A glacier is a large, long-lasting river of ice that is formed on land and moves in response to gravity and undergoes internal deformation. Glacier ice is the largest reservoir of fresh water on Earth, mainly in Antarctica and Greenland, and second only to oceans as the largest reservoir of total water. Glaciers can be found on every continent, including on the greater Australian continent. Glaciers are permanent bodies of ice and compacted snow that have become dense and heavy enough to flow under their own weight.

Geologic features created by glaciers include end, lateral, ground and medial moraines that form from glacially transported rocks and debris; U-shaped valleys and corries (cirques) at their heads, and the glacier fringe, which is the area where the glacier has recently melted into water.

There are two main types of glaciers: alpine glaciers, which are found in mountain terrains, and continental glaciers, which cover large areas of continents. Most of the concepts in this article apply equally to alpine glaciers and continental glaciers.

A temperate glacier is at the melting point throughout the year from the surface to the base of the glacier. The ice of Polar glaciers is always below the freezing point with most mass loss due to sublimation. "Poly-thermal" or "sub-polar", they have a seasonal zone of melting near the surface and have some internal drainage, but little to no basal melt.

Thermal classifications of surface conditions vary so glacier zones are often used to identify melt conditions. The dry snow zone is a region where no melt occurs, even in the summer. The percolation zone is an area with some surface melt, and meltwater percolating into the snowpack, often this zone is marked by refrozen ice lenses, glands, and layers. The wet snow zone is the region where all of the snow deposited since the end of the previous summer has been raised to 0 °C. The superimposed ice zone is a zone where meltwater refreezes at a cold layer in the glacier forming a continuous mass of ice.

The smallest alpine glaciers form in mountain valleys and are referred to as valley glaciers. Larger glaciers can cover an entire mountain, mountain chain or even a volcano; this type is known as an ice cap. Ice caps feed outlet glaciers, tongues of ice that extend into valleys below, far from the margins of those larger ice masses. Outlet glaciers are formed by the movement of ice from a polar ice cap, or an ice cap from mountainous regions, to the sea.

The largest glaciers are continental ice sheets, enormous masses of ice that are not affected by the landscape and extend over the entire surface, except on the margins, where they are thinnest. Antarctica and Greenland are the only places where continental ice sheets currently exist. These regions contain vast quantities of fresh water. The volume of ice is so large that if the Greenland ice sheet melted, it would cause sea levels to rise some six meters all around the world. If the Antarctic ice sheet melted, sea levels would rise up to 65 meters.

Plateau glaciers resemble ice sheets, but on a smaller scale. They cover some plateaus and high-altitude areas. This type of glacier appears in many places, especially in Iceland and some of the large islands in the Arctic Ocean, and throughout the northern Pacific Cordillera from southern British Columbia to western Alaska.

Tidewater glaciers are glaciers that flow into the sea. As the ice reaches the sea pieces break off, or calve, forming icebergs. Most tidewater glaciers calve above sea level, which often results in a tremendous splash as the iceberg strikes the water. If the water is deep, glaciers can calve underwater, causing the iceberg to suddenly explode up out of the water. The Hubbard Glacier is the longest tidewater glacier in Alaska and has a calving face over ten kilometers long. Yakutat Bay and Glacier Bay are both popular with cruise ship passengers because of the huge glaciers descending to them.
 

• All About Glaciers - Links to glacier research and projects, glacier facts, questions and answers, a glossary, gallery, bibliography and links to glacier information on the Web.

• Centre for Glaciology, University of Wales, Aberystwyth - Research focus is on glaciology and glacial geology. Staff expertise and research areas include relation between glaciers and climate and interactions between glaciers and the marine environment.

• The Crevasse Zone - Glaciological data collected on the Juneau Icefield, Alaska. Focuses on GPS-derived surface velocity, surface elevations, and strain rates.

• Glacier Image Database - A collection of photographs illustrating glaciation and its effects upon the earth's surface.

• Glacier Research Group, VAW Glaciology Section, ETH Zurich - Glacier and permafrost research in the Swiss Alps, Greenland Ice Sheet (Jakobshavn ice stream), and in Spitsbergen.

• Glaciers - Shockwave video demonstrates different types of glaciers, how they form and flow.

• Glaciology, Alfred Wegener Institute - Topics include introduction to glaciology; studies of ice sheet volume and glacier mass balance of glaciers by means of modelling, geophysical reconnaissance and field studies; and reconstruction of climate history from ice core analysis.

• Glaciology at the University of Copenhagen - Climate research through ice core drillings (including logistics and instrumental development), analysis of chemical and physical parameters and modelling. Other topics: Studies of ice rheology and studies of extraterrestrial ice.

• INQUA Commission on Glaciation - The goal of the Commission on Glaciation is to explore and develop scientific cooperation for the study of past glaciation and present glacier processes.

• Institute for Arctic and Alpine Research Research Glaciology Group, University of Colorado - Research and reference information on ice-flow dynamics and calving, sub-glacial and englacial hydrology, permafrost, glaciers and sea-level change, climate change and glacier mass balance, heat exchange within snow packs, and related topics.

• Michael Studinger's Homepage - Geophysical and glaciological research on the Antarctic continent with aerogeophysical measurements.

• Park Geology Tour - Glaciers - Links to U.S. National Parks featuring alpine glaciers, glacial landforms, and Ice Age flood landforms.

• Peter Knight's Glaciers - Information, photos, and links on glaciers, glacial environments and glacial geomorphology. Also additional resources for users of the book "Glaciers".

• Sang De Brabander's Homepage - Describes glaciological research of Antarctic ice sheets and ice streams by numerical modelling: focus on basal processes, including subglacial hydrology and till deformation. Also: field expeditions to Central Asian glaciers.

• Satellite Images: Glaciers - US Geological Survey Satellite Image Atlas of Glaciers of the World.

• Scott Polar Research Institute - University of Cambridge - World Data Center - Directory of European Glaciology

• Stockholm University Glaciology - Describes research activities, the Tarfala Research Station, and glaciers the group has studied; lists group members; and provides photos, data, and publications.

• USGS Benchmark Glaciers - Long-term program to monitor climate, stream runoff, and the motion, mass balance, and geometry of glaciers at a few sites in Alaska and Washington State. Data are used to understand glacier-related hydrologic processes and improve prediction of water resources, glacier-related hazards, and the consequences of climate change.

• World Glacier Monitoring Service (WGMS) - Source of information on glaciers and their fluctuations worldwide. World Glacier Inventory contains data on the spatial distribution of glaciers. Fluctuations of Glaciers and Mass Balance Bulletin document changes over time (changes in mass, volume, area and length of glaciers).