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Glacier Basics

Compiled from two web sites:

    What is a glacier?

    A glacier is a perennial mass of snow or ice that is large enough and heavy enough to flow, like a very thick fluid. Glaciers form wherever more snow accumulates than is lost each year. As new snow accumulates, it buries and compresses the old snow. Under the weight of the overlying snow, the old snow is transformed from a fluffy mass of ice crystals into dense, hard ice. This process occurs on the upper part of a glacier, at higher altitudes, where more snow accumulates than is lost each year. This is called the "accumulation zone", and is typically covered with snow year-round. The glacier is in constant motion, and the ice in the accumulation zone flows down to lower altitudes, which is called the "ablation zone". The ablation zone is located in the lower part of the the glacier where more snow is lost than accumulates. In late summer, when the seasonal snow has melted, the bare ice of the ablation zone is exposed.

    Glacier cross section

    Figure 1 - Cross section of an alpine glacier showing snow being converted into glacier ice (the left side of the figure) and the two major zones of a glacier's surface. The red arrows show the direction and relative speed of different parts of the glacier. The longer the arrow, the faster ice is moving.

    Types of glaciers

    Valley glaciers are commonly found in Washington and Alaska. These are glaciers are confined to a valley (usually a former stream valley). The smallest glaciers are cirque glaciers. These are small glaciers that occupy a bowl-shaped basin at the head of a mountain valley. Cirque glaciers are usually the remnants of much larger valley glaciers.

    The largest type of glacier is a continental ice sheet, that can cover thousands of square kilometers. Only two ice sheets of this type exist on the planet today, one on Greenland, the other in Antarctica. During the Pleistocene Ice Age (15,000 years ago), ice sheets covered nearly all of Canada reaching down into the northern contiguous United States. Much of the Puget Sound was covered with a continental glacier over 1000 m (3300 ft) deep.

    Ice Ages are periods of time when continental ice sheets expanded to cover large areas of the earth's surface. The term can be used to describe long periods of time (millions of years) during which glaciers expanded and retreated during a generally cool period of Earth history. In this sense the Pleistocene Epoch (1.8 million to 11,680 years ago) is the most recent major Ice Age. The earliest known Ice Ages occurred 800 to 600 million years ago when snow and ice covered the entire planet (Snowball Earth), between 460 and 430 million years ago, and from 350 to 250 million years ago.

    Glaciations are single periods of advance where ice sheets reach their maximum extent. There have been at least 6 major glaciations in the last 700,000 years alone, each one lasting up to tens of thousands of years.

    Only the last two glaciations are well documented in North America. The most recent glaciation (the Wisconsin) saw continental ice sheets reach as far south as Wisconsin and Seattle. The previous glaciation occurred about 120,000 years ago, and is called the Illinoian in North America, for it reached as far south as southern Illinois.

    The presence of till (unsorted debris carried by glaciers), glacial striations, erratics (large boulders deposited by glaciers), and glacial landforms are all used to identify the extent of recent glaciations. A more complete record of the Earth's climate history has been reconstructed from ice and ocean sediment cores. Gas concentrations and isotope data from these cores reveal the timing and duration of the cool periods that allowed ice sheets to advance and the warm "interglacial periods" that caused their retreat.

    Types of Glaciers

    Figure 3 - A cross section of a mountain and its surroundings showing the three major types of glaciers.

    Glaciers and Climate

    Climate is the "average weather" in a location. Though climate actually refers to the entire state of the atmospheric system, it is commonly described by temperature or precipitation. Climate determines how much snow a glacier receives and how fast it melts. The part of the year when glaciers gain more ice than they lose is called the accumulation season. The part of the year when glaciers lose more ice than they gain is the ablation season, generally from June through September. During a cool, wet year, glaciers gain more snow than they lose, causing the glacier to thicken, and subsequently, advance. During warm, dry years, they melt more snow and ice than they receive, causing them to retreat.

    Although glaciers always flow downhill, the idea of glacier "retreat" may give the impression that a glacier can move uphill. In fact, a glacier is in retreat when the rate of movement downhill cannot keep up with the rate of melting. The glacier is melting back faster than it is moving downhill, so it is said to "retreat". By contrast, when a glacier advances, its downhill flow is greater than the melt at the terminus. The rate of glacier response has to do with the volume of the glacier and the rate of mass exchange (accumulation and ablation of snow and ice). For example, for the same size glacier, small rates of mass exchange (like glaciers in polar regions) result in slow responses and high rates (like glaciers in the U.S.) cause fast responses. Smaller glaciers response more quickly than larger glaciers. The position, orientation, or elevation of a glacier affects the magnitude of glacier change in response to climate.

    Glaciers as Landscapers

    Much of the landscape has been shaped by the continuous growth, disappearance, and regrowth of glaciers. In the high mountains these glaciers have carved deep valleys and numerous small basins, while in the lower altitudes they have left a variety of small hills and ridges whenever they have receded. At all altitudes they rerouted streams and created numerous lakes and ponds.

    Glaciers carve landscapes in two ways. First, they pluck rocks from bedrock by either freezing on to it or by ripping rocks from subglacial ledges. The plucked rocks are dragged by the moving ice over the rock remaining in place. The rock studded ice acts like sandpaper to grind bedrock. While glaciers have deeply eroded many valleys, many ridge tops have been unaffected.

    In addition to eroding bedrock, glaciers landscape the terrain by depositing rocks into ridges called moraines. Figure 5 - A cross section of a glacier showing plucking and abrasion.

    Meltwater streams flow from the down valley end (terminus) of the glaciers. The streams originate as meltwater and rain on the glacier surface. The water makes its way into the interior of the glacier through crevasses that are commonly found on the ice surface. Occasionally, surface water drops into "moulins" or circular shafts that descend vertically from the surface into the glacier. Once in the body of the ice water flows in naturally formed cracks or tunnels.

    Eventually, the water reaches the bottom of the glacier where it flows between the rock and the ice. The water may flow within saturated sediments that are sandwiched between the ice and rock, or in small cavities formed on the downslope side of bumps in the bedrock. The water eventually ends up in tunnels which appear as streams at the end of the glacier. Under certain circumstances, the stream exit can expand into a large ice cave. The streams then travel across rock strewn fields, which are stream-modified glacial deposits called outwash plains.

    Streams flowing from glacier termini are clouded with fine sediment giving the streams a milky appearance. This sediment (glacial flour) is created by the glacier grinding its rocky bed. This happens when rocks carried by the glacier, imbedded in the ice at the glacier bottom, scrape along the bottom and act like sandpaper to grind down other rocks.

    Rock debris covers the ablation zone of some glaciers. The rocks fall from the valley walls directly onto the glacier and then are carried down valley by the glacier. Eventually the rock is deposited at the end or along the sides of the glacier. Where the rock piles up at the end of the glacier, a ridge forms, which is called a moraine . If the ridge forms on the side of a glacier it's called a lateral moraine; if it forms at the end of the glacier it's a terminal moraine. The occurrence of moraines are important because they are used to determine the size of glaciers during earlier, colder times. Old lateral and terminal moraines are very common in Puget Sound.

    Figure 6 - A cross section of a typical alpine glacier showing the location of lateral and terminal moraines.