2.4: Gravity Mass Movements
It does not always take moving water to get sediment transport to happen. Sometimes materials move under the direct influence of gravity … this is the process of gravity mass movement . Fluid may play a role by reducing internal friction and supporting grains, but its not what’s driving movement. We will consider the three main types of gravity mass movements described below.
Rockfall
Rockfalls happen when gravity causes rocks to free fall or roll down steep slopes. Fluid is not needed for this motion to occur, but it's presence can lubricate clasts and thus encourage movement.
Video \(\PageIndex{1}\): Rockfall in the Swiss Alps
Slides, Glides, and Slumps
Although it might be important to tease them apart if you were doing surficial mapping, we will consider slides, glides, and slumps together. All of them represent movement of material that happens when there is shear deformation along discrete planes with little internal deformation.
Sediment Gravity Flows
Sediment gravity flows are the final category of gravity mass movements and because they are often preserved in the sedimentary record we will explore them in more detail and talk about the different types. In all cases, they requires massive internal deformation and separation of the parent mass into discrete particles to form a mixture of particles and the fluid. Generally speaking, this is a very fluid type of motion where grains supported by collision with each other and by the fluid. The main distinguishing factor between the different types is the mechanism by which the particles are supported.
Mud flows and debris flows
Mudflows and debris flows (we will treat them as synonymous) happen when a dense muddy matrix (a slurry of sediment and water) is capable of moving and supporting large clasts. They can be subaerial or subaqueous. Although we cannot embed it in this page, there is a great video of an active debris flow available on Youtube via Storyful at https://youtu.be/Fsh5E9m3PrM?si=DSFONNDTEzY305d7 .
Debris flow deposits are typically poorly sorted, have a sandy/muddy matrix, show little internal organization, and often have large “floating” clasts.
Grain flows
Grain flows happen when cohesionless (dry) sediment is supported in the air by grain-to-grain collisions. Grain flow deposits are generally thin (a few cm thick), lobe-shaped in 3D, and are typically sandy with massive or inverse grading.
Video \(\PageIndex{3}\): Grainflow in dry desert sand.
Liquefied Flows
Disturbances like earthquakes or crashing storm waves can cause saturated sediments to compact in the subsurface. When this happens, the upward movement of intergranular water can temporarily supports sediment grains to create liquefied flows . The resulting deposits typically have massive soft-sediment deformation (flame structures, load structures, convolute laminae) and possibly sand volcanoes at the surface.
Video \(\PageIndex{4}\): Liquidized/fluidized flows interacting with large particles and layered sediment.
Video \(\PageIndex{5}\): Eruption of sand volcanoes after an earthquake in New Zealand.
Turbidity currents
Turbidity currents happen where sediment is held aloft by turbidity within the fluid. The result is that you have a relatively dense fluid with sediment and water that flows in response to gravity, typically downslope through less dense clean water. The resulting deposits show normal grading (coarsest grains at the bottom) and an overall fining upwards trend. These coarse-fine successions are commonly referred to as Bouma sequences . Remember from earlier in this chapter that Stoke's Law tells us that the biggest particles settle out first.
Video \(\PageIndex{6}\): Turbidity current in a flume.
Readings and Resources
- Dasgupta, P. and Manna, P., 2011, Geometrical mechanism of inverse grading in grain-flow deposits: An experimental revelation, Earth-Science Reviews, v. 104, no. 1-3, p. 186-198. https://www.sciencedirect.com/science/article/pii/S0012825210001376#f0025
- Middleton, G.V. and Hampton, M.A., 1976, Subaqueous sediment transport and deposition by sediment gravity flows in Stanley, D.H. and Swift, D.J.P. (eds.), Marine sediment transport and environmental management, Wiley, New York, p. 197-218.
- Geological Digressions, Sedimentary structures: Turbidites - https://www.geological-digressions.com/2020/04/09/sedimentary-structures-turbidites/
- USGS, Landslide Types and Processes - https://pubs.usgs.gov/fs/2004/3072/fs-2004-3072.html