Lecture 25

Glaciers and Ice Ages

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Lecture Notes

• Today about 10% of the continents are covered in ice; during the last ice age, which ended about 11,000 years ago, approx. 30% of the continents were covered in ice. It's been estimated that the New York City area was once under ~ 250 meters of ice. The retreat of glaciers was taken as evidence that the Earth's climate had changed.
• An ice crystal is considered a mineral (occurs naturally, is an inorganic solid, made of H2O, has a regular crystal structure).
• We can consider snow as sediment, and compacted snow that sticks together as a sedimentary rock. Following this reasoning, ice at the surface of a pond can be considered as an igneous rock. Glacier ice is a metamorphic rock. Under pressure preexisting ice recrystallizes to a solid state - molecules rearrange to form new crystals.
• Glaciers are sheets or streams of ice that last all year and flow under gravity.
• Two categories of glaciers: Mountain glaciers are in or close to mountainous regions. Continental glaciers that spread over large areas of the continental crust - i.e. on Greenland and Antarctica.
• For a glacier to form: 1) it must be cold; 2) an accumulation of snow; 3) snow on a slope that is gentle enough (< 30 deg) so snow doesn't avalanche and will not blow away.
• The transformation of snow to glacier ice is slow and requires layers of younger snow lying on and compacting the older snow. With increasing load the air is squeezed out of the snow. Points of the hexagonal snow crystals that are in contact melt (pressure solution, see ch 11). Snow transforms into firn, a packed granular material, with 25% air. Under pressure firn grains melt at contact points and ice transforms into a solid mass of blue ice with some air trapped in bubbles.
• Meltwater accumulated at the base of glaciers reduces friction between the ice and the rock beneath and helps the glacier move. Glaciers also flow internally by deforming plastically, or by ice crystals sliding past one another. This is the dominant style of movement for glaciers that do not have a wet bottom. Glaciers move downhill at rates from 10 to 300 m/year. Due to friction between the ice and rock, the center part of the glacier moves the faster than the sides; the top moves faster than the bottom.
• Ice in a glacier becomes plastic at a depth of about 60 meters. Large cracks, crevasses, only form above the brittle-ductile transition.
• Glaciers thicken with snowfall, shrink by ablation - removal of ice by submilination, or evaporation of ice to vapor, melting and calving. If snowfall exceeds loss, the glacier advances - the toe, or the leading edge, moves downslope; otherwise the glacier retreats. As a glacier retreats, the ice inside the glacier continues to flow downslope.
• Large pieces that calve off glaciers that have reached the ocean are called icebergs.
• Sea ice, e.g. that covering the Arctic Ocean, can also break and supply pieces of drifting ice. The Arctic ice has been disintegrating due to global warming.
• As glaciers move they erode the rock beneath. Large rocks carried along by glaciers can scrape the rock beneath and leave scars, called striations, that indicate the direction of flow. Rocks can also fall from cliffs surrounding the glacial valley and be incorporated into the glacier.
• Valleys eroded by rivers have a "V" shape. Valleys eroded by glaciers have a "U" shape. Fjords are submerged glacial valleys.
• Rocks that fall onto glaciers are carried along the surface. Lateral moraines are stripes of rock deposited along the sides of the glacier. Rock deposited at the toe of a glacier is end moraine; also known as glacial till. Rock can also be carrled beneath the glacier and deposited at the end (toe) by running water.
• Ice loading and glacial rebound: A large ice sheet (< 50 km in diameter) causes the land beneath to subside (glacial subsidence). The lithosphere is elastic, and can deform under this load. The asthenosphere is hot/ductile and flows out of the way. When the ice sheet melts, the crust rebounds. Glacial rebound is still occurring (e.g. in North America) in areas covered by ice > 10,000 years ago.
• More fresh water is held in glaciers than in any other reservoir (e.g. lakes, rivers). During the last ice age, much more water was held in glaciers and global sea level was ~ 100 meters lower than it is today. Coastlines migrated seaward, in some areas by > 100 km. With the glaciers melting worldwide, global sealevel continues to rise.
• Continental glaciers depress the lithosphere and can block rivers. In sum, glaciers can profoundly alter the drainage of a continent.
• Glaciations: studies of submarine sediments in the 1960's revealed changes in the assemblages of plankton. Different plankton species survive in cold water than in warm water. Because of this, it is possible to track changes in water temp. by examining what plankton species are preserved in the sediments. During the Pleistocene epoch, 20 to 30 glacial advances were inferred by cold-water plankton species. This work was refined by studies of oxygen isotopes in calcite (from plankton).
• Causes of ice ages
• Long-term causes: Continents have to drift near the poles for snow to accumulate; and must lie well above sea-level. Global ocean circulation, that can bring warm water from the equator to the poles, must be interfered with - by the continents blocking the currents. CO2 is a greenhouse gas, and traps heat. For an ice age to occur, CO2 content in the atmosphere must be low.
• Short-term causes: Milankovich cycles - The eccentricity of the Earth's orbit around the Sun gradually changes in a cycle taking about 100,000 years. The tilt of Earth's axis relative to the plane of the ecliptic varies on a time scale of 41,000 years between 22.5 deg and 24.5 deg. The Earth's spin axis also precessees on a time-scale of about 23,000 years. These three factors combine to alter the amount of insolation. At times of high insolation an ice age doesn't occur, or are less likely; at times of low insolation, an ice age is more likely. These factors don't tell the whole story as they cannot increase or decrease the global temperature by more than about 4 deg. Celcius. Inferred temperature changes are greater.
• Other factors are believed to be important as well - e.g. the Earth's albedo (or how much of the Sun's energy is reflected back into space). Also, thermohaline circulation in the oceans - if it is interrupted, this could trigger an ice age. Also, as we will see in the next lecture, the content of CO2 in the atmosphere is key. <\ul>