Chapter 14 - Mass Wasting

creep details

aided by alternate expansion and contraction of surface material - freezing and thawing, imperceptibly slow - cannot be seen

stable slope angle

angle of repose, different for various materials

landslides occur..

as a result of earthquakes, rain

rockslide

blocks of bedrock break loose and slide downslope

soil

combination of regolith, organic matter, water and air

slow

creep, solifluction

slow mass wasting

creep, solifluction

unconsolidated material

debris, mud, earth

type of material

debris, mud, earth, or rock

role of water

diminishes particle cohesion (friction), adds weight

earthquakes as triggers

dislodge rocks and unconsolidated material

mass wasting

downslope movement of rock, regolith and soil via gravity

slump

downward sliding of mass of rock or unconsolidated material as unit along curved surface

type of motion

fall, slide, or flow

fall

free-falling pieces

slopes form

from mountain building and volcanic processes

rockslide details

generally very fast and destructive, occurs where rock strata are inclined or joints and fractures in the rock are parallel to slope, triggered by rain or melting snow - common in spring

creep

gradual downhill movement of soil and regolith

solifluction

gradual flow of saturated surface layer underlain by impermeable layer (permafrost)

control of mass wasting

gravity, other factors overcome inertia to enable mass wasting to occur

distinct from erosion b/c

it does not require a transporting medium

regolith

layer of rock and mineral fragments produced by weathering

slope materials weaken over time b/c

long-term weathering, infiltration of water

damage

mass wasting caused by earthquakes can cause more damage than the earthquake itself

when slope becomes unstable..

mass wasting occurs

translational slide

material moves along a flat surface - joint, fault, bedding plane

slide

material moves along a surface as a coherent mass

flow

material moves as a viscous fluid

debris flow details

mudflows, often confined to canyons and channels, common in semiarid regions - limited vegetation, rate of flow depends on slop and water content

earthflow details

occurs on hillside in humid area, water saturates soil, material breaks away leaves scar that forms tongue or lobe that flows downhill, highly viscous - slower than debris

slump details

often consists of multiple blocks, occurs along oversteepened slopes, materials above held in place by material below, anchor at base is removed, unstable material above reacts to gravity

permafrost

permanently frozen ground (below zero Celsius for two or more years), ice underground affects behavior at surface, thawing produces unstable ground (slide, slump, subside)

removal of vegetation

plant roots bind soil, becomes unstable (?)

solifluction (soil flow)

promoted by a dense clay hardpan or impermeable bedrock layer, common in regions underlain by permafrost, can occur on gentle slopes

debris flow

rapid flow of soil and regolith containing large amount of water

rate of movement

rapid or slow

prevent mass wasting

revegetation, regarding terraces, reduce subsurface water, prevent undercutting (relocate river channel), prevent undercutting (add rip rap), construct safety structures (retaining wall, rock bolts)

mountains are eroded by

rivers and glaciers to form steep, unstable slopes

bedrock

rock

in order for mass wasting to occur, there must be..

slopes

mass wasting and erosional processes...

slowly lower land surface

rapid mass wasting

slump, rockslide, debris flow, earthflow

fast

slump, rockslide, debris flow, earthquake

effects of mass wasting and running water produce

stream valleys

rotational slide

surface of rupture is concave upward, descending material moves downward, with outward rotation

processes based on

type of material, type of motion, rate of movement

earthflow

unconfined flow of clay-rich soil on hillside after heavy precipitation

lahar

volcanic mudflow, triggered by volcanic activity - eruption

liquefaction

water-saturated surface materials behave as fluid-like masses that flow, caused by earthquakes

process that follows

weathering