Ch. 8: Mass Movements, Wind and Glaciers

■  Every day the landscape around us undergoes changes.

■  Changes that are powered by gravity alone are called “mass movement” or “mass wasting.”

■  Mass movement includes events like landslides, mudflows, avalanches, rock falls, and the slow downhill creeping of soil.

■  What influences these movements? ■  The material’s weight and resistance to sliding is a

big factor. (Clay, for example, is very slippery.) ■  How wet is the area? Water acts as a lubricant. ■  Was there a trigger, like an earthquake?

Types of Mass Movements ■  Mass movement is classified by the types of material

in motion and the speed of the motion. ■  The slowest type of mass movement is creep. ■  Creep is what it sounds like, the slow steady downhill

movement of soil. It may take years for it to become obvious.

■  How can we tell? ■  Creep is fastest when the slope is steep and the ground is wet.

Types of Mass Movement

�Pistol butt� tree trunks result from soil creep.

Types of Mass Movement: Flows

■  In some mass movements, Earth materials flow as if they were a thick liquid.

■  These flows may be very slow or 100 mph, depending on the slope and the amount of water mixed in.

■  Mudflows are mixtures of mud and water and can be very fast!

Types of Mass Movement: Flows

This is a volcanic mud flow called a lahar. It occurs when hot ash from an eruption melts snows and ice on the volcano..

Types of Mass Movement

■  Mudflows are common in volcanic regions or in mountainous desert areas which experience flashfloods.

■  Slides occur when surface soil and rock layers move downhill as a unit, parallel to the slope.

■  Landslides are mixtures of rock and soil while rockslides are chunks of rock moving downhill.

■  During a rockslide, a sheet of rock moves downhill on a sliding surface. The sheet of rock breaks into chunks on the way downhill.

Types of Mass Movement: Slides

landslides

Types of Mass Movement:Slumps

■  When a mass of material in a landslide rotates and slides along a curved surface, a slump results. Slumps are usually much slower than slides or flows.

■  As with other mass movements, slides and slumps can be triggered by an earthquake or a heavy rainfall.

■  Landslides of snow that occur in steep mountainous areas are called avalanches.

Types of Mass Movement

Types of Mass Movement

■  Rock falls occur at high elevations, steep cliffs, or steep road cuts.

■  The pile of rock at the bottom of the cliff is called talus.

Mass Movement Affects People

■  Human activities can contribute greatly to mass movement.

■  We can…. –  Build heavy buildings on unstable slopes... –  cut roads into unstable slopes... –  allow pools and other water-bearing containers to

leak…. –  live in high risk areas.

Mass Movement Affects People ■  Mudflows kill many people annually. When

we live in steep areas with a rainy climate, trouble is certain!

Mass Movement Affects People ■  Preventative actions include:

– draining hillsides with pipes or wells which pump out excess water...

– putting rock barriers at the feet of steep road cuts in the mountains…

– covering unstable rocky road cuts with heavy netting to channel falling rocks to the base of the cliff…

– planting vegetation on road cuts…. – putting in drainage culverts under roads to

channel water away. 13

Wind ■  Wind can only pick up very light materials, like dust,

but can carry it to great heights. This blowing away process is called deflation.

■  Heavier particles, like sand, can only be picked up to low heights, and are more often rolled along the ground in a process called saltation.

■  Since wind blows away the lighter particles, what is left behind on the desert floor are larger pieces , pebbles and larger rocks. This material too heavy to blow away is called desert pavement.

■  Wind-carried sand smoothes and polishes rocks in a process called abrasion.

Wind ■  The results of abrasion are rocks called ventifacts,

literally in Latin “made by the wind.”

Wind ■  Most people think of desert as full of sand dunes. ■  In reality, a typical desert is less than 10% dunes! ■  To make dunes you need an abundant supply of

sand and steady winds. ■  There are different types of dunes, but the most

common are the barchans. ■  Which way do you think the wind is blowing?

Wind

■  Dunes travel, or “migrate”, as sand grains roll up the gently sloping windward side of the dune and tumble down the steep slip face.

Wind

■  Dunes can sometimes grow to hundreds of feet in height.

■  Sand Mountain, Nevada, is a great example of a sand dune piling up against a mountain range.