Geology of the Hawaiian Islands Class 17 9 March 2004 Any Questions? Mass Wasting, Mudflows Chapter 7 Mass wasting Transportation of large masses of rock Downslope movement of rock and soil debris under the influence of gravity Very important kind of erosion Mass wasting Gravity is the driving force of all mass wasting Mass wasting Can range from small-scale scale processes soil creeping down slope To much larger-scale processes huge landslides Mass Wasting Processes Talus Talus Accumulation of rock debris at the bottom of a slope Angle of Repose Talus Formed of loose rubble; Stands at the angle of repose (about 30-35 ) 1
Angle of Repose Steepest angle at which fragments will stand without further sliding or rolling down hill Controlled by: coarseness and size of fragments uniformity of size of the fragments The larger and more uniform the size of the fragments, the steeper the angle of repose Talus Occur when a large piece of rock and/or soil breaks off and slides down hill Often initiated by earthquakes and by very heavy rainstorms Initiated when rock/soil originally held in place by internal cohesion suddenly loses that cohesion Form on slopes that are steep enough for the weight of the surficial material to overcome the cohesive force and fail 2
Steep Headwall Jumbled-up Toes Tendency to slide increases with increasing surface slope Addition of water promotes sliding by adding weight and by reducing cohesion Friction along the slide surface controls the speed of the downslope movement Spoon-shaped sliding surface Addition of water reduces friction along the surface and allows the mass to slide faster Some slides move as coherent mass Others break up and the material inside becomes jumbled and disorganized Blackhawk Landslide, Southern California Slide moved 8 km down a slope of 2.5 Slide is 100 m thick Moved on cushion of air; moved 8 km in about a minute = 400 km/hr (225 mph) Debris flows Occur when the rock/soil mass loses coherency and lots of water is involved Debris becomes mixed up completely and flows as liquid mud Often carry large clasts Can be very destructive More destructive in explosive volcanic terranes where ash blankets the slopes and then becomes unstable 3
Soil Creep Mass movement that moves very slowly Weak soils on steep slopes move slowly down hill Mass movement in dry regions Example: leeward sides of the islands Most important processes: Gravity fall Sliding of rock fragments Down-slope creep of soil Mass movement in dry regions Chemical weathering less effective Less soil Fragmented rocks not badly decomposed Mass movement in dry regions Infrequent large rain storms erode large amounts of material in a day or two are not very common Mass movement in wet regions Weathered rock held together by plant roots Dominated by soil avalanches Soil cover on steep slope pulls loose and slides downward 4
Mass movement in wet regions Most slides small (less than 100 m long downslope,, less than 30m wide, and a few meters thick) Occur most often after very heavy rains Certain areas are more prone than others Usually because of local geological factors If bedding dips downslope, a landslide is more likely to develop Certain areas are more prone than others Usually because of local geological factors Reducing vegetation that was stabilizing the soil Over watering (decrease friction) OK to cut Dangerous to cut Cutting a slope during construction can cause a landslide Large-scale slumping and sliding Has affected all of the islands Confirming evidence has been discovered in the past 10 years Major landslides have removed very large pieces of the volcanoes 5
adjacent to the Hawaiian Ridge Rotational slumps (non- disintegrative) Debris avalanches (disintegrative) adjacent to the Hawaiian Ridge Rotational slumps Broad (up to 110 km) Short (30-35 35 km long Thick (about 10 km) Steep adjacent to the Hawaiian Ridge Debris avalanches Narrow (a few 10 s of km wide) Long (up to 250 km) Happened quickly in one event adjacent to the Hawaiian Ridge Slumps occur during the major shield building stage Once volcanism stops, the islands are probably stable adjacent to the Hawaiian Ridge Caused by: Steep slopes Young slopes of volcanoes very steep Become even steeper when the flanks of the volcanoes are inflated by volcanic activity adjacent to the Hawaiian Ridge Caused by: Volcanic activity Earthquakes Shake the ground and cause failure 6
Molokai Oahu Tuscaloosa Ko'olau Volcano 1,120 km 3 Moved 90 km Tuscaloosa Seamount 2.5 km Thick 16 km Wide 28 km Long Kilauea Buttressed on north by Mauna Loa South flank is free surface Caldera 7
Hilina Slump Off south flank of Kilauea About 5200 km 2 in area About 100 km wide Headwall is SWRZ of Kilauea on NW; Kilauea ERZ on the east GPS Measurements Reference 1975 Kalapana Earthquake Magnitude 7.2 3 m of vertical movement 8 m of horizontal displacement Questions? Thursday Field trip to Manoa Landslide Meet at Faculty Housing across from Manoa Market Place If you are taking GG 101 Lab you don t need to go on this trip Take Rainbow Shuttle BEFORE 8:45 am There WILL be questions on Exam 2 about this trip Next Tuesday Local Landslide Hazards Next Thursday Exam 2 Will cover everything since Exam 1 8
Questions? 9