Velocity and Surges Flashcards
What methods are used to measure glacier velocity?
GPS
TLS- terrestrial laser scanning
UAVs
Remote sensing
How can TLS be used to measure glacier velocity?
Scan and rescan it at another time and see the differences.
What are limitations in using TLS and UAVs to measure glacier velocity?
You need relatively small glaciers
What is a benefit of using UAVs to measure glacier velocity?
Detailed velocity measurements
What kind of technology can be used for measuring the velocity of larger areas of glaciers?
Remote sensing- SAR (Synthetic Aperture Radar) interferometry
How does remote sensing- SAR (Synthetic Aperture Radar) interferometry work?
It is a technique that is used to record the velocity of a glacier by comparing different satellite radar images of the same site.
You need a number of passes over the same location, and then your record the difference
Glacier velocity is never constant…but it can be divided into two types.
Continuous
Discontinuous
What is a continuous glacier?
One which is travelling at a similar speed most of the time.
Slow- cold based glacier- <30 m/a
Fast- warm based glacier- 50-100 m/a
What is a discontinuous glacier?
Discontinuous- change their speed dramatically
Surging glaciers >10,000 m/a
Ice streams >400 m/a
What are short term speed up events?
These are usually melt water driven but sometimes tidal driven.
Often called stick slip motion- small thing that happens everyday- Tidal modulated twice per day.
What are surges?
Surges are a dramatic glacier advance (Meier and Post, 1969).
They have a long quiescent phase of normal velocity.
…and then a short surge phase glacier 10-10,000 x faster
Period (10-100 years)
How much of the worlds glaciers surge?
Less than 1%nworlds glaciers surge (concentrated in certain areas- Alaska, Iceland etc).
What is the morphology of a surge?
They have a wave like bulge because the top part of the glacier is flowing so much faster than the lower part of the glacier.
How can we tell if a glacier has surged or not?
Amalgamation of features… Push moraines Crevasse filled ridges Hummocky moraines Concertina eskers
What is a typical surge?
Occur at all scales
Valley glacier
Ice stream in ice sheets
Found both on both rigid and deforming bed glaciers
Both warm and subpolar glaciers
Rigid bed model
Short surge cycle- 15 years- 1 year suddenly active- high velocities 40-60 m per day not year
This is because there is a build up of high water pressure in the basal water system.
This leads to increased basal sliding- these cavities become linked and the surge front advances as it destroys the tunnel system, and the higher water pressure makes the cavities.
Deforming bed model
The front of the glacier is frozen and the rest is warm based- the surge bulge forms at this boundary.
The subglacial water is flowing through the till
There is a barrier to sliding (frozen toe) stopping the water flowing through the till.
Water pressure builds up and suddenly passes through threshold and glacier shoots forward.
Murray et al, 2000, 2003 model
At the start of the cycle, the glacier is cold based (polythermal).
Ice builds up in surge front causing thickening until pmp is reached in part of the glacier- turned into warmbased.
Basal meltwater is produced, which leads to high pore water pressures and weakening of the till.
Accelerated deformation of the till produces frictional heating, more melting, sliding
This causes the ice to thin, causing refreezing and the surge ends.
For a glacier to maintain a steady state…?
Mass flux through the system must be in equilibrium with surface mass balance
Energy fluxes must maintain constant enthalpy (ice temperature and water content)
So cold dry glaciers
Small glacier can keep in balance- don’t surge
Larger ones can’t keep in balance- may surge
What criterion can be used to distinguish between glaciomarine and glaciotectonic sedimentation?
The glaciomarine environment is dominated by…
sedimentary processes, related to…
Sediment supply
Water content
Distance from the glacier margin.
What criterion can be used to distinguish between glaciomarine and
Glaciotectonic deformation, however, is dominated by…
deformational processes, related to… Effective pressure Shear strain Nature of the subglacial material Distance from the glacier margin.
What are glacio-marine sediments?
Glacio-marine sediments are deposited by glacial metlwater in an ocean environment.
What are glaciotectonic landforms?
The surface or morphologic expressions of subsurface structures resulting from glacial deformation of bedrock.
In the daily records of surface velocities…
What is base velocity?
What are speed up events?
There is no glacial melt as temperatures are below zero.
Days where the temperature is above zero and melting occurs.
What are the models?
Rigid bed model
Deforming bed model
Murray et al, 2000, 2003 model
