Lecture 4: Glacier Motion

Question 1

Why do glaciers move?

Answer 1

• Distribution of mass around the ELA (glacier flow must balance the amount of accumulation and ablation taking place)
• Physical properties of ice

Question 2

What are balance velocities, and what equation can be used to find Q(x) the discharge through a cross section?

Answer 2

Balance velocities are the mass needed to be removed from accumulation to ablation zone in 1 year to maintain equilibrium.
Q(x) = Σ(WxBx) where Wx is the width of the cross section and Bx is the specific net balance.

Question 3

What process is occurring if measured velocity is greater than the balance velocity?

Answer 3

Thinning

Question 4

What is glacier motion dependent on? And what does it result from?

Answer 4

Dependent on pressure and distribution of water at the bed, and results from the permanent strain of the ice and glacier bed in response to stress.

Question 5

Stress is the result of weight of the ice and slope of the bed, but what are the two types?

Answer 5

Normal and Shear

Question 6

Stress can be longitudinal or concentrated, give examples of what might cause each.

Answer 6

Concentrated = due to undulations at the bed.
Longitudinal = pushing/pulling of ice.

Question 7

The strain that results as a response of the ice / glacier bed to stress can be elastic or permanent, but what are the 3 mechanisms of strain?

Answer 7

• Deformation of ice
• Deformation of the glacier bed
• Sliding at the ice/bed interface.

Question 8

Glens law model the deformation rate of ice (Σ) as equal to At^n where A and n are constants that decrease with temperature. What is t?

Answer 8

Shear stress

Question 9

Describe the process of ice deformation.

Answer 9

Exhibited by all glaciers, it results from movement within or between individual ice crystals (creep). Due to cumulative effect of strain, velocities are greatest at surface.

Question 10

When might ice deformation occur as fracture, not creep?

Answer 10

When ice cannot deform due to amount/direction of stress.

Question 11

Bed deformation occurs under what condition? What does it result in?

Answer 11

Where glaciers are underlain by soft sediments (so motion is via sediment deformation).
Can result in high rates of subglacial erosion and deposition, and may be responsible for unstable glacier motion.

Question 12

Where are strain rates highest in each of the mechanisms of strain?

Answer 12

Bed deformation = highest at top of deforming layer.
Ice deformation = highest at surface.
(Basal sliding only describes movement between glacier sole and bed)

Question 13

What is the condition for basal sliding to occur, and what two forms might it take?

Answer 13

Basal ice must be at the pmp.

Can occur via regelation sliding or enhanced plastic flow.

Question 14

Describe the process of regelation sliding.

Answer 14

Melting on upstream side of obstacles and freezing on downstream. Refreezing releases latent heat, carried back to the melting side through the obstacle.

Question 15

What are the constraints on obstacle size during basal sliding?

Answer 15

Obstacle must be big enough to cause sliding, but small enough to allow heat conduction.

Question 16

Summarise enhanced plastic flow.

Answer 16

Creep enhanced near obstacles due to higher strain rates. Therefore ice accelerates over bumps, with larger bumps leading to higher strain rates.

Question 17

Give the three effects of water pressure on sliding.

Answer 17

• Submergence of small bed roughness features
• Increasing local stress over obstacles by accumulating in lee side.
• ‘Hydraulic Jacking’ where increased water pressure leads to increased sliding.

Question 18

How is medial morraine formed?

Answer 18

Glacier motion drags rock from Nunatek.

Question 19

Describe velocity variations within glaciers.

Answer 19

• Velocities increase from bed to surface, and to the ELA (although variations in width complicate this pattern)
• Velocities are highest on the centre line in valley glaciers (and ogives - seasonally moving glaciers)

Question 20

Why do glaciers flow at a range of velocities?

Answer 20

• mass balance gradients
• ice temp
• bed conditions (water pressure, soft/hard beds, topography)

Question 21

What is typical of slow flowing glaciers?

Answer 21

They are cold-based and so motion occurs via ice deformation only

Question 22

What velocity of flow occurs on ice sheet interiors?

Answer 22

Slow

Question 23

Jakobshavn Isbrae is a fast flowing glacier, but what is it an example of, and what other feautres are fast flowing?

Answer 23

• Jakobshavn Isbrae = polythermal outlet glacier of an ice sheet (warm bed and high ice flux)
• Ice streams (due to bed deformation)
• Surge type glaciers (i.e. Variagated glacier)

Question 24

Jakobshavn Isbrae in Greenland has recently experienced acceleration from its calving front. What explanations for this speed up are there?

Answer 24

• enhanced basal lubrication due to seasonal meltwater
• break up of floating ice tongue
• reduced overburden pressure = more flow
• melt effect of warm ocean waters increased extent of thinning inland.

Question 25

How does the wind interact with Western Greenland, and what does this cause?

Answer 25

Wind over sub-polar gyre forces warm water into fjords, and melts bottom of ice tongues.

Question 26

The Pine island glacier is responsible for 10% of what two things?

Answer 26

• Drainage of West Antarctica

- Observed global sea level rise

Question 27

Grounded Pine Island Glacier has thinned by up to 1.6m/year, but how far has its grounding line retreated, and where may this retreat stop in future.

Answer 27

Already retreated 30km, may stop after another 200km where next bedrock rise is encountered.

Question 28

Short lived events of surge type glaciers are still debated over for their cause. What are their two main phases?

Answer 28

• Surge/active phase = rapid transfer of ice from upper to snout, dramatic advance of ice front but overall thinning.
• Quiescent phase = ice builds up in reservoir are, snout stagnates, albates in situ.