Weathering
Sedimentary Cover
Sedimentary Cover
+ Sedimentary rocks form a veneer over
much older
igneous and metamorphic “basement”
rocks.
+ This veneer varies in thickness from 0
to 20 km
Types of Weathering
Weathering
+ Physical – Mechanical breakage, disintegration.
+ Chemical *- Decomposition by reaction with water. Produces
dissolved matter and an insoluble residue.
+ Biological – Action of organisms often combines physical and
chemical weathering processes.
+ Coffee metaphor for weathering:
! Physical – Grinding coffee beans.
! Chemical – Leaching coffee with hot water
8 Generates dissolved material (coffee), and
8 Depleted residue (coffee grounds).
Rock Disintegration
• + Weathering –
Processes that
• break-up solid rock.
• ! Weathered Granite
• ! Fresh Granite
Rock disintegrates to form
sediments which can form
sedimentary rocks.
Physical Weathering
aka mechanical weathering
-Mechanical breakup with no
change in mineral composition.
-Often occurs with chemical
weathering, except at climate
extremes.
-Physical weathering dominates in
polar and desert regions.
-Everywhere else, chemical
weathering dominates.
+ Types of Physical Weathering
-Jointing (release of overburden
pressure)
-Frost Wedging
-Root Wedging
-Thermal Weathering
-Alternate wet / dry
Jointing
• Rocks expand when
rock erodes off of
the top of a
formation from
reduction in
pressure.
• + Rocks develop
cracks called joints.
• + Igneous rocks
crack in parallel
layers,
Frost Wedging
+ Water enters a rock
fracture, freezes in
fractures.
+ Ice expansion wedges
the fracture open.
+ During thaw, water
infiltrates deeper into
crack.
+ Repetition results in
rock disintegration.
+ Dominant process in
mountainous regions.
Frost wedging
-Rocks wedged loose
by freeze-thaw
accumulate as talus
at the base of steep
slopes.
- Talus (broken rock)
accumulate at the
bottom of the slope
Chemical weathering
-Breaks minerals into
chemical components.
-Requires water “the
universal solvent”.
-Significant in humid
(water-rich) landscapes.
-Chemical weathering
virtually absent in
deserts.
- 3 major processes
1. Dissolution
2. Oxidation
3. Hydrolysis
Types of Chemical Weathering
Dissolution-Some minerals (halite, gypsum, calcite) dissolve.
-Acidity (i.e. acid rain) enhances this effect.
Oxidation- A reaction whereby a metal loses electrons.
-Important in decomposing minerals.
- Rusting is a familiar example of oxidation
Hydrolysis-Breaking (lysis) by water (hydro).
-Water breaks apart elements that hold silicates together.
Alteration minerals (residues) such as
-Clay minerals
-Iron oxides (rust)
Biological weathering
• Mosses and plants
release acids during
decomposition that
cause rock to
deteriorate
Effects of weathering
Unstable minerals
decompose.
Rock volume is
reduced.
Corners and edges
attacked rapidly =
spheroidal
weathering.
Chemical Weathering in Granite
Chemical Weathering in Granite
+ Biotite and feldspar weather to clay minerals.
+ These minerals are removed by running water.
+ Mineral removal causes the rock to crumble.
+ Quartz, resistant to weathering, is left over.
Weathering Rates
-Temperature and wetness extremes
Hot and Wet – Chemical weathering at a maximum
Cold and Wet – Physical weathering at a maximum
Hot and Dry – Physical weathering dominates
Cold and Dry – All weathering at a minimum
-Biological effects – Active biota accelerates
weathering.
-Geologic past – Rates of weathering have varied with
atmospheric and biotic changes.
Differential weathering
- Rock masses do not weather
uniformly.
-This results in many unusual
and spectacular rock
formations,
landforms and landscapes.