Ice Ages Past and Future
Review Sheet for Exam 1
Fall, 1999
This review sheet should not be taken as a complete review of the materials in this course.
I suggest first reviewing the Summary and the Keywords of the Hamblin reading on page 35 of the coursepack. Also, look at the summary tables at the end of the Bennet and Glasser chapters in the coursepack. For example, look at Table 6.2 p. 69-70, Table 9.2 p. 103-104, and Table 9.3 p. 104. Only review the features from these tables that we discussed in class. For specifics on these features, refer to the chapter readings preceding the respective summary tables.
Features I've discussed in class:
From Table 6.2 p. 69-70 (Erosional Features)
Striations
Chattermarks (a specific variety of Friction Crack, the example I used in class falls under the category "reverse crescentic gouge" which is convex up-ice.)
Roches Moutonnee
Subglacial Meltwater Channels (See Eskers)
Ice Marginal Meltwater Channels
Cirques
Tunnel Valleys
From Table 9.2 p. 103-104 (Glacial Depositional Landforms)
Moraine (don't subdivide as in reading, just know "end moraine" and for valley systems, know lateral and medial moraines)
Drumlin
From Table 9.3 p. 104 (Landforms of Glacial Meltwater)
Outwash fan
Braided River
Kame (Delta and Moulin)
Esker
In addition to these landforms, learn what sedimentary deposits are formed by ice, running water, and lakes. The DNR Bulletin #4 has a summary of the landforms and deposits left by continental glaciers, then discusses the evolution of the Great Lakes. The Two Creeks Forest Bed pictures and captions summarize what these deposits tell us about the glacial and lake history:
http://www-personal.umich.edu/~hoaglund/TwoCreek/twocreek.html
Headings follow the syllabus, but I have a few sections of general questions and definitions.
I have only answered the questions that were inspired by students of the present and past semesters. Try to answer them on your own first, then see how your answer compares.
Lecture 1 Overview to glacial systems:
1. Define the following: glacier, ice sheet, ice cap, pack ice, ice shelf, ice stream, piedmont glacier, alpine or valley glacier, cirque glacier, horn, arete, tarn, lateral moraine, medial moraine, end moraine.
2. Visit on the web:
http://www-personal.umich.edu/~hoaglund/courses.html
3. Define accumulation zone, equilibrium line, ablation zone, snow, firn (neve), ice.
4. How does ice flow through a glacier (from where to where?).
5. What is meant by glacial advance? glacial retreat?
Lecture 2: Some physics of glaciers and glacial flow
Comment:
Glaciers are controlled by climate. The greater the contrast with altitude between the mass balance in the accumulation zone and the mass balance in the ablation zone, the greater the amount of ice that flows through the system. Generally, there are three equations that describe the flow of this ice through the glacier and the glacier's resultant shape. You do not have to work any of the equations, but you will want to know what they say about glaciers. 1) The "profiling equation" relates the thickness of the ice to distance from the ice margin, generating an ice shape. It was derived from the Nye equation (see below) by assuming a constant shear stress at the base of the ice. Under the conditions of constant basal shear stress, the slope of the ice must increase as the thickness decreases. Both the Nye equation and the Glen law go into determining how fast the ice is deforming, i.e. flowing. 2) The Nye equation determines shear stress ("directed pressure") and then 3) the Glen law uses that shear stress to determine strain-rate. Add up the strain rates from the base to the top (or a point inside the ice) and you can get ice flowing velocity at the top (or the point inside the ice). But remember, there is also basal sliding and bed deformation. The observed velocity will be the sum of the sliding velocity, the bed deformation velocity, and the flowing velocity.
1. What is mass balance? What is the mass balance gradient? Which climate has more ice flowing through the system, maritime climate (relatively warm climate near the ocean) or continental climate (relatively cold climate interior of a continent)?
2. What does the "ice profiling" equation describe or relate? What does this say about the profile shape of a glacier?
answer: The ice shape (profile) is predictable if you know the "profiling constant" (The constant itself varies for different glaciers depending on the temperature of the ice, the flow velocity, and the amount of water on the bed which are all interdependent). One can get an idea of the thickness of the ice assuming or knowing a profiling constant. The "1/2 parabola" is centered on the x-axis (parabolas are usually displayed centered on the y-axis, so maybe we should call it a "square root function"). In mathematics any "polynomial" of degree (that is power) 2 (or 1/2) is called a "quadratic." Fancy words that all mean the same thing. For us, what's important is that we can determine ice thicknesses just knowing the minimum distance along a glacial flow path from the point of interest to the nearest ice margin.
3. What does shear strain mean? Shear strain rate? What do the variables in the Glen law stand for? How does the Glen law relate to the velocity of ice measured on the surface?
answer: Shear strain is the amount the ice deforms (changes shape) due to a shear stress (think "directed pressure") that acts parallel to a surface. In our glacier example, the surface is a layer of ice parallel to the bed. Shear strain rate is the rate at which the ice deforms, so it is an amount of deformation per time. Glen's law for ice determines what the shear strain rate will be for a given shear stress for a single ice layer. Because one layer rides on the layer below it, to determine velocity at a point (say the ice surface) on the thickness profile, you have to add up all the deforming layers through the ice thickness to that point (the whole thickness if you're talking about the ice surface velocity) to determine the velocity of the ice at that point.
The above is only the velocity from internal deformation of the ice. You have to add the basal sliding velocity (and velocity associated with deformation of the bed, if any) to get the observed velocity of the glacier.
4. What 3 variables determine whether ice will deposit or erode?
We did not cover this as thoroughly as I have covered it in the past. I will not ask this question on the exam, but for your own interest:
answer: Ice thickness, velocity, and amount of water at the bed (and whether it's freezing or melting) all interact to determine whether a glacier is depositing or eroding. In general, higher ice velocity, thick ice, and abundant freezing water correspond to an increase in glacial erosion, all else being equal. However, the story is very, very complicated involving interaction and feedbacks among the three variables. As the ice thickness increases, increased load (normal) stress increases clast contact pressure (see Boulton model, pg. 85, chapter 5 of Bennet and Glassner) increasing abrasion, and increased shear stress increases velocity. Therefore, one would expect erosion to increase, and often it does. But increased clast contact pressure due to increased load (normal) stress (due to the increased ice thickness) eventually causes the clast to deposit (i.e. lodge). In addition, increased ice thickness can insulate the base of the glacier affecting the thermal condition of the basal ice, which in turn affects the amount of water present at the base of the ice.
5. What do the Glen law and Nye equation define and what is the difference
between them?
answer:
The Glen law relates the shear strain rate (see above, it's the cursive E with the dot on it) to the shear stress (the "directed pressure") causing ice to deform parallel to the bed.
The Nye equation relates the shear stress (the "directed pressure") to ice thickness and the amount of slope.
6.What is the effect of thickness on shear strain rate?
Simple answer: All else equal, the greater the thickness, the greater the shear strain rate.
Detailed answer: For the same slope, an increase in ice thickness increases the shear stress at a point in the ice (by the Nye equation) which in turn increases the strain-rate at that point in the ice (by the Glen law).
7. What is the effect of water at the base of the ice on the velocity of the ice?
Simple answer: All else equal, the more water, the faster the ice.
Detailed answer: Water at the base of the ice increases the basal sliding velocity of the ice. This is added to the flow velocity within the ice to generate the total velocity observed on the surface.
8. What is meant by the pressure-melting point? Pressure-melting effect?
answer: There is a decrease in the melting point of ice with increasing ice thickness (about 1 degree centigrade for every kilometer). The base of a three kilometer thick ice sheet will melt at approximately -3 centigrade, provided there is a source of heat. See #9
The pressure-melting effect is the melting of ice due to compressing (squeezing) ice. If ice is near the pressure-melting point and is squeezed, the new, higher pressure lowers the melting point with the result that some melting occurs. See #9,#10
9. What are these possible sources of heat at the base of a glacier?
answer: Heat sources for the base of a glacier are geothermal and frictional basal inputs, and the transfer of latent heat conducted from where it is liberated elsewhere from freezing water. The latent heat sources are not actually from the base of the glacier, but from surface meltwaters that reach the base of the glacier. The pressure melting effect is NOT a source of heat, but can be a reason for water at the base of an ice sheet.
special conservation of energy note: Latent heat for pressure melting is absorbed from excess heat in the remaining ice, so the remaining ice cools from the old melting temperature to the new one. Latent heat goes in and out, and work energy goes in and out, but no energy is created or destroyed. However, under a glacier, some of the latent heat can be "arrested," conducted away for use elsewhere. The ice under a glacier is mostly equilibrated in terms of pressure. It takes changes in pressure to squeeze or work ice, so pressure-melting is not common. However, pressure-melting of ice is significant around obstacles, like when a Roche Moutonnee forms.
Bottom line: Basal ice with a temperature near its pressure-melting point is more "sensitive" to heat contrasts and will more readily melt producing water at the base of the ice.
10. What creates water at the base of a glacier?
answer: basal heat sources (geothermal and frictional heat), latent heat liberated from surface meltwaters reaching the base, and the pressure melting effect.
11. Could you explain subpolar glaciers in terms of their pressure melting point?
answer: Sub-polar glaciers are "in-betweens." A temperate glacier has an ice temperature profile near the pressure melting point all the way down, at least at some time during the year. Most valley glaciers are temperate. A polar glacier is well below the pressure melting point all the way down, but may have some melting near the base. An example is the Antarctic ice sheets, which are formed and exist in a polar climate that very rarely gets above freezing.
The sub-polars (in-betweens) have temperate, polar, and "in-between" temperature profiles depending on the location of the profile on the icesheet. An example is Greenland's ice sheet. The ice sheets that covered North America during the Ice Ages were sub-polar.
It is easier to generate water at the base of the ice (from heating) if the ice is near the pressure- melting point, as in a temperate glacier.
Some General questions
1. What is till?
answer: Till is an unsorted, massive to crudely stratified sedimentary deposit, deposited by glacial ice (as opposed to running water). The key to recognizing till is its unsorted character, almost unique (geologically) to glaciers. See Guide to Till handout.
2. What three zones in a glacier transport sediment?
3. How is the basal zone further subdivided?
4. Define compressive flow
5. Define extending flow
6. What are transverse crevasses? Where do they form?
7. What are longitudinal crevasses? Where do they form?
8. What do crevasses say about ice flow and ice flow direction?
Subglacial Erosion and Deposition (Till I)
1. What are suspension and traction zones in ice?
answer: Traction and suspension zones make up the basal ice zone. Basal ice (about 1 to 3 meters thick measured from the bed up into the ice) is carrying debris. However, this debris is only in contact with the bed in the traction zone, which is right where debris is grinding the bed. The traction zone is the ice-bed interface, measured in centimeters off the bed. In contrast, debris in the suspension zone is not in contact with the bed, and is therefore not being scoured or scouring.
2. What is a drumlin?
3. Is a drumlin erosional, depositional, or both? How can you tell the direction of ice flow when the drumlin formed?
4. What is a Roches Moutonnee
5. Is a Roche Moutonnee erosional, depositional, or both? How can you tell the direction of ice flow when the Roches Moutonnee formed?
6. What type of till is deposited subglacially by active, flowing ice?
7. What are the two main processes of glacial erosion?
Glacial hydrology and Ice Contact Stratified Deposits
1. What two characteristics are imparted to sediments deposited by running water?
2. What is a moulin?
answer: A moulin is a shaft through the ice where surface meltwater can "cascade" through the ice reaching the base of the ice. Since this water is carrying sediment, a "moulin kame" might form at the base of this shaft. Moulins typically form (or at least get started) where two crevasses intersect.
3. Define kame (name two types). How does a kame form?
4. Define esker (name two types based on channel position relative to the ice-bed interface). Can an esker erode down and into bedrock?
5. Define tunnel channel, i.e. tunnel valley. Why are tunnel valleys so large? What source of water is tapped? Why doesn't this water leave the glacier equally all along the ice margin?
6. Do tunnel valleys fill with sediment? Why do the sediments of tunnel valleys and associated outwash fans make good groundwater aquifers?
Supraglacial Deposition (Till II), ...
1. What are shear planes? How does a glacier get debris to the supraglacial position?
2. Define kettle.
3. Define moraine.
4. Which moraines are formed by continental glaciers? Which moraines are formed by valley glaciers?
5. What is hummocky topography? What other name is used to describe it?
6. void...not covered in class: How might one distinguish a crevasse fill ridge from an esker?
7. What type of till is deposited by stagnant, wasting, dead ice?
8. What type of till is slurried and re-worked by water in slumps?
...Outwash and Lake Plains
1. What two characteristics are imparted to sediments deposited by running water?
2. define: outwash, outwash fan (sandur)
3. What are braided streams?
4. Where is the coarsest sediment in an outwash fan, closest to the position of the old ice margin, or further?
5. define: varve, proglacial lake, pluvial lake, lacustrine clay
6. What does pluvial mean?
Great Lakes Geologic History
1. Define outlet, isostatic uplift, ice lobe.
2. Which way did the Great Lakes drain when partially occupied by ice?
3. #2 ... when ice free?
4. Are paleo-beaches of the same age at the same elevation in northern and southern Michigan?
5. What events are recorded by sediments exposed in the bluff outcrop of the Two Creeks Forest Bed?