Chapter 6: Weathering

Endogenic Processes

What is the geological processes that originate inside of the Earth’s crust?

Exogenic Processes

What are geomorphic processes that originate at/or near the Earth’s surface?

(Mass wasting, weathering, erosion and deposition of materials by fluvial, glacial, coastal, aeolian processes)

Weathering

What is the set of exogenic processes that alter the physical and chemical state of rock material at or near the Earth’s surface?

Erosion

What is the pick up and transport of material (usually weathered to some extent) by some agent such as water, wind, ice, gravity and waves?

Physical weathering

What type of weathering is

* mechanical breakage of rock by stress, thermal expansion and contraction, frost wedging, etc.
* dominant in cold and dry environments?

Chemical weathering

What type of weathering is

* decomposition of rock structure by chemical reactions that result in chemical changes to the rock
* dissolution and clay production
* dominant in hot and wet environments
* much faster rates than its counterpart?

Quite resistant to physical and chemical weathering, so they end up becoming clastic rocks

What happens to quartz when weathered?

Turn into clays with ions carried away in the process

What happens to feldspars when chemically weathered?

Less stable than their counterpart mineral

Undergo more chemically weathering, producing clay minerals and iron oxides

What happens to mafic minerals when weathered?

Dissolved and ions carried in solution

What happens to calcite, gypsum and halite when weathered?

Weaken rock structure

Provide greater surface area for other processes

Why do joints and fractures occur in rocks when weathering happens?

More surface area

What does more weathering equal?

* Disintegration of rock, stresses along pre-existing joints or fractures
* No chemical alteration
* Smaller particles from larger
* Often results in angular surfaces
* Thus resultant particles reflect original texture and structure of rock
* Happens in cold and dry environments

What are the features of physical weathering?

1. sheet jointing
2. frost wedging
3. thermal expansion
4. salt crystal growth
5. plant wedging

What are the processes of physical weathering?

S

F

T

S

P

Sheet joining/unloading

What is the type of physical weathering where

* uplift and erosion result in a vertical expansion of the rock surface and the formation of fractures as the overlying confining pressure is reduced
* fractures parallel to the surface form **exfoliation sheets**

Spheroidal weathering (onion skin)

What type of weathering has

* smaller scale unloading processes likely acting in concert with other weathering processes, such as hydration or thermal expansion

Frost wedging

What type of physical weathering has

* Ice crystal growth (ppt or sublimation)


* 10% increase in water volume upon freezing
* pressure (100 lb/sq in) on surrounding surfaces
* Exploits existing joints
* A common process in climates with freeze-thaw cycles, alpine areas, arctic areas,
* Block fields (felsenmeer) surface with angular blocks likely due to weathering in situ
* Talus slopes

Thermal expansion

What type of physical weathering has

* Heating and cooling results in differential expansion and contraction
* Changes in the volume of rock material leads to stresses in the rock and eventual breakage of the bonds within the rock

Salt crystal growth

What type of physical weathering has

* salt crystallization as water evaporates in pores and fractures exert mechanical stress that weathers rock and monuments
* most effective in arid regions, marine coastlines

Tafoni weathering

What type of salt crystal growth weathering has

* Natural rock cavities are often found in the arid environment or coastal areas
* Various mechanisms of formation, but salt heaving is a common mechanism

Plant wedging

What type of physical weathering

* numerous biological forces including growth of plant roots


* Seed exploits crack in rock, as root grows, it exerts force on surrounding rock,
* Wind can blow over the tree, resulting force blasts rock apart

* Formation of new substances, often softer
* Finer, particle sizes
* More soluble


* Occupy larger volume than original
* Usually occurs along planes of weakness, joints or fractures
* Results in rounded surfaces
* Most occur in the presence of water or air (hot, wet environments)

What are the features of chemical weathering?

1. Solution
2. Hydration
3. Hydrolysis
4. Oxidation
5. Chelation

What are the processes of chemical weathering?

(SHHOC)

Solution

Which process of chemical weathering is

* water dissolving some minerals that are naturally soluble
* Rainwater is weakly acidic but SO2 and N2 produce stronger acids that are effective in dissolving minerals
* involves a CaCO3 equation

Hydration

Which process of chemical weathering is

* a mineral absorbing water molecules and changing the chemical bonds to produce a different structure

Hydrolysis (1)

Which process of chemical weathering is

* involving chemical reactions (H+ and OH-) from water to enter the crystal lattice and replace other ions, changing the composition and forming a new mineral
* ions are freed from the crystal lattice, and go into solution, resulting in minerals being often weaker and more soluble
* makes them least resistant to weathering

Hydrolysis (2)

What chemical weathering process makes feldspars into clay minerals?

The water splits into hydrogen (H) and hydroxide (OH). These two change the chemical composition and size of minerals in a rock. In feldspar, the water + orthoclase, are in a dissolution equation, turning them into kaolinite (clay), silicic acid and potassium (K). This forms clay!

How does hydrolysis turn feldspars into clay?

Oxidation

What is the chemical weathering process where:

* minerals combine with O2, resulting in a loss of electrons

Acid Rock Drainage

What is the oxidation of rocks with sulfide minerals