Gravity processing

Question 1

Define gravity processes

Answer 1

The process by which regolith, sediment, and rock moves downslope under the force of gravity

Question 2

How do gravity processes occur?

Answer 2

When gravity’s effect overcomes a slopes resistance

Question 3

What dictates slope resistance?

Answer 3

Slope gradient and internal friction

Question 4

Define angle of repose

Answer 4

The steepness threshold under which the material is stable and over which the material will flow under its own gravity

Question 5

Why are landscapes more vulnerable to landslides after heavy rain?

Answer 5

Because water provides lubrication between the grains, making them weaker

Question 6

What happens as a material’s internal strength increases?

Answer 6

Its angle of repose increases

Question 7

Where do gravity processes dominate?

Answer 7

Where there is a gradient

Question 8

Give the name of gradients found on land and in the oceans

Answer 8

Topographic relief and bathymetric relief

Question 9

Where is topographic relief most abundant?

Answer 9

Uplifting orogens

Question 10

Where is bathymetric relief most abundant?

Answer 10

Continental margins

Question 11

What two mechanisms can trigger gravity processes?

Answer 11

Steepening the gradient over the angle of repose and reducing the natural angle of repose by decreasing the material strength

Question 12

Give two examples of how a gradient can be steepened over the angle of repose

Answer 12

Undercutting (cliffs, valley walls, river banks) and depositional steepening (marine settings)

Question 13

Give two examples of how the natural angle of repose can be reduced by decreasing the material strength

Answer 13

Adding lubricating water to air-filled pore spaces and earthquake induced liquifaction

Question 14

Describe the process of earthquake induced liquifaction

Answer 14

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.

Question 15

What dictates the style of a mass wasting event?

Answer 15

Material strength

Question 16

Debris flows and turbidity currents are characteristic of what strengths of material?

Answer 16

Fluid (the weakest)

Question 17

Describe fluid materials

Answer 17

Usually unconsolidated, recently deposited sediment

Question 18

Describe debris flows and turbidity currents

Answer 18

A mixture of sediment and fluid. Can move downslope under their own gravitational potential because they are denser than the surrounding medium

Question 19

What are two differences between between debris and turbidity flows?

Answer 19

Debris flows are laminar and turbidity flows are turbulent

Question 20

What determines whether a gravity flow will be a debris or turbidity flow?

Answer 20

Reynold’s number (Re)

Question 21

What type of flow is the result of a high Re number and why?

Answer 21

Turbulent because materials with a lower viscosity are more likely to become turbulent

Question 22

What controls a material’s viscosity?

Answer 22

The number of grain collisions

Question 23

Describe a mixture with higher viscosity

Answer 23

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.

Question 24

When do debris flows occur?

Answer 24

When the volume and mass of sediment is greater than that of pore fluid (which is always water)

Question 25

How are the grains in a debris flow kept apart?

Answer 25

Liquifaction, where high pore fluid pressure is maintains. This pressure is high enough to overcome friction between particles, allowing flow movement.

Question 26

What does a debris flow’s high viscosity allow it to do?

Answer 26

Transport large clasts at low velocities because the clast has a very low settling velocity (Stoke’s Law)

Question 27

How does the movement of a flow stop, or ‘freeze’?

Answer 27

When the friction between particles overcomes pore fluid pressure.

Question 28

What does the freezing of a debris flow produce?

Answer 28

Debrite

Question 29

Why are debrites unsorted?

Answer 29

Because the particles are unsorted when the flow stops

Question 30

Describe a debrite

Answer 30

Poorly sorted, matrix supported deposition of debris flow. Is massive and has randomly oriented clasts

Question 31

When does a turbidity current occur?

Answer 31

When the volume and mass of a sediment is less than that of the pore fluid.

Question 32

Why do flows sink in a turbidity flow?

Answer 32

Because the sediment-fluid mixture is more dense than the surroundings

Question 33

What are the two zones of turbidity currents?

Answer 33

The body and the head

Question 34

Describe the body of a turbidity current

Answer 34

High density, less turbulent, fast moving. Feeds into the head

Question 35

Describe the head of a turbidity current

Answer 35

More turbulent, which entrains ambient air and water, decreasing the density. Is fed by the body.

Question 36

What causes turbidity flows to decelerate?

Answer 36

The density difference between the flow and ambient conditions

Question 37

What happens when the turbidity flow starts to decelerate?

Answer 37

The coarsest low is deposited

Question 38

Describe a turbidite

Answer 38

The well-sorted, normally graded deposition of a turbulent flow

Question 39

Describe the observation potential of topographic relief

Answer 39

Easy to see in modern deposits but low preservation in sedimentary rocks

Question 40

Describe the observation potential of bathymetric relief

Answer 40

Hard to see in modern deposits but high preservation in sedimentary rocks