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Lecture

EARTHSC 1G03 Lecture Notes - Frost Heaving, Spheroidal Weathering, Crystallization


Department
Earth Sciences
Course Code
EARTHSC 1G03
Professor
Maureen Padden

Page:
of 2
CHAPTER 8: WEATHERING AND SOIL
Processes that alter rocks: weathering, erosion and transportation
Weathering: destructive processes that change the physical and chemical character of the rock
at or near the surface
Erosion: physical removal of rock particles by an agent such as running water or glaciers
Transportation: movement of eroded particles by agents such as rivers, waves, glaciers, or wind
WEATHERING
Alteration of Rocks:
Mechanical weathering: physical disintegration; break rock into smaller pieces
o Increases surface area
o Increases chemical weathering rate
Chemical weathering: decomposition of rock from exposure to atmospheric agents
o Proportional to the surface area
Effects:
Spheroidal weathering: rock has been rounded from initial blocky shape b/c weathering acts
quicker and more intense on corners than surface; example of “small scale exfoliation”
Differential weathering: varying rates of weathering in an area b/c differential resistance among
rocks to weathering
o Ex. Of resistant type: sandstone
o Ex. Of less resistant: shale
When weathered, shale forms gentle slopes of eroded rock debris called talus
MECHANICAL WEATHERING
Pressure release: reduction of pressure on top of a body of rock; causes rocks at depth to crack as they
expand upwards
Sheet joints: fracturing parallel to rock surface or in concentric layers develop parallel as outer
part of rock expands more
o Exfoliation: Process of spalling off of rock layers from the top
o Exfoliation domes: large, rounded landforms developed in massive rock by exfoliation
Frost Action:
Mechanical effect of freezing water on rocks
o Frost Wedging: water expands as it freezes in joints; this wedges the rock apart
Northern climates or mountainous regions
o Frost heaving: lifts rock and soil vertically
Evident in winter with bulging ground surface
Salt Crystallization:
Occurs when saline solutions enter cracks/joints and evaporate, leaving salt crystals; when
heated, salt crystals exert pressure on rock and decompose it
o Arid climates
B/c stronger heating results in stronger evaporation and more salt
crystallization
o Coasts
Other processes:
Plant growth roots growing in cracks thereby breaking up rock
o Break up of rock increases passageways for water and air to enter
Burrowing animals burrow through cracks
Extreme temperature change
Humans
CHEMICAL WEATHERING
Chemical reactions which decompose rocks and minerals thereby altering their original composition
Direct solution total dissolution of rock
o Ex. Calcite goes into solution when exposed to CO2 and H2O
Hydrolysis insoluble residue and dissolved ions
Oxidation oxygen combines with rock
o Iron turns to iron oxides (hematite, limonite)
Acids acids give off hydrogen ions when they break down in water; these ions disrupt the
orderly arrangement within minerals
o H+ ions substitute for positive ions (Ca+, Na+, or K+)
o Ferromagnesian mineral include positive ions Al, Fe, Mg, Ca are replaced by H+ due to
their weak bonding
o Example. Feldspar turns to clay mineral when exposed to carbonic acid
WEATHERING RATES
Rocks of different compositions and origin weather at different rates. Minerals are weathered
chemically in reverse order to the sequence of crystallization (Bowen’s reaction series)
Least resistant Most resistant
CLIMATE AND WEATHERING
Chemical Weathering:
o Rocks chemically weather faster in humid climates due to availability of water
o Increased temperature results is optimal
Physical weathering: Temperature:
o Cool temperatures result in freezing and thawing, promoting physical weathering
o Steep slopes promote rapid disintegration
Olivine
B/c it’s a
ferromagnesian
mineral
Pyroxene
B/c it’s a
ferromagnesian
mineral
Horneblende
Biotite
Orthoclase
Quartz
b/c strength
of silicon-
oxygen bond