Revised 8 / 06 (Monroe 6th ed.)

Weathering of Rock at the Surface of the Earth (Ch. 6)




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

Basalt and granite

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




These are the processes that tear down the mountains and save us all from nose bleeds

This stuff can be mighty impressive

Carve valleys

Rivers and glaciers

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

Strickler's 3rd Law of GeoFantasy

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

Rock type

Granite vs. limestone tombstones

Limestone in humid vs. arid climates

Pollution level

A new twist compliments of human activities

EXAMPLE: Egyptian Obelisk


Soil Development and Loss

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!!!


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


Weathering: an overview

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

Biological activity

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)


Mechanical Weathering

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

Surface Unloading

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


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


Chemical Weathering

Over the course of geologic time everything gets dissolved

See Strickler's 3rd and 4th Laws of GeoFantasy

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)


High temperatures act as a catalyst


Dissolving of rocks by weak acids over a long period of time

Stronger acids obviously dissolve rocks faster

Don't usually occur in nature

Organic acids

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


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



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


Biological Weathering

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


Inter-relationships between weathering types

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)


Relative Rates of Weathering

Rates vary depending on minerals and rocks

REVIEW: several factors contribute to surface weathering


Dry --> moist --> wet --> saturated

Cold vs. temperate vs. hot

Rock type

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

Bowen's Reaction Series

Mafics weather quicker, felsics weather slower

Sandstone - quartz is all that's left after repeated cycles of weathering



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