3. GLACIAL MASS BALANCE I

Transcription

1 Glacial Geology 3. Glacial Mass Balance I 3. GLACIAL MASS BALANCE I 40 Points Objective: to learn basic concepts related to glacier mass balance. You should be able to: Describe where, how and when accumulation and ablation occur, Identify and explain the factors influencing the rate at which snow transforms to ice; Determine how accumulation, ablation, and the rate of transformation of snow to ice affect total glacier mass; Calculate water equivalency; and, Calculate mass balance gradients and relate mass balance gradient to flow rates, to particular environments, and to accumulation and ablation rates. Read: Bennett & Glasser (2009) Chapter 2 pp ; Chapter 3 pp Davies, B. (2017) An introduction to glacier mass balance. AntarcticGlaicers.org Fill in Table 3.1. [6] Region where accumulation/ablation dominates (higher/lower elevations) Why is process dominant here? TABLE 3.1 Comparison of Accumulation and Ablation Accumulation Ablation List ways in which accumulation (inputs)/ablation (outputs) occur. What time of year does accumulation/ ablation dominate for midlatitude glaciers? Why is process dominant in that season? What time of year does accumulation/ ablation dominate for tropical glaciers? Why is process dominant in that season? 2. Based on the information in Table 3.1, how might we expect the total mass of a glacier to change (increase, decrease, stay the same) if the following changes in climate occur? [1] The climate gets warmer The climate gets cooler The climate gets wetter The climate gets drier K.A. Lemke UWSP 17

2 3. Glacial Mass Balance I Glacial Geology 3. What might happen to the total mass of ice (increase, decrease, not change) in a glacier if: [1] mass of ice accumulated = mass of ice ablated? mass of ice accumulated > mass of ice ablated? mass of ice accumulated < mass of ice ablated? 4. If we are monitoring a glacier to determine whether the total mass of ice has increased or decreased from one year to the next, what changes in the glacier might we observe if the total mass of ice had: [2] Increased? Decreased? 5. Although snow may accumulate on a glacier surface during the accumulation season, the snow must transform to ice for the glacier to grow. Two key factor determine the rate at which snow transforms to ice. List these two factors, state how/why each factor affects the rate of transformation, and state the ideal conditions for a rapid transformation for each factor. [6] Factor 1: How/why does this factor affect the transformation of snow to ice? What conditions will cause the most rapid transformation of snow to ice? Why will this condition result in a rapid transformation? Factor 2: How/why does this factor affect the transformation of snow to ice? What conditions will cause the most rapid transformation of snow to ice? Why will this condition result in a rapid transformation? 6. Again, although snow may accumulate on a glacier surface during the accumulation season, and although the snow may transform to ice, the total mass of a glacier may not increase if ablation occurs. Ablation occurs on the glacier surface, within the glacier (internally), and at the glacier base. [2] What are the two key factors affecting surface ablation? K.A. Lemke UWSP

3 Glacial Geology 3. Glacial Mass Balance I What are the two key factors affecting internal and basal ablation? Knowing that firn is snow that has survived a summer melt season, should we find firn only in the accumulation zone, only in the ablation zone, or in both the accumulation and ablation zones? Explain why. [2] 8. Scientists often measure mass balance at the end of the accumulation season to determine the amount of mass added to, and again at the end of the ablation season to determine the amount of mass lost from a glacier. Because fresh snow, old snow, firn and ice do not all have the same density (see Table 3.2), scientists calculate mass balance in terms water equivalent the volume of water added to or lost from the glacier if the snow, firn or ice melted. Scientists assume the total volume of water added or lost is spread evenly across the glacier surface and thus report mass balance measurements in depths (meters) of water equivalent. For example, if 1 m 3 of water is added to a surface covering 1 m 2, the water depth over that area would be 1 m. The densities listed in Table 3.2 are just approximations; actual densities will vary. TABLE 3.2 Density Approximations Substance kg/m 3 % Water Substance kg/m 3 % Water Fresh snow Granular firn Damp fresh snow Wet firn Settled snow Glacier ice Wind-packed snow Liquid Water Source: Scientists measured material depths at four sites. Calculate the water equivalent depth, in meters, for samples 1-4. [4] Sample 1: fresh snow depth 2 m Sample 2: settled snow depth 1.5 m Sample 3: granular firn depth 1 m Sample 4: glacier ice depth 3 m For samples 5-8, calculate the density of the sample and then decide what type of material your sample contains based on the values in Table 3.2. Pay attention to your units! Sample 5: size = 3000 cm 3 ; weight = 470 g Density Material Sample 6: size = 3000 cm 3 ; weight = 795 g Density Material Sample 7: size = 3000 cm 3 ; weight 1825 g Density Material Sample 8: size = 3000 cm 3 ; weight = 2130 g Density Material K.A. Lemke UWSP 19

4 3. Glacial Mass Balance I Glacial Geology 9. By definition, net mass balance: at the equilibrium line equals zero (accumulation = ablation), in the accumulation zone is positive (accumulation > ablation), in the ablation zone is negative (accumulation < ablation). Differences in mass balance between the accumulation and ablation zones create a slope on the glacier surface. In Figure 3.1, note that the bottom glacier has a steeper slope than the top glacier due to differences in the mass added and lost. This surface slope allows ice to flow from the accumulation zone to the ablation zone; the steeper the surface slope, the faster the flow. The mass balance gradient determines the surface slope. Scientists calculate the mass balance gradient (MBG) as the change in mass balance across the equilibrium line: FIGURE 3.1 Accumulation, Ablation and Glacier Surface Slope Accumulation zone Mass added GLACIER ICE Accumulation zone Mass added GLACIER ICE Equilibrium line Equilibrium line Ablation zone Mass lost Ablation zone Mass lost MBG = cchaaaaaaaa iiii mmmmmmmm bbbbbbbbbbbbbb cchaaaaaaaa iiii eeeeeeeeeeeeeeeeee. (3.1) To help visualize mass balance gradient (slope), we can graph the mass balance at a set distance above and below the equilibrium line (Figure 3.2) using water equivalent measurements (Table 3.3). The purple dots in Figure 3.2 represent the amount of mass gained or lost at a particular elevation and the blue line represents the mass balance gradient (surface slope) across the equilibrium line. The first set of measurements provide an example. Calculate the mass balance gradient for the remaining glaciers listed in Table 3.3 and graph the mass balance gradients as shown in the example. [10] Mass Balance ( 100) Figure 3.2 Mass Balance Gradient Graph Equilibrium Line Example Glacier 1 Glacier 2 Glacier 3 Glacier 4 Glacier 5 Table 3.3 Mass Balance Gradient Data and Calculations Glacier Elevation Annual Mass Mass Balance Gradient Across the Equilibrium Line (m) Balance (mm w.e./yr) (mm w.e./m) Example (lower part of accumulation zone) 500 ( 200) = 700 mm +3.5 mm (equilibrium line) MBG= = = 200m m (upper part of ablation zone) K.A. Lemke UWSP

5 Glacial Geology 3. Glacial Mass Balance I 10. Which of the six glaciers has the steepest mass balance gradient? [2.5] Which of the six glaciers should have the fastest flowing ice? As ice flows downhill, what will happen to the gradient? (increase/decrease) What impact will this change in gradient have on the flow rate? Which of the six glaciers should respond fastest to climate change? 11. Compare and contrast glaciers with steep mass balance gradients to glaciers with flatter mass balance gradients by completing Table 3.4. [3.5] Table 3.4 Comparison of Glaciers with Steep and Flat Mass Balance Gradients Mass Gains Losses Flow Sensitivity to Location Climate Balance (large/ (large/ Rate climate change Coastal or Interior Warm + Wet (maritime) Gradient small) small) (fast/slow) (high/low) Temperate (midlat) or Polar Cold + Dry [continental) Steep Flat: K.A. Lemke UWSP 21

6 3. Glacial Mass Balance I Glacial Geology 22 K.A. Lemke UWSP