Gravity processing Flashcards
Define gravity processes
The process by which regolith, sediment, and rock moves downslope under the force of gravity
How do gravity processes occur?
When gravity’s effect overcomes a slopes resistance
What dictates slope resistance?
Slope gradient and internal friction
Define angle of repose
The steepness threshold under which the material is stable and over which the material will flow under its own gravity
Why are landscapes more vulnerable to landslides after heavy rain?
Because water provides lubrication between the grains, making them weaker
What happens as a material’s internal strength increases?
Its angle of repose increases
Where do gravity processes dominate?
Where there is a gradient
Give the name of gradients found on land and in the oceans
Topographic relief and bathymetric relief
Where is topographic relief most abundant?
Uplifting orogens
Where is bathymetric relief most abundant?
Continental margins
What two mechanisms can trigger gravity processes?
Steepening the gradient over the angle of repose and reducing the natural angle of repose by decreasing the material strength
Give two examples of how a gradient can be steepened over the angle of repose
Undercutting (cliffs, valley walls, river banks) and depositional steepening (marine settings)
Give two examples of how the natural angle of repose can be reduced by decreasing the material strength
Adding lubricating water to air-filled pore spaces and earthquake induced liquifaction
Describe the process of earthquake induced liquifaction
Pore space is reduced by the sieving of water saturated sediment. Water escapes upwards due to high pore pressure. Repeated sieving reduced pore pressure faster than it can be lowered. The high pressure holds the grains apart, reducing internal friction.
What dictates the style of a mass wasting event?
Material strength
Debris flows and turbidity currents are characteristic of what strengths of material?
Fluid (the weakest)
Describe fluid materials
Usually unconsolidated, recently deposited sediment
Describe debris flows and turbidity currents
A mixture of sediment and fluid. Can move downslope under their own gravitational potential because they are denser than the surrounding medium
What are two differences between between debris and turbidity flows?
Debris flows are laminar and turbidity flows are turbulent
What determines whether a gravity flow will be a debris or turbidity flow?
Reynold’s number (Re)
What type of flow is the result of a high Re number and why?
Turbulent because materials with a lower viscosity are more likely to become turbulent
What controls a material’s viscosity?
The number of grain collisions
Describe a mixture with higher viscosity
They are concentrated in grain with less pore fluid, therefore have more grain collisions. They have a higher viscosity and a Re below 1. They produce laminar (debris) flows.
When do debris flows occur?
When the volume and mass of sediment is greater than that of pore fluid (which is always water)
How are the grains in a debris flow kept apart?
Liquifaction, where high pore fluid pressure is maintains. This pressure is high enough to overcome friction between particles, allowing flow movement.
What does a debris flow’s high viscosity allow it to do?
Transport large clasts at low velocities because the clast has a very low settling velocity (Stoke’s Law)
How does the movement of a flow stop, or ‘freeze’?
When the friction between particles overcomes pore fluid pressure.
What does the freezing of a debris flow produce?
Debrite
Why are debrites unsorted?
Because the particles are unsorted when the flow stops
Describe a debrite
Poorly sorted, matrix supported deposition of debris flow. Is massive and has randomly oriented clasts
When does a turbidity current occur?
When the volume and mass of a sediment is less than that of the pore fluid.
Why do flows sink in a turbidity flow?
Because the sediment-fluid mixture is more dense than the surroundings
What are the two zones of turbidity currents?
The body and the head
Describe the body of a turbidity current
High density, less turbulent, fast moving. Feeds into the head
Describe the head of a turbidity current
More turbulent, which entrains ambient air and water, decreasing the density. Is fed by the body.
What causes turbidity flows to decelerate?
The density difference between the flow and ambient conditions
What happens when the turbidity flow starts to decelerate?
The coarsest low is deposited
Describe a turbidite
The well-sorted, normally graded deposition of a turbulent flow
Describe the observation potential of topographic relief
Easy to see in modern deposits but low preservation in sedimentary rocks
Describe the observation potential of bathymetric relief
Hard to see in modern deposits but high preservation in sedimentary rocks
