8 Weathering

8/10/2019 8 Weathering

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Weathering

• Physical weathering – breaking larger rocks into

smaller rocks without changing the minerals.

– Produces clastic sediment that will eventually make

clastic sedimentary rocks• Chemical weathering – chemical reactions that

cause some minerals to break down; new

minerals are created through the process as well

as dissolved ions. – Produces chemical sediment that will eventually make

chemical sedimentary rocks



Physical Weathering:

fracture along zones of weakness

A joint is a fracture where the rocks on either side have not moved. In afault the rocks on either side have moved.



Fig. B.5a


frost wedging

Water expands when it

freezes, which can expand

fractures in the rock and can

push blocks in the direction of

expansion.

Most effective in locationswith frequent freeze-thaw

cycles, e.g. early spring and

late fall in New England.



Physical Weathering

http://localhost/var/www/apps/conversion/tmp/Animations/PhysWeatherppt.html

http://localhost/var/www/apps/conversion/tmp/Animations/PhysWeatherppt.html




differential thermal expansion

• Some minerals expandmore than others for anygiven temperature change.

• In this case, the blackmineral will expand morethan the blue minerals, soit pushes them apart.

• Most effective inenvironments where thereare large day-nighttemperature differences.

warming



Physical Weathering: exfoliation

• When rocks form deep in the

Earth they form under a great

deal of pressure.

• As the overlying rock is

removed by erosion, the

amount of pressure they are

subjected to decreases and

they expand vertically. This

forms cracks that are parallel tothe ground surface.

• Most effective in places that

are undergoing erosion at the

ground surface.

Depth = 5 km

Vertical expansionHorizontal cracks



Physical Weathering: exfoliation

Fig. B.4a



Chemical Weathering: Hydrolysis

1. water + carbon dioxide → carbonic acid

2. feldspar + carbonic acid + water →

clay minerals + dissolved ionsincluding the bicarbonate ion

3. ions washed to ocean

4. organisms use bicarbonate ion to buildshells (calcium carbonate)





Oxidation Reaction

1. The iron in mafic

minerals (e.g. pyroxene

or olivine) is Fe2+.

2. When these minerals

are exposed to oxygen,

the iron they contain is

converted to Fe3+.

3. This oxidized iron is

converted to iron oxideminerals such as

hematite (Fe2O3).



Some Examples

of

Iron Oxidation



Iron Oxides

The rocks in this environment are coated with iron oxide minerals and give the

landscape a very red appearance. Note that the iron oxides were formed millions ofyears ago in a more humid environment.



Dissolution

1. water + carbon dioxide

→ carbonic acid

2. calcite + carbonic acid

→ dissolved ions +carbon dioxide

The rock is completelydissolved.

Fig. B.6b



Controls on Chemical Weathering Rate:

1. Rock type

Fig. B.8dIn general felsic minerals are more resistant to weathering than maficminerals. Marble is even less resistant than some mafic minerals.




2. Surface Area

Fig. B.7a

If there is more surface area, then there are more places for the reactions to

occur and the overall rate of weathering is faster.




3. Climate

• Chemical reactions are faster at warmer

temperatures

• Water is required for the reactions to take

place.

hot/humid > cold/humid > hot/dry > cold/dry




4. Presence of a soil

• CO2 concentrations are ~10x higher in the

soil than in the atmosphere. Therefore,

more carbonic acid is formed in the soil.

• Also, soils retain water like a sponge so

stay wet long after the rain.




5. Time

• Weathering rates of fresh rock are slow,because there are few fractures and low surfacearea.

• As physical weathering processes create microand macro fractures, the rate of weatheringincreases because there is more surface areaexposed.

• Later, the rate of weathering slows again as allthe easily-weathered minerals have beenremoved from the material, leaving on the moreresistant ones.



The Earth’s Thermostat

Ocean Crust

limestone sediment

CO2H2O + CO2

bicarbonate ion

Water and carbon dioxide react to

form carbonic acid which corrodes

rock creating the bicarbonate ion.

These ions are washed into theocean where they are transformed

into limestone sediment.

Some (but not all) of this limestonesediment is subducted. The

subducted limestone will melt, the

carbon from the limestone remains

as CO2 in the magma, which will

eventually escape back into the

atmosphere.

Limestone

sediment



The Earth’s Thermostat

• Something warms Earth

• Chemical weathering willaccelerate.

• Faster weathering

removes CO2 from theatmosphere faster thanvolcanic eruptions* areputting it back into theatmosphere.

• The amount of CO2 in theatmosphere decreases,so the climate cools backto its original state.

• Something cools Earth

• Chemical weathering willslow.

• Slower weathering

removes CO2 from theatmosphere more slowlythan volcanic eruptions*are putting it back into theatmosphere.

• The amount of CO2 in theatmosphere increases, sothe climate will warm up.

* The rate of volcanic eruptions does not depend on Earth’s climate and doesnot change much, i.e. it is nearly constant.



End result of Weathering

= Clastic Sediments

and dissolved ions(which are chemical sediments)

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8 Weathering