Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/661453
Bill Langer is a consulting research geologist who spent 41 years with the U.S. Geological Survey before starting his own business. He can be reached at Bill_Langer@hotmail.com CARVED IN STONE 36 AGGREGATES MANAGER / April 2016 D uring the summer of 1974, four of my colleagues at the U.S. Geologi- cal Survey and I spent the summer studying modern glaciers in Alas- ka. One of the glaciers we visited was the Mendenhall Glacier, located about 12 miles from Juneau. The Mendenhall is a lake-termi- nating, calving glacier. That is geology-speak meaning the glacier terminates (ends) in a lake where large chunks of ice calve (break off) from the glacier and float off as icebergs. When people talk in geologic time, a gen- eration (about 25 years) is a blink of an eye. Sometimes, a lot can happen during that blink. My parents were born in 1913. The world was just coming out of the Little Ice Age, a cold climate period from the early 14th century through the mid-19th century when mountain glaciers around the world, including Alaska, grew tremendously. From about 1750 until just before my parents' birthdays, the Mendenhall appeared to stand still. This was because the forward movement of the ice front (due to the accumulation of snow and ice in the Juneau Ice Field) had been about the same as the ablation (sublimation, calving, and melting of ice) at the front of the glacier. The advancing ice acted like a giant conveyor belt carrying large amounts of dirt to the front of the ice. As the front of the ice melted, the dirt the ice was carrying was deposited as gigantic piles of boulders, gravel, sand, silt, and clay called a moraine complex. The Mendenhall Glacier began to retreat during the warmer climate following my parents' birthdays. The moraine complex dammed meltwater from the glacier, form- ing Lake Mendenhall. As the glacier continued to retreat, the lake became longer and longer. From the time my parents were born to when my wife, Pam, and I were born (cir- ca 1947), the ice retreated 1.2 miles, or about 186 feet per year. From our birthdays to the birth of Kimberly and Rob (our daughter and son-in-law circa 1970), the ice front retreated about 0.3 miles; a much slower rate of about 69 feet per year. Then the ice began melting faster. By the start of the next generation (Donovan and Delaney circa 2004), the glacier had retreated another 0.6 miles, or about 93 feet per year. Since then, the rate of retreat has continued to accelerate, and by 2014, the ice front retreated another 0.5 miles or 240 feet per year. Overall, the gla- cier has retreated 2.6 miles in 100 years; about 137 feet per year. I have projected where the ice front might be if and when Donovan or Delaney have kids. Maybe they will take them to the Mendenhall Glacier to see what can happen in the blink of an eye. Author's note: If you ever get to Juneau, you should stop by the Mendenhall Glacier Visitor Center. You can plan your trip by visiting www.mendenhallglacier.net. A lot can happen during a geologic blink of an eye. The Mendenhall Glacier Mendenhall Glacier, 1974. Retreat of Mendenhall Glacier, by generation. Aerial image taken 2014.