Revised 8 / 06 (Monroe 6th ed.)
Including...
Introduction
Soil Development and Loss
Weathering: an Overview
Mechanical Weathering
Chemical Weathering
Biological Weathering
Inter-relationships Between Weathering Types
Relative Rates of Weathering
General Review: a regional view
Earth has 2 levels
Below and above sea level
Earth has 2 types of rock
Earth has 2 processes
Construction and Destruction
Over the course of geologic time these 2 are in balance
Tectonics: the study of earth processes which result in the creation and deformation of magma and rock
This builds the earth as we know it
It's what we've studied up to this point
The destructional processes are next
Weathering
Erosion
Transportation
These are the processes that tear down the mountains and save us all from nose bleeds
This stuff can be mighty impressive
Carve valleys
Basically erode the land and transport the debris to the beach
But first the earth has to break it up because the mountains are too big to move in one piece
Surface conditions directly affect rock exposed at and near the surface
Several factors contribute to surface weathering
Water content: dry --> moist --> wet --> saturated
Temperature: cold vs. temperate vs. hot
And all possible combinations
Granite vs. limestone tombstones
Limestone in humid vs. arid climates
Pollution level
A new twist compliments of human activities
EXAMPLE: Egyptian Obelisk
Surface weathering also results in soil development (Monroe: fig. 6-14 thru 18, pg. 183 thru 186)
Soils develop very slowly
Can be considered "Our most valuable mineral resource"
Without plain dirt, life as we know it...
Soils reflect the rock material from which they are derived
Different parent materials clearly result in different types of soils
We won't spend a bunch of time on soils
It's good stuff, but a class unto itself
It's being lost at an alarming rate (Monroe: fig. 6-22, pg. 189)
Many books dwell on how we are building on agricultural lands
We can always tear them down
Possibly more important is loss through increased erosion
The result of stupidity, ignorance, and/or greed
Permanent loss
At least as far as our time scale is concerned
Several ways to damage/destroy earth's soils
Overgrazing - "desertification"
Sub-Sahara Africa
Overuse - "saltification"
Lower Colorado River area
Central Valley, California
Over cutting - "rapenpillagization"
Pacific NW logging practices
Improper farming methods - blowitawaytion"
Midwest: 10' to 20' lost since mid-1800's in some areas
Or more, locally
Change from strip to contour plowing helped (Monroe: fig. 6-23, pg. 190)
But still lots being lost
"Every time the surface is disturbed..."
Think about the 4-wheel drive ads on TV
Glorifying off-road "fun and excitement"
4X4's for everyone!!!
DIGRESS TO: C&G Pass and KGO
EVERY cloud of dust is more topsoil being lost
How fun is starvation
We're in trouble
And often focus on the wrong target
The scars left by early day loggers and miners are truly insignificant when viewed against the excess of modern practices
Much more on this when we get to erosion
Top several meters of earth's crust
Actual depth highly variable
Fracturing due to unloading can extend much deeper (see below)
The surface exposes most rocks to a different physio-chemical environment from where they formed
Similar in concept to the metamorphic process
A fundamental change in mineralogy due to changing environmental conditions
It is possible to view surface weathering as just another type of metamorphism
In this case resulting from the absence of heat and pressure
And you end up with dirt instead of rock
Three basic types of weathering
Water is a major player in all three
Water is called "the hidden devil in the ground"
Physical break-up of rocks
Results in smaller pieces with the same chemical composition
Decomposition and/or dissolution by chemical activity
Results in smaller pieces with different chemical composition
Not really a process of it's own
Can lead to both mechanical and chemical weathering
Very few areas where its all one kind or another
However, there are many areas where one type will predominate (see below)
Also called physical weathering
Smaller pieces / same stuff
Two main types
Fracturing and Abrasion
Like the difference between a hammer and sandpaper
Results in clastic sediments of all sizes
Smaller than sand to huge blocks of rock
Generally results in angular fragments and surfaces
DIGRESS TO: angular, sub-rounded, and rounded
Usually joint controlled
But how do the joints get there?
This is also a surface effect - rock at depth is real solid
Allow entry of fluids and roots
Lots of situations can lead to mechanical weathering
In general terms, anything which disturbs the surface and moves material will lead to some mechanical weathering
Rocks in a river
Obviously, the energy of the river directly affects what is being moved
DIGRESS TO: bed load vs. suspended load
Rivers in flood stage have greater energy
Can move larger pieces with greater force
Therefore the potential for mechanical weathering is greater
Moving sand and silt acts like sandpaper on the larger rocks in the river bed
Also on each other
Rocks hitting other rocks can break
Making smaller pieces which can then become part of the suspended load
Rock falls
Usually break up when they hit bottom
Ice - the big one
Water has highly unusual properties
REVIEW: water and magic
Stress the density anomaly
Water expands 9% when it freezes
If it freezes in a confined space it can exert a tremendous outward force against walls
Up to 4.3 million pounds/ft2
This should be enough to break just about anything!
Water seeps into cracks in rocks
Freezes from top down
Seals off the last remaining area where expansion could take place
DIGRESS TO: imagine if it froze from bottom up
No cracking at all
How would this affect the weathering process in the higher mountains?
Widens cracks and pries rocks apart
Called frost wedging (Monroe: fig. 6-3, pg. 172)
Doesn't happen everywhere
Likes cold areas with large daily temperature fluctuations
Most effective when daily/weekly temperature range spans 32 deg. F
Process slows when temperature never climbs above freezing
Or never drops below freezing
Salt Crystal Growth
Most waters contain dissolved "salts"
Lots of different materials, not just NaCl
Water in soil or rocks starts to evaporate
Increases the concentration of the dissolved minerals
Leads to crystallization
Can increase pressure and break-up rock or other materials
EXAMPLE: Fence posts in Bonneville Salt Flats
Generally most effective in arid regions
Probably due to less water
More frequent fluctuations above and below the saturation threshold
Most rocks form at depth where the pressure is greater than near the surface
As the rock is tectonically moved upward, the decrease in pressure causes the rock to expand
Leading to fractures and joints
Exfoliation (Monroe: fig. 6-4/5, pg. 173/174)
Most common in granitic rocks
And others with a uniform, homogenous texture
Looks like an onion skin
Peels off rock layers
At right angles to the direction of pressure release
Can expand out into valley walls
DIGRESS TO: Zimbabwe - ancient city
Rock bursts
Potential in new mine workings
Deadly!
Temperature changes
Used to be thought capable of causing exfoliation due to diurnal temperature fluctuations
Needs some moisture to set it off
Alters and swells "susceptible minerals"
Probably the mafics (WHY?)
Basically a physical weathering process with a chemical catalyst
Extremely high temperatures can also lead to fracturing
Forest fires, nuclear bombs, Texas chili farts
Over the course of geologic time everything gets dissolved
Occurs in all environments
Most effective in hot and humid areas
High temperatures and lots of water
Abundant organics to make acids (see below)
Minerals chemically react when exposed to water and heat
Results in smaller pieces AND different stuff
Makes rounded fragments and surfaces
Water need to be able to touch the minerals
Surface area VERY important! (Monroe: fig. 6-10, pg. 180)
EXPLAIN FULLY
High temperatures act as a catalyst
Solution
Dissolving of rocks by weak acids over a long period of time
Stronger acids obviously dissolve rocks faster
Don't usually occur in nature
Generally weak but very common in nature
Carbonic acid most common
H2O + CO2 --> H2CO3
CO2 is produced by the decay of organic material
Combines with groundwater to make carbonic acid
And directly from the atmosphere
Combines with rain water
ALL RAIN WATER is actually acid rain!!!
Carbonic acid may be weak but it is real hard on calcium carbonate
Limestone, dolomite, marble
CaCO3 + H2CO3 --> Ca+2 + 2HCO3-1
Ions are removed by percolating solutions
Can completely dissolve large amounts of limestone
Results in lots of amazing topography
Karst topography
DIGRESS TO: sink holes, etc.
Stronger acids can be produced locally
Sulfuric acid (H2O + SO2 --> H2SO4)
Groundwater in contact with sulfide minerals
EXAMPLE: Tri-State mining area, Yukon well water, TAB Acid Spring
Oxidation
Transfer of electrons and the addition of oxygen
Your basic rust (Monroe: fig. 6-8, pg. 179)
Occurs to any and all iron bearing minerals
Common reaction: 4FeO + O2 --> 2Fe2O3 (limonite)
The oxygen comes directly from the atmosphere or from carbonic acid
Commonly associated with sulfide mineral deposits
Gossan
Hydrolysis
Transfer of electrons and the addition of hydrogen
Commonly attacks feldspar minerals
EXAMPLE: feldspar (orthoclase) + hydrogen (from carbonic acid) + water --> Kaolinite (clay) + silicic acid + potassium ions
2KAlSi3O8 + 2H+ + 9H2O --> H4Al2Si2O9 + 4H4SiO4 + 2K+
Hydrolysis makes clay (end up as shale)
Clays - very small with 1 perfect cleavage (like the micas)
Which clay is formed depends partly on the climate
Humid: Kaolinite (used in soft ice cream)
Arid: Montmorillonite - a water-poor clay
Works well as a soil additive (swells and holds water)
Also works great as kitty litter
Hydrolysis also liberates ions to be used by plants
Such as potassium as illustrated above
Weathering resulting from the action of organic materials
Not really a process of it's own
Can lead to both mechanical and chemical weathering
Many possibilities
Lichen, fungus, mold, etc. (Monroe: fig. 6-6a, pg. 174)
Secrete acids which "etch" the rock
Make subtle irregularities
The plant can use these as anchors
Plant roots
Can grow into cracks - force them apart (Monroe: fig. 6-6b, pg. 174)
Breaks rock - a mechanical process
Increases surface area - leads to increased chemical attack
Organic acids
These can chemically weather the minerals
Organic debris
Falls and decomposes
Releases carbon which can combine with water and the atmosphere to make acids
It's like a cycle (remember the hydrologic cycle? Same idea)
Biologic / physical (EX: tree roots in cracks)
Biologic / chemical (EX: root acids)
Chemical / physical (EX: decomposed granite)
Physical / chemical (EX: Fracture and abrasion lead to increased surface area)
Abrasion and "rock flour"
Chemical / biological (EX: leads to soil formation)
And back to biologic weathering
Core stones in granitic rocks (Monroe: fig. 6-1a, pg. 170) (Monroe: fig. 6-12, pg. 181)
Rates vary depending on minerals and rocks
REVIEW: several factors contribute to surface weathering
Climate
Dry --> moist --> wet --> saturated
Cold vs. temperate vs. hot
Granite vs. limestone
Limestone in humid vs. arid climates
Pollution level
Differential weathering (Monroe: fig. 6-2, pg. 171)
Rocks/minerals which weather rapidly, erode rapidly
Weathering rates directly affects topography we see
DIGRESS TO: why is Grants Pass in a valley?
Many common minerals weather quickly
Related to the order of crystallization
Mafics weather quicker, felsics weather slower
Sandstone - quartz is all that's left after repeated cycles of weathering
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