chapter 15
In areas where permafrost exists, what lessons have been learned from building structures upon the permafrost?
Structures built on permafrost must accommodate for solifluction and subsidence during summer The Alaskan Pipeline is suspended above the ground to prevent melting of permafrost Many Alaskan rail lines have affected the permafrost causing subsidence and discontinuance of the rail line
solifluction-
a well saturated slope that may flow downslope at a rate of up to a few millimeters each day; common wherever water cannot percolate into deep layers; thus saturating the slopes surface. common in regions underlain by permafrost.
where is the greatest risk for mass wasting events & why
all states experience damage from MWEs The greatest lies in mountainous areas: heavy precipitation, snow melt, earthquakes
where vegetation is lacking MW is enhanced
conditions favoring MW may exist for a long time without an even occurring- all it needs is a trigger
what can accelerate creep
creep is accelerated by any type of disturbance: raindrop impacts, plant roots, burrowing animals
lahars-
debris flows composed of volcanic materials from the flanks of volcanoes- can occur during eruptions or when a volcano is quiet
slumping
downward sliding of material as a unit along a curved rupture surface, doesn't travel fast or far very common type of MW, occurs commonly on oversteepened slopes upper portion held in place by material at the bottom as the bottom material is removed, the upper portion becomes unstable
can oversteepening occur in consolidated materials?
even in solid rock, oversteepening can produce unstable slopes, and eventually, mass wasting
what must exist for mass wasting to occur
for MW to happen there must be a slope
slow moving type of MW: CREEP
gradual downhill movement of material; locations that experience a daily freeze/thaw.
what controls mass wasting events
gravity is the controlling force
fall- types of motion
involves freefall of individual pieces, common on steep slopes where loose material cannot remain stable
what is the condition of a slope prior to reaching its point of instability?
long before MW occurs, the slope weakens and becomes more susceptible to gravity. The slope remains stable by becomes nearer to being unstable. The slope reaches its point of instability; this is when the mass wasting event begins. sometimes, a trigger can be determined for the MWE
what is the angle of repose
loose sediment (unconsolidated, granular particles) assumes a stable slope. the steepest angle at which material remains stable
slide- types of motion
mass movements where there is a distinct zone of weakness that separates the sliding material from the stable material beneath
how does it differ from erosion
mass wasting does not require a transporting medium (water, wind, glacial ice)
why are stream valleys much wider than the streams that are creating them
mass water + running water= stream valleys; if only running water carved stream valleys they would narrow
at what rate do most mass wasting events occur
most MW is imperceptibly slow. millimeters- centimeters of movement each year
how are these constantly being created
mountain building and volcanism both create slopes by constantly changing an area's elevation.
Over steepened slopes in mass wasting
naturally occurring in a variety of locations (waves pounding against a cliff)
rockslides
occurs when blocks of bedrock break loose and slide down a slope one of the fastest and most destructive MWE
flow- types of motion
occurs when material moves downslope in the form of viscous fluid, most flows are saturated with water and move in the shape of lobes or tongues
what is a talus slope-
piles of rock fragments along the base of a steep slope from which the material originated
removal of vegetation-
plants deter erosion and contribute to slope stability root systems bind the soil together plants also shield a slope's surface from raindrop impacts
where does mass wasting occur
rapid events are more common on younger, unstable, steep mountains. very large mass wasting events are possible on these steep, unstable slopes
what has the fastest rate of movement
rock avalanche-up to 125 mph
debris flows in semi arid conditions
semiarid regions randomly receive cloudburst and/or rapidly melting snow, creating sudden floods. causes loose materials to be washed out into nearby stream channels. results in a flowing tongue of well mixed mud, soil, rock, & water.
what is cohesion
shear strength (internal friction) of a rock/sediment
categories of MWE:
slumps rockslides debris flows earthflows
what is a trigger
the actual event that initiated the MWE
what is mass wasting
the downslope movement of weathered materials directed by the influence of gravity
what is the ultimate destination for all sediment
the sea
what are the different types of mass wasting events classified by
type of material- did the descending mass begin as a sediment(unconsolidated) or as bedrock(consolidated) type of motion- how did the descending mass move downslope
debris flows
very rapid MWE involving the flow of highly saturated unconsildated materials. sometimes called mudflows. tend to follow canyons and stream channels
factors that play a role in MW
water, over-steepened slopes, removal of vegetation & earthquakes
water in mass wasting
when pore spaces become filled with water, the cohesion between sediment grains is destroyed allowing grains to slide past one another
what is an alluvial fan
when reaching the end of its channel, the flow spreads out as a fan-like deposit onto the valley floor
earthflows-
when saturated, material may detach forming a lob/teardrop shaped mass flowing downslope. viscous flows that involve less water than that of debris flows common in humid areas during heavy precipitation or snowmelt. not confined to a pre-existing channel
solifluction occurs in the material above the permafrost called the active layer
when the active layer thaws in the summer, the meltwater cannot percolate into the permafrost. the active layer becomes saturated and slowly flows
rate of movement- classifying mass wasting-
wide range or rates; very rapid-extremely gradual
can mass wasting events occur without an obvious trigger
yes
earthquakes are common triggers for MWE
EQ's can dislodge and redistribute an enormous amount of earth
creep cannot be observed in action (too slow)
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rate of flow depends on slope angle and water volume
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slow MWE can actually move more material over a larger area than a rapid MWE
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without the continuous creation of slopes, mass wasting would eventually stop
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