Presentation on theme: "Chapter 15 – Mass Wasting Mass wasting – the downslope movement of rock, regolith, and soil due to the influence of gravity. Does not require presence."— Presentation transcript:
Chapter 15 – Mass Wasting Mass wasting – the downslope movement of rock, regolith, and soil due to the influence of gravity. Does not require presence of water to move this material- but water may facilitate movement. Landslides and mudslides are among the most spectacular and deadly of geologic events.
Peru, 1970 – Offshore earthquake (remember subduction zone), triggered rockfall on vertical face of Nevado Huascaran. After falling 1 km, the rock mass shattered and pulverized, tons of rock and ice raced down mountainside, into a valley, killing 20,000 people. In addition to the original rock material, as the avalanche progressed, it picked up additional debris. A portion of the debris cleared a 650 to 1000 ft. high ridge.
Landforms are shaped when exposed rocks are weathered and the weathered products are removed. Erosion by water is usually gradual, except during and following intense rainfall events, when mass-wasting events may play a role in landform development. Usually, rugged, geologically young areas are most prone to spectacular mass-wasting events. In a Continental Arc setting, inland from a subduction zone, the Andes Mts. are continually growing. 3
Over time, as the landscape matures, the slopes become less steep (approaching equilibrium) and the terrain becomes more subdued. Larger mass-wasting events are replaced by smaller, more local events. Western US, geologically younger, more active – more prone to mass-wasting events. Eastern US mass-wasting usually is associated with slope disturbances in mountainous areas. Example: Ellijay, GA 2003. 4
Controlling Forces of Mass-Wasting Gravity – Friction - Cohesion When Gravity overcomes Friction & Cohesion, movement can either be slow and gradual or rapid, when there is a triggering event. Triggering Events can include: Over-steepened slopes Removal of Vegetation 5
Earthquakes/other vibrations No apparent reason Water is heavy and it acts as a lubricant. When soil becomes saturated, the weight overcomes the internal friction holding the mass on the slope. Landforms over time become stabilized. Slopes are often “held up” by resistance at the base of the slope. When base is undercut, support is lost. More triggering events: 6
Natural slope that has reached equilibrium Roadcut at base of slope removes support, slope “needs” to regain equilibrium by mass- wasting. Roadcut
A stable slope of unconsolidated, particles, the slope angle is called the “angle of repose”. Angle of repose varies from 25 to 40 degrees. Larger, angular particles support the steepest slopes. If slope becomes destabilized, debris will move downslope to re-establish stability. 8
An example of oversteepening – Valley-fill material deposited during slope stabilization (erosion and redeposition). Flash floods due to 22” of rainfall scoured the valley. 9
When vegetation is removed by logging, fire, clearing for cropland, etc., anchoring effect of roots is lost. Or, conversely, on a modified slope (construction area, etc.), that is newly grassed, immature grass may not have deep enough roots to stabilize slope. Earthquakes or other vibrations – Can disrupt internal friction of “locked” rock fragments. Gentle, long-term vibrations caused by trucks or cars could also play a role, near highways. 10
Earthquakes can also cause liquefaction (on slopes and flat areas), i.e., the loss of the soil’s structural integrity. Sometimes landslides happen with no apparent warning. Old Man of the Mountain, New Hampshire http://www.cs.dartmouth.edu/whites/old_man.html Mass Wasting Processes are classified based on the: Type of material The kind of motion displayed The Velocity of movement 11
Materials – Unconsolidated soil (regolith) – Debris flow, mudflow, etc. Bedrock – Rock slide, Rock fall Type of motion Fall – freefall, material leaves the slope Slide – mass remains coherent and moves along a definable slope Flow – mass moves downslope as a viscous flow
Rate of movement – Rock avalanches are generally the fastest, reaching up to 125 miles per hour. When mass starts moving down steep slope, air becomes trapped beneath avalanche, serves as a cushion, lessens friction with ground. Most mass-wasting events are generally slow, sometimes as slow as downslope “creep” which may be measured in millimeters per year.
Slump - mass of rock or unconsolidated soil moves as a unit along a curved surface (glide plane). Rupture is often spoon- shaped and occurs because of over-steep- ening (perhaps with water and/or vibrations acting as the trigger). Figures 15.11 and 15.12 show slumps. 15.12 shows backward tilt of upper surface. 14
Rockslide/Debris slide – usually takes place where strata are tilted or joints and fractures parallel surface. More common after rain or during spring, when water lubricates slide surface. If material is unconsolidated, debris slide is used instead. Examples: rockslides after earthquakes, etc.. When strata are tilted, weakness of shale or clay may contribute to slippage (Fig. 15.14, pg. 458) 15
Outline map of Franklin Mts., El Paso, TX showing major faults and landslide blocks. The sedimentary rocks dip (tilt) about 35 degrees to the west. The landslide blocks are just huge rock slides. Most of the western slides likely slid upon shale layers. 16
Debris Flows/Mud Flows – contain large amounts of water, generally tend to follow canyons and valleys. Debris Flows in Semi-Arid regions – soil & regolith are washed into stream channels. Consistency ranges from wet concrete to a soupy mixture. When flow leaves confinement of steep, narrow canyon, the sediment is rapidly deposited in a fanlike manner.
Miniature version of an alluvial fan below. Alluvial fan deposits are usually very poorly sorted, i.e., there is a wide range of grain sizes.
Lahars – Volcanic mud flows. Triggers – Water saturation of unstable ash along flanks of large, composite volcanoes. Sudden melting of tons of ice and snow during eruption. Examples – Toutle River (Mt. St. Helens), Mt. Pinatubo (after eruption), Nevado del Ruiz, Colombia destruction of town of Armero – 25,000 dead. 19
Earthflows – more common to hillsides in humid areas. Breakaway of heavily saturated soil leaves a rounded scar and produces a flow downslope from the slumped area. In Figure 15.17, the earthflow is associated with an upslope slump. These are generally slower than earlier described mudflows. Earthflows likely lose more of their coherence than a slump. 20
Slow mass-wasting events – Creep – may be caused by alternating events freezing/thawing or wetting/drying. Creep may be promoted also by saturation of the soil by water. Solifluction – “Soil flow” – when underlying conditions (tight clay, frozen permafrost) do not allow the downward drainage of water in saturated soils.
Underwater landslides – flanks of large, underwater volcanoes and volcanic islands. Also occur at the margins of continental shelves. May occur as simple slumps, or may become “turbidity flows” that build broad, underwater sediment fans. In areas subject to repeated turbidity flows, distal deposits may build up as a series of thin, graded bedding sequences, called “turbidite sequences”.