A glaciation (or ice age) occurs when the Earth’s climate is cold enough that large ice sheets grow on continents. There have been four major, well-documented glaciations in Earth’s history: one during the Archean-early Proterozoic (~2.5 billion years ago), another in late Proterozoic (~700 million years ago), another in the Pennsylvanian (323 to 300 million years ago), and the most recent Pliocene-Quaternary glaciation (Chapter 15). A minor glaciation that occurred around 440 million years ago in modern-day Africa is also mentioned by some authors. The best-studied glaciation is, of course, the most recent. The Pliocene-Quaternary glaciation is a series of many glacial cycles, possibly 18, during the last 2.5 million years. There is especially strong evidence for eight glacial advances within the last 420,000 years as recorded in the Antarctic ice core record . The last of these, known in popular media as “The Ice Age” but known by geologists as the last glacial maximum, reached its height between 26,500 and 19,000 years ago [10; 17]. Follow this link to an infographic that illustrates the glacial and climate changes over the last 20,000 years ending with the human influences since the Industrial Revolution.
Causes of Glaciations
Why do glaciations occur? The causes include both long-term and short-term factors. In the geologic sense, long-term means a scale of 10’s to 100’s of millions of years and short-term means a 100 to 200,000-year scale. Ideas about long-term causes of glaciations over geologic time include the positioning of continents near poles by plate tectonics and the Wilson Cycle and changes in ocean circulation due to re-positioning of the continents such as the closing of the Panama Strait. Short-term factors are more recognizable for the most recent Pliocene-Quaternary Glaciation and are most relevant to today’s anthropogenic climate change, but may have taken place in the earlier glaciations.
Short-term causes of glacial fluctuations are attributed to cycles in the rotational axis of the Earth and in Earth-Sun relations due to variations in the earth’s orbit called Milankovitch Cycles. These cycles affect the amount of incoming solar radiation, and changes in carbon dioxide in the atmosphere. During the Cenozoic, carbon dioxide levels steadily decreased from a maximum in the Paleocene causing a gradual climatic cooling . As the climate cooled, the effects of the Milankovitch Cycles began to influence climate with regular cycles of warming and cooling. Milankovitch Cycles are three orbital changes named after the Serbian astronomer Milutin Milankovitch. The three orbital changes are the wobbling of Earth’s axis called precession with a span of 21,000 years, the angle of Earth’s axis called obliquity with a span of about 41,000 years, and variations of the distance from the sun in Earth’s orbit around the sun referred to as eccentricity with a span of 93,000 years . These orbital changes created a glacial-interglacial cycle of 41,000 years from 2.5 to 1.0 million years ago and a longer cycle of ~100,000 years from 1.0 million years ago to today (for detail, see this chart). The combination of these three Milankovitch Cycles changes the angle at which the sun’s energy strikes the surface of the earth near the poles and the amount of energy (insolation) received by Earth (for detail, see this chart). As the climate cooled during the Cenozoic Era, the subtle changes in energy received by the planet were expressed as a warmer and cooler climate cycle, thus the glacial-interglacial cycles.
This chart illustrates the effect of the Milankovitch Cycles.
This video summarizes ice ages: their characteristics and causes.
Sea-Level Change and Isostatic Rebound
Since glaciers are ice located on land (not floating in the ocean), when glaciers melt and retreat two things happen, sea-level rises globally and the land rises locally due to isostatic rebound. Melting glacial water runs off into the ocean and sea-level worldwide will rise. For example, since the last glacial maximum about 19,000 years ago  sea-level has risen about 400 feet (125 meters) . An overall global change in sea level is called eustatic sea-level change. More water in the ocean causes a eustatic sea-level rise. Another important factor causing eustatic sea-level rise is thermal expansion. According to basic physics, thermal expansion occurs when a solid, liquid, or gas expands in volume when the temperature increases. Watch this 30-second video demonstrating thermal expansion with the classic brass ball and ring experiment. About half of the eustatic sea-level rise during the last century has been the result of thermal expansion, the rest from the melting of glaciers [21; 22].
However, tectonics and isostatic rebound can move the land up and down. The change of sea level as it relates to a more local continental landscape is called relative sea-level change. The relative sea-level change includes both the vertical movement of the eustatic sea-level and the vertical movement of land, so that the sea-level change is measured relative to the land. Therefore, if the land rises a lot and sea-level rises only a little, then the sea-level would appear to go down.
The lithosphere can move vertically as a result of two main processes, tectonics, and isostatic rebound. Tectonic uplift occurs when tectonic plates collide as discussed in the Plate Tectonics chapter. Isostasy describes the equilibrium that exists for the earth’s lithosphere, where denser lithosphere “sinks” lower on top of the asthenosphere and less dense lithosphere “floats” higher on the asthenosphere. Isostatic rebound is when some weight is removed from the continental lithosphere causing it to “float” higher on the asthenosphere. Erosion can remove this weight very slowly or the relatively rapid removal of glaciers can remove a lot of weight in a short amount of time. Melting glaciers removes weight from the continental lithosphere causing it to rise or “rebound” from being previously depressed. Most glacial isostatic rebound is occurring where glaciers recently melted (19,000 years ago) such as Canada and Scandinavia. Glacial isostatic rebound causes the relative sea-level to fall or rise less quickly as seen from the land. Isostatic rebound also occurred in Utah when the water from Lake Bonneville was removed . Isostatic rebound is still taking place wherever Ice Age ice or water bodies were present on continental surfaces. Its effects can be seen in terraces forming on river floodplains that cross these areas.
This map shows the rates of vertical crustal movement worldwide. Note that the greatest upward movement occurs in regions affected by recent glaciation, isostatic rebound. Also, note that crustal depression has also occurred in adjacent regions as subcrustal material displaced by isostatic lowering from the weight of the ice has flowed back in under the rebound.
Rates of isostatic rebound worldwide, greatest in regions of recent glaciation.
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).
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).