Types of Glaciers
Many types of glaciers exist, with fairly loose or imprecise definitions. An ice sheet is a continent-scale mass of ice that spreads in all directions. An ice cap or ice dome is a smaller version of an ice sheet, sitting on a mountain top or high plateau, and also spreading in all directions (or at least in several directions). For pretty glaciers flowing down from the mountains, different people may use different terms: mountain glacier (it flows down a mountain), valley glacier (it flows down a valley on the side of a mountain), and plain old glacier. An outlet glacier drains an ice sheet or ice cap between rock walls, and an ice stream is a fast-moving “jet” of ice within an ice sheet or ice cap flowing between slower-moving regions of ice. But if an ice sheet is drained by a fast flow with ice on one side and rock on the other, is that fast flow in an ice stream or an outlet glacier? Classifications such as this help us talk about things, but are not precise.
Flowing Solids and Hot Ice
Dr. Alley has spent months of his life living on the great ice sheets of Greenland and Antarctica. (And Dr. Anandakrishnan, who has worked so hard on this course, has spent a lot more time on the ice sheets than Dr. Alley has!) Eating and sleeping and working at -30º, it is hard to think of ice as being a hot material, but that is exactly what it is.
Recall that heat is the vibration of atoms or molecules in a material, and that in most solids including ice, the atoms or molecules are arranged in regular, repeating patterns. Melting of ice occurs when the typical molecule vibrates fast and hard enough to break free from the bonds that tie it to its neighbors and escape from that regular arrangement. When a material is almost hot enough to melt, the atoms are vibrating almost hard enough to break free from their neighbors and move around, so it is relatively easy with a little extra push to move a few molecules at a time past their neighbors. The gravitational stresses caused by the surface slope of a glacier supply that little extra push, and the ice deforms (primarily by dislocation glide, for those of you with materials-science backgrounds). When a material is not nearly hot enough to melt, the molecules are not even close to vibrating hard enough to break free from their neighbors, a whole lot of extra push is required to move molecules, and moving even a few molecules at a time is very difficult. The material then deforms elastically, or it breaks, but it does not creep and deform permanently in the way that a glacier does.
Most people measure temperature on a scale that gives “nice” numbers (something between 0 and 100) for typical daytime temperatures, so that talking about the temperature is easy for us. But, there are other temperature scales that make more sense in physics. If you slow the vibrations of molecules by cooling them, you can imagine that there must be some temperature at which vibration stops because all the heat has been removed. We call that temperature “absolute zero” or just zero in an absolute temperature scale. (Yes, in a quantum world, the Heisenberg uncertainty principle means that the last tiny bit of vibration can’t really be removed, but absolute zero comes darn close, so live with it.) If we set the zero on our temperature scale to this “absolute zero,” and then use degrees that have the same size as in the commonly used Celsius or Centigrade scale, we get the Kelvin scale. Ice melts at 273ºK and water boils at 373ºK; there are 100 degrees between melting and boiling in Kelvin, just as in Celsius. (The Rankine scale uses Fahrenheit-sized degrees and absolute zero as zero, with ice melting at 460ºR and water boiling at 640ºR, but almost nobody uses Rankine any more, so you are welcome to forget you ever heard about it. Or, you can practice sniffing disdainfully.)
As a general rule, little or no permanent deformation (creep) occurs when the temperature (in Kelvin or Rankine!) is less than about half the melting temperature, and creep occurs rather easily when temperatures exceed about three-quarters of the melting temperature. The coldest mean-annual temperature on Earth today is about eight-tenths of the melting temperature of ice (that is 217ºK, which is also -56ºC or -69ºF, in case you still like old-fashioned thermometers). Most ice is as close to melting as is red-hot or even white-hot iron being worked by a blacksmith. This is why glaciers usually flow rather than breaking—although breaking is still possible where deformation is very fast and where the pressure is very low, producing crevasses. So, the next time you are tempted to “pull down your pants and slide on the ice,” remember that ice is a “hot” material, even if you may not look very hot when you’re through. (We recommend that you don't "pull down your pants and slide on the ice," for many reasons related to public decency, avoidance of frostbite, and not sliding over a cliff or falling into a lake.)
As noted in the chapter, glaciers that are frozen to their beds don’t erode much, but if the basal ice is at the melting point, glaciers can erode very rapidly. Such thawed-bed glaciers have three ways to erode: plucking, abrasion, and subglacial streams.
Ice is an unusual material—higher pressure lowers its melting point rather than raising it (because ice becomes smaller when it melts; the tinker-toy-structure of ice has much open space, and squeezing ice tends to force it to become denser water, whereas most materials contract as they freeze so that higher pressure favors the solid). If a glacier is sliding across a bump in a bed, ice will tend to melt on the upglacier side of the bump where the pressure is higher. The meltwater will flow around the bump to the downglacier side, where the lower pressure will allow the water to refreeze. The heat given up by the refreezing will be conducted back through the bump, to allow more melting. But, you may remember that melting and freezing can open cracks in a rock. So, a glacier sliding over its bed can work rocks loose, and then freeze those rocks onto its base, in a process known as plucking. (When water spreads over the bed of a glacier in the spring as melting on the surface starts to feed water downward, the friction with rock that holds the ice back becomes concentrated on smaller regions of the bed not lubricated by the water, and this stress concentration breaks rocks, causing most plucking.)
Once glacier ice contains rocks at the bottom, it is like sandpaper—it drags those rocks over other rocks, scratching and polishing and knocking loose smaller rocks. This process is called abrasion. If you examine rocks on the walls of Yosemite, many still retain a polished appearance with parallel scratches or striations, showing where abrasion was active. Bumps are polished on one side—the upglacier side—but may be rough and jagged on the downglacier side where rocks were plucked off of them.
Melting of glaciers can produce a lot of water. The toe of a fast-melting glacier may supply more water to streams than does a similar-sized region in the rainiest place on Earth. The glacier acts to collect snowfall from a big area and take the snow to melt in a much smaller area, and trees and grass do not grow on glaciers to use the melt but they do grow on ground to use rainfall. Glacier melt usually flows down holes in the glacier, called moulins, that often form at the bottoms of crevasses. (Some brave or foolhardy people like to go caving in moulins after they drain during the winter.) The moulins eventually reach the glacier bed, where they feed large, steep, fast-moving streams. These erode in the same ways as streams outside of glaciers. Glaciers with much melt water usually cause erosion to be faster than in nonglaciated regions. Fluctuations in water pressure, as moulins fill with water during daytime melting and drain as melting slows at night, contribute to cracking rocks for plucking.
When Is the Next Ice Age?
In the text, we noted that the history of ice ages generally has involved 90,000 years of cooling, followed by 10,000 years of warming, then repeat. The rate of cooling initially is slow, and some people prefer to refer to 10,000 years of warmth followed by cooling. The northern hemisphere has been in the not-much-change/slight-cooling phase for almost 10,000 years already, and you might expect that we are ready to drop into the next ice age. Some people have suggested that humans have already headed off that ice age, or that global warming is a good thing because it will head off the ice age.
The 100,000-year pacing of a 90,000-year/10,000-year world is linked to interaction of the different orbital cycles, but the 100,000-year cycle in the out-of-roundness of the orbit is important. The orbit goes from nearly round to more squashed and back in about 100,000 years. But, there exists a slower modulation that takes about 400,000 years. The orbit goes nearly round, a little squashed, nearly round, more squashed, nearly round, even more squashed, nearly round, not as squashed, nearly round, barely squashed, repeat, with the nearly-rounds spaced 100,000 years apart. We are in the barely-squashed part now, and the last time that the orbit was in the barely-squashed mode, the warm time of the ice-age cycle lasted 30,000 years rather than 10,000 years. Climate models have confirmed that this should be our natural future; another 20,000 years of warmth (or maybe 40,000 years) before the next ice age starts. However, human burning of fossil fuels may extend the warmth beyond the next 20,000-40,000 years.
Also, note that the 19,000-year cycle noted in the text is an oversimplification. There is instead a “quasi” periodicity ranging from 19,000 to 23,000 years, as we mentioned briefly, and this was calculated by Milankovitch and is observed in the data collected to test Milankovitch's calculations, beautifully confirming his predictions.
Central Pennsylvania and Glaciation
During at least one old glaciation (probably over 1 million years ago), ice dammed the West Branch of the Susquehanna River and formed a lake in the Lock Haven area of Pennsylvania. If that lake filled to the next lowest bedrock outlet (into the Juniata River along the Bald Eagle Valley at Dix), then the water would have lapped at the steps of Old Main on Penn State’s University Park campus. There is no evidence of such a large lake, and before the lake filled all the way, it probably drained through failure of the ice dam, but we’re not sure. With ice so close, however, the State College area was cold during the ice ages.
Isotopic Ratios of Dead-Bug Shells
In the main text, you learned how the changes in ice volume control the isotopic composition of water in the ocean, and how we can reconstruct the ice-age cycle from the history of shell isotopic compositions in a sediment core because the shells record the water isotopic composition. As usual, things are a bit more complicated than that. Shell isotopic composition also is affected by temperature. Bigger ice gives heavier isotopic ratios in shells, and colder temperatures also give heavier isotopic ratios in shells. (At high temperature, both heavy and light atoms have plenty of energy to get up and go; at low temperatures, the heavy ones tend to get stuck in shells while the light ones can jump out.) Because both colder and bigger ice favor isotopically heavier shells, it is hard to tell how much of the signal in a shell is from temperature or from ice volume.
One way around this is to go to a place that is really cold today; the water was above freezing during the ice age (there were shells living in it…), so there the signal must be primarily one of ice volume. Other approaches include finding additional paleo-thermometers, such as estimating the temperature from the species living in a place and leaving their shells, or using changes in other “contaminant” ratios in shells that depend on temperature. Yet another way is that there is water in spaces in mud, and the water in some sediments is from the ice age, so just measure the isotopic composition of that water.
The result of this is that isotopic ratios did change because there was much more ice during the ice age than today, and because most places were much colder during the ice age than today.