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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