Laboratory Manual for Physical Geology. Mass Wasting

Transcription

1 Laboratory Manual for Physical Geology Mass Wasting Overview... 2 Materials Needed... 2 Factors that Affect Mass Wasting... 2 Triggers of Mass Movements... 5 Types of Mass Movements... 6 Recognizing Mass Movements Calculating Gradient and Slope Angle Gros Ventre Landslide Slumgullion Landslide Exercises Mass Wasting Page 1 of 23

2 OVERVIEW Mass wasting, also known as slope movement or mass movement, is the geologic process by which soil, sediment, and rock move downslope under the force of gravity. In mass wasting, material is moved directly by gravity, in contrast to erosion where material is moved by water, wind, or ice, and may move partly as a result of gravity. Mass wasting events constantly put property and human safety at risk. One event alone, in fact, can set off multiple landslides. According to USGS estimates, the 2010 Haiti earthquake triggered 4,000 to 5,000 landslides. Data gathered about mass wasting events such as these are valuable to geologists and engineers in reducing risks. In this lab, you will gain experience in interpreting basic data, images, and topographic maps related to mass wasting events so you can assess risk factors as well as the effects of these events in various geographic settings. MATERIALS NEEDED 100 Topographic Maps Calculator Ruler Hand lens Pencil and eraser Printouts (from lab manual or downloadable pdf) o Questions on Gradient and Slope for Marysvale o Questions on Gros Ventre Landslide o Slumgullion Contour Map o Graph paper for Slumgullion Topographic Profile o Questions on Slumgullion Landslide FACTORS THAT AFFECT MASS WASTING Mass wasting is affected by many factors, including: steepness of slope (slope angle) strength of material cohesion of material friction between fragments degree of weathering water saturation climate vegetation underlying geology Forces acting to move material downslope increase as the slope angle increases. The force of gravity is always vertically downward, but that force vector can be visualized as two smaller forces at right angles, one parallel to the slope and pointed downhill and one perpendicular to the slope and pointed into the slope. Mass Wasting Page 2 of 23

3 The force perpendicular to the slope holds the material together and prevents movement. The force parallel to the slope acts to move the material downslope. When the slope angle increases, more of the gravitational force is split into the parallel force and less into the perpendicular force, so steeper slopes increase the likelihood of mass movement. Figure 10.1 Effect of gravity and slope angle on slope stability. The slope material s strength, cohesion, and the internal friction between fragments together determine the slope s shear strength, which resists slope failure. Solid rock is very strong and cohesive so it can stand in vertical cliffs, but loose sand is not cohesive and moves downslope easily. Angular fragments can lock together producing a large amount of internal friction, increasing the shear strength, while rounded fragments can roll over each other and have less internal friction, decreasing the shear strength. Clay minerals are platy and can slide over each other very easily, particularly when wet, decreasing the shear strength. A measure of the shear strength of loose material, such as sand or gravel, is its angle of repose, which is the angle from horizontal that the material holds when poured onto a surface to form a cone. When a slope equals this angle, its shear strength exactly matches the force pointed downslope. The angle of repose determines the maximum slope a loose material can hold before it fails by mass movement. The angle of repose varies, depending on the material properties. Material Properties and Angle of Repose Sorting... Poor sorting increases the angle, since grains can pack together. Grain Shape... Greater angularity increases the angle; angular grains lock together. Grain Roughness.. Rougher surfaces increase the angle by increasing friction. Grain Size... Larger grains increase angle slightly, though less than other effects. Grain Density... Denser materials have lower angles (weak effect). Cohesion... Some minerals have attractive forces that hold grains together, thus increasing the angle. Water... Water has strong effects on both increasing and decreasing the angle, depending on saturation and material type. Table 10.1 Mass Wasting Page 3 of 23

4 A small amount of water increases cohesion. This is demonstrated in building sand castles by using damp sand to make walls and dribbling wet sand to form tall spires. The steep slopes are held together by surface tension at water-air interfaces between grains. When sand is fully saturated with water, there are no water-air interfaces to cause surface tension. The water then acts as a lubricant between the grains and provides a buoyant force on the grains, which greatly decreases the shear strength. Even without waves lapping at the walls, sand castles collapse to a low angle if gently saturated with water. The angle of repose of loose, well sorted, well-rounded quartz sand is about 33, but only when water saturated. Consequently, eolian cross bedding is much steeper than cross bedding in fluvial deposits. Water is important also because it greatly decreases the strength of materials with high clay content. Clay minerals hold water molecules on their surface, which causes them to lose cohesion when wet. Internal friction is also reduced because the platy particles can slide easily over each other. Some clay minerals can absorb water into their crystalline structure and swell, further decreasing the strength of the clay mineral. Therefore, clay and shale beds are often the slippery layer along which overlying strata slide downslope. Water also increases the weight of soil or debris, increasing the load on a slope and increasing the likelihood of mass movement. Climate is an important factor because it controls weathering as well as the supply of water and the type of vegetation. As weathering decomposes and breaks down rock, the shear strength of rock is reduced and its susceptibility to mass wasting begins to increase. The deeper the weathering zone extends, the higher the chances of mass movement and the larger the potential size of landslides. In areas where temperatures and rainfall are high, such as the tropics, significant weathering may extend tens of meters below the surface. In arid or semiarid areas, the weathering zone is much shallower, but loose sand and gravel and broken rock accumulate at the surface, making it susceptible to mass wasting. Vegetation usually helps prevent slopes from moving. Root systems reduce the amount of water in the soil through transpiration, bind the soil together, and hold soil to the bedrock. These factors all increase the shear strength of the slope material, which can help stabilize a slope. Many mass movements take place after vegetation has been removed by brush or forest fires or human activity. Plants also increase the rate of infiltration of rainfall into the soil, decreasing the chance of rapid runoff that can trigger flows of loose surface debris. However, if the roots do not penetrate deeply, they may not effectively increase the shear strength. The weight of vegetation and infiltrated water in such cases can actually increase the likelihood of slope failure. The bedrock geology of an area affects slope stability as well. If the bedding or foliation of underlying rocks dips in the same direction as the slope, it provides potential failure surfaces, increasing the risk of mass wasting. It also allows water to more easily percolate along the bedding planes and decrease friction and cohesion between adjacent rock layers. The presence of clay layers can make this problem even more severe. Bedding or foliation dipping opposite the slope is much more stable. Mass Wasting Page 4 of 23

5 Bedrock geology is also important in determining the steepness of slopes that form in an area. The type of bedrock is an important control on the type of soil and the depth of weathering in an area. For example, volcanic rocks weather rapidly and deeply, producing thick soils rich in swelling clays. Shales weather and erode rapidly while carbonates are susceptible to chemical weathering. Fracturing breaks bedrock into fragments that can move downslope, provides potential failure surfaces, allows infiltration of water, and makes the rock more susceptible to weathering. TRIGGERS OF MASS MOVEMENTS Natural events as well as human actions can trigger mass wasting. Earthquakes, heavy rainfall, snowmelts, erosion, wildfires, volcanic eruptions, rock weathering, and road construction are just a few potential triggers of mass wasting. Some conditions in nature change over time and can trigger mass movement where a slope had previously been stable. Shaking during earthquakes often triggers mass movements by decreasing forces that hold material together and by providing forces (both upward and laterally) that trigger landslides. Earthquakes often liquefy water-saturated mud and sand. Since they shake large areas, earthquakes can trigger many mass movements simultaneously, causing great damage and loss of life. Changes in water saturation and vegetation can trigger slope instability. Many regions experience more frequent mass movements during rainy seasons or during longer term wetter cycles, such as El Nino events in western North America. The shear strength of most materials decreases slowly over time due to chemical weathering. Slopes can also be destabilized by physical weathering, such as freeze/thaw cycles that trigger rockfalls or roots that pry fractures open. Erosion at the bottom of a slope along stream banks, lake sides, and sea cliffs, can increase slope. Solid cliffs of rock can also be undercut, promoting collapse. In regions of active plate tectonics, such as coastal California, slopes can increase as anticlines grow or thrust faults advance. In volcanic regions, slopes can increase as the volcano grows or as magma chambers below are inflated. Mass Wasting Page 5 of 23

6 Figure Slump in clay-rich soil in Fargo, North Dakota. Snow cleared from the roadways was dumped on top of the slope, which overloaded the slope and saturated the soil when it melted, triggering the slump. Photo by D.P. Schwert, North Dakota State University. Humans create artificially steep slopes that materials cannot support by making roadcuts, adding materials to the top of slopes, and removing materials from the bottom of slopes. Irrigation or removing or changing vegetation are two other common human activities that can destabilize slopes. Humans also change drainage conditions by building retaining walls that trap groundwater. The increasing water saturation behind the wall can make the slope unstable and push the wall over. Under natural conditions, a material s load is carried by grain-to-grain contacts with friction between the grains maintaining the slope. Addition of rock or buildings or other material to a slope by humans adds weight that increases the load. The added weight also increases the water pressure within the slope material, decreasing its shear strength. This reduces slope stability and can ultimately lead to slope failure, which is often instantaneous and almost always unexpected. TYPES OF MASS MOVEMENTS During mass wasting events, material can move downslope in many ways. Although the process can change along the length of a mass movement or over time, mass wasting events are generally categorized by the dominant type of movement as: Falls mostly vertical, falling movement Slides sliding movement of coherent blocks Flows churning, flowing movement In addition, mass movements are classified by the material involved: Rock relatively unweathered, solid rocks Debris coarse, loose materials (mostly sand, gravel, and boulders) Soil fine, loose materials (clay, silt, and sand) - called earth as in earthflow Different forms of mass wasting are also categorized by the rate of movement, which is extremely important in risk assessments and the amount of water in the landslide material. These are not easy to determine for ancient landslides, however. Falls Falls are extremely rapid, almost vertical movements of broken material. They mostly involve rock and such an event is called a rockfall. These events commonly occur along steep canyons, cliffs, quarry faces, and road cuts and generally result from failure along joints or bedding planes in the bedrock. Rocks can fall vertically or they can rotate away Mass Wasting Page 6 of 23

7 from a cliff face and thus gain considerable horizontal momentum. These are sometimes classified as topples. Earthquakes or blasting, frost wedging, chemical weathering, and undercutting are common triggers. Rockfalls range in size from a few small rocks to massive collapses involving millions of tons of debris that block highways and destroy buildings. They are a common hazard in mountainous regions where the steepest slopes occur. Loose, angular rocks broken by rockfalls and deposited at the bottom of cliffs and rugged slopes are called talus or scree and form talus slopes. Where rockfalls are funneled down a ravine or crack, the talus spreads out at the base to form talus cones. These slopes exhibit the angle of repose of the talus, which can be steeper than 35 because the poorly-sorted, angular fragments can lock together. Slides Figure 10.3 Rockfall with talus accumulation at base. A slide involves movement of coherent blocks of material along one or more surfaces of failure. The material, which can be soil, debris, or rock, may move as a single unit or break apart into multiple blocks as it travels. Two main varieties of slides are recognized: slumps and rock slides. A slump is a sliding movement along a concave (spoon-shaped) failure surface. At the top or head of the slump, the block moves mostly downward, forming one or more scarps. Because the material follows an upward curved surface, the block is rotated backward toward the hillside as it moves downhill. In the middle of a slump the material moves mostly parallel to the slope. At the bottom or foot, a slump is shoved out from the slope, protruding upward. The overall movement of slumps therefore includes rotation of the mass around a horizontal axis, so these are also called rotational slides. The top part of a slump mass is under extension, forming cracks and separating into blocks, while the lower part is under compression, as shown by transverse folded ridges (which trend across the landslide) and thrusts and longitudinal cracks (which trend parallel to the landslide). The toe of a slump is thrust upward onto the former ground surface and then slides downward along this surface. Many slumps break into multiple pieces, creating stair-stepped slices with each slice sloping back toward the hillside. These backward sloping steps covered by backward tilted trees are sure indicators of a slump. Slumps often occur on slopes of unconsolidated or weakly consolidated materials and involve just earth or debris. They range in size from tens of square feet, with a single scarp and a simple rotated block, to many acres with many scarps and individually moving blocks. Mass Wasting Page 7 of 23

8 Slumps involving rocks are common around mesas capped by resistant rocks, such as sandstone or lava, on top of steep slopes of weak shale or altered volcanics. The heavy cap rock is broken into slices and tilted back toward the mesa as it moves downslope along a concave failure surface in the weak strata below. These types of slumps are also called toreva blocks for a village on the Second Mesa of the Hopi Indian Reservation of Arizona. The rate of slump movement ranges from extremely slow, moving over the course of weeks to years, to moderate rates which can be detected over several hours. Slumps are often triggered by the removal of material near the bottom of a slope, which steepens the slope and decreases support for the upslope material. This local steepening can occur naturally, as in sea cliffs and river banks, or by human activity, such as in roadcuts or housing developments. Removal of the toe of a slump that moved onto roadways or lawns often triggers further movements. An increase in water saturation of soil or debris often causes slump events. Many of the slope failures along the southern California coast are slumps triggered by seasonal rainfall. A slide, in the restricted definition, occurs when coherent blocks of material move downslope along a planar surface. Because the surface is planar, there is little rotation, and these movements are sometimes called translational slides. Rock slides, the most common variety, often occur where the local slope and underlying bedrock dip in the same direction. Clay or shale layers acting as lubricated slide surfaces are frequently involved. Slides also occur where fractures are parallel to the slope. As with slumps, rock slides are often triggered by the removal of material from the base of a slope, causing loss of support. Rock slides are rapid to very rapid events and can involve very large volumes of rock. Slides can also happen to earth (soil) when coherent blocks of turf slide down hillsides, leaving large planar scars behind. This happens when roots do not penetrate deep enough to anchor the soil or when the soil is developed on smooth bedrock. Flows A flow is a mass movement in which the material moves like a fluid. These events range from extremely slow, where the material moves by slow plastic deformation, to fast, where the material moves smoothly in a laminar fashion or by chaotic, turbulent motion. Flows are classified by the type of material and the rate of movement with the amount of water also being an important consideration. Creep is the extremely slow downhill movement of shallow materials. The greatest movement occurs at the surface, causing utility poles, fenceposts, and gravestones to tilt downslope and retaining walls to tilt and topple over. Since the process is slow, it may take years for any movement to be detected. However, creep is more widespread, moves more material, and causes more monetary damage than any other mass wasting type. Mass Wasting Page 8 of 23

9 Trees grow as they are tilted by creep and develop a sharp upward curve at their base to keep growing vertically. Since the movement is greatest at the surface, creep is often restricted to soil and debris, but even thin vertical strata and foliated rocks can become tilted in a downslope direction. Figure 10.4 Creep tilts fence posts and makes trees and shrubs bend upward to grow vertically. Photo by D.P. Schwert, North Dakota State University Creep can occur anywhere and in any climate, including polar areas where it is aided by freezing and thawing. Alleviating the problem is difficult and generally requires secure attachment of materials to the underlying bedrock or completely avoiding the affected area. Mudflows move rapidly (up to 80 km per hour) and contain at least 50% silt and clay size particles combined with up to 30% water. Mudflows are common in arid and semiarid regions where they are triggered by heavy rainstorms that rapidly erode bare dirt surfaces. They are also created by the erosion of loose volcanic ash. Mudflows can form from other mass movements, such as slumps that can dewater and squeeze out mudflows from their toes. Debris flows are composed of larger size particles than mudflows and contain less water. These flows move slower than mudflows, but because of high sediment content, they are dense and viscous and therefore capable of transporting large objects such as large boulders, houses, and bridges. Debris flows of volcanic materials are termed lahars and those that are actually generated by eruptions may be warm to hot. Figure 10.5 Deposit of mudflow in California on March 3, Note that the deposit is flat and fine-grained, with only sticks on the upper surface. The mudflow did not move the vehicles or carry much coarse debris. Figure 10.6 Although often called mudflows, the flows that were triggered by the 1980 eruption of Mt. St. Helens are more properly termed debris flows or lahars. The twisted girders of a highway bridge and the deposit littered with large rocks show these flows carried huge objects because they had great viscous strength and density. Mass Wasting Page 9 of 23

10 Earthflows contain less water than either a mudflow or debris flow and consequently move much slower. They flow like a thick, viscous mass. Clay, fine sand and silt, and finegrained, pyroclastic material are all susceptible to earthflows. In many cases, a mass movement that begins as a slump or rock slide may change into an earthflow further downslope. Figure 10.7 Earthflow on Mission Pass in the California coastal ranges. The lateral lines on the hillside show creep. Photo Credit: B. Bradley, University of Colorado Debris avalanches are extremely rapid flows of coarse to huge rocks and debris. Needing high relief to form, they usually occur in mountainous regions and often incorporate snow and chunks of ice from glaciers, although water is not essential for their ability to flow. Debris avalanches may begin as rock slides or as large slumps, but gain speed and turn completely fluid. They often have enough momentum to climb hills and valley walls and can even be launched into the air where they flow over such barriers. The horrific mass wasting catastrophes at Nevados de Huascarán, Peru, in 1962 and 1970, were of this type. RECOGNIZING MASS MOVEMENTS Most mass movements are too small to be seen clearly on detailed topographic maps (1:24,000 scale), but large events can form distinctive landforms that are discernible. Much of the evidence used to determine types of mass wasting events requires aerial photos and field inspection since it cannot be seen on regular topographic maps. Large Mass Wasting Events on Topographic Maps Rockfalls... Generally leave areas of fresh rock on the side of a mountain or cliff. Debris accumulates below the rockfall in a talus cone that has a steep angle of repose (20 degrees or more). Earth and Debris Slides... Have internal fault scarps and thrust faults, showing they moved as coherent blocks with shear zones between them. Often leave the failure surface exposed as a long planar, sloping scar. Rock Slides... Break up into large angular jumbled blocks. Often leave the failure surface exposed as a long planar, sloping scar. Flows... Can be distinguished by the appearance of folds and they spread out greatly at the bottom. Extremely fluid mud and debris flows have flat, smooth tops marked by flow lines. Mass Wasting Page 10 of 23

11 Figure 10.8 Large rockslides leave unvegetated scars on mountainsides. Madison Slide, Madison Canyon, MT ACME Mapper Figure 10.9 Slumps leave curved scarps. Grand Mesa, CO ACME Mapper Figure Large slumps produce descending steps with back-tilted knobs or ridges and most deposits are hummocky. Chuska Mountains, NM ACME Mapper Figure Depressions in hummocky terrain are often filled with small lakes. Bonneville Slide, Columbia River Gorge, WA ACME Mapper Mass Wasting Page 11 of 23

12 Figure Landslides tend to smooth over scarps and steep canyons in mountainous areas. Lost River Range, ID ACME Mapper Figure Landslide deposits are usually rougher than surrounding alluvial fans Blackhawk Slide, Lucerne Valley, CA ACME Mapper CALCULATING GRADIENT AND SLOPE ANGLE Gradient Gradient is the change of elevation over a unit of distance. It usually has units of feet per mile or meters per kilometer, depending on the most convenient unit system for the area. A gradient of 10 feet/mile means the elevation changes 10 feet over a distance of a mile, although the gradient may only apply to a much shorter distance. The gradient is calculated as the ratio of rise (the change of elevation) over the run (the horizontal distance). Using a contour map, the gradient between two points is determined by the following method: 1. Use contour lines to determine the elevation at each point. 2. Use the horizontal scale to measure the distance between the same two points. 3. Divide the elevation change by the horizontal distance. Example: Calculate Gradient The upper elevation is 5,200 feet and the lower elevation is 5,000 feet, with a distance of 0.5 mile between them. Gradient = (Upper Elevation Lower Elevation) / Horizontal Distance Gradient = (5,200 feet 5,000 feet) / 0.5 mile = 200 feet / 0.5 mile = 400 feet / mile Mass Wasting Page 12 of 23

13 A gradient representing the slope you would have to climb or descend to go from one point to another on some arbitrary path can be calculated between any two points on a topographic map. When dealing with slope stability and mass wasting, however, it is important to select points that are directly down or up slope from each other, such as the top and bottom of a hill. If using intermediate points on a mountainside, the horizontal distance between the two points should be roughly at right angles to the contour lines. Slope Angle The angle of the slope is related to the elevation change and horizontal distance by this equation from trigonometry: Tangent of angle θ = Opposite side of triangle / Adjacent side of triangle which is the same as Tangent of angle θ = Rise / Run which is the same as Tangent of angle θ = Elevation Change / Horizontal Distance so θ = Arctangent (Elevation Change / Horizontal Distance) Unlike gradient, the elevation change and the horizontal distance must be in the same unit of measure. They can both be in feet, meters, miles, or kilometers. The method to determine the slope angle from a topographic map is: 1. Use contour lines to determine the elevation at each point. 2. Use the horizontal scale to measure the distance between the same two points. 3. Change the horizontal distance to the same unit as the elevation, if necessary. 4. Divide the elevation change by the horizontal distance. 5. Calculate the arctangent (inverse tangent) of this number using a calculator. Example: Calculate Slope Angle The upper elevation is 5,200 feet and the lower elevation is 5,000 feet, with a distance of 0.5 mile between them. Horizontal Distance = 0.5 mile = 0.5 mile * 5280 feet / mile = 2640 feet θ = Arctangent (Elevation Change / Horizontal Distance) θ = Arctangent (200 feet / 2640 feet) = Arctangent (0.076) θ = 4.3 degrees Mass Wasting Page 13 of 23

14 GROS VENTRE LANDSLIDE One of the world's largest mass wasting events in recent times was the Gros Ventre landslide near Jackson Hole, Wyoming, that occurred on June 23, After several weeks of heavy rain, approximately 50 million cubic yards of sedimentary rock slid down the north face of Sheep Mountain, crossed over the Gros Ventre River, and raced up the opposing mountainside a distance of 300 feet. The landslide created a huge dam over 200 feet high and 400 yards wide across the Gros Ventre River, backing up the water to form Lower Slide Lake. On May 18, 1927, a portion of the landslide dam failed, resulting in a flood that was 6 feet deep for at least 25 miles downstream. The small town of Kelly, six miles downstream, was wiped out and six people were killed. Figure Topographic map of the Gros Ventre, Wyoming landslide, to scale. N part from USGS Topographic Quadrangle: Shadow Mountain, WYO, 1968, S part from USGS Topographic Quadrangle: Blue Miner Lake, WYO, 1968 Mass Wasting Page 14 of 23

15 The slide occurred where strata of late Paleozoic age dip steeply to the north toward the Gros Ventre River. The river flows parallel to the strike of these beds and is eroding down and removing support from the strata dipping toward the river. The strata are the permeable Tensleep Sandstone, underlain by the Amsden Shale, which rests on the resistant Madison Dolomite. The Amsden Shale provided a failure surface, probably when the clay became unstable and slick as it mixed with water from heavy rains and spring snow melt. Movement of the subsequent rock slide was very rapid, as witnessed by a few cowboys on the opposite side of the valley who said they were lucky to escape on horseback. South North Figure Cross section from south to north of the Gros Ventre landslide. SLUMGULLION LANDSLIDE The Slumgullion landslide is located in the San Juan Mountains of southwestern Colorado. The landslide formed approximately 700 years ago on the south flank of Mesa Seco, which is composed of Tertiary andesitic breccia and tuff, hydrothermally altered to clay minerals, and capped by resistant layers of welded ash-flow tuff with basalt lava flows on top. The mesa collapsed south, leaving a large scarp. Then the landslide turned west and moved several miles down the steep mountainside to the Lake Fork of the Gunnison River. Here it spread out upstream and downstream, dammed the river, and created Lake San Cristobal, the second largest natural lake in Colorado. Upslope, the landslide altered the drainage pattern, diverting unnamed creeks on the north side and Slumgullion Creek on the south side to flow westward along the landslide margins. Rock fragments of various sizes mixed with clay and sparsely covered with tilted trees gave the mountainside a jumbled, hummocky appearance that led prospectors to name the feature for slumgullion stew, a common miner s meal. Mass Wasting Page 15 of 23

16 Figure Slumgullion landslide location. Figure The active nature of the Slumgullion landslide is shown by disturbed soil, hummocky topography, freshly exposed rock and soil, and the jumbled orientation of trees on the landslide surface. A second event, which began about 300 years ago, started near the top of the mountain and is moving on and over the older, larger, inactive flow. This second flow continues to move as much as 20 feet (6 m) per year. Measurements indicate the velocity of movement varies seasonally. Movement increases during daily cycles of low atmospheric pressure, which act to pull water upward in the slide material. Figure Detailed map of the active portion of the Slumgullion landslide. Contours of the rate of movement are in black. Mass Wasting Page 16 of 23

17 The entire landslide area is 6.8 km long, averages about 400 m wide and has an estimated average depth of 60 m. The active portion is about 3.9 km long, averages about 300 m wide, and has an estimated average depth of about 14 m. Colorado highway 149 was wisely located to descend the mountain to the south of the slide instead of on the slide itself. The road had to cross the landslide, however, so a relatively short section is located below the active, upper portion of the landslide. So far the highway has not been badly disrupted. The portion from the lake uphill past the highway that diagonally crosses the landslide is inactive. Above the highway is a prominent light-colored bulge that marks the toe of the active part of the landslide, which extends from there up to the prominent scarp at the top of photograph. Figure Lake San Cristobal is at the lower right. The top of the active portion of the Slumgullion landslide is broken by tensional features, such as normal faults and tension cracks, while the toe has compressional thrust faults and features indicating lateral spreading. (Trees have actually been split apart at the roots.) The middle part of the flow, where it is narrowest, has the greatest velocity and is flanked by ridges showing horizontal shear. Above the narrow part are small thrust faults that form as the landslide mass squeezes into the constriction. Below the narrow part, the landslide is extended, forming pull-apart basins. The toe of the active portion of the landslide is steep where it has thrusted up and onto the lower, older landslide surface. Mass Wasting Page 17 of 23

18 Gradient and Slope for Marysvale, UT Mass Wasting Lab Student Name Section EXERCISES Assignment: Part 1: Use the online activity Calculate Slope and Gradient to complete the table for Marysvale, Utah. Part 2: Answer questions on Gros Ventre landslide. Part 3: Draw a topographic profile of the Slumgullion landslide. Part 4: Answer questions on Slumgullion landslide. READ THIS BEFORE YOU BEGIN 1. View the lab videos for: Mass Wasting: Lab Intro Mass Wasting: Gros Ventre Rock Slide 2. Review the material in the lab manual chapter for mass wasting. Part 1 - Gradient and Slope Measurements Table for Marysvale, Utah Record your answers from the online activity, Calculate Slope and Gradient. Location 1 Center of Section 21, in the SE township Gradient (ft/mile) Slope Angle 2 W half of Section 15, in the SE township 3 Center of Section 28, in the NE township 4 E half of Section 21, in the NE township 5 6 From the elevation 6209, along the unimproved road at the S edge of Section 12, SW township, to center of Section 8, under the number 8, SE township From the hill marked 6868, along the W edge of Section 5, to the W shore of Middle Spring Lake, Section 2, both in the SE township Mass Wasting Page 18 of 23

19 Gros Ventre Landslide Mass Wasting Lab Student Name Section Part 2 - Questions on Gros Ventre Landslide 1. The gradient for the pre-slide surface was calculated as 1282 ft/mile in the video presentation. What is the pre-slide slope angle, to the nearest tenth degree? 2. The post slide angle of repose was calculated as 657 ft/mile. What is the post-slide slope angle, to the nearest tenth degree? 3. What role did the Amsden Shale play in the Gros Ventre landslide? 4. How would you classify the Gros Ventre landslide? Explain why. 5. Now that the Gros Ventre landslide has occurred, is the south side of the Gros Ventre valley immune from further landsliding? Explain why. Mass Wasting Page 19 of 23

20 Slumgullion Landslide Topographic Profile Mass Wasting Lab Student Name Section Part 3 - Slumgullion Landslide Topographic Profile Using the Slumgullion Contour Map, construct a topographic cross-section down the length of the landslide. 1. Construct the topographic profile from points A to B to C. The profile is not straight, but is bent at B, in order to follow the center of the landslide. 2. Record the horizontal scale, the vertical scale, and the contour interval on your profile. 3. Calculate the vertical exaggeration and record it on your profile. 4. Mark significant features the river, road, and the positions of A, B, and C. 5. Interpret the location of the scarp at the head of the slide and label it on your profile. 6. Interpret the location of the toe of the active portion of the landslide and label it on your profile. Remember where it is located relative to the road and look for a steeper slope. 7. On your profile draw a line representing the bottom of the upper, active portion of the flow, using the thickness given in the lab chapter. 8. On your profile draw a line representing the bottom of the lower, inactive portion of the flow, using the thickness given in the lab chapter. Mass Wasting Page 20 of 23

21 Slumgullion Contour Map Mass Wasting Lab Student Name Section Mass Wasting Page 21 of 23

22 Mass Wasting Page 22 of 23

23 Slumgullion Landslide Questions Mass Wasting Lab Student Name Section Part 4 - Questions on Slumgullion Landslide 1. What is the estimated volume for the entire landslide area, in cubic meters (m 3 )? 2. What is the estimated volume of the second flow, in cubic meters (m 3 )? 3. What causes the velocity of movement in the second flow to vary seasonally? 4. What is the post-slide angle of the entire landslide? 5. How would you classify the active portion of the Slumgullion landslide? 6. There are currently lakeside home sites available near the foot of the Slumgullion landslide. Developers have installed retaining walls to provide additional support for the base of the slope, anchored these to bedrock, and backfilled with crushed rock to provide for water collection and drainage. What recommendation would you make to any potential buyers? Mass Wasting Page 23 of 23