most stable mineral group are
Clay Minerals. After clay minerals note that mineral stability follows Bowen's Reaction Series in reverse order of the the crystallization sequence
Major Silicate minerals in Igneous and Metamorphic Rocks become
Clay minerals or Aluminum Oxides and Iron Oxides. EX: Feldspars & Micas breaking down by Hydrolysis to form clay minerals. and Ferromagnesian minerals breaking down by Oxidation reactions to form Iron Oxides.
Factors Influencing Chemical Weathering
Climate, Rock Structure, Organic Activity, Time, Mineral & Rock Composition
Soil Profile
Consists of the vertical arrangement of all Soil Horizons down to the parent rock material
Gray, gypsum-rich limestone capping messa and pedestal rock, in semi-arid cimate near
Ghost ranch, NM
Bowen's Reaction Series is known as the
Goldich Dissolution series in case of weathering
Bio-Mechanical Weathering by Marine Boring organisms in limestone at
Phra Nang Headland, Thailand
Original Minerals: Calcite
Product: Ca, Co, Ions
Original Minerals: Muscovite Mica
Product: Clay Minerals, K ions
Original Mineral: Feldspar
Product: Clay Minerals, K, Na, Ca ions
Original Mineral: Iron-bearing Silicates (olivine, pyroxene, amphibole, biotite)
Product: Clay minerals, Iron Oxide
Original Minerals: Quartz
Product: Quartz
Organic Activity or Bio-Mechanical Activity
Tree roots and burrowing/boring animals disintegrate materials
Turrets
Vertically elongated blocks of rocks that originate from weathering along strongly developed vertical joints
E-Horizon
Very fine-grained iron and aluminum compounds are removed(elevated) & moved to lower horizontal (B-horizon). This leaves coarser materials, in the A-horizon. The -horizon is composed of mostly resistant sand and silt particles and often has a bleached appearance.
Primary substances necessary for Chemical Weathering
Water Oxygen Hydrogen Ions
C-Horizon
Zone of Partially Decomposed Rock. Would observe some original minerals in this horizon, but the felspars and micas and ferromagnesians would be beginning to break down for clays and iron oxides respectively. This is a transitional zone between the Bedrock below and the developing soils above.
R-Horizon
Zone of unweathered bedrock "fresh as a daisy"
Paleosols
a buried soil that formed at some time during the geologic past. Some typical mineral or mineral like substances that are commonly found in paleosols include limonite, bauxite and caliche. Some paleosols can be dated and used to reconstruct and interpret pateoclimates and atmospheric conditions
Talus
a coarse, angular rock material that accumulates at the base of a slope or cliff. Results from a combination of frost wedging and rock falls
Hoodoos
a fantastic column, pinnacle, or pillar of rock produces in a region of sporadic heavy rainfall by differential weathering and erosion of horizontal strata.
Weathering Pits
a shallow depression on flat or gently sloping exposures of granite due to dissolution in impounded shallow pools of water
What is the primary source of hydrogen ion?
dissociation of carbonic acid that forms as a result of dissolution of carbon dioxide in water
Those minerals that crystallize at ___ temp will be ___ stable at the surface and thus the most susceptible to chemical weathering. _____ will be least susceptible to chemical weathering and most stable
higher. least. Quartz
Tafoni or Cavernous Weathering
honeycomb texture formed by cavernous weathering on the surface of some sandstones or granites, which forms due to salt crystallization & subsequent thermal expansion and contraction on rock surface's in coastal or desert environments.
Hoodoos in
in Bandelier National Monument, NM
Mechanical Weathering
in the physical breakdown or disintegration of rocks and minerals into smaller and smaller pieces with no chemical change
Soil Erosion
is the beginning of sediment transport in the Rock Cycle and so is a natural process but is often enhanced up to tenfold by human interaction with the natural environment, especially poor agriculture practices, off road vehicles, construction and vegetation conservation
Chemical Weathering
is the decomposition of rocks & minerals into new minerals and substances due to water, oxygen, and hydrogen ions.
Rock Stability: Limestone and Marble weathering
is very dependent on climate
Dissolution breaks down
limestone, dolostone, marble, rock salt and rock gypsum
Soil erosion can be reduced by
maintain vegetative cover, tilling parallel to contour, reducing overland runoff, increasing infiltration and maintaing crops or vegetation year round.
The Master Soil Horizons are distinguished from each other based on
major compositional and textural differences. Generally, compositional changes are marked by distinct changes in color w/in the soil profile
Regolith
mantle of loose, unconsolidated, in situ weathered rock material which rests on solid, unaltered rock.
O-Horizon
mostly organic material or humus
Frost Action or Frost Wedging
need moisture, permeability in materials and temps that fluctuate across the freezing point. increases in volume when water freezes to ice physically breaks rock into smaller pieces often produces Talus.
Type: Hydrolysis
no gas removed Needed: water and hydrogen ions
Felsic Igneous rocks are
not as stable as quartzite, but are relatively insusceptible to chemical weathering b/c they are composed of primarily low temp minerals from Bowen's reaction series
Calcification
occurs in Arid climate zone & produces incomplete weathering and nutrients in the A-horizon and precipitates calcium carbonate & calcite in the B-horizon. Produces Pedocals.
Podzolization
occurs in humid temp climate sone and produces complete weathering in the A-horizon, a distinct E-horizon, iron oxides and clay minerals accumulate in the B-horizon and nutrients in the O-horizon. Produces Podzols.
Dissolution
Chemical reaction whereby ions pass from a mineral into a solution
How many textural classes exist in the soil texture triangle?
12
Salt Weathering in a Sandstone at
Abiquiu, NM
Thermal Expansion & Contraction
Alternate heating and cooling of rock surfaces under direct solar heating physically disintegrate rock material.
Pedestal rock in
Bisti-De-Na-Zin Wilderness, NM & Abiquiu, NM
Type: Dissolution
CO2 removed Needed: water and hydrogen ions
Duricrust that form only under semi-arid to arid climate
Calcrete Duricrusts (calcium carbonate) & Gypcrete Duricrusts (Gypsum)
State Soil of NC
Cecil Series
Oxidation
Chemical reaction in which oxygen as an oxidizing agent causes a loss of electrons by an atom or ion and thus involves an increase in oxidation number.
Hydrolysis
Chemical reaction w/ water in which an O-H bond is broken
Chemical weathering is like a
Differentiation process in that it breaks rocks and minerals into both soluble and insoluble substances
Hardened upper layers of horizons in Paleosols are referred to as
Duricrusts
Thermal Expansion & Contraction: rock
Expansions form parrallel to the boulder surface crack at 90o to the boulder surface form due to contraction. Rock layers gradually peel off due to alternating expansion and contraction
Oxidation breaks down
Ferromagnesian minerals in Mafic Igneous Rocks and their metamorphic equivalents
Exfoliation Domes in Granite at
Ko Samui, Thailand
Turrets in Granite at
Ko Samui, Thailand
Weathering Pits in Granite at
Ko Samui, Thailand
The formation of Tors or Castle Koppies
Large corestones are initially produced by chemical weathering along joints/fractures in massive rocks like Granite. Sharp corners are worn off by chemical decomposition. The horizontal joints are caused by stress/pressure release where as vertical joints are tectonic origin. If the area experiences uplift-induced erosion, erosion moves the granular weathered debris between corestones
Salt Weathering >> Crystal Growth
Mechanical disintegration of rocks due to growth of salt crystals in pore spaces in rocks. Most often occurs in semi-arid to arid environment where evaporation is a major process or in coastal environments.
Relationship between Mechanical and Chemical weathering
Mechanical weathering breaks rocks and minerals down into smaller and smaller pieces, which provide more surface area for chemical weathering to take place on. So the 2 types actually work together.
Igneous and metamorphic rocks form most stable to least stable
Most-Quartzite, granite, basalt-least
Soil
Natural body consisting of layers (horizons) of mineral and/or organic constituents of variable thickness,which differs from the parent material in physical,chemical, mineralogical and biological properties.
Soil Horizons
O A E B C R
Talus Slope that results from Frost Action and Rock Falls in fractured quartzite near...
Taos, NM
Tepee Buttes at
Tent Rocks National Monument near Cochiti, NM
Bauxite
aluminum oxides concentrated enough that they become ore for aluminum, which forms from intense topical weathering of aluminum rich source rocks, usually felsic igneous rock.
Tors & Castle Koppies
an isolated pile of boulders or jumbled pile of highly jointed blocks formed by a 2 stage process of weathering and erosion
Tepee Butte/Rock
an isolated, residual conical hill, formed by a capping of resistant rock that protects the underlying softer material form erosion
Pedestal Rock
an isolated,erosional mass of rock supported by or balanced on a pedestal formed by differential weathering and erosion.. when the erosional mass of rock is wide and the stem is thin they are also called mushroom, toadstool, or balancing rocks.
Rocks that are rich in quartz
are extremely stable or least susceptible to chemical weathering. such as Quartzite or Quartz Sandstone in any Climate
Laterites
are tropical soils in which intense leaching removes everything except very insoluble residues of aluminum and iron oxides. If the aluminum oxides become concentrated enough these soils may become ore for aluminum called Bauxite
Why would you expect to find the thickest soil profiles in the tropics?
b/c the primary conditions that promote rapid chemical weathering and soil formation are warmth and moisture, which are both abundant int he tropics.
Water in weathering: Mechanical weathering
b/c the water molecule expands by about 9% in volume upon freezing it can also promote mechanical weathering
Water in weathering: Chemical weathering
b/c water is polar molecule, it is a very good solvent
Stress or pressure or unloading can
continue to considerable depths and cause Exfoliation Domes and Sheeting, especially in Granites.
Hydrolysis breaks down
feldspar and micas in many types of igneous and metamorphic rocks
Mineral & Rock Composition: Bowen's Reaction Series
high temp minerals are less resistant to & low temp minerals are more resistant to chemical weathering
B-Horizon
often called the Zone Of Accumulation b/c clay minerals, iron oxides, and carbonates accumulate here. I humid-temperate climates, clay minerals & iron oxides accumulate in semi-arid climate, carbonates accumulate. Most of the materials found in this horizon are completely changed form their original rock composition w/ exception of quartz
Stress Release/Pressure release
opens large fractures in rocks and allows other processes to operate. Rocks originally form under high confining pressure, which reduces the volume of the rock. These fractures are usually large or small joints parallel to the out crop surface
The most stable minerals are
oxide minerals such as iron oxides (hematite,limonite,goethite) aluminum oxides (Gibbsite) & silica oxides (quartz)
Salt Weathering > Crystal Growth
produces Tafoni or Cavernous weathering features
Duricrusts that form under sub-tropical to tropical climate conditions
silcrete, Ferrecrete and alcrete
General info about soil formation and soil profiles
soils and soil horizons form in relatively short time spans. These changes occur from the top down and can occur in a few tens to a few hundreds of years. In less humid regions soil can take thousands of years to develop.
The source of energy for weathering is...
solar energy/radiation
A-Horizon
some humus but mostly mineral matter. Soluble materials are leached from the lower A-horizon are precipitated in the B-horizon in arid-semiarid climate. Referred to as Zone Of Leaching.
Both Mechanical Weathering and Chemical Weathering occur under...
surface/near surface conditions
Tafoni or Cavernous weathering in
the Entrada Sandstone near Abiquiu, NM
