Weathering
Weathering
• Weathering is the combined action of all
processes, both physical and chemical,
which reduce or change the original rock to
something smaller in size.
The Process of Rock Break-up
Copyright © Christopher Hauser 2002
Weathering
• In physical weathering rocks are fractured and
broken apart
• In chemical weathering minerals are transformed
from types stable when the rocks were formed to
types stable at surface temperatures and pressures
• Weathering acts to form regolith the weathered
material situated above the original solid unaltered
bedrock
Physical/mechanical weathering
• Two types
– Those related to volumetric change of the rock
mass itself
– Those related to volumetric change of the
material introduced into voids and fissures
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Volumetric change of rock mass
• Pressure release or unloading is a process
that relieves the confining pressure on the
underlying rock as the rock is brought
closer the surface by erosion of the
overlying layers
Unloading
• Unloading causes the
rock to crack in layers
more or less parallel to
the surface creating a
type of joint called
sheeting structure
Copyright © Frans Kwaad 2002
(Near Rennes, western France)
Exfoliation dome
• Sheeting structure
developed over a
single large knob or
hill produces an
exfoliation dome
Sierra Nevada, California
Exfoliation
• On a small scale micro
fissures can provide
lines of weaknesses
along which
exfoliation can occur
• Often referred to as
onion skin weathering
Guyana.
Brazil
Near Bayona, NW Spain
2
Expansion/Contraction
• Insolation weathering
can cause the break up
of rock resulting from
thermal expansion and
contraction (also
called thermal fatigue)
Carbonaceous sandstone
Agulhas Park, Struis Bay. (Thermal
fatigue due to veld fires)
Sandstone
• Hydration weathering
is the breakup of rock
due to expansion and
contraction when clay
minerals absorb water
(often referred to as
slaking). Typical of
Bokkeveld and Ecca
shales in SA
Dolerite
Ancha Mountains, Arizona
Volumetric change of introduced
material
• Two types of
processes
– Effects of crystal
growth of ice and salt
– Stresses introduced by
biological activity
Water Absorption
Copiapo, Chile
Crystal Growth
• Frost action is the
repeated growth and
melting of ice crystals in
the pore spaces and rock
fractures
• Water penetrate into
joints and along bedding
planes in the rock and
can break the rock
through block separation
Monument Valley (Canyonlands
National Park, Utah)
3
Block Separation (Monument
Valley, Utah)
Block Separation (Wolfberg Gorge
Cedarberg, Clanwilliam))
Crystal Growth (cont.)
Tafoni formed by granular
disintegration, (Copiano,Chile)
• In cold climates frost
action results in a
surface cover of
angular fragments that
completely blankets
the bedrock beneath –
this “rock sea” is
called a felsenmeer.
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Different forms of tafoni
Debris slopes
Sandstone, Arizona
Granite, Chili
Granite, Skeleton Coast, Namibia
Sandstone, Australia
Salt crystal growth
• Frost action on steep
cliffs of bare rock,
detaches rock
fragments that fall to
the cliff base to form
talus or debris slopes
or debris cones
Vioolsdrift, Northern Cape
Salt crystal growth
• In dry areas the
evaporation of water
may precipitate salts
and enhance the
growth of salt crystals
• These can produce
grain by grain
disintegration of
granular rock types
Enon conglomerate, Oudtshoorn
5
Temperature and salt crystal growth
(Skeleton Coast Namibia)
Salt crystal growth: the special case
of the Uniab desert pavements
Salt crystal growth and granular
disintegration in desert environments
Desert Varnish
• Granular
disintegration close to
the base due to salt
crystal growth
• Very common in
desert environments
such as Walvis Bay
and Swakopmund
• Desert varnish refers to
the very thin stain or hard
glaze commonly occurring
on exposed rock surfaces
in deserts
• Origin related to chemical
precipitation, biological
action of micro-organisms
and bacteria on dust that
settles on the rock surface
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Chemical weathering
• Main processes
discussed earlier –
oxidation, hydrolysis
and carbonation
• Chemical weathering
more active under
warmer and more
moist conditions
• May form corestones
Case of the Hugenote tunnel
• Deep chemical weathering of Cape granite
and accumulation of debris on Paarl side of
the mountain caused tremendous trouble to
engineers during construction of the
Hugenote tunnel
• Special soil freezing technique was
developed in SA to combat the problem
Chemical weathering
• Hydrolysis and
oxidation can be active
in tropical and
subtropical climates
to depths of up to
100m
• A main concern to
construction engineers
Formation of bornhardts
• Most bornhardts
(inselbergs) are
formed in two stages
– Firstly due to
subsurface weathering
through hydrolysis
– Secondly the
weathered material is
exposed and eroded
thereby exposing the
inselberg
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Carbonation
• Leads to the formation
of limestone caverns
and underground
drainage
Carbonation (on small scale features
Elephant skin weathering,
Vioolsdrift
Fluting near Lüderitz
Paleosolution forms, Derbyshire, UK
Weathering pits near
Keetmanshoop, Namibia
Effect of weathering in urban areas
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