R.M. Clary, Ph.D., F.G.S..

Department of Geosciences

MississippiStateUniversity

CHAPTER 17

CENOZOIC GEOLOGIC HISTORY:

THE PLEISTOCENE AND HOLOCENE EPOCHS

OUTLINE

INTRODUCTION

PLEISTOCENE AND HOLOCENE TECTONISM AND VOLCANISM

Tectonism

Volcanism

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 ProglacialLakes

WHAT CAUSED PLEISTOCENE GLACIATION?

The Milankovitch Theory

Short-Term Climatic Changes

GLACIERS TODAY

PLEISTOCENE MINERAL RESOURCES

SUMMARY

CHAPTER OBJECTIVES

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 Earth’s 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 Earth’s rotation and orbit.

Important Pleistocene mineral resources include sand and gravel, diatomite, peat, and placer deposits of gold.

LEARNING OBJECTIVES

To exhibit mastery of this chapter, students should be able to demonstrate comprehension of the following:

plate tectonics’ 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 Lakes’ geologic history

the proposed hypotheses for the causes of glacial and interglacial periods

CHAPTER SUMMARY

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).

Enrichment Topic 1. Louis Agassiz and the History of Geology

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 moutonnées, he changed his beliefs from the diluvial theory to support Agassiz’s 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 Agassiz’s evidence. It was more often the case that the early hypotheses were met with rejection, and only after considerable debate were they accepted. Brock, “The Rev. William Buckland, the First Palaeoecologist,” 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 17.1Continental Glaciers, Ice Caps, and Valley Glaciers

Figure 17.2Pleistocene Uplift and Tectonism

Figure 17.3Pleistocene and Recent Volcanism

3.Pleistocene glaciers covered about 30% of the land surface, and were widespread on the Northern Hemisphere continents.

Figure 17.4Centers 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 17.5Pleistocene Glaciers in North America

Figure 17.6Climatic Changes During the Cenozoic

Figure 17.7Pollen 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 17.8The Greenland Ice Sheet

Figure 17.9Erosion 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 17.10Origin of End Moraines and Outwash

Figure 17.11Terminal and Recessional Moraines in the Mid-Continent Region

Figure 17.12The Geologic Evolution of Cape Cod, Massachusetts, and Nearby

Areas During the Ice Age

Figure 17.13The Bering Land Bridge

7.The tremendous weight of Pleistocene glaciers caused isostatic subsidence of Earth’s crust. When the glaciers melted, isostatic rebound began and continues even now in some areas.

Figure 17.14Glaciers 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 glacier’s margin. The Great Lakes formed when advancing glaciers deeply eroded the area. As the glaciers melted and retreated, the lakes formed with water.

Figure 17.15PleistoceneLakes in the Western United States

Figure 17.16Glacial LakeMissoula was a ProglacialLake, Whereas Lake

Bonneville was a PluvialLake

Figure 17.17Four 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 Earth’s rotation and orbit bring about climatic changes that produce glacial-interglacial intervals.

Figure 17.18According to the Milankovitch Theory, Minor Irregularities in Earth’s 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 17.19Glaciers in the Cascade Range

Enrichment Topic 2. The CO2 Culprit

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. (“The Holocene CO2 Rise: Antropogenic or Natural?” 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. (“CO2 as a Primary Driver of Phanerozoic Climate,” 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. (“No Easy Way Out of the Greenhouse,” 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, “Dead Heat: The Health Consequences of Global Warming Could Be Many,” 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 ecosystem’s 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,

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 “could shift from being a carbon sink that takes greenhouse gases out of the atmosphere to becoming a net emitter of carbon dioxide.”Eilperin, “Study Ties Tree Deaths to Change in Climate,” 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.

LECTURE SUGGESTIONS

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 USadministrations 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 Earth’s 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.
  1. Why is it important that tectonic processes continue on Earth? Have students hypothesize what would happen if tectonic processes on Earth ceased.
  1. What will the future hold for the Earth’s continents if present-day tectonics continues? Discuss the possible arrangement of Earth’s 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.
CONSIDER THIS
  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?
  1. 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?
  1. How can each one of us make a difference to combat human influence on climate?
IMPORTANT TERMS
cirque / interglacial stage / outwash
continental glacier / isostasy / pluvial lake
end moraine / isostatic rebound / pollen analysis
glacial stage / Little Ice Age / proglacial lake
glacier / Milankovitch theory / U-shaped glacial trough
ice cap / moraine / valley glacier
ice-scoured plain / neoglaciation / varve

SUGGESTED MEDIA

Videos

  1. Glaciers, Earth Revealed #23, Annenberg/CPB
  2. Living Glaciers, Educational Images, Ltd.
  3. Signs of Ancient Ice Sheets, Educational Images, Ltd.
  4. A Visit to St. Helens, Educational Images, Ltd.
  5. A Geology Field Trip through the Appalachians of Pennsylvania, Educational Images, Ltd.
  6. The Ice World, Discovery Channel
  7. The Living Edens, Yellowstone: American’s Sacred Wilderness, PBS Home Video
  8. Mystery of the Megaflood, WGBH Boston
  9. Supervolcano, The Discovery Channel
  10. Treasured Places in Peril: Global Warming Impacts of the Southeast, Films for the Humanities and Sciences
  11. Can We Save Planet Earth, Open University
  12. The Truth About Climate Change, Open University
  13. Eruption of Mount St. Helens, IMAX
  14. Fire Below Us: Remembering Mount St. Helens, National Geographic
  15. Glaciers and Icecaps: The Melting, Ambrose Video Publishing
  16. Secrets of Glacier National Park, Discovery Communications, Inc.
  17. Glaciers, Ambrose Video Publishing
  18. Global Warning?, The History Channel
  19. Journey to 10,000 B.C., The History Channel
  20. Secret Yosemite, National Geographic
  21. Secret Yellowstone, National Geographic
  22. Ring of Fire, IMAX
  23. Global Warming, NOVA
  24. Earth in the Time of Climate Change, Classroom Encounters

Slides and Software

  1. Ice & Its Effects on the European Landscape, slide set, Educational Images
  2. Signs of Ancient Ice Sheets, Educational Images, Ltd.
  3. Explore Climate Change CD-ROM, Geological Society of America

CHAPTER 17 – ANSWERS TO QUESTIONS IN TEXT

Multiple Choice Review Questions
1.b / 5.a / 9.d
2.d / 6.c / 10.a
3.a / 7.c
4.e / 8.b
Short Answer Essay Review Questions

11. Minor irregularities in Earth’s rotation and orbit are sufficient to alter the amount of solar radiation that Earth’s receives at any given latitude and can affect climate changes. When variations in Earth’s orbit and axial tilt combine to decrease net solar radiation, a glacial onset occurs. Orbital eccentricity, the tilt of the Earth’s 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 glacier’s margin. An example of a proglacial lake was LakeBonneville.

15. Any moraine deposited at a glacier’s 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 glacier’s origin.

16. Isostatic rebound occurs when melting of large quantities of ice or erosion reduces

the load on top of the Earth’s crust. Then, the crust slowly rises or rebounds.InScandinavia, 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.