14.5: Ice Age Glaciations

A glaciation (or ice age) occurs when the Earth’s climate becomes cold enough that continental ice sheets expand, covering large areas of land. Four major, well-documented glaciations have occurred in Earth’s history: one during the Archean-early Proterozoic Eon, ~2.5 billion years ago; another in late Proterozoic Eon, ~700 million years ago; another in the Pennsylvanian, 323 to 300 million years ago, and most recently during the Pliocene-Pleistocene epochs starting 2.5 million years ago. Some scientists also recognize a minor glaciation around 440 million years ago in Africa.

The best-studied glaciation is, of course, the most recent. This infographic illustrates the glacial and climate changes over the last 20,000 years, ending with those caused by human actions since the Industrial Revolution. The Pliocene-Pleistocene glaciation was a series of several glacial cycles, possibly 18 in total. Antarctic ice-core records exhibit especially strong evidence for eight glacial advances occurring within the last 420,000 years [16]. The last of these is known in popular media as “The Ice Age,” but geologists refer to it as the Last Glacial Maximum. The glacial advance reached its maximum between 26,500 and 19,000 years ago [10; 17].

Former ice sheets, present during the Last Glacial Maximum event (or the last ice age) in North America, are called the Laurentide Ice Sheet.

Figure \(\PageIndex{1}\): Maximum extent of Laurentide Ice Sheet.

Causes of Glaciations

Glaciations occur due to both long-term and short-term factors. In the geologic sense, long-term means a scale of tens to hundreds of millions of years and short-term means a scale of hundreds to thousands of years.

Long-term causes include plate tectonics breaking up the supercontinents, moving land masses to high latitudes near the north or south poles, and changing ocean circulation. For example, the closing of the Panama Strait and isolation of the Pacific and Atlantic Oceans may have triggered a change in precipitation cycles, which combined with a cooling climate to help expand the ice sheets.

Figure \(\PageIndex{2}\): Oxygen isotope measurements as a proxy for temperature over the past 65 million years. Lowest levels indicate glaciations.

Short-term causes of glacial fluctuations are attributed to the cycles in the Earth’s rotational axis and to variations in the Earth’s orbit around the Sun which affect the distance between Earth and the Sun. Called Milankovitch Cycles, these cycles affect the amount of incoming solar radiation, causing short-term cycles of warming and cooling. During the Cenozoic Era, carbon dioxide levels steadily decreased from a maximum in the Paleocene, causing the climate to gradually cool [18]. By the Pliocene, ice sheets began to form. The effects of the Milankovitch Cycles created short-term cycles of warming and cooling within the larger glaciation event.

Milankovitch Cycles are three orbital changes named after the Serbian astronomer Milutan Milankovitch. The three orbital changes are called precession, obliquity, and eccentricity. Precession is the wobbling of Earth’s axis with a period of about 21,000 years; obliquity is changes in the angle of Earth’s axis with a period of about 41,000 years; and eccentricity is variations in the Earth’s orbit around the Sun leading to changes in distance from the Sun with a period of 93,000 years. [19]. These orbital changes created a 41,000-year-long glacial–interglacial Milankovitch Cycle from 2.5 to 1.0 million years ago, followed by another longer cycle of about 100,000 years from 1.0 million years ago to today (see Milankovitch Cycles).

This video summarizes ice ages: their characteristics and causes.

Sea-Level Change and Isostatic Rebound

When glaciers melt and retreat, two things happen: water runs off into the ocean causing sea levels to rise worldwide, and the land, released from its heavy covering of ice, rises due to isostatic rebound. For example, since the last glacial maximum about 19,000 years ago [17] sea-level has risen about 400 feet (125 meters) [20]. A global change in sea level is called eustatic sea-level change. During a warming trend, sea-level rises due to more water being added to the ocean and also thermal expansion of sea water. About half of the Earth’s eustatic sea-level rise during the last century has been the result of glaciers melting and about half due to thermal expansion [21; 22]. According to basic physics, thermal expansion occurs when a solid, liquid, or gas expands in volume when the temperature increases. Watch the 30-second video demonstrating thermal expansion with the classic brass ball and ring experiment.

Relative sea-level change includes vertical movement of both eustatic sea level and continents on tectonic plates. In other words, sea-level change is measured relative to land elevation. For example, if the land rises a lot and sea level rises only a little, then the relative sea level would appear to drop.

Continents sitting on the lithosphere can move vertically upward as a result of two main processes, tectonic uplift and isostatic rebound. Tectonic uplift occurs when tectonic plates collide. Isostatic rebound describes the upward movement of lithospheric crust sitting on top of the asthenospheric layer below it. Continental crust bearing the weight of continental ice sinks into the asthenosphere, displacing it. After the ice sheet melts away, the asthenosphere flows back in and continental crust floats back upward. Erosion can also create isostatic rebound by removing large masses like mountains and transporting the sediment away (think of the Mesozoic removal of the Alleghanian Mountains and the uplift of the Appalachian plateau), albeit this process occurs more slowly than relatively rapid glacier melting.

The isostatic rebound map below shows rates of vertical crustal movement worldwide. The highest rebound rate is indicated by the blue-to-purple zones (top end of the scale). The orange-to-red zones (bottom end of the scale) surrounding the high-rebound zones indicate isostatic lowering as adjustments in displaced subcrustal material have taken place.

Figure \(\PageIndex{3}\): Rates of isostatic rebound worldwide, greatest in regions of recent glaciation.

Most glacial isostatic rebound is occurring where continental ice sheets rapidly melted about 19,000 years ago, such as in Canada and Scandinavia. Its effects can be seen wherever Ice Age ice or water bodies are or were present on continental surfaces and in terraces on river floodplains that cross these areas. Isostatic rebound occurred in Utah when the water from Lake Bonneville drained away [23]. North America’s Great Lakes also exhibit emergent coastline features caused by isostatic rebound since the continental ice sheet retreated.

References

10. Laabs, B. J. C. et al. Chronology of latest Pleistocene mountain glaciation in the western Wasatch Mountains, Utah, U.S.A. Quat. Res. 76, 272–284 (2011).

16. Augustin, L. et al. Eight glacial cycles from an Antarctic ice core. Nature 429, 623–628 (2004).

17. Clark, P. U. et al. The Last Glacial Maximum. Science 325, 710–714 (2009).

18. McInerney, F. A. & Wing, S. L. The Paleocene-Eocene Thermal Maximum: A Perturbation of Carbon Cycle, Climate, and Biosphere with Implications for the Future. Annu. Rev. Earth Planet. Sci. 39, 489–516 (2011).

19. Hays, J. D., Imbrie, J. & Shackleton, N. J. Variations in the Earth’s Orbit: Pacemaker of the Ice Ages. Science 194, 1121–1132 (1976).

20. Fleming, K. et al. Refining the eustatic sea-level curve since the Last Glacial Maximum using far- and intermediate-field sites. Earth Planet. Sci. Lett. 163, 327–342 (1998).

21. Church, J. A. et al. Changes in sea level. , in: J.T Houghton, Y. Ding, D.J. Griggs, M. Noguer, P.J. Van der Linden, X. Dai, K. Maskell, and C.A. Johnson (eds.): Climate Change 2001: The Scientific Basis: Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel 639–694 (2001).

22. Wigley, T. M. L. & Raper, S. C. B. Thermal expansion of sea water associated with global warming. Nature 330, 127–131 (1987).

23.Max D Crittenden. New Data on the Isostatic Deformation of Lake Bonneville. GEOLOGICAL SURVEY PROFESSIONAL PAPER 454-E (1963).