Glacier

Introduction

A glacier is a mass of ice larger than 247 acres (one tenth of a square kilometer) that starts on land, but can move out over coastal waters in polar regions where calving (breaking apart) produces icebergs. All glaciers move as the result of their weight and gravity. Glaciers require a specific cold climate to exist.

There are three types of glaciers: alpine glaciers in mountain valleys; piedmont glaciers near the base of mountain ranges where glacier masses have combined; and continental glaciers, which are called ice sheets when they are very large. Continental glaciers have covered vast areas of Earth at different times through history.

When glaciers advance they change the landscape. When they retreat, tell-tale signs are left behind. Paleoclimatologists studying the signs left from retreating ancient glaciers have discovered that massive glaciation occurred fairly regularly. Past glaciation cycles appear to be related to subtle changes in solar radiation. Scientists are actively looking for answers to how human activities that are already causing climate changes will influence Earth's glaciation cycles in the future.

Historical Background and Scientific Foundations

Glaciers start where more snow falls in the winter than melts in the summer. First, a snow field collects. Through successive freezing and thawing the snow becomes grain-sized ice called firn. Over time, the snow field becomes an ice field. When the icy mass gets large enough so it begins to move, it is called a glacier.

Alpine glaciers move down the mountain in channels, scouring the walls to form characteristic U-shaped valleys called cirques. A piedmont glacier forms where several glaciers combine at the foot of mountains. These are common in Alaska.

A glacier picks up surface soil and rocks and carries the load (called till) forward with the ice. When the glacier recedes, till is dropped. Till that is dropped in front of a receding glacier is known as a terminal moraine. The overall landscape left behind a retreating continental glacier is irregular. Glaciated landscape features include hollows that are collectively described as kettle drum topography. Boulders are scarred as they are carried for hundreds of miles or kilometers to new locations. They are called erratics because the rock type is not consistent with the area where the boulders are dropped.

In the mid-1800s, Swiss-born geologist Louis Agassiz (1807–1873) first suggested the concept of continental glaciers when he noticed evidence of glacial changes in the landscapes over large areas of Canada that resembled the glaciated alpine landscapes of Switzerland. Subsequently geologists determined there was not one but a series of periods in ancient history where large glaciers covered up to 32% of the total land area of Earth.

Evidence that there have been eight Ice Age cycles over the past 750,000 years has been collected from deposits left behind on land, and sea floor sediments. Evidence also comes from ice core samples taken from Greenland and Antarctica where continental glaciers that date back to the Ice Ages still cover most of the land area.

Impacts and Issues

Each period of vast continental glaciation was separated by a warmer interglacial period that lasted about 10,000 years. Serbian engineer Milutin Milankovitch (1879–1958) developed a mathematical explanation for the Ice Ages based on variations in solar radiation. The Milankovitch theory is in close agreement with 20,000, 40,000, and 100,000 year cycles of glaciation. The most recent Ice Age began about 70,000 years ago, peaked about 20,000 years ago and ended about 10,000 years ago. Since inter-glacial periods were generally around 10,000 years, Earth could be approaching another Ice Age.

There is a big question about Earth's future that was not considered by Milankovitch. Where climate was controlled by natural cyclical variations in solar radiation in the past 750,000 years, it may not be controlled only by natural causes in the future. Human activities have changed the climate through greenhouse gases emitted from the widespread use of fossil fuels. Carbon dioxide and methane levels are higher in the atmosphere in the twenty-first century than they have been in any previous interglacial period.

WORDS TO KNOW

CALVING: Process of iceberg formation when huge chunks of ice break free from glaciers, ice shelves, or ice sheets due to stress, pressure, or the forces of waves and tides.

GLACIATION: The formation, movement, and recession of glaciers or ice sheets.

ICE AGE: Period of glacial advance.

KETTLE DRUM: Usually termed a kettle: depression or sinkhole left in a mass of rocky debris by a retreating glacier. A large, isolated chunk of ice buried in debris melts, causing a collapse of the overlying debris.

MORAINE: Mass of boulders, stones, and other rock debris carried along and deposited by a glacier.

PALEOCLIMATOLOGY: The study of past climates, throughout geological history, and the causes of the variations among.

PIEDMONT: A region of foothills along the base of a mountain range.

Glaciers along the coast of Greenland are melting at record rates as a result of global warming. Since glaciers contain 70% of Earth's freshwater, when they melt the freshwater changes the dynamics of the ocean water around them. Melting glaciers also cause the sea level to rise. Global warming is adding to sea levels rising as the oceans get warmer and expand. Where glaciers need a very specific cold climate to form, there is no easy answer to the question of when the next Ice Age will start.

See Also Glaciation; Glacier Retreat; Ice Ages.

BIBLIOGRAPHY

Books

Lurie, Edward. Louis Agassiz:A Life in Science. John Hopkins University Press, Baltimore, MD: 1996.

Weart, Spencer R. The Discovery of Global Warming. Harvard University Press, Cambridge, MA: 2004.

Web Sites

“All About Glaciers.” National Snow and Ice Data Center, 2007. < http://www.nsidc.org/glaciers> (accessed August 16, 2006).

“Atmospheric Changes.” U.S. Environmental Protection Agency, 2007. < http://www.epa.gov/climatechange/science/recentac.html> (accessed August 16, 2007).

“Past Climate Cycles: Ice Age Speculations.” American Institute of Physics, 2007. < http://www.aip.org/climate/cycles.htm> accessed August 16, 2007).

“The Physical Environment: Glacial Systems.” University of Wisconsin Stevens Point, 2007. < http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook> (accessed August 16, 2007).