Lecture 12: Weathering and Erosion

Sediments and sedimentary rocks are the product of numerous physical processes such as:

Weathering, transport and ultimately deposition (and subsequent diagenesis)

Weathering describes

A series of processes that take place as a rock is exposed to the elements over a long period of time

What is mechanical weathering

Rocks are broken down as a result of prolonged physical processes

What is chemical weathering

Rocks are broken down as a result of chemical reactions

What is erosion

Mechanical process that loosens and transports sediment from the place of weathering downhill (usually water or ice)

What is an example that shows rocks have variable resistance

Cliffs of the Grand Canyon are made of rocks standing after less resistant materials are weathered and eroded away

Erosion is the removal of

Weathered material

What are some examples of physical weathering

Zones of weakness (joints/cracks form along preexisting planes of weakness), frost wedging (water expands as it freezes in joints this wedges the rock apart and occurs in northern climates or mountainous regions), sheeting and exfoliation, spheroidal, mineral crystallization, and biological activity

What is exfoliation

Mechanism of physical weathering- exhumed rock expands to compensate for pressure release, discrete sheets of rocks form along exposed surface

What is spheroidal mechanism of weathering

The peeling of small rock bodies into onion-like layers; physical and chemical weathering around joints (small-scale exfoliation)

What is root wedging

Mechanism of physical weathering: as trees anchor themselves to the bedrock in search of nutrients, smaller pieces of unconsolidated material are inevitably created, these pieces eventually become exhumed and are integrated into the sedimentary cycle

Why does stability increase with decreasing crystallization temperatures

As we move down Bowen’s reaction Series, the structure of silicate minerals become more complex and three-dimensional (3D structures usually correlate with increased resistance to weathering)

What are some ultramafic rocks

Peridotite and komatite

What are some mafic rocks

Gabbro/basalt

What are some intermediate rocks

Diorite and andesite

What are some felsic rocks

Granite and rhyolite

What is plagioclase feldspar in BRS

Continuous series of crystalization

Olivine properties

Chemical formula (Mg, Fe)SiO, no cleavage and one single tetrahedron as silicate structure

Pyroxene group (Augite) properties

Chemical formula (Mg, Fe)SiO, two planes of cleavage at 90 degrees, and single chains as silicate structure

Amphibole group (Hornblende) properties

Chemical formula: Ca(Fe.Mg)SiO(OH), two planes at 60 and 120 degrees cleavage, and double chains as silicate structure

Biotite properties

Chemical formula: K(Mg,Fe)ALSiO(OH), one plane as cleavage, and sheets as silicate structure

Muscovite properties

Chemical formula: KAI(ALSiO)(OH), one sheet of cleavage and sheets as silicate structure

What minerals are the Micas rocks

Biotite and Muscovite

Orthoclase properties

Chemical formula: KAISiO, two planes at 90 degrees for cleavage, and three dimensional network as silicate structure

Plagioclase properties

Chemical formula: (Ca,Na)AlSiO, two planes at 90 degrees for cleavage, and three dimensional networks for silicate structure

What are the feldspar minerals

Orthoclase and Plagioclase

Quartz properties

Chemical formula:SiO, no cleavage, and three dimensional networks as silicate structure

At what angle does an oxygen atom covalently bond with two hydrogen atoms

105 degrees

Water plays a major role in

Physical and chemical weathering

I simple solution, minerals go into solution without

Precipitation of other minerals

Chemical reaction of simple solution evaporites (gypsum)

CaSO * 2H2O —> Ca2 + SO + water

Chemical reaction of simple solution carbonates (calcite)

CaCO + HCO —> Ca + 2(HCO-)

Rainwater can dissolve

Carbonate rocks completely over time (dissolved constituents run off into surface and subsurface water (ex: marble- carbonate rocks)

Less resistant minerals, such as feldspar,

Partially dissolve (react) in water (what does not dissolve is changed into clay mineral- minerals with low solubility such as quartz are not changed)

In hydrolysis,

Cations are replaced by H+ ions and controlled by pH (dissolution of olivine forms talc or serpentine and conversion of feldspar to kaolinite)

Oxidation and reduction reactions:

Involves a species losing an electron and gaining one

Pyrite chemical weathering example of oxidation and reduction

(FeS) oxidation 9reaction in oxygen and water) forms FeO (hematite) = rust

Ferric Iron example of oxidation and reduction chemical weatherinf:

Fe + e- = Fe2

What are the six mechanisms of chemical weathering

Simple solution, carbonate rocks, silicate rocks, hydrolysis, oxidation and reduction reactions, and hydration/dehydration reactions

Hydration/dehydration reactions involve the addition or removal of

Water to achieve equilibrium

What are two products of a hydration/dehydration reaction

Gypsum from hydration of anhydrite and limonite from conversion of hematite

Common example of hydration/dehydration reaction

Soy

What are the weathering products of Iron-bearing silicates (Olivine, pyroxene, amphibole, and biotite)

Clay minerals and Iron oxide

What are the weathering products of feldspar

Clay minerals (K, Na, Ca ions

What is the weathering product of quartz

Quartz

What is the weathering product of muscovite mica

Clay minerals (K ions)

What are the weathering products of calcite

Ca, CO ions

Water are the four factors controlling weathering rates

Parent rock (rocks like carbonates weather more easily), Climate (chemical weathering is favoured by warm humid and tropical climates and physical weathering is favoured by cold climates), soil (traps water and acidifies it further enhancing weathering, and time (the longer the exposure to the elements the greater the progress of weathering)

Sedimentary grains are seperated based on what two things

grain size and shape as well as density

What sorts materials very well

Water and wind (transport mode)

What can transport all size ranges and is not a very effective sorter

Ice

What are the coursest grains

Glacial erratics