Weathering, Erosion, and Mass Movement Flashcards
Define weathering
physical breakdown and chemical alteration of rock at or near Earth’s surface
What are the two kinds of weathering?
Mechanical and Chemical
Mechanical Weathering
physical forces breaking rock into smaller pieces
Chemical weathering
chemical transformation of rock into new compounds
True or False: Both kinds of weathering work simultaneously and reinforce each other?
True
True or False: Mechanical weathering increases surface area?
True
What is the relation between mechanical and chemical weathering
as mechanical weathering breaks rocks into smaller pieces, more surface area is exposed to chemical weathering
Types of mechanical weathering
- Frost wedging
- Sheeting/unloading
- Biological Activity
- Salt crystal growth
Frost wedging
- water that freezes in cracks enlarges the cracks
- lenses of ice increase in size as they attract liquid water
Unloading/Sheeting
- unloading leads to sheeting
- concentric slabs break loose when large masses of igneous rock are exposed due to erosion and glaciation.
How does an exfoliation dome form?
-continued weathering causes slabs to fall off and form a EXFOLIATION DOME
What are joints?
Joints are fractures produced by contraction during the crystallization of magma
Biological growth
- PLANT ROOTS grow into fractures in rocks, expanding cracks
- BURROWING ANIMALS move fresh material to the surface, enhancing mechanical and chemical weathering
Types of Chemical Weathering
- Dissolution
- oxidation
- hydrolysis
- spheroidal weathering
What is the most important agent in chemical wearthing?
Water
Three things about Dissolution
A small amount of acid in water is responsible for the corrosive properties
- CO2 dissolved in rain=carbonic acid
- Calcite (CaCO3) is particularily susceptible to weakly acidic solutoin
Three things about Oxidation
rust (iron oxide) forms when O2 combines with Fe
- water catalyses the rxn
- important in decomposing ferromagnesium minerals (olivine, pyroxene, hornblende, and biotite)
Two things Hydrolysis
- any rxn with water
- silicates (clay) decompose via hydrolysis
True or False: Clay minerals are the most abundant product of weathering
true
Spheroid weathering (3)
1) water penetrates jointed rocks
2) chemical waethering decomposes minerals and enlarges joints
3) rocks are attacked around the corners and edges, producing spherical shapes and fragments
What influences the rate of weathering
Rock Type and Climate
- silicate minerals weather in same order as Bowen’s rxn series (Olivine –> pyroxine –> amphibole –> biotite etc)
- warm/moist climates enhance weathering
- cold/dry climates inhibit
Differential Weathering
uneven weathering due to different mineral composition
Examples of differential weathering
Soil
-portion of regolith that supports growth of plants
Define regolith
Mineral fragments that cover the surface of earth
Impact of human activity on soil
soil erosion (loss of topsoil)
-enhanced by deforestation and farming
soil conservation
-planting in patterns that follow contour of landscape reduces rate of water runoff
-grassed waterways between crops prevetns formation of gullies and traps soil washed from cropland
True or False: Weathering and erosion are slow processes (snail)
true
Define Mass Movement
movement of bedrock, rock debris, or soil downslope due to gravity
-ie. landslide
What is the main driving force of mass movement?
erosion agents (weathering, erosion etc)
Gravitational Driving Force
F(drive)=ma=mg
Friction and Cohesion RESISTING FORCE
F(res)= mu(fric)mgcos(theta)
F(res)=basal friction+internal strength
Shear deformation
internal deformation (flow) due to gravity
Factors that influence internal strength of rock debris, sediment, or soil
1) electrostatic forces (ie. clay particles)
2) friction @ grain contact=f(angularity, grain size)
3) pore pressure
4) surface tension
(wet sand–>dry sand–>damp sand) (in order of increasing cohesion)
Define Angle of repose
increase with grain size
- fine sand (round): 35 degrees
- fine sand (angluar): 40
- course sand: 45
What makes a slope SUSCEPTIBLE to mass movement
1) slope angle
2) relief
3) bedrock type (ie. jointed, fractured, bedded)
4) thickness of debris (ie. soil cover)
5) climate
- a) ice (freeze-thaw cycle)
- b) water/moisture
- c) rain
- d) vegetation (low cover)
What TRIGGERS mass movement
- heavy rainfall/snowmelt
- addition of buildings
- earthquakes/explosions/eruptions
- excavation/road construction/ river erosion
Falls
Rock: rockfall, very fast
Debris: debrisfall, fast-very fast
Slide
Rock: rockslide, fast
Debris:
-slump (rotational), slow-moderate
-debris slide (translational), slow-fast
Flow:
Rock: -creep, very slow (10 cm/yr) Debris: -creep, very slow (,10 cm/yr) -Earthflow/gelifluction/debris flow/mudflow/quick clays, mod-fast -avalanche, very fast -
creep
very slow
Mechanisms:
-wet/dry cycle
-freeze/thaw cycle
gelifluction
in colder climate: H2O saturated debris moves over permafrost during warmer months
Debris flow
course material (boulder/gravel) is predominant, supported by matrix of finer sediment
mudflow
mix of debris and water, confined by chanel
- faster/further than debris flow
- high water content
Where do mudflows occur
flanks of volcanoes, steep deforested mountain sides, steepened cliffs of unconsolidated fine sediment
True or False; Pyroclastc flows (ash, hot gases, sediment) turn into mudflows (sediment and water) downstream
true
Quickclays
- clays deposited in laciomarine environments
- decreased stability when salt is leached out by freshwater
Clay mineral formation
orthoclase feldspar + acid+ water = clay + potassium + soluble silica
(2KAlSi3O8 + 2H+ + 9H2O = Al2Si2O5(OH)4 + 2K+ + 4H4SiO4)
What is the relative rate of weathering of common minerals?
most stable to least stable;
Iron oxide, aluminum hydroxides, quartz, clay minerals, musovite mica, potassium feldspar, boilite, albite, amphiboles, pyroxene, anorthite, olivine, calcite, halite.
