L2: Glaciers Mass Balance Measurement

What is the accumulation vs. ablation zone?

Accumulation zone - part of the glacier with snow for the whole year.

Ablation zone - part of the glacier where snow and some of glacial ice has melted.

What feature separates the accumulation and ablation zones?

Equilibrium line - accumulation and ablation rates are equal here. Terminus often remains in the same place when in equilibrium as accumulation flows into ablation.

How can accumulation and ablation be used to tell when glacial mass is balanced?

When accumulation is equal to ablation mass is balanced. Greater accumulation equals a more positive mass balace and vice versa.

What is the ELA?

The altitude on a glacier where accumulation is equal to ablation. Approximated by the late summer snow line. ELA in warm climates may move upwards - indicators of climate change.

What is the mass balance gradient of a glacier?

The spatial pattern of net accumulation and ablation at different elevations along the surface of a glacier. Net accumulation increases at higher elevations, and decreases down-glacier for example.

What does glacial mass balance indicate?

Flow rate. A steeper mass balance gradient suggests a faster flow, and higher accumulation/ablation rates.

What factors might affect mass balance?

• Avalanches

• Shading

• Rock/landslides on to the glacie

• Cloudiness/fog

• Surface meltwater ponds/lakes

What is the most common factor which may cause mass balance gradients to be non-linear?

Debris cover - (above to 2cm) insulates ice underneath, whereas any thinner may cause melting to increase. E.g., in volcanic environments thin soot/ash will increase melting but thick layers of tephra will insulate.

What processes control ice formation and accumulation?

• Wet snow is melted and refrozen - grains of snow are compacted and rounded.

• Dry snow undergoes sintering (bonds form under heat).

What is firn?

The intermediate stage between fresh snow and glacier ice - air passages in snow are cut off and density increases to approach 917 kg/m3.

What other factors may contribute to glacier growth at local scales?

• Wind-blown drift;

• Freezing rain or refreezing meltwater;

• Avalanches; and

• Condensation.

What are the two key ablation processes?

• Calving - marine-terminating glaciers, affected by water depth, temp, tidal variation, crevassing, water discharge.

• Melting - mostly at surface.

What is the grounding line, and what is produced when these coincide with the calving front?

• The point at which the glacier begins to float.

• A dynamic ‘tidewater glacier’ is produced when these coincide with the calving front.

What factors influence melting (ablation process)?

• Albedo - high albedo equals low ablation;

• Thin debris cover - increased absorption and re-radiation - increased ablation;

• Thick debris - increased protection - decreased ablation.

What are three classic features formed by supraglacial debris cover?

• Dirt cones;

• Glacier tables and cryoconite holes; and

• Supraglacial debris.

How is glacial mass balance measured?

Inputs and outputs over the mass balance year must be measured, with the difference in these then being expressed in mm/m. Can be +ve or -ve.

What is a direct method of measuring glacial mass balance?

• Measure accumulation using snow pits to obtain snow depth and intensity;

• Measure ablation using a network of stakes to obtain ice surface changes.

World Glacier Monitoring Service collates all glacier measurements worldwide using reference glaciers and the stakes and pit method.

What is a geodetic method of measuring glacier mass balance?

• Aerial photographs are used to measure area and altitude changes across a range of different timescales;

• Drones and satellite measurements are used; and

• Elevation changes tracked on DEMs.

Useful for ice sheet mass balance measurements.

What is a climatic method of measuring the mass balance of glaciers?

• Meterological data used to calculate precipitation (input) and surface energy balance (melting);

• Climate models commonly used;

• Calving/discharge needs to be added where known.

What is a gravimetric method of measuring mass balance?

• Satellites measure changes in the gravitational pull of large ice masses.

• New method.

What is a mass-budget method of measuring glacial mass balance?

• Model or measure the accumulation from snow-fall and then the discharge/flow of ice at the margin;

• Requires assumptions about the rate of snow to ice conversion (firn compaction); and

• Requires good knowledge of glacier cross section.

What are the conclusions of this lecture?

• ELA separates the accumulation and ablation zones;

• Accumulation occurs mainly by snow (precipitation, windblown, or avalanched) being compressed to ice;

• Ablation processes are complex and depend on glacier type;

• Calving is the main ablation process worldwide but surface melt is more important outside of antarctica;

• Measurements of mass balance indicate glacier health and can be used to understand sea-level contributions;

• Mass balance can be measured in six ways (directly, geodetic, hydrologically, climatologically, gravity, and mass budget).