drainage channels within the glacier. The glacier itself may be several hundreds of meters thick; its structure may be further complicated by the presence of medial moraines, composed of rocky debris that snake across many valley glaciers. The moraine patterns are good indicators of past dynamical instabilities.

Extensive geophysical studies have been conducted on temperate glaciers in the Pacific Northwest such as the Blue and Columbia Glaciers. The Bering and Malaspina Glaciers, located along the Gulf of Alaska, are examples of surging glaciers with highly crevassed surfaces, complex subglacial hydrology, and surface and internal moraines. Each has been intensively studied with surface, airborne, and spaceborne remote-sensing techniques.

Polar Ice Sheets

Greenland and Antarctica are blanketed by the last of Earth's great ice sheets. Continental in size, the ice sheets are characterized by complex dynamics driven in part by external climate forcing and by spatial and temporal variations at the glacier bed and at internal boundaries. The dynamical processes manifest themselves on the ice sheet surface by the presence of exotic structures such as ice streams. These are rivers of ice within the ice sheet, hundreds of kilometers long, that discharge ice from the interior ice sheet toward the floating ice shelves and eventually to sea. The margins of ice streams are heavily crevassed and are strong targets for microwave radar, and they can effectively attenuate signals from high-frequency radar (Figure 4.1).

Antarctic ice streams ride over a bed lubricated by subglacial water. The nature of the bed enables the ice streams to move at speeds of several hundreds of meters per year, whereas nearby ice frozen to the bed may move at speeds of only tens of meters per year. Greenland ice streams (such as the Jacobshavn Glacier) apparently flow via the deformation of a basal layer of relatively warm ice — the combination of warm basal ice and the presence of extensive surface crevassing makes Jacobshavn one of the last important glaciers to resist detailed sounding of the glacier bed.

Ice sheets preserve an important stratigraphic record of past changes in climate and dynamics. The record takes the form of vertical and horizontal gradients in density, temperature, crystal size, crystalline fabric, impurity content, and deformation rate. Local vertical variations in these properties can lead to stratigraphic horizons that are detectable and apparently continuous for more than hundreds of kilometers (Figure 4.2).

Polar Ice Shelves

Ice shelves are enormous slabs of floating ice that are fed by a combination of ice flow from the interior ice sheet and accumulation on the ice sheet surface. The largest ice shelves are found in Antarctica. Both the Ross and Filchner-Ronne Ice Shelves are about the size of Texas. Ice thickness ranges from about 800 m near the grounding line to about 250 m near the calving margin. Water-layer thickness beneath the ice shelves varies from a few meters near the grounding line to hundreds of meters.

A few ice-shelf-like environments have been identified recently in northern Greenland. For example, Peterman Glacier occupies a long fjord. Much of the length of Peterman Glacier is floating on ocean water that fills the fjord. Pockets of water upstream of the grounding line are also believed to exist based on the strength of radar returns from deep subglacial valleys (Figure 4.3).

The interior structure of ice shelves can be complex. Moraine material deposited on the surface of East Antarctica's outlet glaciers, for example, is carried downstream and buried, only to show up as a strong scattering layer in radio-echo sounding data. Rifts through the ice shelf can form near grounding lines or around ice rises. Upwelling brine is forced horizontally through lower-density firn (i.e., granular ice formed by the recrystallization of snow) near the surface, forming a layer nearly opaque to radar. These brine layers are carried downstream and can completely obscure the ice bottom from radar. Bottom and surface crevasses can tear through a significant thickness of ice. Once identified, the crevasses can be useful indicators of the stress regime within the ice shelf (Figure 4.4).

Ice-thickness gradients of the ice shelf and currents within the subglacial ocean can plate large thicknesses of sea ice onto the base of the ice shelf. Direct measurement has shown a 6-m-thick layer of briny sea ice on the bottom of the southeastern Ross Ice Shelf. Several hundred meters of sea ice are believed to be accreted onto the

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