SCIENTIFIC RESULTS 



25 



NORTH rOI.AR CAP ICE 



Polar caj) ice is the end product of all the forces Avhich develo]) a 

 very stronn". massive ice cover in the central north ])olar sea. This 

 covering- constitutes about TO per cent of the polar basin; approxi- 

 mately li.OOO.OOC) square miles. Fields and floes break away from the 

 main i)olar core, either to mix with the inshore ice over the conti- 

 nental shelves, or to be discharaed throufjh one of the several ocean 

 straits. The «rreat sea-ice cap, \\\Hm closer examination (see fig. 11, 

 ]). '20), has the same ireneral shape as the ])()lar basin, with its marfjin 

 closely i)arallelini2: the course of the l.OOO-meter isobath. Like the 

 dee])er })art of the ])olar l)asin. the elliptical-sha})ed cap lies much 

 closer to the Greenland-Xorth American side than it does to Europe 

 and Asia, with its long axis running from Si)itsberoen to Point 

 Barrow, its center, often called the pole of inaccessibility (see fig. 14, 

 p. 27). offset about -fOO miles toward Alaska. 



POLAR Cap Ice 



FiGiiiK lij — The polar cap ice at the North I'ole on May 12, 1!)26. In suimiier. 

 small polynyas are stated to be found every 5 to 10 miles. (Photograph taken 

 from the dirigible Xorge by L. Ellesworth. i 



This hub of the permanent sea ice is continually fed by pack ice 

 around its periphery, and reinforced by accretional freezing and 

 snowfall, all at a rate during the last century at least, equal to the 

 coml)ined j)rocesses of melting, crushing, and discharge through the 

 various exits. Freezing t)n the underside of the cap })r()gresses 

 throughout nine months of the year, most rapidly during the winter. 

 Examination shows that tlie underside of an ice sheet is often rough 

 and bitish-like in character. This is the transition zone between ice 

 and water. The thickness of the solid sheet depends mainly upon 

 two factors, viz. the condition of heat and the magnitude of the 

 forces that I'aft and ]iile u]) the ice. Sea water solidifies in the 

 Arctic Ocean to an axcragc (l('i)th of GI/2 feet.^° But winds and cur- 



'■'■ The maximum thickness of hummocked ice observed by Nansen in the Arctic Ocean 

 was 11 feet 10 inches, but he sets the mark of S feet as tlie estimated mean thickness of 

 the entire cover. Stefansson (1922). north of Banks Land, reports the thickness of old 

 fields, free of pressure ridges and rafting, as 12 to 14 feet. 



