denudation: rivers and ice...weathering def: breakdown of rock and minerals by physical pressures...
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Chapter 2. Denudation: Rivers and Ice
DENUDATION: process that lowers level of
land
- caused by rivers, glaciers, waves & wind
- involves processes of WEATHERING &
EROSION
Weathering
Def: breakdown of rock and minerals by
physical pressures and chemical reactions
2 types: 1. Physical 2. Chemical
1. Physical Weathering
- also known as Mechanical Weathering
- rocks broken into smaller fragments
- involves forces in physical environment
Types of Physical Weathering
I. Expansion/Contraction
- rock expands when heated suddenly
- contracts when cooled quickly
- results in cracking of rock
Types of Physical Weathering (cont)
II. Exfoliation
- associated with release of internal pressure within
rock
- caused by erosion of overlying material
and heating and cooling
- outer layers of rock “peel away” like layers of
an onion
Exfoliation
Types of Physical Weathering (cont)
III. Frost Fracture
- in climates experiencing temp fluctuations
- extreme heat and cold causes cracks in rocks
- water seeps into cracks and freezes
- water expands during freezing
- cracks widen until rock splits apart
Frost Fracture: Talus Slope
Frost Fracturing
Types of Physical Weathering (cont)
IV. Root Wedging
- called PLANT ACTION
- plant roots pry rock joints apart
V. Burrowing animals
- expand existing rock cracks through tunneling
Root Wedging
Root Wedging
Chemical Weathering
Def: breakdown of rock minerals by
chemical reaction
- usu related to action of rainwater when
it comes into contact with rocks
- 3 main processes occur:
i. Solution
ii. Hydrolysis
iii. Oxidation
Chemical Weathering (cont)
I. Solution
- contaminants in rainwater create
carbonic acid
- erodes minerals in rock causing it to decompose
- ex limestone
- called CARBONATION
Carbonation: This form of weathering occurs wherever seeping ground
water circulating through the sandstone dissolves the calcium-carbonate
cement between the individual grains, causing the rock to crumble to sand.
Chemical Weathering (cont)
II. Hydrolysis
- carbonic acid in water dissolves silicates
in rock producing soft clay
- clay turns to soil
Hydrolysis: iron-bearing silicate minerals weather to
form clay
Chemical Weathering (cont)
III. Oxidation
- oxygen in water reacts with metallic
minerals in rock
- reaction produces new minerals called
oxides
- reddish-brown stains on rocks are iron oxides
or RUST
- pale-greenish stains are copper oxides
Oxidation
Erosion
Def: the breakdown of rock into smaller fragments
and the movement of these fragments to other
locations.
- moved by wind, water, ice, gravity
- running water a main agent of erosion
- rain water flows over Earth’s topography
- collects in RIVERS
Erosion and Gravity
Erosion (cont)
- land area from which a river receives its water
called DRAINAGE BASIN
- basins of different rivers separated by heights
of land called DIVIDES
Drainage Basin
Example of a Watershed
River Erosion
- river system erodes high land and carries
material to fill lower land
- rate at which area is lowered or eroded depends
on a # of factors:
1. Resistance of underlying rock
2. Volume of water
3. Vegetative cover
4. Gradient of river bed
River Erosion (cont)
- rivers concentrate energy and erode river bed
both vertically and laterally
-VERTICAL EROSION: river erodes bottom of
river channel
- LATERAL EROSION: erosion of the river banks
- this produces MEANDERS in river channel
River Erosion (cont)
- erosional processes include:
1. Hydraulic Pressure -- water pressure
2. Corrosion -- erosion of rock by solution
3. Abrasion -- grinding away of rock by rock
fragments and particles carried in water
Life Cycle of a River
- as rivers erode and lower the landscape, its
drainage pattern and channel shape change
- rivers are thought to pass through stages
- Youth, Maturity and Old Age
- at each stage, river has certain characteristics
Youth Stage
- youthful rivers typically found in highland
or mountainous areas
- steep, V-shaped valley
- straight channel & uneven riverbed
- fast, turbulent flow; high erosional force
- rapids & waterfalls common
Mature Stage
- high relief has been lowered
- river bed has gentler slope, slower river flow
- river may have many branches or TRIBUTARIES
- river valley broad, flat with well-developed
FLOODPLAIN
- valley widens and MEANDERS more evident
Floodplain
Risks of Living on a Floodplain
Old Age Stage
- relief around river extremely flat, valley very broad
- river bed has almost no slope, slow flow
- muddy water due to poor drainage
- meanders very pronounced
- OXBOW LAKES form
Formation of Oxbow Lakes
Formation of Oxbow
Depositional Features of Rivers
- As agents of erosion, rivers transport weathered material
- silt, sediment, sand, gravel, rocks carried along
- this material eventually gets deposited
- Where?
- along river bed, banks and at mouth of river
- DELTA is depositional land form
Delta Formation
- every river carries its STREAM LOAD
- materials it transports
- when energy of river lessens, materials get dropped
- energy greatly reduces at mouth of river
- this is where river enters its reservoir (lake, sea, ocean, etc.)
- sediments build up here -- form DELTA
- pattern of sediments-- big items deposited first
Conditions That Favour Delta Formation
- large load of sediment
- shallow sea at river mouth
- sheltered coasts with weak tides and currents
- absence of large lakes along the course of the river
which will siphon off the load
Types of Deltas
3 types:
1. Arcuate Delta
- symmetrical, fan-shaped
ex. Nile River Delta
2. Digitate Delta
- asymmetrical; bird’s foot shape
ex. Mississippi River Delta
3. Estuarine Delta
- no particular shape
- seen as tidal mud flats at low tide
Nile Delta: Arcuate
Mississippi Delta: Digitate
Rio de la Plata Estuary in South America
Rio de la Plata Estuary in Argentina and
Uruguay looking west - east
River Erosion Slide Show
Geography - River Erosion » SlideShare (share powerpoint presentations online, slideshows, slide shows, download presentations,.url
Glaciation
- ~ 1 million years ago, earth experienced drop
in average temperature
- no agreement on how or why temperatures changed
possible reasons include:
1. Change in earth’s orbit
2. Decreased output from sun
3. Volcanic dust blocking out sun
Glaciation (cont’d)
- as earth temps decreased, water cycle changed
- more water trapped on Earth as snow
- snow built up over time and formed ice
- called GLACIERS
- 2 types: 1. Continental 2. Alpine
Glacier
Alpine (Mountain) Glacier
Continental Glacier
Continental Glaciation
- large sheets of ice cover large land areas in
high latitudes during cooling periods
- called glaciers ADVANCING
- when temp on earth warm, ice sheets melt
- called RETREATING
-exs. Greenland and Antarctica
Antarctica
Greenland
Greenland From the Air
Continental Glaciers (cont’d)
-as glaciers retreat, depositional landform features
created by meltwater
- these features include:
- outwash plain
- terminal moraine
- erratic
- drumlin
- esker
Outwash Plain
Outwash Plain
Outwash Plain
Terminal Moraine
Erratics
Drumlin
Drumlin
Esker
Esker
Alpine Glaciation
- occurs in high elevations
- glaciers grow on side of mountain
- gravity pulls glacier down through valleys
- creates U-shaped valley and other features:
lateral, medial, terminal moraine
cirque
arete
hanging valley
fiord
Alpine Glaciation
Lateral Moraine
Lateral Moraine
Medial Moraine
Terminal Moraine
Cirque
Cirque
Cirque and Tarn
Arete
Hanging Valley
Hanging Valley
Hanging Valley
Fiord
Western Brook Fiord
Gros Morne
Western Brook Fiord
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