Presentation on theme: "Glaciers Glaciers are piles of snow that flow under their own weight. They are capable of enormous erosion and deposition, mainly because of the near-lack."— Presentation transcript:
Glaciers Glaciers are piles of snow that flow under their own weight. They are capable of enormous erosion and deposition, mainly because of the near-lack of limits on transportation and their close association with freeze-thaw cycles that produce much physical weathering. Many Northern landmasses were sculpted by glaciers. v 0035 of 'Glaciers' by Greg Pouch at :56:03 LastSavedBeforeThis :48:10 13Glaciers.ppt
Glaciers Processes 3 Glacial Processes > Rheology 4 Glacial Processes > Formation 5 Glacial Processes > Flow 6 Causes of Glaciers Products Erosional 7 Erosion > Continental 8 Erosion > Alpine 9 Diagram of alpine erosional features 10 Paternoster and Rosary Depositional 11 Deposition > Materials 12 Deposition > Features 13 Continental Deposition Figures II 14 Meanwhile, back in the tropics…
Glacial Processes > Rheology A glacier starts moving when somewhere in the glacier, stresses exceed the plastic limit of snow at that P-T. The upper part of the glacier is elastic and brittle. Crevasses form in the upper part and do not continue into plastic part The lower part of the glacier deforms ductilely (plastically or viscously) in response to ice-top elevation. Glaciers are plastic or very viscous, and can transport enormous quantities of sediment, without regard for the size of the particles.
Glacial Processes > Formation Stacks of snow (glaciers) form where more snow falls than melts/evaporates. Glaciers are favored by abundant snowfall and low evaporation and melting rates, as is found at high elevations and in polar regions, especially near oceans. Most active glaciers are not terribly cold. Snow and Ice When snow falls, there are usually intricate crystals with lots of air (powder). During diagenesis and lithification, pressure solution of points and compaction result in coarse granular corn snow. More pressure solution and cementation result in firn. Finally, once most of the air is driven out and crystals starts getting bigger, it becomes glacial ice, which is a lot like schist. The region where more snow falls than melts is the zone of accumulation (covered by clean, white, snow every winter), and the region where more snow melts than falls is the zone of ablation (dirty snow where snow is melting, leaving mud). The line separating these two zones is called the snow line, and this shifts with the weather. Glacial ice is mainly compact grains, and often has rock particles entrained. It is a lot like snow-banks in a parking lot, especially after a few days of warm weather.
Glacial Processes > Flow Snow is like mashed potatoes: a small bit of it is very much a rigid solid, but in a big enough pile, it flows like a fluid or plastic. (By the way, rocks are also like mashed potatoes in this sense, except cold low- pressure rocks are elastic-brittle.) A glacier is like mashed potatoes being dumped over some area (analogy in trouble) and oozing out from there, with mice eating them. How far the potatoes get is determined by how fast the potatoes are coming in and how fast the mice eat them. If there are lots of mice, the outermost part of the potato-glacier recedes, even though the potatoes are moving away from the potato-source-region. Similarly, a glacier is a balance of snow accumulation and ablation. Even when the glacier's terminus is retreating, the ice itself is advancing from the zone of accumulation. The main force driving movement is height of the upper surface of the glacier. Ice flows from points of high ice-top to low ice-top, which might not be related to ice-bottom. Pressure solution as well as crystal deformation are significant in ice flow.
Causes of Glaciers In the recent past, there have been four major ice ages with warm inter-glacials between. This happens with surprising regularity. There have been at least three other glacial epochs further in the past we know of. (The odds of preservation seem low, so there could be others we don't know about.) Milankovitch cycles (variations in insolation due to perturbations in earth's tilt, ellipticity and timing of solstices relative to perihelion) cause observable variations in sedimentation through much of the rock column. They correspond well with ice ages, but don't explain the lack of glaciation during most of earth history Circum-polar seas Presently, there is a nearly land-locked polar ocean, and a circum-polar continent. Other glacial ages also seem to have had a land-locked polar ocean. This seems to be the other necessary condition. This is probably related to the efficiency of circulating heat on a planetary scale. (During times when there was easy circulation from poles to equator, temperatures seem to have been fairly even. Giants/Milankovitch/milankovit ch_3.html
Erosion > Continental Continental glaciation This refers to glaciers that cover big areas (ice caps) or huge areas (ice sheets) and are not confined by channels. They flow under their own weight with little regard to underlying topography. They do not leave spectacular scenery. –Polished surfaces are due to abrasion by fine glacier sediment –Striations are grooves cut by large, resistant chunks that are transported by glacier. –Chatter marks are crescent holes that look like a chip was knocked out with a hammer. –Removal of regolith and soil and transportation towards terminus. These erosional features are also common in alpine glaciation. Because glaciers eventually retreat, the erosional areas often have later depositional features, so erosional features with no depositional features are rarely seen. Continental glaciation erosion features are common in most of Canada, WI, MN, UP of MI…
Erosion > Alpine Alpine Glaciation refers to glaciers that form at high altitude in mountain chains. Under these conditions, the glaciers move in stream-like fashion down channels, widening and deepening them. Cirque: a semi-circular depression formed in the upper-most reaches of a glacial valley. It looks like an amphitheater, or the hole left by an ice-cream scoop. This is a basic features in alpine glacial terrain. Glacial Valley/U-shaped valley: a steep-walled, flat bottomed valley formed by glacial erosion. A truncated spur results where a ridge gets cut off by a valley glacier. Glaciers don't zigzag as easily as water, and tend to sort of bulldoze through hills rather than follow the path of least resistance. Arête: a sharp ridge separating two glacial valleys. The rock left after a bunch of cirques are removed Côl: a low pass in an arête. Two cirques intersecting through an arête. Horn: a sharp peak formed where cirques have cut away. Tarn: a rock-bottomed lake occupying a depression caused by glacial erosion. A tarn is found in a cirque. Paternoster lakes: a string of usually-circular lakes connected by streams in a glacial valley. Hanging valley: a valley whose elevation is higher than the main valley it empties into. (Glaciers flow according to ice-top elevation, not ice-bottom.) Fjord/fiord: a U-shaped valley that is now filled with seawater, occurs where a glacial valley has been submerged. Common in Norway and the UK and Cana da’s maritime provinces..
Paternoster and Rosary "Pater noster" is Latin for "Our Father". Medieval Swiss thought that paternoster lakes looked like the Our Father/Mystery beads on a rosary. Figures from
Deposition > Materials Materials –Till is unsorted debris deposited by a glacier, and is largely unstratified. The co-occurrence of pebbles and clay, and rather dense, massive clay, are indications of till. –Rock flour is the finely-pulverized material in till: rock flour often becomes wind-deposited loess. (High winds and vast exposed floodplains are common with glaciers: the ice melting varies seasonally and diurnally, and the temperature contrast causes high winds, so eolian re-working of glacial sediments is very common.) –Any rock that is moved far from its point of origin by a glacier is a glacial erratic These can be as big as buildings, or as small as pebbles, although the term is usually reserved for boulders and bigger. –Outwash is the coarse material resulting from re- working of till by meltwater. The fines are carried off by water or wind. Outwash is coarse, permeable, and stratified. The streams of meltwater are usually braided and often leave extensive outwash plains. –Because glacial streams and lakes are strongly influenced by weather and melting, they deposit sediments with strong annual variations in composition and size known as varves. Mommy Rock, Daddy Rock and Baby Rock at Yosemite
Deposition > Features Any deposit of till on, or left by, a glacier is a moraine. –Lateral moraines form at the sides of valley glaciers. When they merge, they become medial moraines alpine –An end moraine is a hill that results when the glacier’s advance is steady for several years. A terminal moraine is the one furthest out, and all the ones inside of that are recessional. –A ground moraine is a thin, extensive deposit, also known as a till plain. These have rolling topography, and most of the Midwest is till plain covered with loess. –Sometimes, a chunk of ice melts slowly and gets separated from the glacier and deposition occurs around it before it melts. The hole that is left behind is known as a kettle. Most ponds and small lakes in the Midwest are kettles Drumlins are hills that are steep in the up-ice direction, symmetrical left-ice to right-ice. They look soft of like blue-whales pointing northish. They are composed of till (the drumlins, not the whales) In-Glacier streams There are streams underneath, in, and on top of glaciers, mostly underneath. –The meandering, sandy channel deposits are known as eskers: this is basically a sandy ridge that meanders like a stream. –If there is depression in the top of a glacier, sediment deposits in it and the resulting hill left when the glacier melts is a kame.
Continental Deposition Figures II
Meanwhile, back in the tropics… Ice ages have profound geological effects, even far away from the glaciers. The climate is generally wetter and a bit cooler, and more even. Huge quantities of water are tied up in the ice, resulting in decreases in sea level (and changes in isotopic composition) and exposure of much continental shelf as dry land. Many continental shelves were exposed during the last ice age, and continental sediments are found offshore. An ice sheet weighs a lot, depressing the underlying plate. After the ice melts, the plate gradually rebounds. Many areas that are currently desert received much more rainfall, resulting in extensive lakes, such as Lake Bonneville in Utah (Great Salt Lake is the only remnant). Many rivers that are fairly minor now carried huge amounts of rainfall, and much water infiltrated into aquifers that now receive little recharge, such as Egypt’s Nubian Sandstone.
Glaciers Glaciers provide some of the most breathtaking (rugged erosional) and some the most uninteresting (flat and bumpy depositional) landscapes in the world. Many Northern landmasses were sculpted by glaciers. The recent glaciations locked up enough water to significantly affect sea-level, and so had major effects away from the glaciers. Glaciers generated a huge variety of landforms, both erosional and depositional. They are capable of enormous erosion and deposition, mainly because of the near-lack of limits on transportation and their close association with freeze-thaw cycles that produce much physical weathering. Glaciers have moved truly amazing amounts of material (100+m thick till in most of the Midwest) in a 'short' time From