Glacier
A glacier islarge, long-lasting flowice thatformed on landmovesresponsegravity. The glacier fringe isarea whereglacier has recently melted. Theretwo main typesglaciers: alpine glaciers, whichfoundmountain terrains,continental glaciers, whichassociatedice agescan cover large areascontinents. Glacier ice islargest reservoirfresh water on Earth,second only tooceans aslargest reservoirtotal water. Glaciersfound on every continent except Australia. This article will concentrate on Alpine glaciers, but most ofconcepts also applycontinental glaciers exceptscalequite different.
Austria's longest glacier,Pasterze, winds its 8 km (5 mile) route atfootAustria's highest mountain,Grossglockner.
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A numbergeologic featuresassociatedglaciers, including moraines that form from glacially transported rocksdebris atterminus ofglacier; distinctive U-shaped valleyss ;cirques (cwms) at their heads.
The upper part ofglacier that receives most ofsnowfallcalledzoneaccumulation. The snowfall heresufficiently heavyexertdownward forcecause deep erosionrockthis area, often leavingbowl or amphitheater-shaped depression calledcirque. Onopposite end ofglacier, at its foot or terminal end, iszonedeposition (also calledzonewastage orzoneablation) where upwardlateral forces predominatedepositionsediment occurs. Between these two zones islineequilibrium wheredownward erosive forces ofzoneaccumulation andupward deposition forces ofzonedeposition cancel. Erosive lateral forcesnot canceled; therefore, glaciers turn v-shaped river-carved valleys into u-shaped glacial valleys.
Glacial morainesformed fromdepositionmaterial fromglacier andexposed afterglacier has retreated. These features usually appear as linear moundstill, which ispoorly-sorted mixturerock, gravelboulders thatwithinmatrix offine powdery material. Terminal or end morainesformed atfoot or terminal end ofglacier, lateral morainesformed onsides ofglacier,medial morainesformed downcenter. Less obvious isground moraine which often blanketssurface underneath much ofglacier downslope fromlineequilibrium. Other features formed by glacial deposition include distinctive streamlined hills known as drumlins,long snake-like ridges formed by streambeds under glaciers, known as eskers. Glacial meltwaters contain rock flour, an extremely fine powder ground fromunderlying rock byglacier's movement.
The Upper Grindelwald Glacier andSchreckhorn, at Grindelwald, Switzerland.
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So-called "stosslee erosional features"formed by glaciersshowdirectionits movement. Long linear features that followdirectionmovementcalled glacial striationsdivots inrockcalled chatter marks. These two featuresboth left onsurfacesstationary rock that were once underglacierwere formed when loose rocksboulders inice were transported overrock surface. Transportfine-grained material withinglacier can smooth or polishsurfacerocks, leadingwhat geologists call glacial polish. Glacial erraticsrounded boulders that were left bymelting glacier andoften seen perched precariously on exposed rock faces after glacial retreat.
Some glaciersquite small, while others may be tenskilometerslength. Today, theyfound on very high mountainsequatorialmid-latitude regionsprogressively lower as one approachespoles. GreenlandAntarcticaheavily glaciated, topointbeing almost entirely covered by ice. Glaciationthis extentcalled continental glaciation.
The downstream endcontinental glaciers often flows intosea. Asice reachesocean,breaks off, forming icebergs. Glaciersreally riversice that move slowly downhill. Evenvery cold climates, there may be unglaciated areas, which receive too little precipitationform permanent ice.
The snow from which glaciers formsubjectrepeated freezingthawing, permittingformation offormgranular ice called nevé. Underpressure oflayersicesnow above it, this granular ice fuses into solid glacial ice. Glacial ice contains minute air bubbles asresult, giving itdistinctive blue tint dueRayleigh scattering. The lower layersglacial ice flowdeform plastically under this pressure, allowingglacier aswholemove slowly likeviscous fluid. Glaciers do not needslopeflow, being driven bycontinuing accumulationnew snow at their source. The upper layersglaciersmore brittle,often form deep cracks known as crevasses asflex. These crevasses make travel over glaciersrisky proposition. Glacial meltwaters flow throughoutunderneath glaciers, carving channels inice similarcavesrockalso helpinglubricateglacier's movement.
During ice ages, continental glaciers may be as much as 1500 meters thick. A more extreme instanceglacial growth may have occurred duringSnowball Earth period. Inpast several centuriesEarth's glaciers have generally been retreating, often dramatically.
See also: West Antarctica
