Sedimentary rocks are rocks formed the accumulation and consolidation of sediments. Some sedimentary rocks are composed of particles weathered from pre-existing rocks.
Sediments- weathered rock materials that are deposited by water, ice, or wind, precipitated from concentrated chemical solutions or produced by organic processes.
1. detrital (clastic) sediments
broken and weathered rock material (sedimentary, igneous, meatamorphic) that has not been dissolved. Clastics make up more than 75% of all sediments
on Earth's surface.
2. chemical precipitates
formed by the precipitation of concentrated solutions. Examples: rock salt, rock gypsum and chert
3. biochemical
formed by organic process. Examples: calcite shells(limestone, chalk), pant deposits in swamps (coal)
Characteristics of sediments can be used to determine geologic history
1. composition
depends on mineralogy of source rock, duration of weathering and the rate of burial.
2. size and sorting
a. size depends on the mineralogy of the source rock, distance traveled and the method of transport. Classisfied using the Wentworth scale.
>2mm -gravel (conglomerate and breccia)
2mm-1/16mm- sand (sandstone)
<1/16mm- clay/silt (mud, mudstone,shale)
b. sorting is controlled by the method of transport and the distance
well-sorted- all the same size and composition
poorly-sorted- not all the same size and composition
3. roundness is controlled by the method of transport. The more round the grains are the more the rock has been transported from its original environment.
4. color- dependent on the original mineral color, weathered mineral color, and organic material
Types of weathering
Mechanical weathering: physical change in size of the rock material without change in the composition
1.frost wedging
Water moves into open spaces (fractures) in the rock and freezes; once it freezes the rock will break apart because water expands 9%. Most important in
temperate regions.
2. unloading(pressure release)
Large crystalline bodies are exposed by erosion resulting in their expansion and fractures forming layers. Example: Stone Mountain granite
3. thermal contraction and expansion
Daily heating and cooling will weaken and break the rock material. Most important in desert areas.
4. biologic activity
Usually done by plants and animals. Examples: root-wedging, burrowing and humans Chemical weathering: results in a change in the composition of the rock material; *water is the most important agent.
1. solution
change from a solid to a liquid
2. oxidation
The process of combining with oxygen ions. A mineral that is exposed to air may undergo oxidation. Most important in ferromagnesium minerals hematite
and limonite (iron oxides).
3. hydrolysis
reaction with water such that water is incorporated into the new mineral. Example: the weathering of feldspar will produce kaolin and ions in solution.
Diageneis- all the changes that the sediment undergoes after its initial deposition.
Lithification- process where a sediment is converted to a rock
1. compaction
rearrangement of particles due to increasing weight of overlying rocks.
2. cementation
precipitation of crystalline materials between particles. The most common cements are silica, calcite, and iron oxides.
3. mineralogic changes
the breakdown of less stable grain(breakdown of feldpars forming a clay matrix); usually caused by diagenesis
Sedimentary rock types
Detrital (clastic) rocks
conglomerate- rounded gravel-sized particles
breccia- angular gravel-sized particles
sandstone- sand-sized particles mostly composed of quartz
siltstone- silt-sized partilces
shale- fine-grained, composed mainly of silt and clay *most abundant sedimentary rock
Chemical Precipitates
limestone- CaCO3
dolomite- MgCO3, form by the direct precipitation of sea water
chert- direct precipitation of sea water
Evaporites form by the evaporation of sea water
halite-- rock salt
gypsum-- rock gypsum (CaSO4-2H2O).
anhydrite-- loss of water in the atomic structure(CaSO4)
Biochemical
coquina- loosely cemented shell material (coral)
chalk- soft, porous limestone made of micrscopic marine organisms
chert- siliceous deposits of diatoms
coal- oraganic deposits formed in swamps
Sediments- weathered rock materials that are deposited by water, ice, or wind, precipitated from concentrated chemical solutions or produced by organic processes.
1. detrital (clastic) sediments
broken and weathered rock material (sedimentary, igneous, meatamorphic) that has not been dissolved. Clastics make up more than 75% of all sediments
on Earth's surface.
2. chemical precipitates
formed by the precipitation of concentrated solutions. Examples: rock salt, rock gypsum and chert
3. biochemical
formed by organic process. Examples: calcite shells(limestone, chalk), pant deposits in swamps (coal)
Characteristics of sediments can be used to determine geologic history
1. composition
depends on mineralogy of source rock, duration of weathering and the rate of burial.
2. size and sorting
a. size depends on the mineralogy of the source rock, distance traveled and the method of transport. Classisfied using the Wentworth scale.
>2mm -gravel (conglomerate and breccia)
2mm-1/16mm- sand (sandstone)
<1/16mm- clay/silt (mud, mudstone,shale)
b. sorting is controlled by the method of transport and the distance
well-sorted- all the same size and composition
poorly-sorted- not all the same size and composition
3. roundness is controlled by the method of transport. The more round the grains are the more the rock has been transported from its original environment.
4. color- dependent on the original mineral color, weathered mineral color, and organic material
Types of weathering
Mechanical weathering: physical change in size of the rock material without change in the composition
1.frost wedging
Water moves into open spaces (fractures) in the rock and freezes; once it freezes the rock will break apart because water expands 9%. Most important in
temperate regions.
2. unloading(pressure release)
Large crystalline bodies are exposed by erosion resulting in their expansion and fractures forming layers. Example: Stone Mountain granite
3. thermal contraction and expansion
Daily heating and cooling will weaken and break the rock material. Most important in desert areas.
4. biologic activity
Usually done by plants and animals. Examples: root-wedging, burrowing and humans Chemical weathering: results in a change in the composition of the rock material; *water is the most important agent.
1. solution
change from a solid to a liquid
2. oxidation
The process of combining with oxygen ions. A mineral that is exposed to air may undergo oxidation. Most important in ferromagnesium minerals hematite
and limonite (iron oxides).
3. hydrolysis
reaction with water such that water is incorporated into the new mineral. Example: the weathering of feldspar will produce kaolin and ions in solution.
Diageneis- all the changes that the sediment undergoes after its initial deposition.
Lithification- process where a sediment is converted to a rock
1. compaction
rearrangement of particles due to increasing weight of overlying rocks.
2. cementation
precipitation of crystalline materials between particles. The most common cements are silica, calcite, and iron oxides.
3. mineralogic changes
the breakdown of less stable grain(breakdown of feldpars forming a clay matrix); usually caused by diagenesis
Sedimentary rock types
Detrital (clastic) rocks
conglomerate- rounded gravel-sized particles
breccia- angular gravel-sized particles
sandstone- sand-sized particles mostly composed of quartz
siltstone- silt-sized partilces
shale- fine-grained, composed mainly of silt and clay *most abundant sedimentary rock
Chemical Precipitates
limestone- CaCO3
dolomite- MgCO3, form by the direct precipitation of sea water
chert- direct precipitation of sea water
Evaporites form by the evaporation of sea water
halite-- rock salt
gypsum-- rock gypsum (CaSO4-2H2O).
anhydrite-- loss of water in the atomic structure(CaSO4)
Biochemical
coquina- loosely cemented shell material (coral)
chalk- soft, porous limestone made of micrscopic marine organisms
chert- siliceous deposits of diatoms
coal- oraganic deposits formed in swamps
Pictures of some sedimentary rocks:
For more information please visit:
http://geology.com/rocks/sedimentary-rocks.shtml
Video:
http://www.videojug.com/film/geology-sedimentary-rocks
http://geology.com/rocks/sedimentary-rocks.shtml
Video:
http://www.videojug.com/film/geology-sedimentary-rocks