6 - Weathering and Erosion

what is a sedimentary rock?

rocks made of sediment

how is sediment created?

mechanical and chemical weathering in situ

how are igneous rocks turned into sedimentary?

weathering, deposition, and erosion/transport

what is weathering?

provides raw material for sedimentary rock through mechanical and chemical weathering in situ

what is mechanical weathering?

breaking down rock physically

what are the 4 types of mechanical weathering?

1. frost wedging

2. salt crystal growth

3. sheeting

4. biological activity

what is frost wedging?

when water seeps into fractures in rock, water freezes and expands, causing widening of fractures and chunks to break off

what is salt crystal growth?

salt water seeps into fractures, water evaporates and salt crystals forms, growth of crystals widens fractures further

what is sheeting?

removal of overlying rock/soil that was causing a confining pressure, relieving downward pressure causing outward expand and perpendicular fracture to expansion (granite batholith)

what is jointing?

a form of sheeting:

• break of rock due to contraction - occurs during cooling/crystallization of magma or mountain building

what is biological activity (in terms of mechanical weathering)?

• plant roots penetrating fractures

• burrowing animals remove rock material - clam boring

• humans - road cuts, mining, construction, etc.

what is chemical weathering?

breaking down of rock components/minerals, altering minerals and releasing components to the environment, weakening outer rock

chemical weathering makes rocks more susceptible to:

: mechanical weathering, as outer rock gets weakened exposing more rock to be weathered

what are the 3 forms of chemical weathering?

1. Dissolution

2. Oxidation

3. Hydrolysis

what is dissolution?

polar water molecules attract and remove ions from crystal structure

• most minerals insoluble in water, so need acid to increase corrosive capability of water

• decaying organisms release organic acids

what is oxidation?

ion loses an e^-, common with Fe = rusting

• rusting occurs when Fe freed from crystalline structure

what is hydrolysis

water loosening - H⁺ ions from dissociated water or other source replace cations in crystal structure

• destroys orderly crystalline structure

what happens in silicate mineral weathering?

• multiple chemical weathering processes occur to same minerals

• end product are mainly clay minerals

• dissolved silica formed by hydrolysis used in organisms like shells

• weathering to quartz is minimal due to it’s resistance - makes up sandy beaches

fracturing =

more weathering = more fracturing

what is spheroidal weathering?

rounded appearances of rock outcrops

• corners most vulnerable, areas most weathered

what are the factors controlling weathering?

rock type and climate

how does rock type impact weathering?

• mineralogy - sensitivity of ions in crystal to chemical weathering

• strength - interlocking crystals more resistant than cemented crystals

• structure and loading history - more stress and loading experienced = easier to weather

• thermal expansion = heating and cooling causes more fractures

how does climate impact weathering?

• temperature

• moisture availability

• vegetation

• soil/groundwater chemistry - more acidic more weathering

what is erosion?

transport in a medium (something carrying material)

what is fluvial transport?

erosion occurring in rivers/moving bodies of water with high flow

what is dissolved load?

ions dissolved in water during chemical weather, ions precipitated when chemistry changes

what is suspended load?

solid particles carried in water

what is bed load?

material that is carried along the bed, rolling, sliding, saltation, if flow is fast enough, will move big particles a little

what is saltation?

“bouncing” - transferring kinetic energy by bumping into other particles

• bumped particles carried in suspension a short distance

what is aeolian erosion?

suspended and bedload (same as fluvial), only small particles carried by wind other than HURRICANE winds

what is glacial erosion?

sediment carried by glacier, all sizes of particles transported (erratics)

what is mass wasting?

when material travels downslope via gravity after slope destabilization

what causes slope destabilization?

slope too steep, intense rainfall, undercut, earthquakes

what is precipitation deposition?

when dissolved ions come out of solution from cooling, evaporation, or exchanged with ions in host rock

what is bed load deposition?

basically already deposited, large grains roll/slide/sand grains move saltation

what is suspended load deposition?

depends on river capacity and competence

what is stream capacity?

max load a stream can move per unit time

what is stream competence?

streams ability to transport particles based on time

when does deposition occur in fluvial deposition?

when stream competence drops

what is aeolian deposition?

only smallest grains transported, sand-sized grains move via saltation, large particles stay deposited unless hurricane winds occur

what is glacial deposition?

all sizes deposited when ice melts

what are the depositional environments?

large grains = mountains

sand-sized grains = plains and beaches

silt and clay = lakes and oceans