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GEOLOGY 12
WEATHERING
Assignment # 24
April 18, 2006
Weathering = process that changes the physical and chemical properties of rocks at and
near the earth’s surface.
- breaks down rocs that are stationary, or moving
- breaks down rock into smaller particles
- crystals of any rock can be loosened and altered to new minerals by weathering
- usually a very slow process
- the more a surface is weathered, the more surface is exposed for more
weathering
2 types of weathering:
- mechanical and chemical weathering usually work together
1) mechanical (physical) weathering = larger rock fragments break down into
smaller particles no chemical change or weathering involved
e.g. ice gets into rock cracks, expands, splits rock
plant roots get into rock cracks and split rock (roots into house
foundations, sewer pipes, tree roots uplift side walk slabs, etc)
- chemical weathering usually attacks the contacts between mineral grains
- the larger the crystals, the quicker weathering can occur
- the finer the crystals, the longer it takes for weathering to break down the rock
- rock is broken down but no chemical changes occur to rock particles,
rock crystals such as quartz, feldspar don’t change
2) chemical weathering = decomposition of rock by water and atmospheric gases
- when chemical weathering occurs, usually new chemical compounds form
- crystals that are chemically weathered are changed into different minerals
e.g. feldspar - clay
Erosion = is the picking up or physical movement of rock particles by agents such as
water, wind, glaciation.
- most eroded rock particles have usually been weathered. However, some rock
can be eroded without having been weathered; e.g. stream erosion
Transportation = movement of eroded particles by water, wind, ice
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Effects of Weathering:
- gravestones: inscriptions can’t be read (page 105, fig. 5.1 a & b)
- building stones rounded and eaten away especially limestone building material
- granite = strong silicon-oxygen bonds, weathering takes longer
rock outcrops: most noticeable effect of weathering = discoloration of rock material
- discoloration can easily be seen in new rock outcrops, in NS the red color of the
water and the red color in the rock is due to the iron in rock
- field geologists use hammer to get to non-weathered rock samples
soil is produced from rock weathering
salt in ocean result of millions of years of weathering (point: human blood has
approximately the same salinity as ocean salt water)
- weathering produces all the soil on the planet - all soil used for farming is a result of
weathering
- some mineral deposits e.g. gold are result of weathering and erosion
Types of physical weathering:
Spheroidal weathering = weathering both mechanical and chemical on edges of
rock more intense
rounding corners, resulting compared to flat faces of rock
(page 105, fig. 5.2)
Differential weathering = rate of weathering varies in different areas and different
types of a rock resulting in some unusual formations (page 106, fig. 5.3 and 5.4)
Mechanical Weathering:
A) Frost action:
1) Frost wedging: water gets inside rock fractures (joints), freezes, ice
expands up to 9% volume, expands joint, allows more water in, expands joint more until
rock shatters rock over time. (Page 107 fig. 5.5)
- areas that have a lot of temp variations (freeze - thaw) have rocks that are
shattered; e.g. mountain tops, northern climate areas
2) Frost heaving: lifts rock and soil vertically
- I winter ice forms under rocks and soil, as ice expands it heaves rock up and also
heaves soil up; e.g. frost heaves roads and breaks road material.
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B) Pressure Release: e.g. batholith: as batholith rises towards crust, granite cools and
solidifies,
lots of pressure from above by crust and overburden which can be
thousands of feet thick
as overburden eroded (unloading) and as batholith continues
to rise the top part of batholith undergoes less pressure
results in batholith material
cracks due to less pressure
leading to pressure weathering process
(Page 107, fig. 5.7)
sheet joints = outer surface expands , rock material underneath does not = fractures
along the surface of rock
sheets or slabs of rock peel off in layers (p. 108, fig. 5.8)
- if sheets of rock break off and due to gravity fall or slide down a hill or dome =
exfoliation
- examples of exfoliation domes = e.g. Sugar Loaf, Rio de Janeiro - Brazil
and Yosemite National Park (Page 108, fig 5.9)
C) Plant growth: roots, animal burrows, extreme temperatures
Assignment Questions for students to do:
Page 120: Questions # 1, #2 and #5
CHEMICAL WEATHERING
- rock decomposition vs (mechanical) rock fragmenting/shattering
- transforms rocks and minerals exposed to water and air into new chemical products
- crystals form under the earth without much water and without much oxygen compared to
the surface when the rocks are exposed
- exposed minerals achieve equilibrium with surface air and oxygen usually resulting in
new minerals
Oxygen:
- a nail rusts due to oxygen and water: iron + oxygen = iron oxide
- oxygen reacts with many minerals on the surface to change the rocks color = road rock
outcrops - rocks after being exposed to air turn color, usually a darker
Acid:
- most effective chemical weathering process
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- acids are chemical compounds that give off hydrogen ions (H+)
(ion has a positive charge and becomes a substitute in other minerals
and disrupts the arrangement of atoms in a mineral, thus changing the
original mineral i.e. decomposes the original minerals)
and
hydrogen ions disrupt the orderly arrangement of atoms in most minerals
- strong acids = sulfuric and hydrofluoric acids - usually associated with volcanic eruptions
- these acids are strong and can kill trees, other vegetation and react with rocks in
the area
- most important natural source of acid is carbonic acid = carbon dioxide + water
(H2CO3) carbonic acid
- carbon dioxide in air dissolves in rain
weak acid (acid rain)
pollution and acid rain - modern day pollution from industry releases great amounts of
carbon dioxide into the atmosphere; therefore when it rains - a weak carbonic acid is result
- the more carbon dioxide is emitted, the stronger the acid rain becomes - the more damage
the acid rain can cause on soils, plants, rocks, building materials, and the stronger the
underground and above ground streams become re acid which leads to more plants dying,
etc.
** Assignment: make a poster re how acid rain forms - effects of acid rain - future
outlook for agriculture and vegetation in general if acid rain continues to increase. **
- weak acid rain falls onto soil - percolates into soil - chemically weathers soil material
- some effects of acid weathering = development of caves underground
weather building materials
discolor metal statues (e.g. copper turns green, bronze
turns black)
feldspar turns into clay (rocks that contain a lot of
feldspar in soil eventually turn into clay in the soil)
- the ease at which rocks are chemically weathered depends upon the chemical bonding of
the minerals that make up a rock:
quartz = strong bonds therefore weathering takes longer
diamond = extremely strong bonds therefore one of hardest mineral known
(e.g. diamonds are concentrated in pipes since other material is weathered
and diamonds are not; therefore as weathering and erosion coninue on
surrounding material, diamonds accumulate
e.g. S.W. Africa (Namimbia west coast diamond areas)
e.g. northern Canada diamonds found in extinct volcanic pipes and in
glacial deposits
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H+ ions break atomic bonds of crystals, the stronger the bond, the harder
it is to break those bonds = longer time it takes to chemically weather a rock
- chemical weathering varies in different parts of the world due to different climates,
especially rain amounts
Class Assignment: Page 120, #4, #6, and #7
SOIL
Soil = layer of weathered unconsolidated material overlying the bedrock
- different soils exist in different horizons or layers
soil scientists definition = weathered, unconsolidated material that contains organic matter
and is capable of supporting plant growth = regolith
- a mature soil takes hundreds of years to create
Soil formation = rate of soil formation depends upon:
- rainfall
- temperature
- slope (drainage and erosion)
- original rock material
loam (a type of soil) = equal amounts of sand, clay and silt
- usually well drained
- very fertile
topsoil = upper and top part of soil material
subsoil = underneath soil that is stony and lacks organic matter
importance of clay minerals and quartz:
- quartz turns into small grains of sand that makes soil loose (as compared
to clays) for good aeration and drainage of water
- clay minerals help hold water and plant nutrients in soil
Soil Horizons = as soil develop/mature, certain distinct layers are produced and can be
distinguished by appearance and chemical composition (Page 115, fig. 5.19)
O horizon = uppermost layer; has decomposed organic material and ground
vegetation growing on it
A horizon = dark in color, lots of organic material, below surface
vegetation, contains humus (decomposed vegetation material which
contributes to organic acids that leach into lower soil layers)
zone of leaching = lower part of A layer, carries dissolved chemicals down,
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leaching may make lower A layer light in color compared to upper
portions
B horizon = made up of leached materials from A layer, often clay in
texture, red or brown in color
C horizon = incomplete weathered parent material and is the start of the
bedrock material
- soil classification: there are 12 large groups of soils called orders, these are
distinguished according to characteristics of the soil layers (Page 116, Table 5.3)
Residual Soil = soil that has developed from weathering of rock directly beneath it
Transported Soil =soil not developed from bedrock, but transported regolith from other
areas
e.g. loess (wind deposits), river deposits (Nile)
Basalt soils = (e.g. many parts of Nova Scotia)
- basalt is fine grained
- no large crystals of quartz
- lots of feldspar
- when feldspar weathered = clay material lacking sand (silica/quartz) particles
- this soil drains poorly, therefore not very good for farming
- most of NS interior has spruce trees growing on soil formed from basalt type of
rock and most of NS interior poor farming
- volcanic lava is rich in silicate material, breaks down easily, porous soil formed,
good drainage, breaks down easy, lots of plant nutrients, lava soils good for
agriculture: e.g. Hawaii - grow lots of fruits, etc.
Canadian and USA mid western plains = soil took 10,000 years to form of glacial
deposits
Class Assignment: Page 120, #8, #9, #10, and #11
Organic Activity = worms, ants, other insects and plants break up soil material, make
holes
allows water into soil
contributes to further soil breakdown
- dead material adds carbon dioxide gas to soil
weathering
Soils and Climate:
leads to further chemical
climate affects soil thickness and character
- wet climate leads to leaching downward
- dry climate leads to water leaching up bringing minerals up, especially salts
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hardpan =
cemented soil particles, usually clayey
- hard to plant things
- poor drainage
- usually found in very wet climates
- in dry climates top layer becomes hardpan due to precipitation of
minerals to top as water evaporates
laterites = soils where temperatures are high and lots of rainfall, thus soils are very leached,
usually red in color due to iron oxides (e.g. African soils in west Africa, S. Africa,
etc.)
- leaching of certain soils consolidates minerals e.g. bauxite (Aluminum)
aluminum oxide is not soluble and over time collects and remains on top of
bedrock
resulting in bauxite deposits
- since bauxite requires tropical conditions to form, there is very little bauxite
mined in USA and Canada
e.g. bauxite that is transported to Alcan Aluminum smelters in Lac St Jean area if
Quebec all comes from Guiana in South America. Why don’t they smelt
aluminum in Guiana = need lots of electrical power and PQ has that
- laterite soils very unproductive vs heavy jungle vegetation
- why does such vegetation grow on very unproductive laterite soils:
the jungle vegetation grows from the top layers of humus covering the soil
- when jungles are cut down, the laterite soils loose any of their fertility by leaching
caused by heavy rain falls. As a result, even though there is lots of land, once jungles are
cleared, soils are only good for a few years, after that they require a lot of fertilizer, or the soild
just disappear.
(e.g. Brazil - cut down tropical rain forest in Amazon Basin for 1) minerals and 2)
agriculture and beef production (beef for USA markets for who = hamburgers; what will
happen in a few years to the area re farming, climate (rain forest gone, effects on climate)
Class Assignment = Page 120, questions: #12 - 22
Special Assignment Questions (will be checked)
Page 120 (Expanding Your Knowledge section) #3