Chp 5: Weathering and Erosion
I. Define Weathering and Erosion Weathering - “The decomposition and disintegration of rocks
and minerals at the Earth’s surface by mechanical and physical
processes. Weathering processes involve very little or no movement
(or removal of) decomposed earth material.
Erosion – “The removal of weathered rocks and minerals from
the place where they formed. Forces or transporting agents involved
in moving disintegrated earth materials are:
Water – running water such as streams, rivers, etc.
Wind – prevailing winds, tornadic storms, sea breezes, etc.
Gravity – The influence of gravity causing landslides, avalanches, etc.
Ice – in the form of glaciers
II. Modification of the Earth’s Surface Weathering, erosion, and transportation of earth materials
Surface processes continually wear away rocks and landforms
In geologic time they combine to wear away entire mountain ranges, reducing them to
flat, low-lying plains.
Weathering Effects quite apparent….
p.133
Wind and water leave curious rock formations behind due to erosionArches National Park, UT
Fig. 5-CO, p.120
Dust storms in Central Plains
during 1930’s
Fig. 5-21, p.141
Gravity effects- loose rocks slide down hill…
Heavily weathered granitic rock
Fig. 5-1a, p.122
Breakdown into smaller particles…
Fig. 5-1b, p.122
Chp 5: Weathering and Erosion
III. Types of General Weathering Mechanical (or Physical) Weathering – The physical disintegration of
rock into smaller and smaller pieces. The chemical composition of the
rocks and minerals are not altered. The particles formed are called
clastics (meaning “broken”)
Chemical Weathering – occurs when air and water react chemically
with rocks to alter their composition and mineral content. The final
products not only differ physically from the parent material, but they are
different chemical substances. (i.e. Limestone dissolving by acid rain
releasing its calcite content as ions.)
Differential weathering - Rocks weather by both mechanical and chemical processes
occurring together. Since rocks are not homogenous in composition, usually parts
weather at different rates. Called differential weathering resulting in an uneven surface.
Spheriodal weathering – Because of differential weathering, the surface of rocks is
many times sharp and angular, or cuboidal. These corners formed on the rocks are
“attacked” from all three sides resulting in a “rounding” of the angular piece.
This is spheriodal weathering.
Differential weathering of limestone in Spain
Fig. 5-2b, p.123
Honeycomb weathering – coastal CA
Fig. 5-2c, p.123
Exfoliation dome in central CA
Fig. 5-4b, p.125
Chp 5: Weathering and Erosion
IV. Types of Mechanical (Physical) Weathering Frost wedging – Water expands upon freezing. If water seeps into
cracks in the rock and freezes, the ice formed exerts pressure along the
crack, expanding the crack, or breaking off a piece of rock. Many times
the broken piece remains in place until the spring thaw, resulting in areas
(such as mountain passes) of rock fall hazard. The loose angular rock
debris at the base of mountains and cliffs is termed talus.
Salt Cracking – Whenever salt water evaporates, the salts reform crystals. If water
containing dissolved salts enters a crack in the rock and then evaporates, the pressure
created by the newly forming salt crystals can break the rock. This is salt cracking
and is common in deserts and shorelines. This is why it is not a good idea to salt
driveways or sidewalks to rid them of ice. The concrete will eventually break apart.
Abrasion – This is the mechanical wearing and grinding on rock surfaces by friction
and impact with other rock materials. This gives the rocks a rounded appearance.
This occurs in flowing water, wind actions (i.e. natural sand blasting), and glaciers
Water enters cracks in rock, freeze and thaw breaks rock apart…
Fig. 5-3, p.124
Fluids seep along fractures and aid in breakdown of rock into particles
Fig. 5-10, p.132
Chp 5: Weathering and Erosion
Organic Activity – Plant roots can crack rock material by the hydraulic pressures
associated with root growth. Burrowing animals can contribute to rock disintegration.
Pressure Release Fracturing – Rocks buried deep within the earth are under the pressure
of the overburden (country rock). As the overburden is eroded away, internal pressures
of a granite pluton cause it to expand. This causes the surface of the granite to split and
crack forming sheets and blocks of rock at the surface in a process known as exfoliation.
This may also occur in rocks that are porous such as feldspar rich granites. Water may
be “absorbed” by the feldspars causing them to swell and crack. This process of swelling
by the addition of water is called hydration. This is one of the processes that can turn
feldspars into kaolinite, a major clay-forming mineral.
Thermal Expansion and Contraction - Heat causes matter to expand and cold causes
matter to contract. Surface rocks exposed to the intense heat of the daytime sun heat up
and expand. At night when it is cooler, the rocks contract. This constant expansion and
contraction over many years causes the rocks to break apart. Enchanted Rock in central
Texas was named so because of the cracking sounds it is supposed to make
during this process.
Tree roots grow into crack
in rock and contribute to
erosion of that rockOrganic erosion
Fig. 5-6a, p.127
Sheet joint formed by expansion in granite..hammer is 30cm long
Fig. 5-5, p.126
Stone Mountain is a large exfoliation dome in Georgia
Fig. 5-4c, p.125
Slabs of granite bounded by sheet joints-note slabs are inclined
Down
Fig. 5-4a, p.125
Chp 5: Weathering and Erosion
V. Types of Chemical Weathering 1. Oxidation – reactions with oxygen – rusting:
4 Fe + 3 O2  2 Fe2O3
iron + oxygen = iron oxide
Oxidization reactions are common in nature and usually turns useful material into wastes.
This is most common in iron bearing mafic minerals such as olivine, amphibole, and
biotite.
2. Corrosion – reactions involving oxygen, water, and CO2 found in the air and water.
Combinations of these can cause corrosive chemical conditions that can chemically
weather rocks.
3. Weathering by Solution – dissolution whereby ions disperse into water. I.e. rivers
flowing across limestone can dissolve Ca+ and CO3- and carry these ions away.
4. Acids and Bases – Acids are solutions with an abundance of free hydrogen ions (H+),
while bases are solutions that have an abundance of free hydroxyl ions (OH-). Acids
and bases dissolve minerals by pulling atoms out of crystals. Carbonic Acid (H2CO3)
is formed in abundance in nature whenever CO2 dissolves in some rivers and streams
Lichens growing on rocks derive nutrients from the rock and
contribute to weathering
Fig. 5-6b, p.127
Weathering effects enhanced by increasing surface area of any particle:
Basic premise here is that the more surface area that is available for these
processes to act upon, the faster the process will act and the more likely
the rock will be broken apart…(weathered)… surface area increases, but
the Volume stays the same!!
Fig. 5-11, p.132
Chp 5: Weathering and Erosion
5. Acid Rain – During storms, Nitric Acid, (H2NO3) is formed by
lightning breaking apart N2 in the atmosphere into N + N. This
combines with water to form nitric acid causing rainwater to
naturally become slightly acidic with a pH of 5.5 – 6.5. Pollutants
in the atmosphere such as sulfur dioxide gasses can also contribute
to acid rain.
Soil formation is a prime end product of weathering and erosion
SOILS
Fig. 5-15, p.136
Soil Formation- function of climate and vegetation
Fig. 5-16, p.137
a. BrownExpansive soils
b. Expansive
soil in Dallas
Fig. 5-20, p.140
a. laterite: deep red soil found in tropics
b. Slash and burn agriculture
Fig. 5-18, p.138
Table 5-2, p.143
Contour plowing
Fig. 5-23, p.143
pH of soil
Areas where acid rain is a problem
p.131
Underside of rock showing scaly white material known as ‘caliche’
Fig. 5-17a, p.137
Alkali soil in California-white material is NaCO3-very few plants live
Fig. 5-17b, p.137
Chp 5: Weathering and Erosion: Summary
Weathering - “The decomposition and disintegration of rocks
and minerals at the Earth’s surface by mechanical and physical
processes.
Erosion – “The removal of weathered rocks and minerals from
the place where they formed. By these agents:
water
wind
gravity
ice
Types of Weathering:
1. mechanical-physical disintegration into smaller pieces; no chemical change
2. chemical- air and water react to change chemical composition and size
3. differential- both chemical and physical, creates uneven surface
4. spheroidal- processes attack exposed surfaces, create a rounded surface
Chp 5: Weathering and Erosion
Types of mechanical weathering:
1. frost wedging-water enters crack, freezes, expands, pushes crack open
2. salt cracking-salt left behind when water evaporates expands, breaks
open rocks…occurs in deserts and shorelines.
3. abrasion-friction and impact between grains, rivers, glaciers and wind
4. organic activity- plant roots and burrowing organisms break rocks
5. pressure release- as buried rocks are exposed due to erosion, their
overburden is removed leading to release of pressure and they
expand or crack. Exfoliation and hydration are 2 examples of this…
6. thermal expansion and contraction: heat and cold-expand, contract
Types of Chemical Weathering:
1. oxidation- reaction with oxygen…basically is ‘rusting’
2. corrosion- reaction with oxygen, water and CO2
3. solution- dissolution of ions into water; rivers dissolve carbonates…
4. acids and bases- carbonic acid formed in rivers; dissolves minerals
5. acid rain-naturally occuring-lightning and Nitrogen; pollution contribu
Chp 5: Weathering and Erosion
Primary end products formed by weathering and erosion are-rock particles which turn into sedimentary rocks after
transportation
-soils which have important economic implications for human
survival (i.e. growing crops, mining certain minerals (bauxite,
kaolinite, etc)
Human impact-soil management schemes
-exacerbate acid rain into a problem in some areas
-mining and other activities make erosion worse than would
normally occur…
Fig. 5-2a, p.123
Fig. 5-7, p.128
Fig. 5-8, p.129
Fig. 5-9, p.129
Fig. 5-12, p.133
Fig. 5-13, p.134
Fig. 5-14a, p.135
Fig. 5-14b, p.135
Fig. 5-19, p.139
Fig. 5-22a, p.142
Fig. 5-22b, p.142
Fig. 5-22c, p.142
Table 5-1, p.134