Transcript Formation of Sedimentary Rocks

Formation of Sedimentary Rocks
Formation of Sedimentary Rocks
 Sedimentary rocks account for approximately 5% of the
crust. The upper 16 km of Earth.
 Regardless of this low percentage, approximately 75% of
all rocks exposed at the surface is sedimentary rock.
 Factors leading to the formation of sedimentary rocks
include;
1) Erosion and weathering
Reference: Tarbuck
2) Transportation
and Lutgens Pages 41
3) Deposition
- 42
4) Lithification
Erosion and Weathering
 In reference to the Rock Cycle, the origin of sedimentary
rocks begins with the processes of weathering and erosion.
 Pre-existing rocks can be broken down both mechanically
or chemically to produce sediment.
 Mechanical Weathering happens when a material is broken
into smaller pieces which still retain the characteristics of
the original material. Example: freeze – thaw action.
Water freezing cracks
fragments of rock
Mechanical weathering
 Frost wedging
Erosion and Weathering
 In reference to the Rock Cycle, the origin of sedimentary
rocks begins with the processes of weathering and erosion.
 Pre-existing rocks can be broken down both mechanically
or chemically to produce sediment.
 Chemical Weathering happens when a material is altered
by chemical action and a new material is produced which is
chemically different than the original material.
Examples: Oxidation reactions - Iron rusting
Decomposition of limestone (statues and headstones)
Chemical weathering
Chemical weathering:
Processes that break rock components and
internal structures of minerals, making new
minerals
Most important processes:

dissolution
 oxidation
 hydrolysis
WATER: most important agent
Chemical weathering
 Dissolution
Dissolving minerals by a
liquid agent (such as water)
How?
Many minerals are water-soluble
example: Halite (salt)
Adding acid (H+) increases corrosiveness
example: carbon dioxide and rain
Chemical weathering
 Oxidation
Chemical reaction that causes loss of electrons
Example
Iron (Fe) + water (H2O)
oxygen (O) combines with Fe to form hematite (Fe2O3)
Oxidation decomposes important minerals:
mineral
olivine
pyroxene
hornblende
formula
(Mg,Fe)2SiO4
(Mg,Fe)SiO3
Ca2(Fe,Mg)5Si8O22(OH)2
where
Earth’s mantle
Earth’s mantle
Continental rocks
Chemical weathering
 Hydrolysis
Decomposition of granite
Granite (mainly quartz + potassium feldspar)
+
carbonic acid (water + carbon dioxide)
 Liberates potassium from feldspar
 makes kaolinite
 quartz
- important plant nutrient
- this left over mineral is clay
- very stable
- main inorganic part of soil
- very stable, glassy appearance
- carried to sea: beach sand, sand dunes
- cements to form sandstone
Chemical weathering
 Alterations
chemical weathering that causes physical
changes to rocks
Rock corners are eroded more rapidly than sides
(more surface area at corners)
Chemical weathering
 Alterations
“ spheroidal weathering ”
T&L Figure 5.11
Transportation of Sediment
 Erosional agents (such as, water, wind, and ice,) influenced
by gravity, transport the sediment to new locations where it
is deposited.
 Sediment is broken down during the transport phase. This
process is called Abrasion.
 Three main agents tend to transport the sediment, these
being;
1) Water (rivers and groundwater)
2) Ice (glaciers)
Wind Blown Sand
3) Wind
Deposition of Sediment
 Eventually the sediment is deposited in lakes, river
valleys, seas, and more frequently the oceans.
 In dry areas of the world, sediment (sand and silt)
can accumulate in areas called deserts.
Lithification of Sediment
 Following deposition, sediment may become lithified,
which means turned to solid rock.
 Sediment is commonly lithified by two processes;
1) Compaction:
when the sediment size is small (silt and clay), the weight
from the layers of sediment above can compact the lower
layers forming solid sedimentary rock.
Shale
Round Pebbles
Conglomerate
Lime (Calcite)
Sand
Silt and Clay
Limestone
Compaction
Sandstone
Shale
Lithification of Sediment
 Following deposition, sediment may become lithified,
which means turned to solid rock.
 Sediment is commonly lithified by two processes;
2) Cementation:
Over long period of time mineral matter dissolved in groundwater
pass through pore spaces and cement (stick) sediment particles
together to form solid sedimentary rock.
Sediment Particles
Water with dissolved minerals,
(calcite or silica) pass through
pore spaces
Cement