Chapter 15 Mass Wasting

1. 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.

15. 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)

19. 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.

20. 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.

22. 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�.