Continental glaciers and Ice ages
Land forms resulting from continental glaciers: Continental scale glaciation creates interesting opportunities for ice to interact with large volumes of sediment. Results include:
- Drumlin: Unlike roche moutonee, these are hills made of reshaped glacial till.
- The steep end is on the side of the ice's approach
- The shallow end points in the direction of ice movement
- Kame: Hills of stratified drift that form when a stream deposits sediment in a hole in the glacial ice.
- Esker: Long sinuous ridge of stratified drift. Results from sediment deposited an under-glacier stream.
- Kettle lake: This is essentially the opposite of a kame. When a block of glacial ice is stranded by a retreating glacier, it prevents sediments from being deposited. When if finally melts, a depression is left that fills with water. Think of Minnesota - the land of 1000 lakes. these lakes are mostly kettles formed during the retreat of a continental ice sheet.
Cretaceous was a greenhouse world. From the Eocene onward, there has been some evidence of valley glaciation, at least in parts of Antarctica. It seems that the switch from greenhouse to ice house conditions occurred during that interval. During last 1 - 2 million years, the situation has become extreme, with major continental glaciations alternating with interglacials. The interval from 2 mya to 10,000 years ago is called the Pleistocene Epoch.
Pleistocene glacial sequences: During mid- 19th century, Louis Agassiz first recognized widespread glacial deposits in Europe, and proposed former "ice ages" By turn of century, geologists had identified four major glacial intervals, which were punctuated by interglacials. These periods were pieced together by careful mapping a study of glacial landforms.
The Oxygen isotope record: During the late 20th century, a new technique allowed us to refine this sequence.
Condition 18,000 years ago during the last glacial maximum:
- The method:
- There are several stable (i.e. non-radioactive) isotopes of oxygen. The most importatn are 16O and 18O.
- When ocean water evaporates, molecules containing the lighter isotope of oxygen, 16O, are more likely to "take off."
- Normally, this wouldn't cause any permanent change in the oceans' isotopic chemistry because the molecules would return tot he ocean soon enough as rain. During an ice age, however, the water is locked up in glacial ice, so the oceans become isotopically heavy.
- Now the magic: Critters like foraminiferans are constantly sampling the oceans' oxygen by building it into their CaCO3 shells. When they die and fall to the bottom, they create a record of oceanic oxygen isotopes during their lives.
- We can reconstruct the ocean's isotopic history by looking at the ratio of oxygen isotopes present in foraminiferan shells deposited at different times. That ratio, in turn, tells us how much water was locked up as continental ice.
- The result:
Apparently there were closer to thirty distinct glacials and interglacials.
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Causes of the ice ages:
Ice ages seem to be driven by three basic considerations: Milankovitch cycles, continental configurations, and positive feedback.
The Ice-Age is still with us:
- Milankovitch cycles: In the 1920s, the Yugoslavian meteorologist Milutin Milankovitch realized The Earth's movement through space is subject to three kinds of cycles:
Each cycle influences the severity of winters to some degree. When their coldest intervals line up, the Earth is susceptible to much colder winters. This, in itself, is not enough to make an ice age.
- Orbital eccentricity: The orbit around the Sun is an ellipse that changes shape (becoming more and less circular) in a cycle of 100,000 years.
- Axial inclination: The axis of rotation is tilted. The angle of tilt varies from 21.5 deg. to 24.5 deg. in a cycle of 41,000.
- Axial precession: The axis of rotation wobbles around an axis like that of a toy top. So, today the axis points toward Polaris, the north star, but in earlier times, it didn't. One full precessional wobble takes 23,000 years.
- Continental configuration has two important effects:
This has been true during:
- If snow falls in the ocean, it melts, even if it is cold. Ice ages happen when there are large continental masses near the poles that can catch the falling snow.
- The modern pattern of ocean circulation erquires continents that form barriers to the free flow of waters near the equator. During the Eocene, tropical water could flow around the Earth without leaving the tropics. The development of the Isthmus of Panama and the collision of Africa with Eurasia forces water to circulate near the poles.
- The Cenozoic, when the northern continents are close to the North Pole and Antarctica is at the South Pole.
- The Late Paleozoic, when the southern half of Pangea lay over the South Pole.
- Positive feedback: The real key is that the factors that cause ice ages form a positive feedback loop.
- When Milankovich cycles line up, the Earth has cold winters, causing increased snowfall.
- When snow begins to accumulate on continents, it reflects more sunlight back into space, reducing the Earth's temperature further.
- As it gets even colder, less fallen snow melts over the summers and continental glaciers spread, reflecting more sunlight and further lowering the temperature.
In this and many other lectures, we discuss various natural processes and patterns as if the Earth were at equilibrium. The fact is that it isn't. We can list ways in which equilibration is still occurring:
The future: Had the industrial revolution not occurred, this would still be a complex topic. What is known is that:
Now for something cool.
- The warmest period of the Holocene Epoch (the last 10,000 years) occurred roughly 6,000 years ago. At this point Earth had its highest recent sea levels. Since then, Earth has cooled and sea levels dropped slightly. Extrapolating, we would think we were headed toward a new ice age in maybe 100,000 years.
- But the industrial revolution has returned to the atmosphere CO2 that was locked up over millions of years during the late Paleozoic. The predictable result is an increased greenhouse effect. Indeed, over the last century: