Decay of Earth’s Rocks (weathering):
the Key to our Beautiful Earth
“Weathering” (Wx...not this…which
is why we call it rock decay or stone decay)
1. What is the difference between weathering and erosion?
Weathering: breaking down of rock in situ (in place); the decay of rock (rock decaying).
Erosion: transport of weathered material.
**Weathering = the beginning to the changing of the earth’s surface!**
study of how & why earth changes; a
combination between Geography and Geology.
2. Joints: Rock weaknesses give weathering a foothold (not what you think)
A. Joints - regularly spaced fractures or cracks (fissures) in rocks
o Example of “The Rock” at Morro Bay that show no offset across the fracture (fractures that show an offset are called faults).
o Joints create free space in rock by which other agents of chemical or physical weathering can enter...
o What you use in rock climbing...
o How do they form?
o Contractional Cooling/Thermal Expansion
o Extrusive igneous: Master Joint at Joshua Tree NM
o Faulting/Folding/Tectonics - Cracking the Rocks
o Yosemite Valley vertical joints
3. Physical (or mechanical) Decay Processes (disintegration of rocks and minerals by a physical or mechanical process)
o Upon freezing, there is an increase in the volume of the water (~11%)
o As the water freezes
it expands and exerts a force on its
surroundings, and also the hydraulic pressure exerted by water deep inside
the crack as the freezing water at the top of the crack seals the top:
o Frost wedging is more prevalent at high altitudes and latitudes where there are many freeze-thaw cycles
o If you can’t imagine frost weathering in Antarctica, I’m not sure I can help...
o Mt Fitzroy (Patagonia) is an example of beauty from joints separated by frost weathering
o See shattering at the top of Mt Whitney (highest point in lower 48)
B. Pressure release: when a massive rock (granitic, some sandstone) forms under great pressure, quick erosion causes pressure release shells to “pop off” in slabs:
Please do NOT call it exfoliation:
o Exfoliation is what happens to your skin ... erosion of millimeter- to several inch-scale flakes
o The correct term is: pressure release shells
o Road cut in Yosemite
o Hiking Half Dome in Yosemite
o Royal Arches in Yosemite Valley
o TWO main processes:
o Crystallization exerts pressure - as water percolates through fractures and pore spaces it may contain ions that precipitate to form crystals. As these crystals begin to form, they can expand over large areas, exerting an outward force that can expand further and weaken rocks
o Heating and cooling can also exert pressure - salts expand and contract more than regular silicate minerals
o Death Valley visit you can see all morphological expressions
(salt decay is the most common way to make these forms)
Alveoli (finger-ish size):
Tafoni (fist-ish and bigger)
o Petra, Jordan
o Large sandstone caves!
Basal decay: notches made around the base of rocks
“Mushroom rock” story...
Larger granitic “mushroom” rock in Arizona...
WIND IS WIMPY: sidebar on why wind is often a poor explanation for weird forms
o Wind explains sand dunes, but not weird rocks
o Salt weathering is often explained by wind, but...
o Good old fashioned mechanical & chemical decay can create interesting forms, such as:
o What you might see at Castle Rock in Santa Cruz
o How to tell the difference?
o Use the “smooth as a baby bottom” (or “smooth as Professor Allen’s head”) test... wind smoothes (polishes) rocks, while mechanical & chemical decay roughens them!
D. Thermal Expansion and Contraction
1. Early desert observers thought that centuries of heating and cooling could split rocks
o Many of these clean splits are due to calcrete cracking, and just dissolved...lots of evidence in arid regions
o Daily heating and cooling of rocks does not seem to have an effect, BUT...
o Forest or grass fire may cause expansion and eventual breakage of rock
o Fire is most effective when the water content is high
o Fire most often causes millimeter-size flaking
o The flaking continues after the fire, as millimeter-size flakes split off previously decayed grains
E. Expansion and contraction from Wetting and Drying
F. Pressure of Roots - Plant roots can extend into fractures and grow, causing expansion of the fracture; Growth of plants can break rock
NOTE: There’s usually more than one decay process at work...
Practice time: How many types of physical weathering can you identify?
Connecting PHYSICAL ROCK DECAY with what’s next:
So...physical weathering uses joints (weaknesses in the rock) to split the rock into increasingly smaller pieces...
That rock cracking helps speed up chemical weathering processes speed up by increasing surface area by doubling surface area every time rock is split...
4. Chemical Decay Processes (chemical alteration or decomposition of rocks and minerals) Intro Movie
A. Dissolution: mineral is completely dissolved by the water
o Here is a movie of what happens when you put a crystal of salt in water
o Here is a view at the atomic level (scanning tunneling microscope) of a mineral dissolving before your eyes...
o Most common type of dissolution is of limestone rock (e.g. making caves)
o First, Make Carbonic
o Then, Dissolve the Limestone:
o What you can see when you “drop acid”
o Example of limestone dissolution (at scale of centimeters, called karren)
o How dissolution can decay and build rocks
SIDE BAR ON ACID RAIN
Rain and deposition -- within clouds (including fog) sulfur dioxide (SO2)
and oxides of nitrogen (NOx) can form acidic
particles when they react with water:
See building decay from sulfuric acid reactions from acid rain:
B. Hydrolysis: H+ or OH- replaces an ion in the mineral; and important process in the production of clay minerals
o SEM view (SEM = scanning electron microscope) of turning orthoclase into clay
o SEM view of platelet structure of clay
C. Hydration or Dehydration: addition/removal of H2O to a mineral
o What does it look like at the molecular level?
o Can be seen at the microscopic level in the form taken
o When it happens to a giant body of rock: it blisters
D. Oxidation: (think rust)
o Since free oxygen (O2) is more common near the Earth’s surface, it may react with minerals to change the oxidation state of an ion
o This is more common in Fe (iron) bearing minerals, since Fe can have several oxidation states, Fe, Fe+2, Fe+3
o Deep in the Earth the most common oxidation state of Fe is Fe+2
o Gives rocks a reddish color (“rubification”)
o Causing mechanical stresses (synergism between physical and chemical weathering)
o Reverse Reaction is Reduction, but need bog conditions of anoxic environment to generate lots of reduction at Earth’s surface
E. Chelation and Biological Organic Acids
SO WHAT DOES CHEMICAL DECAY LOOK LIKE?
LANDSCAPE OF CORE STONES AND TORS:
3rd: Tors are piles of core stones: as grus washed away, core stones stack on one another...
LANDSCAPE OF SPHEROIDAL FORMS:
o Sometimes taken too far?
Domes made when joints are spaced far away:
LANDSCAPE OF PITS:
o If you are interested, work with Tom Paradise at University of Arkansas, THE World Expert!
5. Tell Me Again, Why I should Care?
Well...ROCK DECAY MAKES NEW MINERALS that are important to your lives....
A. Al & Fe Oxides such as...
o Aluminum oxide Al2O3 that goes into aluminum cans
B. Mobile Cations (key nutrients for plants)
o Once in primary igneous minerals, released by chemical weathering...
o Calcium > Sodium > Magnesium > Potassium
o Affect pH (Acidity) and very important in determining mobility
o Stable in all terrestrial environments except extremely alkaline and tropical humid soils
o For those who have had geology, Quartz is the last to crystallize and is very stable at Earth’s surface
D. New Crystals (secondary minerals) such as...
o Variety of other rock coatings on rock faces
E. Weathering rind
o A rock may show an outer weathered zone and an inner unweathered zone in the initial stages of weathering
o The outer zone is known as a weathering rind that sometimes has a color change
o As weathering continues the thickness of the weathering rind increases, and thus can sometimes be used as an indicator of the amount of time the rock has been exposed to the weathering process
F. Case Hardening helps define and protect caverns by creating a hard outer shell...but even the smallest fissures can lead to decay from the inside out…
o People try and create it artificially to help with rock stability, but all it takes is a little fissure and...
G. The key to Life on Earth: Clay Minerals
o What you drive over for fun...
o Movie of feldspar turning to clays
o More and more evidence that clay is the key to the development of life on earth
Practice: What types of decay can you identify in this image?
6. What’s correct in this textbook diagram? (NOT this class’s textbook...)
Extra: Thinking like G.K. Gilbert (famous American geomorphologist):
o Transport Limited landscapes: vegetation cover, where the rate of transport limits the rate of erosion
o Alp Shoulder example
o Weathering-limited landscapes: deserts (less vegetation), where the rate of rock decay limits the rate of erosion