Historical Geology
INTRODUCTION
PLEISTOCENE STRATIGRAPHY
Terrestrial Stratigraphy
Deep-Sea Stratigraphy
ONSET OF THE ICE AGE
Climates of the Pleistocene and Holocene
Glaciers—How Do They Form?
GLACIATION AND ITS EFFECTS
Glacial Landforms
Changes in Sea Level
¬PERSPECTIVE Waterton Lakes National Park, Alberta, and
Glacier National
Park, Montana
Glaciers and Isostasy
Pluvial and Proglacial Lakes
WHAT CAUSED PLEISTOCENE GLACIATION?
The Milankovitch Theory
Short-Term Climatic Changes
GLACIERS TODAY
PLEISTOCENE MINERAL RESOURCES
The following content objectives are presented in Chapter 17:
¬ The Pleistocene and the Holocene or Recent epochs encompass only the most recent 1.8 million years of Earth history.
¬ The Pleistocene Epoch, lasting from 1.8 million years to 10,000 years ago, is best known for widespread glaciers but was also a time of continuing orogeny and volcanism.
¬ Much of our information about Pleistocene climates comes from oxygen isotope ratios, pollen analyses, and the distribution and coiling directions of planktonic foraminifera.
¬ Pleistocene continental glaciers were present on the Northern Hemisphere continents as well as Antarctica, and thousands of small glaciers were in mountain valleys on all continents.
¬ Sea level fell and rose during the several Pleistocene advances and retreats of glaciers, depending on how much water from the ocean was frozen on land.
¬ The tremendous weight of continental glaciers caused EarthÕs crust to subside into the mantle and to rise again when the glaciers wasted away.
¬ Many now-arid regions far from glaciated areas supported large lakes as a result of greater precipitation and lower evaporation rates during the Pleistocene, and numerous other lakes formed along the margins of glaciers.
¬ A current widely accepted theory explaining the onset of ice ages relies on irregularities in EarthÕs rotation and orbit.
¬ Important Pleistocene mineral resources include sand and gravel, diatomite, peat, and placer deposits of gold.
To exhibit
mastery of this chapter, students should be able to demonstrate comprehension
of the following:
¬ plate tectonicsÕ role in shaping the paleogeography and climate of the Pleistocene and Holocene epochs
¬ the methods used to determine Pleistocene climatic changes
¬ characteristics and recognition of the glacial and interglacial stages in North America
¬ the methods used to determine oceanic temperature changes and difficulties in correlation of marine and terrestrial deposits for the Pleistocene Epoch
¬ effects of glaciation upon Earth's surface
¬ the North American Great LakesÕ geologic history
¬ the proposed hypotheses for the causes of glacial and interglacial periods
1. The most recent part of geologic time is the Pleistocene (1.8 million to 10,000 years ago) and the Holocene or Recent epochs (10,000 years ago to the present).
When Louis Agassiz first proposed his hypothesis of glaciation, he encountered geologists who could not conceive of glacial processes having once operated in an area where ice was no longer present. One geologist who ascribed to the concept of diluvium—or the sedimentation processes caused by an ancient, Noachian deluge—was William Buckland, the first professor of geology at Oxford. However, Buckland kept an open mind, and after Agassiz showed him erratics, moraines, striations, and roches moutonnˇes, he changed his beliefs from the diluvial theory to support AgassizÕs glacial theory. It was reported that, at an 1840 meeting of the Geological Society of London, Buckland admitted that he set out from Neufchatel to discount the glacial theory, but had instead become converted through AgassizÕs evidence. It was more often the case that the early hypotheses were met with rejection, and only after considerable debate were they accepted. Brock, ŅThe Rev. William Buckland, the First Palaeoecologist,Ó Biologist, 1993 v.40 n.4 p.149-152.
2. Although
the Pleistoceone is best known for widespread glaciers, it was also a time of
volcanism and tectonism.
Figure 17.1 Continental
Glaciers, Ice Caps, and Valley Glaciers
Figure
17.2 Pleistocene
Uplift and Tectonism
Figure
17.3 Pleistocene
and Recent Volcanism
3. Pleistocene glaciers covered about 30% of the land surface, and were widespread on the Northern Hemisphere continents.
Figure 17.4 Centers
of Ice Accumulation and Maximum Extent of Pleistocene
Glaciers
4. At least four intervals of extensive Pleistocene glaciation took place in North America, each separated by interglacial stages. Fossils and oxygen isotope data indicate about 20 warm-cold cycles occurred during the Pleistocene.
Figure 17.5 Pleistocene
Glaciers in North America
Figure
17.6 Climatic
Changes During the Cenozoic
Figure
17.7 Pollen
Analysis and Climate
5. Areas far beyond the ice were affected by Pleistocene glaciers: Climate belts were compressed toward the equator, large pluvial lakes existed in what are now arid regions, and sea level was as much as 130 m lower than now.
Figure 17.8 The
Greenland Ice Sheet
Figure
17.9 Erosion by
Continental and Valley Glaciers Yields Distinctive
Landscapes
6. Moraines, striations, outwash, and various other glacial landforms are found throughout Canada, in the northern tier of states, and in many mountain ranges where valley glaciers are present.
Figure 17.10 Origin of End Moraines and Outwash
Figure 17.11 Terminal and Recessional Moraines in the Mid-Continent Region
Figure 17.12 The Geologic Evolution of Cape Cod, Massachusetts, and Nearby
Areas During the Ice Age
Figure 17.13 The Bering Land Bridge
7. The tremendous weight of Pleistocene glaciers caused isostatic subsidence of EarthÕs crust. When the glaciers melted, isostatic rebound began and continues even now in some areas.
Figure 17.14 Glaciers and Isostasy
8. During times of ice accumulation, large
pluvial lakes existed in now arid areas because of greater precipitation and
overall cooler temperatures, which lowered the evaporation rate. In contrast to
pluvial lakes, which formed far from the areas of glaciation, proglacial lakes
form where meltwater accumulates along a glacierÕs margin. The Great Lakes
formed when advancing glaciers deeply eroded the area. As the glaciers melted
and retreated, the lakes formed with water.
Figure 17.15 Pleistocene Lakes in the Western United States
Figure 17.16 Glacial Lake Missoula was a Proglacial Lake, Whereas Lake
Bonneville was a Pluvial Lake
Figure 17.17 Four Stages in the Evolution of the Great Lakes
9. Major glacial intervals separated by tens or hundreds of millions of years probably stem from changing positions of plates, which in turn profoundly affect oceanic and atmospheric circulation patterns.
10. According to the Milankovitch theory, minor changes in EarthÕs rotation and orbit bring about climatic changes that produce glacial-interglacial intervals.
Figure 17.18 According to the Milankovitch Theory, Minor Irregularities in EarthÕs Rotation and Orbit May Affect Climatic Changes
11. The causes of short-term climatic changes such as occurred during the Little Ice Age are unknown; two proposed causes are variations in the amount of solar energy and volcanism.
Figure 17.19 Glaciers in the Cascade Range
For many years, researchers have questioned whether humans will have a major impact on climate change. It has long been proposed that rising carbon dioxide levels will cause global warming, and these increased CO2 levels are attributed to the Industrial Revolution and the burning of fossil fuels. However, some scientists offered rebuttals about the impact of humans on CO2 levels, as well as the role that humans play in CO2 production. (ŅThe Holocene CO2 Rise: Antropogenic or Natural?Ó Eos, Jan. 2006, v.87 n.3 p.27.) Several researchers (Royer, Berner, Montanez, Tabor, and Beerling) reviewed geologic records of CO2 and glaciations and demonstrated that CO2 was low (<500 ppm) during periods of continental glaciation, and higher (>1000 ppm) during warmer periods. (ŅCO2 as a Primary Driver of Phanerozoic Climate,Ó GSA Today, March 2004 v.14 n.3 p.4-10) It is predicted by some geologists that twice the CO2 level will exist in the atmosphere by 2100. (ŅNo Easy Way Out of the Greenhouse,Ó Discover, Oct. 2005, v.26 n.10 p.30-31) If the global warming predictions are correct, then humans could possible suffer adverse health effects, including increase in asthma and respiratory ailments, and heat-wave related deaths. (Perkins, ŅDead Heat: The Health Consequences of Global Warming Could Be Many,Ó Science News, July 3, 2004, v.166 n.1 p.10-12)
Enrichment Topic 3. Could Global Warming
Start a Little Ice Age in Europe?
Oddly, global warming could bring about a new ice age in Europe. Europe is warmer than it should be for its latitude. Warm water from the equator travels up North America, and then the Gulf Stream moves eastward across the Atlantic, then south down the European coast. The heat stored in these waters warms Europe. Global ocean circulation is driven by the sinking of cold, dense water in the north Atlantic. If air temperature warms so much that sea surface temperatures do not cool the North Atlantic waters sufficiently, they will remain on the surface instead of sinking. This will shut down ocean circulation, including the Gulf Stream. USA Today (Magazine), June 2002 v.130 n.2685 p.8(2).
Enrichment Topic 4. Slight Changes in
Climate May Trigger Abrupt Ecosystem Responses
The U.S. Geological Survey assessed the implications that a warming Earth would have on North American ecological thresholds. An ecological threshold is the point at which large changes occur that are potentially irreversible. Some of the implications adversely affect humans, including forest dieback, wildfire, and insect infestations. Humans can also increase an ecosystemÕs potential for reaching the threshold, including water use in an area that is affected by drought. Although not enough is known about ecological thresholds, researchers stated that ecosystems will differ significantly. The Environmental Protection Agency, Synthesis and Assessment Product 4.2: Thesholds of Climate Change in Ecosystems, http://climatescience.gov
Forest dieback has also been intensely studied. The death rates of trees in US western forests have doubled over the past 20-30 years. The U.S. Geological Survey attributed the die-back to higher temperatures as well as water scarcity. The scientists warned that if the current rate of tree mortality continues, or accelerates, the U.S. forests Ņcould shift from being a carbon sink that takes greenhouse gases out of the atmosphere to becoming a net emitter of carbon dioxide.Ó Eilperin, ŅStudy Ties Tree Deaths to Change in Climate,Ó The New York Times, January 23, 2009
12. Pleistocene mineral resources include sand and gravel, placer deposits of gold, and some evaporite minerals such as borax.
Global
Warming
1. Discuss the current controversy surrounding global warming. Why has it taken scientists so long to support the idea of a human cause of a global temperature increase? Why is it so difficult for scientists to model climate?
2. Discuss the inherent problems when geologists try to decipher human impact on a climate, when the rock record and other geological data are measured on a much larger geological time scale. Can scientists effectively measure short-term trends—such as those that may exist from the start of the Industrial Revolution?
3. The controversy surrounding global warming has been debated on a national and
political level. Why would some businessmen not want to acknowledge global warming? Who might profit if global warming is a reality? Which countries produce the most CO2? Do these countries have the greatest population? Is it morally acceptable for a few individuals to engage in activities that will influence the entire Earth? This topic offers many opportunities for the integration of cultural, political, social, and moral views.
4. Investigate how different governments respond to the concept of global warming. Do third world countries embrace the concept? Do the superpowers embrace the concept? How have various US administrations responded to requests to voluntarily reduce the amount of CO2 emissions?
Plate Tectonics Continues
1. While the breakup of Pangaea was a central part of the EarthÕs history during the Mesozoic, it is also important to emphasize to students that the process is continuing in the Cenozoic. Stress that the Laramide orogeny continued into the Cenozoic Era, and the mountain-building events that began in the western United States continue today on the western side of the craton. Have students compare the amount of time involved in mountain building in the western United States versus the amount of time involved in mountain building in the Appalachian orogenies.
2. Why is it important that tectonic processes continue on Earth? Have students hypothesize what would happen if tectonic processes on Earth ceased.
3. What will the future hold for the EarthÕs continents if present-day tectonics continues? Discuss the possible arrangement of EarthÕs continents in 1 million years, 10 million years, and 100 million years. Note how difficult it is to predict tectonic arrangements in the geologic future.
1. Consider the global impact of a new ice age. What would happen to seaports? Which major cities of the world would be plowed over by ice? How would agricultural and desert land shift? How would worldwide distribution of vegetation change?
2. Consider the global impact of continued increased global temperatures. What would happen to seaports? Which major cities of the world would be submerged? How would worldwide distribution of vegetation change? How would agricultural lands be affected? How would animal species that are restricted to certain environments cope? What are the consequences for humans?
3. How can each one of us make a difference to combat human influence on climate?
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cirque |
interglacial stage |
outwash |
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continental glacier |
isostasy |
pluvial lake |
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end moraine |
isostatic rebound |
pollen analysis |
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glacial stage |
Little Ice Age |
proglacial lake |
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glacier |
Milankovitch theory |
U-shaped glacial trough |
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ice cap |
moraine |
valley glacier |
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ice-scoured plain |
neoglaciation |
varve |
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11. Minor irregularities in EarthÕs rotation and orbit are sufficient to alter the amount of solar radiation that EarthÕs receives at any given latitude and can affect climate changes. When variations in EarthÕs orbit and axial tilt combine to decrease net solar radiation, a glacial onset occurs. Orbital eccentricity, the tilt of the EarthÕs axis, and the precession of the equinoxes all affect the amount of radiation reaching the Earth.
12. Continental glaciers result in subdued topography, little or no soil, striated and polished bedrock exposures and poor surface drainage, characteristic of an ice-scoured plain. Valley glaciers produce distinctive features such as cirques and broad U-shaped glacial troughs.
13. The Little Ice Age was a cooling trend, with cooler summers, wetter winters, ice cap growth, and more extensive and persistent sea ice, which occurred between 1300 and about 1850. Shortened growing seasons caused famine in some northern latitudes.
In Europe and Iceland, glaciers reached their greatest historic extent by the early 1800s, and glaciers in the western United States, Alaska, and Canada also expanded. Migrations of many Europeans to the New World resulted.
14. A pluvial lake exists during times of glaciation because of the greater precipitation and overall cooler temperatures, especially during the summer, which results in lower evaporation rate. Pluvial lakes form far from an area of glaciation. An example of a pluvial lake was Glacial Lake Missoula.. Proglacial lakes form close to glaciers, where meltwater accumulates along a glacierÕs margin. An example of a proglacial lake was Lake Bonneville.
15. Any moraine deposited at
a glacierÕs terminus is an end moraine.
As a glacier advances, it pushes material ahead of it. At its farthest
point, it deposits its terminal moraine. As it melts back, it dumps material,
with recessional moraines marking the places where it was stationary for a
while. Therefore a terminal moraine would mark the farthest point that the
glacier traveled, and is also the place where outwash forms in meltwater
streams. Recessional moraines would be closer to the glacierÕs origin.
16. Isostatic rebound occurs when melting of large quantities of ice or erosion reduces
the load on top of the EarthÕs crust. Then, the crust slowly rises or rebounds. In Scandinavia, some coastal cities have rebounded enough so that docks built only
a few centuries ago are now far inland from the shore. In Canada, as much as 100 meters of isostatic rebound has taken place during the last 6000 years. When the Great Lakes formed, ice sheets covered the land to the north. As the ice sheets melted, the freed land experienced isostatic rebound.
17. Lake Missoula formed in Montana when an advancing glacier dammed a valley, causing water to fill in behind it. When the ice dam failed, the water rushed out at tremendous velocity and drained south across Idaho and into Washington. In eastern Washington, the floodwaters stripped the soil and carved huge fluvial features. This is called the scablands. This history repeated itself several times.
18. Oxygen isotope ratios (O18 to O16) from deep sea cores show the abundance of glacial ice, because seawater has a higher O18-to-O16 ratio. This is because the lighter isotope, O16, is more easily evaporated, and during times of glaciation, is tied up in glacial ice. The declining percentage of O16 and the rise of O18 is preserved in the shells of planktonic foraminifera, and reveal how the surface water temperatures were fluctuating. Pollen analysis is used to determine past environments. When paleontologists identify the type of plant the pollen came from, they can look at the floral composition of the area and make climatic inferences.
19. The Cascade Range runs through California, Oregon, Washington, and British Columbia. In California, basalt lava flows and cinder cones are present. The volcanoes are still active, as demonstrated by the eruption of Mt. St. Helens in 1980, and recent eruptions of Lassen Peak. (See Chapter 16.) However, there is not a continuous subduction zone along the western margin. The western margin includes transform faults as well, and volcanic activity is not common along transform boundaries.
20. High-resolution stratigraphy is not possible with these deposits. Minor fluctuations in ice sheet position go unrecorded or unnoticed because glacial tillites are generally unfossiliferous, unstratified, and difficult to correlate from one locality to another. Also, the land record is incomplete, as melting and advancing ice sheets may erase the record of a prior glacial advance.
Apply Your Knowledge
1. It appears that the glacier
has a negative budget, or it is experiencing more loss in volume than
gains. The glacierÕs terminus will
retreat with a negative budget.
Also, the glacier can be moving faster because more meltwater is present
that percolates downward and facilitates basal slip. This would result in the
debris moving several hundred meters toward the glacierÕs terminus.
2. From Figure 17.15 in the textbook, we can see that the western area of Nevada was large covered by Pleistocene lakes. The flat surface, and fine sediments (mud) would fit in with this scenario. The benches can be explained as terraces, or different elevations that represent an extended period when the Pleistocene lake stood at that elevation.
3. Pleistocene glaciers deposited moraines as they retreated. Although these sediments were initially deposited by ice, they were modified by wind-generated waves and longshore currents. The curve of Cap Cod resembles a recurved spit, modified by longshore currents that moved from the south to the north.
4. The base surface exhibits signs of glaciation. The linear scratches and polish were probably created by glacial ice moving over the area. The nonstratified mixture of mud, sand, and gravel indicate deposition by a glacier, since—unlike running water—glacial ice does not sort. The large gravel—up to 2 m in diameter—also provides evidence that glacial ice is responsible for the transportation of these sediments. Neither wind nor ice is powerful enough to move such large sediments. The well sorted cross-bedded sand indicates a warmer environment, in which deposition occurred by water. Depending on the size of the cross beds, the sediments may have been deposited by wind, or fluvial processes. The horizontal layers of conglomerate also indicate deposition by water. The clasts are rounded (it is a conglomerate as opposed to a breccia), and the clasts are large, eliminating deposition by wind. These sediments probably represent an interglacial paleoenvironment.
Finally the light and dark clay layers are probably varves, representing an annual deposit within a lake during a glacial event. The boulders within the varves are probably drop stones, released from melting ice. Therefore, the area exhibits signs of glaciation, followed by an interglacial period, and then further glaciation.
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