Transcript MASS MOVEMENTS, WIND, & GLACIERS mass movement

MASS MOVEMENTS, WIND, & GLACIERS
mass movement = when gravity alone causes loose
sediments and weathered rock to move downslope
1. climate determines which materials and how much is
available for movement
2. all occur on slopes
a. most common on slopes greater than 25o that have
annual rainfall of over 90 cm
b. occur more often after heavy
rains
3. range from very slow motion to sudden slides, fall, &
flows
4. factors that affect
a. material’s weight
b. material’s resistance to sliding or flowing
c. trigger that works to shake material loose from slope
such as earthquakes & heavy rains
d. amount of water on slope: too much water loosens
sediments as well as makes these heavier & easier to
move
5. categories:
a. slides = move as block of Earth material
b. flows = movement involves mixing of particles within
moving mass
c. falls = free fall of Earth material
TYPES:
RAPID MASS MOVEMENTS:
1. mudflows = rapid movement of a large mass of mud and
water
a. occur in dry, mountainous areas and sloped, semi-arid
regions that have intense, short-lived rainstorms
b. usually after heavy rains, earthquakes, or volcanic
eruptions
c. spreads out in large fan shape at base of slope
d. mud avalanche = fast and
large mudflow
e. can move down stream
channels at speeds up to
100 km/h
2. landslides = sudden movement of masses of loose rock
and soil down slope of hill, mountain, or cliff
a. generally occur on steep slopes
b. often after heavy rainfall, spring thaws, volcanic
eruptions, or earthquakes
c. relatively thin block of loose soil, rock, and debri
separates from underlying bedrock
d. can carry millions of metric tons of rock and soil
e. can reach speeds up to 200 km/h
f. rockslides = landslide that occurs
when sheet of rock moves
downhill on sliding surface
3. slump = large block of soil and rock become unstable and
move downhill
a. slides along curved slope of surface in one piece
b. occurs in areas that have thick soils on moderate to
steep slopes
c. often recurs in the same place or adjacent slopes
d. trees on blocks are tilted backward so that they seem to
point more uphill
e. common after rains, can be triggered by earthquakes
4. avalanches = landslides in mountainous areas with thick
accumulations of snow
a. usually occur on slopes of at least 35o
b. sun melts surface snow which refreezes at night forming
an icy crust; new snow falls on top eventually building
heavy enough to slip off & slide down slope
c. ~ 10,000 occur each year in mountains of western US
d. can reach speeds up to 300 km/h
Colorado
5 rockfall = fall of rock from steep cliff
a. also called a rock avalanche
b. most rapid kind
c. rocks range in size from tiny fragments to giant boulders
d. commonly occur in high elevations, steep road cuts,
rocky shorelines
e. most often after heavy rains or during earthquakes
f. can form a talus = cone shaped pile of rock fragments at
base of cliff
Canada
Slow Mass Movements:
1. creep = extremely slow, downhill movement of weathered
rock material
a. most effective type
b. water separates and lubricates rock particles allowing
them to move freely and independently of each other
c. slope does not move as a consolidated mass
d. can observe in areas where see
fences or telephone poles tilted
downhill
e. movement occurs at a rate of
about 1 cm/yr
2. solifluction = slow downslope flow of waterlogged materials
over frozen or clay rich subsoil
a. occurs in arctic and mountains areas where subsoil is
permanently frozen
b. mudlike liquid produced when water is released from
melting permafrost during warm season
c. in spring and summer top layer thaws and flows downhill
d. can also occur in warm, humid
areas that have a hard clay
layer and ground remains
saturated year-round
REDUCING RISKS FOR PEOPLE
1. avoid building structures on steep, unstable slopes
2. preventive actions
a. dig series of trenches to divert running water around
slope and control drainage
b. use steel nets to cover steep slopes
c. construct protective fences along highways where
rockslides are common
d. build retaining walls at base of
weakened slopes
3. most only successful in short run
WIND
1. erodes dry land more effectively than moist land
2. can transport particles in any direction
3. normal winds cannot carry particles as those moved by
moving water
4. primarily occurs in areas with little vegetative cover
(deserts, seashores, lakeshores, semi-arid areas)
WIND EROSION:
Carries rock particles of various sizes
1. sand = loose fragments of weathered rocks and minerals
a. mostly quartz
b. range in diameter from 0.06 to 2 mm
2. dust = particles smaller than sand grains
a. less than 0.06 mm diameter
b. mostly silt and clay
c. usually from soil or volcanic eruption
Particle movement
1. saltation = movement of sand by short jumps
a. occurs when wind speed causes sand grains to roll
along ground
b. move in same direction wind is blowing
2. suspension = strong winds cause particles to stay airborne
for long distances
a. dust particles are lifted and carried high into air
and remain in air for several years
b. create large dust storms
Methods and Effects:
1. deflation = removal of small, loose particles (clay or silt)
from ground surface
a. lowers land surface
b. desert pavement = surface of closely packed small rocks
1) left after top layer of soil has been removed
2) protects underlying land from erosion
c. deflation hollow = shallow depression left after layer of
exposed soil is eroded
1) form in areas where natural plant cover is gone
2) common in dry climates
2. abrasion = weathering of rock particles by impact of other
rock particles
a. windblown sand grains strike other sand grains and
rocks breaking off small fragments
b. some rocks will become pitted and grooved from periodic
wind-blown erosion
c. ventifacts = rocks shaped by wind-blown sediments
1) some can be polished smoothed by continued
abrasion
2) can determine direction of wind by shape
3. large masses of rock erode very slowly and only close to
ground
Deposits Formed:
1. dunes = mound or ridge of sand
a. most common type
b. form where soil is dry and
unprotected such as deserts
and beaches
c. characteristics:
1) windward side = side facing wind
2) leeward side (slipface) = steeper slope, sand is blown
over crest to opposite side
3) shape determine by conditions under which it forms:
availability of sand, wind velocity, wind direction,
amount of vegetation present
4) migration occurs when sand blown over crest and
stops when barrier is reached
5) wind velocity determines height: usually 12 to 25 m
range, some reach 100 m
6) dune vegetation helps to anchor coastal dunes
d. types: classification based on shape
1) barchan = crescent shaped
a) most common
b) open side faces away
from wind
2) parabolic = crescent shaped (U-shaped)
a) open side faces into the wind
3) transverse = series of ridges in long, wavelike patterns
a) form at right angles to wind direction
Star dune
4) longitudinal = form in the shape of a ridge
a) lie parallel to the direction of wind
2. loess = thick deposit of yellowish, fine-grained sediments
(silt & clay)
a. accumulation of windblown dust
b. soft and easily eroded
c. can form steep bluffs
d. deposits are extremely fertile
for farming
3. oasis = fertile green area within a desert
GLACIERS
glaciers = masses of moving ice that shape landscape by
eroding, transporting, and depositing huge volumes of rocks
and sediments
1. formation:
a. amount of snowfall in winter exceeds amount that melts in
summer
b. as snow accumulates pressure increases on bottom of
snow pile and snow compacts
c. any snow that melts quickly refreezes, forming glacial ice
d. when snow and ice accumulate to a great enough
thickness, ice will start move downslope due to gravity
e. greatest in regions where temperatures are low and snow
fall amounts are high
2. snowline = elevation above which ice and snow remain
year round
a. lowest level that permanent snows reach in summer
b. elevation varies from place to place
3. snowfield = almost motionless mass of permanent snow
and ice
a. also called an ice field
b. forms by accumulation of snow and ice above the
snowline
c. covers land near the poles and tops of some mountains
at lower latitudes
4. firn = grainy ice
a. formed by partial melting and refreezing which changes
snow crystals into grains of ice
b. usually loosely packed, rounded ice grains
5. glacial ice = mass of interlocking ice crystals
a. pressure of overlying layers become great enough that
ice grains flatten
b. air is squeezed out
c. pack together so tightly that water
cannot move through
d. becomes a bright steel-blue color
6. growth depends on:
a. balance between snowfall received and ice lost by
melting and evaporation
b. increase or decrease in size may be an indicator in
annual climatic change
c. when snow is added faster than it melts → increase in
size
d. when ice disappears faster than snow is added →
decrease in size
7. weather conditions change
the position:
a. warm weather causes it
to melt → retreat
b. cold weather causes it to
advance
TYPES:
1. valley glacier (alpine glaciers) = long, narrow, wedge-shaped
mass of ice
a. form in high, mountainous areas, exist on every continent
b. must be at high elevations where snow remains year round
c. usually in hollows or abandoned river valleys
d. development depends on both temperature and
precipitation
e. flow begins when snow and ice accumulation exceeds 20 m
in thickness
1) speed determined by slope of
valley floor; temperature and
thickness of ice determined by
shape of valley walls
2) sides and bottom move slower due
to friction
3) move less than a few mm per day
2. continental glaciers (continental ice sheets) = huge
masses of ice and snow found near Earth’s polar regions
a. covers an area of 50,000 square km or more
b. covers 10% of Earth’s surface
c. thickest in center causing it to spread outward in all
directions under its own weight
d. can be currently found in Greenland, Antarctica, and
northern Canada
MOVEMENT:
1. often called “rivers of ice”
2. on average moves about 100 m per year
- some move more (km) and some move less (few cm)
3. surge = sudden movement that occurs after long periods of
little movement
4. basal slip = meltwater acts as a lubricant moving entire
mass along bedrock
a. moves by slipping at its base
b. able to work over small
barriers in path by melting
and refreezing
5. internal plastic flow = solid ice crystals slip over each other
causing a slow forward motion
a. pressure of overlying layers cause layers of ice to slide
over each other
b. determine rate of motion by slope and by the thickness
and temperature of ice
1) faster nearer to surface than near the base
2) friction with rock surface slows the flow rate
6. center moves faster than sides (especially valley glaciers)
FEATURES:
1. crevasses = large, deep cracks in surface
a. can be more than 30 m deep
b. usually covered by thin layer of snow
c. break open under the slightest weight
2. ice shelves = ice sheet moves out over the ocean
a. Ross Ice Shelf is largest
b. rise and fall of tides break off large pieces of leading edge
3. icebergs = large blocks of ice that drift into the ocean
a. most of its mass (size) is below surface of water
b. size varies
c. hazardous to ships
EROSION BY:
1. plucking = meltwater freezes and erodes rock by loosening
particles, then lifting and carrying them downslope
2. abrading = scouring of bedrock surface by rock fragments
over which ice moves
a. striations = parallel grooves and scratches formed from
embedded rocks
b. glacial grooves = very large striations
c. glacial polish = silt and sand embedded in glacier base
produces a smooth, shiny finish
d. rock flour = abrasion grinds rocks and other coarse
particles into silt size sediment
e. scratches and grooves run parallel to direction of glacial
movement which can show direction of ice movement
LANDFORMS CREATED:
1. U-shaped valleys
2. cirque = steep walled semicircular depression
a. eroded into a mountain peak
b. one side open to a valley
3. arete = sharp, narrow ridges; form where two cirques on
opposite sides of a valley meet
4. horn = sharp, pyramid shaped peak
a. rock summit eroded by glaciers
b. usually where 3 or more cirques intersect near the
summit
5. tarn = small lake that
forms inside a cirque
6. roches moutonnees = rounded knob of rock
a. smooth, gently sloping side
b. faces direction from which glacier came
7. hanging valleys = tributary glacial valley whose mouth lies
high above floor of main valley
a. form from small tributary glaciers that meet main valley
glacier
b. may see waterfalls form where these join
8. ice sheets erode by leveling landforms, producing fairly
smooth, rounded landscapes
DEPOSITION:
1. erratics = large boulders
a. transported to present location by a glacier
b. composition usually differs from that of bedrock over
which it now lies
2. glacial drift = all sediment deposited by a glacier or its
meltwaters
a. glacial till = mixed debri carried embedded in ice and on
tops, sides, and front edges
1) sediments scraped off by base of glacier
2) left behind when glacial ice melts
b. stratified drift = sediment sorted, deposited, and layered
by glacial meltwater
c. meltwater = water that results from melting of glacial snow
and ice
1) usually milky in color
2) presence of fine rock particles
3. till deposits:
a. moraines = ridges of unsorted rock material on the ground
or on glacier
1) lateral moraine = deposited along sides of valley glacier
as long ridge
2) medial moraine = adjacent lateral moraines join
together
- form dark stripes on glacier surface
3) ground moraine = unsorted material left beneath
glacier when ice melts
4) drumlins = long, tear-shaped mounds of till
a) often found in clusters
b) parallel to direction of glacial movement
c) forms when glacier flows over and reshapes
5) terminal moraine = deposits at the snout or front of a melting
glacier
a) belts of small ridges of till
b) many depression contain lakes or ponds
4. stratified drift deposits:
a. outwash = deposit left by streams flowing from a melting
glacier
b. outwash plain = deposit which lies in front of terminal
moraine
1) sediment spreads out into larger valley or plain
2) crossed by many meltwater streams
c. kettles = depressions across the outwash plain
1) created by melting a large chunk of glacial ice left
buried in the drift
2) when ice melts will form lakes or ponds = kettle lakes
d. eskers = long, winding ridges of gravel and coarse sand
1) form when fill meltwater
tunnels
2) deposited by streams flowing
under melting glacier
e. kames = small mounds of
sand and gravel
ice age = long period of climatic cooling during which
continental ice sheets cover large areas of earth’s surface
1. time of extensive glacial activity
2. earliest one = 600 mya
3. most recent one began 2 - 3 million years ago, retreated
about 18,000 years ago
4. climate changes
a. drop in average temperature of about 5oC
b. have increase in snowfall
5. glacial period = time when ice sheets advance (during cold
periods)
a. during last period covered nearly 1/3 of land surface
b. sea level was 140 m lower than today
6. interglacial periods = time of warmer temperature and ice
sheets retreat
7. causes:
a. Milankovitch theory = small, regular changes in earth’s
orbit and in the tilt of earth’s axis cause ice ages
b. three types of periodic changes occur as earth moves
around sun
1) change in shape of earth’s orbit
2) change in tilt of earth’s axis
3) change in circular motion (precession) of earth’s axis
c. these will change the
amount of solar energy
reaching earth’s surface