GES 121 - Geomorphology & Weathering

geomorphology

study of the shape of the earth, landscape, landforms, and evolution of the shape

what’s there, how it become that, what it will be

collection of landforms, functionally related, creates a landscape (larger spatial scale)

based on erosion, transport, deposition (geomorphic agents)

• erosion: changes surface

• transport: moves material, via water/air/glacially

• deposition: leaves material behind

  • usually deficit, no longer able to move, or to ocean bottom

all can be traced to the sun, exogenic processes need energy

Weathering and Erosion

erosion involves movement; starts transportation, does not need weathering

Weathering: rocks broken down in place (in situ) (happens right there)

Mass Movement: gravity moves debris; agent often involved, but not required

• ex. landslide, rockfall (something giving enough vibration to cause it to move downslope)

Erosion: weathered debris is moved; requires an agent

Types of Weathering

Weathering processes usually occur in concert at varying different rates/times, sometimes at the same time

Physical/Mechanical: imposed stresses break rocks apart (disintegration), no change in composition (FUST)

1. Frost Shattering

2. Salt Wedging

3. Unloading/Exfoliation

4. Thermal Expansion

Chemical: makes weaker, more erodible by altering chemical composition (decomposition), creates a new compound

• faster with warmer temperatures, tends to dominate in tropical climates (tropical rainforests also have water, important) (CHOB)

1. Hydrolysis

2. Oxidation

3. Carbonation

4. Biological

Frost Shattering (Physical Weathering)

water freezes in spaces in rocks, then because ice is less dense than water, it expands and creates pressure (especially when confined), then breaks rock apart

• ex. burst pipe; water expands in confined space, expansive force needs release of pressure, (rocks on landscape also not able to withstand this pressure)

freeze-thaw cycles enhance this; most common in midlatitudes

spring/fall might have daily freeze-thaw cycles/temp flux that increases chances

• ex. mountain environments: south slope melt during the day, even if air temps don’t rise above 0C, so seeps into cracks then freezes at night

  • can cause rock falls, ice holds rock together, spring melts

Salt Wedging (Physical Weathering)

growth of salt crystals in spaces in rocks (instead of ice)

arid environments, dissolved minerals of evaporated water, often salt crystals

often not as effective as frost shattering

Unloading/Exfoliation (Physical Weathering)

overlying material is removed, pressures on underlying layers of rock lessen

on big rock faces/surfaces, often called exfoliation domes, often granite, fairly rare

intruded igneous rocks, expanded pressure

shearing off level sheets of rocks, can’t be cracks in there to start with, because cracks will absorb the pressure

breaks down into smaller pieces of granite

Thermal Expansion (Physical Weathering)

different materials expand and contract at different rates; therefore, extreme conditions will break down rock (fire, water shock)

minimal impact; changes of temperature on landscape are fairly slow, and the pressures can often be taken up by the rocks themselves

episodic, only in special circumstances

Hydrolysis (Chemical Weathering)

if water is added, some minerals expand by absorbing portions of the water

altered composition tending to be softer/weaker

• common: feldspars → clay

  • hydrolyzing rock often turns into clay; if there’s enough of it, we can mine it!

takes place quickly in warm/humid environments (ex. Jamaica, lots of precipitation), important because gives us geographic sense

• bauxite: hydrolyze aluminum rich rocks, creates lots!, mined in tropical climates and processed into aluminum

spheroidal weathering may result; important to know weathering process so don’t get sand eroded flat rock confused for manmade artifact (which would be sharper edged)

works in concert with Physical; break apart, more surface area, more exposed to weathering

Oxidation (Chemical Weathering)

minerals react with oxygen, often rusting

aluminum soils usually yellowish, iron soils reddish

in tropical landscapes with both hydrolysis and oxidating, lots of yellow soil

Carbonation (Chemical Weathering)

water can be acidic, carbondioxide dissolved into water = carbonic acid

• liquid water in atmosphere, CO₂ in atmosphere, forms precipitation and clouds, slightly acidic, more so than regular water at breaking things down

• even more CO₂ in soil than atmosphere because of animal life, etc. → water infiltration dissolves that CO₂, then even more acidic

CO₂ acid in water easily dissolves calcite

generally humid areas (sky water + water available at surface), also warmer temps

Karst topography - streams suddenly disappear into subsurface, big caverns, any soil overtop might collapse → sink hole (especially when limestone is the bedrock)

Weathering itself creating a landform, rare occasion

Biological (Chemical Weathering)

influence of plants and animals;

• ex. burrowing animals (bioturbation) brings stuff to the surface, more subject to weathering

• ex. humans create piles of sand/gravel along sides of highways, erosion taking place, greater gravitational potential/elevation → attacked by rain

  • my ex. gravel'/sand pile beside driveway for several years, driveway breaking apart much faster since

• Chelation: plants can create/secrete strong chelating agents (“claw-like”), grabs onto mineral ions, makes more soluble, then plants consume those dissolved minerals! (not expansive force, but chemical weathering)