Glaciers are slowly moving masses of ice that have accumulated on land in areas where more snowfalls during a year than melts. Snow falls as hexagonal crystals, but once on the ground, snow is soon transformed into a compacted mass of smaller, rounded grains. As the air space around them is lessened by compaction and melting, the grains become denser. With further melting, refreezing, and increased weight from newer snowfall above, the snow reaches a granular recrystallized stage intermediate between flakes and ice known as firn. With additional time, pressure, and refrozen meltwater from above, the small firn granules become larger, interlocked crystals of blue glacial ice. When the ice is thick enough, usually over 30 meters, the weight of the snow and firn will cause the ice crystals toward the bottom to become plastic and to flow outward or downward from the area of snow accumulation.

Glaciers are open systems, with snow as the system’s input and meltwater as the system's main output. The glacial system is governed by two basic climatic variables: precipitation and temperature. For a glacier to grow or maintain its mass, there must be sufficient snowfall to match or exceed the annual loss through melting, evaporation, and calving, which occurs when the glacier loses solid chunks as icebergs to the sea or to large lakes. If summer temperatures are high for too long, then all the snowfall from the previous winter will melt. Surplus snowfall is essential for a glacier to develop. A surplus allows snow to accumulate and for the pressure of snow accumulated over the years to transform buried snow into glacial ice with a depth great enough for the ice to flow. Glaciers are sometimes classified by temperature as faster-flowing temperate glaciers or as slower-flowing polar glaciers.

Glaciers are part of Earth’s hydrologic cycle and are second only to the oceans in the total amount of water contained. About 2 percent of Earth’s water is currently frozen as ice. Two percent may be a deceiving figure, however, since over 80 percent of the world’s freshwater is locked up as ice in glaciers, with the majority of it in Antarctica. The total amount of ice is even more awesome if we estimate the water released upon the hypothetical melting of the world’s glaciers. Sea level would rise about 60 meters. This would change the geography of the planet considerably. In contrast, should another ice age occur, sea level would drop drastically. During the last ice age, sea level dropped about 120 meters.

When snowfalls on high mountains or in polar regions, it may become part of the glacial system. Unlike rain, which returns rapidly to the sea or atmosphere, the snow that becomes part of a glacier is involved in a much more slowly cycling system. Here water may be stored in ice form for hundreds or even hundreds of thousands of years before being released again into the liquid water system as meltwater. In the meantime, however, this ice is not static. Glaciers move slowly across the land with tremendous energy, carving into even the hardest rock formations and thereby reshaping the landscape as they engulf, push, drag, and finally deposit rock debris in places far from its original location. As a result, glaciers create a great variety of landforms that remain long after the surface is released from its icy covering.

Throughout most of Earth’s history, glaciers did not exist, but at the present time about 10 percent of Earth’s land surface is covered by glaciers. Present-day glaciers are found in Antarctica, in Greenland, and at high elevations on all the continents except Australia. In the recent past, from about 2.4 million to about 10,000 years ago, nearly a third of Earth’s land area was periodically covered by ice thousands of meters thick. In the much more distant past, other ice ages have occurred.

冰川就是缓慢移动的巨大冰块，这种冰块是由于每年降雪量大于融雪量不断积累形成于陆地的。雪花降落时是六角晶体，可一旦落在地面，雪花就迅速凝结成大量小而圆的颗粒。由于凝结和融化这些颗粒周围空气空间也随之减少，从而颗粒就会变得更为紧密。雪继续融化、再结冰，并且还要承受上方新的积雪增加的重量，待这些积雪达到一种介于冰片与冰之间的阶段，颗粒物便会再次形成晶状体，这一阶段被称作积雪过程。时间、压力不断增加，并且位于上方的融雪重新结冰，那些较小的积雪颗粒开始变大且与透明的蓝色冰层连结。当这些冰块足够厚（一般是30米以上），积雪的重量就会使底部的冰晶变得具有可塑性，会从有积雪的地方流入或者流出。

冰川是开放的系统，降雪是该系统的补给水源，融雪是该系统的主要输出水源。冰川系统受两个基本气候变量控制：降水和气温。要保持或增加冰川的体积，就必须具备足够的降雪量，以抵消或者超过每年因融雪、蒸发或者以海洋和湖泊中的冰山形式的裂冰的数量。如果夏季温度持续长时间的高温度，上一个冬季所有的降雪都会融化。剩余降雪对形成冰川非常重要。有剩余的积雪就能够积累，并且由于多年积雪形成的压力，将积雪转化为流动冰，深度足以保证冰川流动。按照温度分类，冰川可分为快速流动温带冰川和慢速流动极地冰川。

冰川是地球水循环的一部分，水容量仅次于排名第一的海洋。目前地球上大约有2%的水源处于冰冻状态。2%这个数字可能并不属实，因为全球有超过80%的淡水以冰块的形态存在于冰川中，其中大部分处于南极洲。如果我们估算出全球冰川融化后所释放的水量，储水量必定让人叹为观止。海平面将会上升60米左右。这会显著的改变地球的地理属性。相反，如果另一个冰期到来，海平面会迅速降低。在上一个冰期，海平面下降了120米左右。

当雪降落在高山或者极地地区，便成为冰川系统的一部分。这和降水不同，降水可以迅速回到海洋或者大气中，但降雪要成为冰川的一部分，循环过程非常缓慢。在这里，水会以冰的形态存在几百或上千年，直到作为融水释放进入流水系统。然而，冰也并非完全静止。在巨大的能量的作用下
，冰川在陆地上缓慢移动，甚至切碎最坚硬的岩石，将其吞没、推动、拉拽，最后在离原位置很远的地方沉淀下这些岩石的残余物，在这个过程中它重新改造了地形地貌。因此，冰川创造出了各种各样的地形，冰层脱离表面之后，这些地形能够长时间保持不变。

纵观地球历史，大部分时期内并不存在冰川，但现在10%的地表为冰川覆盖。目前，在南极洲、格陵兰岛，以及除了澳洲以外大陆的高海拔地区都有冰川存在。不久以前，在24万年至1万年前，大约三分之一的地表被上千米厚的冰层定期覆盖。在更遥远的过去，其他冰期也曾出现过冰川覆盖地表的情况。