Download Weathering

Weathering
• Weathering: physical and chemical
breakdown of earth material (rocks and
minerals) at or near the surface.
• Includes disaggregation of parent
materials, erosion and transportation of
resulting clasts and sediments, and leads to
deposition and lithification of these
materials
• Differential weathering: uneven weathering
of heterogenous materials that leads to
unique geologic structures such as hoodoos
and arches
Mechanical Weathering
• Physical processes cause weathering several ways:
• Frost action: freezing and thawing in cracks and
pores in rocks; high altitudes and latitudes
• Pressure release: expansion of uplifted and
exposed intrusive rocks; exfoliation domes such as
Stone Mountain, GA, rock bursts in mines
• Thermal expansion/contraction: differential
reaction by rocks and minerals to daily or seasonal
temperature changes
• Salt crystal growth: occurs primarily in hot, arid
regions
• Biological actions: burrowing, root wedging
Chemical Weathering
• Processes that chemically alter parent material:
• Solution/dissolution: occurs when ions separate in
water due to the solvent properties of H20,
example: dissolution of calcite in limestones by
groundwater
• Oxidation: occurs when oxides are formed by
mineral ions combining with oxygen, example: Fe +
O = hematite (rust)
• Hydrolysis: free ions of H+ and OH- in water can
replace ions in minerals, which are liberated to
undergo oxidation, example: orthoclase weathers
to kaolinite by loss of Al ions
Chemical Weathering
• Chemical weathering operates on the surface of rocks and
minerals, and can be observed in the difference between old
and fresh surfaces of broken rock, controlled by:
• Fractures: produce increased surface area in equal volume of
rock
• Climate: presence of water greatly accelerates weathering.
Humid, warm climates have high weathering rates
• Composition: stable minerals are resistant to weathering,
example: Quartz; less stable minerals rapidly weather
(Bowen’s reaction series)
• One unique form of weathering is spheroidal weathering:
rocks often weather into spherioids due to uneven
weathering. 3X at points compared to flats
Soil
• Regolith: sediments that cover the Earth’s
land surface
• Soil: regolith composed of weathered
minerals, air, water, and organic material;
supports vegetation
• all land-dwelling organisms dependent on
soil
• good farming soil composed of sediments
(sand, silt, and clay), air, water, and humus.
• Humus: carbon derived by bacterial decay
of organic material
Soil profile
• Soil forms in characteristic
layers:
• Horizon O: thin subaerial
layer, predominately organic
material, including humus
• Horizon A: weathered
unstable minerals that
provide nutrients, remnants
of stable minerals such as
quartz grains, a zone of
biological activity.
• Horizon B: where minerals
leached from above
accumulate (zone of
accumulation)
• Horizon C: unaltered to
altered parent material
Sedimentary Rocks
• Composed of sediments from weathering,
chemical precipitation, and clastic material
including fossil fragments
• Sediments are produced by erosion or
chemical means, then transported by
physical processed such as wind, water,
and gravity, and deposited as loose
aggregates of particles.
• Sedimentary particles are classified
according to size: > 2 mm = gravel, 1/16-2
mm = sand, 1/256-1/16 mm = silt, < 1/256 =
clay
Sedimentary Rocks
• Depositional environment: describes the basin in
which sediments are deposited, including: stream
beds, lake bottoms, river deltas, coastal beaches,
continental shelves, and sand dunes
• Variations in transport duration and conditions
produce characteristic sedimentary particles;
abrasion reduces particle size, rounding wears
away sharp angles, and sorting occurs during
deposition
• After deposition, sediments can be lithified by
compaction and cementation to produce
sedimentary rocks
Depositional Environments
Rounding of Sediments
Sedimentary Rocks
• The most common cements in sedimentary rocks are the
minerals calcite and silica, which are derived from
weathering of rocks and transported by water
• There are two major types of sedimentary rocks:
• 1) DETRITAL SEDIMENTARY ROCKS-composed of solid
particles (detritus) derived from weathered parent rocks,
classified according to size and include:
• conglomerate: composed of gravel-sized particles, breccia
refers to a conglomerate with angular clasts
• sandstone: composed of sand-sized clasts of any
composition, most common clast is quartz
• mudrock: general term for sedimentary rocks with silt to
clay sized clasts, varieties include: siltstones, mudstones,
claystones, fissile = shale
Sedimentary Rocks
• 2) CHEMICAL SEDIMENTARY ROCKS: have a
chemical origin and include:
• limestone: the most common carbonate rock,
composed of carbonate mud usually precipitated
from shallow, warm marine waters, and often
containing fossil fragments
• coquina: type of limestone composed of
fragmented seashells
• oolitic limestone: contains small, spherical grains
(ooids), concentric rings of calcite precipitated
around a nucleus
Chemical sedimentary rocks
• Other chemical sedimentary rocks are:
• evaporites: composed of dissolved
substances precipitated from
evaporated water, examples: halite
(NaCl) and gypsum (CaSO4.2H2O)
• chert: microcrystalline quartz deposited
as beds of microfossils (radiolarians and
diatoms), often re-crystallized, includes
flint and jasper
Sedimentary Rocks
• Coal: derived from compressed, altered
land plant remains:
• peat: barely altered plant material
• lignite: more fully altered, low grade coal
(~70% carbon)
• bitumin: or bituminous coal, higher grade
(~80%), with little remaining evidence of
plant material
• anthracite: high grade, high carbon coal (up
to 98% carbon)
Sedimentary structures
• These are textural features formed by
variation in deposition, and include:
• bedding/strata: horizontal layers
• graded bedding: vertical grain size change
• cross bedding: bedding at angles to
depositional surface
• ripple marks: small dunes preserved on
sand deposits, formed by moving fluids (air,
water)
• mud cracks: indicative of subaerial drying
Fossils
• remains of ancient life, found in sedimentary
rocks
• trace fossils: tracks, prints
• body fossils: altered or unaltered remains of
body parts such as bones, teeth or shells
• microfossils: microscopic body fossils
• biostratigraphy: correlation of sedimentary
rocks using fossils
Resources in Sedimentary
Rocks
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clay, sand, and gravel
evaporite minerals: salt, gypsum, fertilizers
placer deposits: gold, silver, uranium
coal
petroleum/natural gas
iron (b.i.f.s)