Ch. 8: Mass Movements, Wind and Glaciers

Transcription

1 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.

2 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!

3 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.

4 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.

5 Types of Mass Movement 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 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.

6 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!

7 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.

8 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?

9 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.