Download WEATHERING AND SEDIMENTARY ROCKS

WEATHERING AND SEDIMENTARY ROCKS
Weathering - Process which acts at the earth's surface to decompose and
breakdown rocks.
Erosion - The movement of weathered material from the site of weathering.
Primary agent is gravity, but gravity acts in concert with running water.
Types of Weathering
1. Mechanical or Physical - the breakdown of rock material into smaller and
smaller pieces with no change in the chemical composition of the
weathered material.
2. Chemical - the breakdown of rocks by chemical agents. Obviously the
chief chemical agent is water which carries dissociated carbonic acid.
Mechanical Weathering
1. Expansion and Contraction - the thermal heating and cooling of rocks
causing expansion and contraction.
2. Frost Action - Water freezes at night and expands because the solid
occupies greater volume. Action wedges the rocks apart. Requires
adequate supply of moisture; moisture must be able to enter rock or soil;
and temperature must move back and forth over freezing point.
3. Exfoliation - process in which curved plates of rock are stripped from a
larger rock mass. Example Half Dome. Exact mechanism uncertain but
probably due to unloading.
4. Other types - Cracking of rocks by plant roots and burrowing animals.
Chemical Weathering
Factors which effect the rate of chemical weathering are:
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Particle size - Smaller the particle size the greater the surface area and
hence the more rapid the weathering
Composition
Climate (See Figure)
Type and amount of vegetation
Chemical Weathering of Rocks
Show Figure and explain formation of carbonic acid
H2O + CO2 ------->> H2CO3
Acid then dissociates and the following happens:
2KAlSi3O8 (feldspar)+ 2H+ + H2O ------->> Al2Si2O5(OH)4 (clay)+ 2K+ + 4SiO2
Weathering of Igneous Minerals
Products of Weathering Figure:
1. Quartz - slow process and largely ineffective. Quartz remains quartz.
Grains are rounded.
2. Feldspar - weathers to clay with the cations Na, Ca, and K going into
solution. Clays that can form include kaolinite (pure aluminum silicate),
illite and montmorillonite. Factors which dictate clay formation are (a)
climate; (b) time; (c) parent material.
3. Muscovite - Same as above
4. Ferromagnesian minerals - weather to clay plus highly insoluble iron
oxides, essentially varieties of limonite (rust).
Rates of Weathering
Studied by S.S. Goldich (Figure) and found to be inverse of Bowen's Reaction
Series. Why? A function of equilibrium, the higher the temperature of formation of
a mineral the more unstable it is at the earth's surface. Hence olivine weathers
the most rapidly.
Soils
Soil - Surficial material that forms due to weathering. Includes an organic
component. Many different soil types. Factors effecting their formation are:
1) Climate
2) Relief
3) Bedrock material
4) Time
Classification of soils varies depending on the classifier. Geologists use a very
simple classification based largely on materials added or removed from the soil
during its formation.
Soil consists of four major zones (horizons) (Figure).
1. O horizon - Organic layer
2. A horizon - Zone of leaching - Cations are
leached from this horizon by strongly acid solutions
generated in the O horizon
3. B horizon - Zone of Accumulation - Cations
leached out of the A horizon accumulate here.
Horizon consists of clays, iron and aluminum oxides.
Deposition due to neutralization of acid solutions.
4. C horizon - Partially decomposed parent material.
Lower most zone.
Soil Types
Pedalfer - Named for the abundance of Al and Fe in the B horizon. Occur in
temperate, humid climates. Lie generally east of the Mississippi River,
correspond with 63 cm/yr rainfall contour.
Pedocal - Named for the accumulation of calcium carbonate in the B horizon.
Characteristic of temperate, dry climates. Lie generally west of Miss River. Poorly
developed A horizon, B horizon is caliche (calcium carbonate).
Laterites - Tropical soils thought to represent the end products of weathering.
Characterized by stark red color and abundance of iron and aluminum oxides
and lesser clay minerals. Requires abundant rainfall.
SEDIMENTARY ROCKS
Sedimentary Rocks - Layered or stratified rocks formed at or near the earth's
surface in response to the processes of weathering, erosion, transportation and
deposition.
Rock Cycle
All rocks discussed in this class are a part of the rock cycle (Figure).
Sedimentary Cycle (Figure)
Processes
1. Transportation - Transporting medium usually water. More rarely wind or
glacial ice.
2. Deposition - Occurs when energy necessary to transport particles is no
longer available. Deposition due to the gentle settling of mineral grains.
Can also be result of chemical precipitation due to changing conditions.
3. Lithification - Involves several steps. All taken together are termed
Diagenesis.
a. Compaction - Squeezing out of water.
b. Cementation - Precipitation of chemical cement from trapped water
and circulating water.
c. Recrystallization - Growth of grains in response to new equilibrium
conditions
Single most important characteristic of sedimentary rocks is layering. Occurs in
response to changes in conditions at the site of deposition. Sedimentary rocks
cover 75% of the earth's surface, but amount to only 5% of the outer 10 km.
Origin of Sedimentary Material
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Derived directly from pre-existing rocks. Ex. quartz.
Derived from weathered products of these rocks. Ex. clay.
Produced by chemical precipitation. Ex. calcite.
First two processes result in detrital or clastic rocks. Third produces nondetrital or
chemical sedimentary rocks.
Minerals of Sedimentary Rocks
1. Clay - Important constituent of mudstones and shales, but occurs in minor
amounts in all sedimentary rocks.
2. Quartz - Most abundant constituent of sandstone. In addition to detrital
quartz, free silica can be chemically precipitated as opal, chalcedony and
chert.
3. Calcite - Chief constituent of limestone. Precipitates from seawater which
is saturated in both Ca+2 and CO3-2. Small changes in both T and P
enough to cause precipitation. Differs from most compounds in that
solubility decreases with increasing temperature.
4. Others
a. Dolomite CaMg(CO3)2 - Most important constituent of dolostone
b. Feldspars - Occur in sedimentary rocks formed by very quick
deposition and burial allowing no time for feldspars to alter to clay.
c. Iron oxides and sulfides - Chemical precipitates dictated by the
environment at the site of deposition.
d. Salts and gypsum - Chemical precipitates occurring in restricted
sedimentary basins under arid climatic conditions. Modern analog
is the Middle East (Red Sea).
e. Volcanic Debris - Glass and other pyroclastic material incorporated
into sediments.
5. Organic Material - Forms coal and gives color to black shales.
Classification of Sedimentary Rocks
Texture - Size, shape and arrangement of particles.
1. Clastic - Formed from broken or fragmented grains (detrital). Rock
appears grainy. Basis of classification of the clastic rocks is the Wentworth
Size Scale which was derived from studies of grain diameters.
Wentworth Size Scale
Boulder
>256 mm
Cobble
64-256 mm
Pebble
2-64 mm
Sand
Silt
Clay
Conglomerate
1/16-2 mm
Sandstone
1/256-1/16 mm
Siltstone
<1/256 mm
Shale
Conglomerate - Detrital rock made up of more or less rounded fragments, an
appreciable percentage of which are pebble size or larger
Sandstone - Consists primarily of grains in the sand size range. Dominant
mineral in sandstones is always quartz. Further subdivide sandstones based on
other minerals present. Quartz sandstone is 99% quartz. Arkose contains both
quartz and feldspar. Graywacke is a garbage sandstone with quartz, feldspar,
mica and rock fragments. Often has a significant fine-grained component and is
poorly sorted.
Siltstone - Rare sedimentary rock composed mostly of silt sized particles. Rare
because dominant mineral is quartz which does not like to get any smaller than
sand size. Many siltstones thought to form by glacial grinding of sand-sized
quartz grains.
Shale - Most common of the sedimentary rocks. Composed primarily of clay
minerals. Often tends to split into flat sheets due to the mica-like cleavage of clay
minerals.
2. Nonclastic (chemical) - Grains are interlocked through crystallization. Has
igneous appearing texture with very little open space.
Limestone - Formed by the precipitation of calcite from seawater. Most form in
marine environments, but also around hot springs, as a crust in desert soils, and
as cave formation.
Dolostone - Composed of the mineral dolomite. Probably starts life as limestone
then is altered to dolostone by Mg-bearing solutions in arid environments.
Evaporites - Formed by partial to complete evaporation of seawater in enclosed
basins. Forms salts and gypsum.
Organic Rocks - Rocks formed by the accumulation of organic material. Ex.
coquina and chalk.
Coal - Rock composed of lithified plant material.
Abundance of Sedimentary Rocks (Figure)
Sedimentary Structures (Figure)
A) Structures formed during deposition
1. Bedding - Layering of sedimentary rocks. Each bed represents a
homogeneous set of conditions of sedimentation. New beds indicate new
conditions. Most layering is parallel, but occasionally it is inclined. These
inclined layers are cross beds. Examples of sedimentary environments in
which cross beds form are dune fields and deltas.
2. Graded beds occur when a mass of sediment is deposited rapidly. The
bedding has the coarsest sediment at the bottom and finest at the top.
Often found forming in submarine canyons. A collection of graded beds is
termed a turbidite deposit. Well exposed in many of the sea cliffs along
So. Cal. beaches.
3. Ripple Marks - Waves of sand often seen on a beach at low tide and in
stream beds.
a) Current - asymmetrical - Rivers
b) Oscillation - symmetrical - Beaches
4. Mud Cracks - Polygonal-shaped cracks which develop in fine grained
sediments as they dry out. Common in arid environments, such as a
desert.
B) Structures formed after deposition
1. Nodule - Irregular, ovoid concentration of mineral matter that differs in
composition from the surrounding sedimentary rock. Long axis of the
nodule usually parallels the bedding plane and seems to prefer certain
layers.
2. Concretion - Local concentration of cementing material. Generally round.
Usually consist of calcite, iron oxide or silica. Can exceed 1 meter in
diameter. Not understood how they form.
3. Geode - Roughly spherical structures up to 30 cm in diameter. Outer layer
consists of chalcedony. Inside lined with crystals. Calcite and quartz the
most common.
C) Other features
1. Fossils - Any direct evidence of past life. Examples are dinosaur bones,
shells of marine organisms, plant impressions, etc.