ess 203 -glaciers and global change - university of …...•please read chapter 5 (pages 65-88) in...

ESS 203 - Glaciers and Global Change

• Volunteer for today’s highlights on Friday ________• Highlights of last Friday’s class – Maddy Weise

Wednesday January 22, 2020

Outline for today• Permafrost• Elastic, viscous, and brittle behavior.• Blue Glacier video

[Start Greenland Ice Cap]

Homework AssignmentFor Friday:• Please read Chapter 5 (pages 65-88) in Frozen Earth.

• In a few sentences, explain why Macdougall thinks James Croll was an “unlikely” scientist.

• In a page or less, outline Croll’s contribution to ideas about why ice ages happen.

http://courses.washington.edu/ess203/

Group Project

• It’s time to be getting some ideas together, and finding partners.

• See GROUP PROJECTS tab on class web site.

• Your goal – write about a cryosphere story that has been covered in the popular press, based on the peer-reviewed scientific literature.

• We will investigate the scientific literature soon. o how to find ito how to read it


Where are we going anyway?• How do glaciers move?• Ice-Age glaciers• How do scientists communicate? (Understanding peer

review and how to approach scientific papers, you can read original sources for projects, for class discussions, and to assess claims in the climate debate.)

• Changing glaciers as climate indicators• Ice cores – archives of past climate changes• Glaciers and sea level• Lake Vostok – giant lake under Antarctica – a biological

refugium?• Future of ice – decades, centuries, millennia

Hard and Soft Ice

Why do glaciologists care about ice temperature?

• Warm ice is "softer" and it flows more readily than cold ice.

• This is just like hot/cold molasses, or the engine oil in your car.

• Ice near 0oC (the melting point) deforms and flows 100 times faster than ice at -20oC.


Seasonally Frozen Ground

What happens to the ground in North Dakota in winter?• Temperature falls well below the freezing temperature

of water for several months.• The ground freezes (but only the upper meter or so,

depending on how long the cold season lasts).

What happens when the air warms up in spring?• The ground thaws again.


PermafrostWhat happens in Alaska, Siberia or northern Canada

where the average temperature of the air over the course of a year can be well below freezing?

• The ground can be permanently frozen to a depth of several hundred meters. (why is it not frozen even deeper?)

• This frozen ground is called permafrost.

There is a whole field of research that studies frozen ground, and its processes and effects on landscape and buildings. This field is called Geocryology.


Active LayerWhat about near the surface? We know that the air can

get warm in the summer.• The upper 1 meter or so of the ground thaws during

the summer, and freezes up again in the winter.This upper layer is called the active layer.

This upper meter can get very wet and boggy, because the water from melting snow and thawing ground doesn't drain into the ground. Why not?

• The ground below the active layer is permanently frozen, and any passageways for water are probably blocked by ice.


Permafrost in northern Manitoba

http://gsc.nrcan.gc.ca/landscapes/details_e.php?photoID=537

• Here, sphagnum peat bogs underlie the boreal forest. • Below a depth of about 60 cm, the bogs remain frozen year-

round. • Intervening fens, the light-toned areas, occupy depressions where

ground ice has melted out.

Lat. 56.743 Long. 93.205 W (same latitude as Scotland)

Photographer: Lynda Dredge GSC Photo: 2001-169

Ice WedgesFrozen ground contracts in winter.• Cracks form repeatedly in the same places.

http://science.nasa.gov/newhome/headlines/ast27jul99_1.htm

• Snow fills the cracks.

• Water freezes in the cracks.

• The ice wedge gets wider.

This wedge was exposed in an eroding stream bank.

Ice Wedge PolygonsFrozen ground contracts and cracks in winter.

http://gsc.nrcan.gc.ca/beaufort/ground_ice_e.php

• These cracks form networks, just like cracks in drying mud.

• There is an ice wedge under each edge of each polygon.

Fossil ice-wedge polygons

Muir Ontario: casts still visible 10,000 years after the ice-age permafrost thawed.

http://www.earth.uwaterloo.ca/services/whaton/waton/polyfig3.html

Frozen Ground

Freezing, shrinking, cracking,luring and trapping snow in winter,refreezing water in summer,we grow fat, engorged through time,we drive deep into the cold Earth, while etching the surface

in crude angular shapesthat reveal our icy veins,

even after we are gone.(anon.)

Photo credit: Tim Haltiginhttp://geog.mcgill.ca/grad/haltigin/


The Curious Scientist: More Group Questions …

Understanding temperatures in the Cryosphere.

• 5-minute discussions• Recorders turn in reports at end of

class.


1. Climate and Flow of Glaciers

• Which glacier flows faster, and why?• Both glaciers are in steady state. Can you say which

ice cap must have the higher accumulation rate in its accumulation area, and the higher ablation rate in its ablation area? Why?

You are a Climatologist studying 2 glaciers. They have identical sizes and shapes.

The other is in Novaya Zemlya (an island in the Arctic Ocean north of Russia), where it is much colder.

One is in Iceland, and it is temperate.

2. Paleo-Geocryology

• Would you expect to have seasonal thawing and freezing of the ground under a thick glacier?

• Why or why not?

You are a geologist studying the last Glacial period (20 ka ago), and you are looking for evidence that permafrost existed then in a particular area.

• What might you look for?• Could you use your results to tell whether there used

to be a thick ice sheet or glacier there during the last Glacial period?


3. Basal Melting and Glacier Motion

• How hot can ice get?A glacier was cold and frozen to its bed. Then it warmed

up, and its temperature at its base rose to the melting temperature, so it was no longer frozen to its bedrock.

• What might happen to the flow speed of this glacier?If the mass balance (accumulation and ablation) did not

change, the glacier will reach a new steady state in which it still transports the same amount of ice past each place, and it will ultimately terminate at the same place.

• What might happen to the thickness of this glacier?


Continuum Mechanics 101 in 101 Minutes

• Elastic Deformation• Viscous flow• Brittle failure

Ways that materials can deform

• Stuck (cold, frozen)• Frictional slip

Materials can also move relative to substrate

• Which ones are important for glaciers?

Elastic DeformationWhat happens when you apply a constant force to a

material like a rubber band?• It can deform elastically when you pull on it.demo: stretched rubber band or spring

• It deforms quickly to a stretched state, then stops deforming until you change the force.

• It returns to its original shape when you stop pulling (or pushing).

demo: bouncing ball

• Do you think this will be important for a glacier?

Do we see Elastic Deformation in Glaciers?

Will an ice cube bounce if you drop it?• Let’s try it sometime …

If you squeeze ice, it can be compressed elastically (and when you release it, it bounces back).

• But … the amount it compresses is very small and hard to demonstrate.*

* Elastic behavior is unimportant for glacier flow.

Viscous Flow

A viscous fluid flows continually as long as a force is applied to it.

Demo 1: Check out silly putty "ice cap" • what is the force?

Demo 2: Pull on putty with lead weight


Brittle Fracture• A brittle solid can fracture if you apply a large enough

force fast enough.[demo: silly putty]


Sometimes a material can be all of the above.• Silly putty?• Earth�s lithosphere? (upper 30 miles or so)• Ice?

What is “evidence”?Scientific evidence consists of observations and experimental results that serve to support, refute, or modify a scientific hypothesis or theory, when collected and interpreted in accordance with the scientific method.*

*Wikipedia

Note the two required factors• Observations made• A theory that is tested by the observations

Glacier-Flow Video

This 11-minute video shows historic time-lapse footage shot at

• Blue Glacier, Mt Olympus• Nisqually Glacier, Mt Rainier

Do these glaciers really move, or change shape, or flow?

• Watch for evidence of viscous flow, brittle failure, and frictional sliding.

Curious Scientists find evidence of glacier behavior

Based on the Blue Glacier video, work with your group to identify and describe as many lines of evidence as you can to support or refute1. Viscous flow2. Brittle fracturing3. Frictional slip

• What evidence did you see for viscous flow in the video?

Closing the tunnel• How long did it take? • Or do we need to run for it?• Tunnel closed in about 20 seconds on film.• Film was speeded up 5,670 times• Actual closure took » 5,670 ´ 20 s = 113,400 s• How many days is this?

Ice flows in glaciers

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Glacier’s Revenge …

With wanton abandon the human scourge assails me.Burrowing relentlessly, deep into my bowels,

with air, water, electricity,Leviathan that I am,

I am powerless to resist their mighty tools.

But always their tenacity will fail.Slowly, slowly, I envelope

all their constructsAnd victory is mine.

(anon.)

Ice fractures in glaciers

We could try hitting an ice cube with a hammer.• What would happen?What evidence did you see for brittle fracture in

the video?Crevasses• Where were they?Jack hammer and chain saw• Did the ice break?Ice sliding off basal ledges• Did it ever break?

What about sliding over the substrate?

Perspectives on Flow and Change

• Kinematic perspective• Dynamic perspective

• Both of these ideas are useful for understanding steady flow or changes, and for predicting flow and changes in many other systems where inventory matters (besides glaciers J).

Input (snow)

Output

Gate

(ice flux)

Kinematic Perspective

• How much volume of ice must a glacier carry by flow past any �gate� in a year, (ice flux) (any gate, not just gate at ELA) in order to evacuate a volume of ice equivalent to all the accumulated snow or ice from upstream in a year?

Until now, we have looked at glacier flow from a kinematic perspective.

• Or if the flux carried doesn’t equal the total upstream accumulation, how fast does the glacier thicken or thin?


We assumed steady state –• Left to its own devices for long enough, the

glacier had already achieved the appropriate combinations of thickness and speed to carry the necessary ice flux.

• (If it had not, it would still be growing or shrinking, and from kinematics we could figure out by how much.)


What did we not need to know?• We did not need to know anything about gravity, or

forces, or ice thickness, or slope of the surface. • We did not need to know anything about how ice

deforms at the microscopic level (crystal properties).• We did not even need to know anything about the bulk

mechanical properties of ice, such as its viscosity, e.g. how soft or pliable is glacier ice, in comparison to motor oil, or honey, or stainless steel …

That we could get this far, is a bit surprising …!

ess 203 -glaciers and global change - university of …...•please read chapter 5 (pages 65-88) in...

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