GEOL 108Lg 1st Edition Lecture 11Outline of Last Lecture- Metamorphic rocks- Steno's rules- Mineral deposits- Carbon cycle and oil - Snowball Earth IntroOutline of Current Lecture- What is mass wasting?a. Mass wasting phenomena, causes-Weathering- Efect of Water- Controls on Mass WastingCurrent LectureMass Wastng LectureMass wastng: Mass wasting, also known as slope movement or mass movement, is thegeomorphic process by which soil, sand, regolith, and rock move downslope under the forceof gravity (e.g. rock slides, landslides).Mass wastng is also referred to by the non-technical term "landslide". Mass wastng is the down- slope movement of a mass of sediment and/or rock due mainly to the force of gravity. The "mass" part of the name implies that some sort of coherent grouping of sediment/rock begins moving downward due to the force of gravity (usually in combinaton with some triggering mechanism such as an earthquake or rapid erosion of the base of a slope). The main cause of mass wastng is gravity.The "wastng" part of mass wastng means that the mountain slope is getting smaller, wastng away. This can occur both rapidly with destructve force and slowly with only a gradual change to the earth’s surface over many years.The force of gravity is downward. As gravity pulls downward on material that’s on a sloping portonof Earth’s surface, a translatonal force is formed within the slope sediment/rock. This force creates shear stress within the slope's material, reducing the slope's strength and making it more prone to mass wastng.Gravity is always in effect, which means that there is always the possibility of mass wastng of a sloped surface. Note that the steeper the slope, the more in line its materialcomponents (sediment and/or rock) are with gravity, so the more likely is mass wastng of that slope. See the diagrams below to better visualize the effects of gravity on slope material.Where do we worry about mass wastng phenomena?-Rockslides, landslides caused by poor road constructon, riverbank collapse, submarine landslides(sitting on the coast), eroding cliff (landslide), debris flow coming down stream channel, rockfall.WeatheringChemical disintegraton of rock by the forces of climate. Part of the formation of sedimentary rocks-Gravestones made from rock that is resistance, one that is not (sedimentary rock) - didn’twithstand weathering.Ex. Granite Image: rocks broken apart by formaton of cracks and pores. Weathering depends on a number of physical and chemical propertesTime: The longer you wait, the more weathering you haveMineral compositoTemperature: Low Temp, less chem. weathering, more chem. weathering at high temperature but less physical weathering.Rainfall, Rainfall Acidity:Topography: the slope of the hillsideThe stuf that comes down the hill is called Talus, debris that forms due to rock faceerosion. Trees can adjust to hillside creep!Rockfalll: Within seconds to minutes, tons of material is transported down the hillside. Landslide San Bernardino Mountains:A landslide that occurred in the San Bernardino mountains was initated as a rock fall from thenorth side of the mountain with a vertcal drop of 4,000 feel. Runout was about 10 km and at a very high velocity.Systematcs:The type of material being transported, water content within the material (saturated with water or just a little water can make a big difference in the material – see lab from this week.Rock SlideBedding planes; intact material comes down the hill, almost as a unityCritical angle at which you can sustain a slope before it begins to slide down - fine sand (35 degrees), coarse sand (40). The rougher stuff, you can pile up steeper.THETA=Atan (friction coefficient) If you are pulling a little sliderfric. coefficient x the normal stress shear stress=fric co. x material stressDifferent kinds of stressesStress= force per areaNow, the role of water makes these things a little more complicated, as mentoned before.When you’re inside a fluid, all the diff. atoms are attracted to each other in all directons. Surface tension is what changes the behavior of sand, for example.Damp sand, which has a little water is MORE cohesive. Dampness binds the sand partcles so that they resist movement.Fricton law depends on what kind of material you have - natural zones of weaknessThe role of water and the role of existng geographical conditons - can predict when landslides will happenRates of downslope movement range from creep to catastrophic.Controls on mass wasting includeangle of slope type of earth material (determines fricton angle - sand vs. coarse pebbles, for example) anisotrophies of earth materials (eg bedding, foliation, jointng) Involvement of waterThe Anisotropy of strength: directon dependency of strength - weak layers and strong layers being pancacked