Earth Science
ademola
Sept21_weathering_erosion.pptxWhat have we done so far?
Deposition of:
Now we’ll look at (Ch 20.8)
Done
Weathering, Erosion and Deposition
Weathering – processes that break down rocks and minerals near Earth’s surface
Erosion – moves Earth materials from their original sites
Deposition- end of the line. Particles come to rest. Build up layers of sediment
Weathering
A. Mechanical Weathering - processes that break a rock or mineral into smaller pieces without altering its composition
B. Chemical Weathering - processes that change the chemical composition of rocks and minerals
Mechanical Weathering
Physically breaking up rocks/minerals
CHEMICAL COMPOSITION DOES NOT CHANGE
1. frost wedging
2. thermal expansion and contraction
3. abrasion by wind, water or gravity
4. plant growth
Mechanical weathering is easier to picture, but not as
overall as important as chemical weathering
Processes and Agents of Mechanical Weathering
1. Frost Wedging – cracking of rock mass by the expansion of water as it freezes in crevices and cracks
http://www.uwsp.edu/geo/faculty/ozsvath/images/frost%20wedging.jpg
Potholes!
Mechanical Weathering
2. Thermal expansion and contraction –
repeated heating and cooling of materials cause rigid substances to crack and separate
Processes and Agents of Mechanical Weathering
3. Abrasion – Scraping
by wind (esp. with blowing sand or dust)
By water (mechanical pounding or
scraping by rock pieces and sediments which are suspended in it)
Wind and Water Abrasion
http://www.gsi.ie/Education/European+Landscapes/United+Kingdom.htm Photo Ref: P211442, "IPR/52-34CW BGS©NERC
Processes and Agents of Mechanical Weathering
4. Plant/Animal Growth – Small roots from plants and trees into cracks in rocks. Roots increase in size force the rock sections apart, increasing the separation and weathering.
Example: Sidewalks buckling/crumbling due to street trees growing underneath
Animal burrows breaking up soft rocks
Plant Wedging
B. Chemical Weathering
dissolving (dissolution)
oxidation
Hydrolysis (not considered here)
Three ways to disintegrate rock
Water is important for all three
Processes of Chemical Weathering
Dissolving (dissolution)
Water, often containing acid from dissolved carbon dioxide, will dissolve minerals from a rock body leaving cavities in the rock. These cavities may generate sinkholes or cave features such as stalactites and stalagmites.
Limestone cave feature result of dissolution
Processes of Chemical Weathering
Oxidation (rusting)
Minerals may combine with oxygen to form new minerals that are not as hard (crumble more easily). For example, the iron-containing mineral pyrite forms a rusty-colored mineral called limonite.
Pyrite Oxidation
http://www.windows.ucar.edu/earth/geology/images/pyrite_sm.jpg
http://www.dkimages.com/discover/previews/965/75014124.JPG
Pyrite
Limonite
Processes of Chemical Weathering
Hydrolysis
Minerals may chemically combine with water to form new minerals. Again these are generally not as hard as the original material.
Factors in Chemical Weathering
Climate – wet and warm maximizes chemical reactions
Plants and animals – living organisms secrete substances that react with rock
Time – longer contact means greater change
Mineral composition – some minerals are more susceptible to change than others
Air pollution- acid rain
A good way to measure effect of air pollution over time?
~8.5 mm/50 yrs = 0.17 mm/yr
Very little reduction for recent years
Older inscriptions show greater surface reduction.
Mechanical or Chemical? Y or N?
| Process | Mechanical Weathering | Chemical weathering |
| Ice wedging | Y | N |
| Plants | ||
| Animals | ||
| Natural Acids | ||
| Plant Acids | ||
| oxygen |
Filled in chart to be available on the review before the midterm
Which produces more sediment (i.e. is more important
In weathering: mechanical or chemical?) (see Ch. 20.8)
Results of Weathering
Weathering produces regolith (“rock blanket”) which is composed of small rock and mineral fragments.
(Earth and Mars)
When organic matter is mixed into this material it is called soil.
(Earth only)
Erosion Transport Agents or Forces
Water
rain
streams and rivers
ocean dynamics
ice in glaciers
Wind
Gravity
What is most important on Earth? Why?
Effects of Liquid Water are most important
Chemical weathering by water produces more sediment
than any other process
Erosion by water. Water is most effective way to transport
products of weathering
Note: abrasion during erosion is mechanical weathering
show “riverstone”
Suppose a planet had no liquid water.
What would you speculate about weathering and erosion
on this planet?
Streams
Flowing surface water will lift and carry small
sediments such as silt and sand.
Stream Erosion and Deposition
Where water moves more swiftly there will be more
erosion.
Where the water slows down, sediments will be
deposited.
Speed of water determines what size particles get deposited
(more on this Wednesday)
Ocean Dynamics
Changing currents (tides) and waves carry away weathered materials.
http://www.dkimages.com/discover/previews/1000/50195183.JPG
http://edge.tamu.edu/waves2001/PC_tour/erosion_files/image002.jpg
Glaciers
Glaciers are large ice fields that slowly flow downhill over time and drag rocks and soil along with them
http://images.encarta.msn.com/xrefmedia/sharemed/targets/images/pho/t628/T628797A.jpg
Wind Transport of Sediments
Transport by Gravity
Weathered sediments can collapse
Technical term: mass wasting
Popular term: rockslides
http://en.wikipedia.org/wiki/Mass_wasting
Finally: Deposition Formation
End result of transport:
Deposition makes layers called strata
(stratified is to be layered)
Definition: eroded particles come to rest.
Water slows down, particles fall out
Wind stops blowing
Glacier stops moving/melts rocks fall out
Example of Strata
Where is this?
What kind of rocks are we looking at?
From Sediments Sedimentary rock
Sedimentary rock is classified by the grain size of
sediments (see table 20.3) this tells you
about how deposited
1. A conglomerate deposited in a fast moving river
Made up of gravel/pebbles
2. Shale deposited in tranquil lake
Made up of tiny mud particles
3. Sandstone is in between
Summary
4 types of mechanical weathering
2 types of chemical weathering
3 erosion agents (counting ice and
water as the same)
How does type of sedimentary rock tell you about
the depositional environment?
Weathering Rate of Marble
Tombstones in Sydney, Australia
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
187018901910193019501970
Inscription Date (Years AD)
Surface Reduction (mm)
Data taken from Dragovitch, D., 1986,
Weathering rates of marble in urban
environments, eastern Australia,
Zeitschrift für Geomorphologie, v. 30,
p. 203-214.
