Structure of Ice Sheets and Cores
Ice sheets are formed from snow. Because an ice sheet survives summer, the temperature in that location usually does not warm much above freezing. In many locations in Antarctica the air temperature is always well below the freezing point of water. If the summer temperatures do get above freezing, any ice core record will be severely degraded or completely useless, since meltwater will percolate into the snow.
The surface layer is snow in various forms, with air gaps between snowflakes. As snow continues to accumulate, the buried snow is compressed and forms firn, a grainy material with a texture similar to granulated sugar. Air gaps remain, and some circulation of air continues. As snow accumulates above, the firn continues to densify, and at some point the pores close off and the air is trapped. Because the air continues to circulate until then, the ice age and the age of the gas enclosed are not the same, and may differ by hundreds of years. The gas ageāice age difference is as great as 7 kyr in glacial ice from Vostok.
Under increasing pressure, at some depth the firn is compressed into ice. This depth may range between a few to several tens of meters to typically 100 m for Antarctic cores. Below this level material is frozen in the ice. Ice may appear clear or blue.
Layers can be visually distinguished in firn and in ice to significant depths. In a location on the summit of an ice sheet where there is little flow, accumulation tends to move down and away, creating layers with minimal disturbance. In a location where underlying ice is flowing, deeper layers may have increasingly different characteristics and distortion. Drill cores near bedrock often are challenging to analyze due to distorted flow patterns and composition likely to include materials from the underlying surface.
Read more about this topic: Ice Core
Famous quotes containing the words structure of, structure, ice and/or sheets:
“Im a Sunday School teacher, and Ive always known that the structure of law is founded on the Christian ethic that you shall love the Lord your God and your neighbor as yourselfa very high and perfect standard. We all know the fallibility of man, and the contentions in society, as described by Reinhold Niebuhr and many others, dont permit us to achieve perfection.”
—Jimmy Carter (James Earl Carter, Jr.)
“With sixty staring me in the face, I have developed inflammation of the sentence structure and definite hardening of the paragraphs.”
—James Thurber (18941961)
“He was high and mighty. But the kindest creature to his slavesand the unfortunate results of his bad ways were not sold, had not to jump over ice blocks. They were kept in full view and provided for handsomely in his will. His wife and daughters in the might of their purity and innocence are supposed never to dream of what is as plain before their eyes as the sunlight, and they play their parts of unsuspecting angels to the letter.”
—Anonymous Antebellum Confederate Women. Previously quoted by Mary Boykin Chesnut in Mary Chesnuts Civil War, edited by C. Vann Woodward (1981)
“Last night you lay a-sleeping? No!
The room was thirty-five below;
The sheets and blankets turned to snow.
MHed got in: Dirty Dinky.”
—Theodore Roethke (19081963)