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Shirley Reimer, age 12, of Linden, Alberta, Canada, for her question:

What processes propel glaciers to move?

A glacier is made of ice which, as we know, is very slippery stuff. When you step on a smooth frozen puddle, your feet slide and glide under your own weight. A glacier is built from snowflakes, which are assembled from tiny ice crystals. Every year enormous snowfalls add extra weight and the tiny crystals are crushed flat. Geologists suspect that their slippery surfaces force a glacier to move under its own weight.

The earth depends on teamwork to perform major projects. For example, several factors work together to make a mighty glacier move as it does. Modern earth scientists think that the job mould be impossible without the help of zillions of slippery little ice crystals, crushed flat by countless tons of frozen snow. As they slither over each other, they cause the great glacier to slide and glide, ooze and spread.

The building of a glacier begins in some region where summers fail to melt the winter snocrfalls. Year by year, the frozen snows accumulate in a thick layer. Its weight exerts enormous pressure on the deeper levels and on the earth below. Some of the massive ice sheets of Greenland and Antarctica exert a pressure of more than seven tons per square foot on the earth beneath recently fallen snowflakes are lacy hexagons, assembled from miniature ice crystals and miniature air pockets. In time, the fluffy flakes become small granules of dense frozen snow. Some of this melts and refreezes. Next winter the surface is buried below new snowfalls.

Year by year, the snowflake crystals are crushed below deeper and heavier layers. Glacial ice is classed as a fragile mineral, unable to support its own weight, When a glacier becomes 200, to 300 feet thick, the slippery crystals in its interior cause the whole mass to move. This theory explains how a glacier is able to move    other factors decide where and how fast it moves.

The great icefield of Antarctica covers hills and hollows over an area of some five million square miles. This mighty mass moves by pushing out from the center, oozing and spreading toward the edges. There its lofty cliffs of ice. are smashed and shattered by the pounding sea. As great chunks deift away in icebergs, more ice spreads out from a valley glacier is usually perched between high peaks. The valley walls prevent the ice from spreading    but gravity helps it to slide down the slope like a frozen river. The' slowpoke glaciers move a few inches per day    while the speeders move 20 to 30 feet.

Geologists poke metal rods into the ice and align them to fixed landmarks beside a moving glacier. They can measure the motion on the surface and around the edges. But mysterious movements go on in the depths of a great glacier    and these are difficult to detect. Often the metal markers are bent because various levels move at different speeds    and possibly slither in different directions.

 

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