Gravity-driven Transport - Mass Wasting:
By comparison, gravity driven mass wasting is simple:
- mineral cementation
- binding by surface tension
- binding by roots
- Irregularities of rock: bedding, foliation, zones of weathering
- Slides, in which unconsolidated material slides downslope (above) and
- Falls, in which moving material, at some point, is suspended in midair.
- Grain flows
- Fluidized sediment flows
- Debris flows
- Turbidity currents
- Grain flows: Cohesionless grains flow downhill under the force of gravity. Fluids (air or water) trapped between grains acts as a lubricant, but unlike in stream flow, doesn't propel the grains. Example: The flow of sand down the slipface of a dune.
Sand volcano from Kuriositas
Tibetan debris flow deposit from Wikipedia.
Turbidity flow from Universidad de Puerto Rico Geological Oceanography
- Earthquakes trigger slumps or other mass wasting events. These kick sediments up into the water.
- The resulting mixture of water and sediment is much denser than plain water, and goes roaring off downslope. This is a turbidity current. It has a life of its own, separate from the landslide that started it. Turbidity currents appear to be capable of carving canyons in deposits of unconsolidated sediments.
- After flowing downhill onto the continental rise - perhaps 100s of km, the current slows and heavier clasts start to fall out, followed by lighter ones and finally including clay particles.
- The result is a sediment layer called a turbidite. These show a fining upward sequence that grades from conglomerate to mud. (Compare with point bar deposit, in which the finest sediment is sand.) Turbidites can be up to 10 m thick, however most are much smaller. They often show sedimentary structures indicating current direction, such as sole marks and flute casts.
Dry Mass Wasting:
The force of gravity acts to move regolith down slope. In consolidated materials, Cohesion - the general tendency of materials to hold together - is the major issue. It's a function of
Wet gravity-driven mass wasting:
Flows - gravity driven movements containing some fluid, can have the characteristics of both dry mass wasting and proper fluid transport. Mass movements in which material flows fluidly, i.e. it loses all traces of its original structure. We recognize four major examples:
Cohesionless grain flow at False Cape State Park, VA