Taku Glacier has been studied for decades, but it took data gathered by a team from the Geophysical Institute to prove that it is the deepest and thickest temperate glacier yet measured in the world.

Using seismic reflection methods to calculate its depth, Geophysical Institute Professor Keith Echelmeyer and doctoral graduate student Matt Nolan discovered that the Taku Glacier is an alarming 1477 meters thick, which is more than four times thicker than previously thought.

Prior to their research, most scientists believed the glacier averaged between 300 and 350 meters thick, based on measurements taken in 1949.

Scientists also believed that only 7 kilometers of the glacier rested below sea level, but Nolan and Echelmeyer's research revealed that at least 35 kilometers, or more than half the length of Taku Glacier, rests below sea level.

"The most alarming part of the discovery is realizing scientists have been using the wrong depth measurements for the glacier for more than 40 years," Nolan said.

Nolan took his first depth measurements of Taku Glacier while enrolled in the Juneau Ice Field Research Program, formed nearly four decades ago to bring students of all ages onto the 55 kilometer-long glacier. To measure its thickness, Nolan used the seismic reflection method, which involves creating a loud noise with dynamite on the glacier and listening for its echo with a seismograph.

Roman Motkya, with the Alaska Division of Geological and Geophysical Surveys, and Dennis Trabant, with the U.S. Geological Survey, assisted in revealing the unusual depth of the Taku Glacier by measuring it with radar. Eventually, however, the team had to resort back to seismic reflection methods because the glacier was too thick to return a radar signal.

Today, the only glaciers in the world known to be thicker than the temperate Taku Glacier are polar glaciers in Greenland and Antarctica, which are composed of ice between 20 and 40 degrees below zero. Temperate glaciers contain ice that hovers near melting point.

Taku Glacier, the largest glacier draining the Juneau Ice Field in Southeast Alaska, gained notoriety in the year 1750 when it blocked the nearby Taku River and created a large lake within its valley.

A repeat performance today would be particularly hazardous because residential developments are located in the valley carved by the Taku River, which starts in Canada and runs into the ocean. In addition to its potential for causing flooding, the unusual past of the glacier has caused many scientists to ponder its future.

At the turn of the century, Taku Glacier was a calving tidewater glacier. During its advance, it built up a high moraine then overrode it. This action ended calving, placed the glacier terminus above sea level, and cut it off from the ocean.

As a landlocked temperate glacier, it distinguished itself for years by advancing at high rates even though other glaciers fed from the same ice field retreated.

Although the ice volume of the Taku Glacier continues to increase, it stopped advancing in 1988. Nolan and Echelmeyer suspect high rates of erosion under the glacier may be contributing to its current standstill.

By comparing recent and historic measurements, they theorize that a weak layer of sediment, which lies between the Taku Glacier and its bedrock, is being eroded at a high rate. The sediment released during the erosion of this layer may be entrenching the glacier into the moraine at its terminus, keeping the Taku Glacier landlocked and preventing it from rising enough to advance.

Their theory is described in detail in a paper recently published in the Journal of Glaciology. The paper was coauthored by Motkya, and Trabant.

Like others, the researchers expect the Taku Glacier to someday resume a gradual advance, but no one is willing to say when.

ECHOES--To discover the depth of Taku Glacier, researchers used the seismic reflection method, which involves creating a loud noise with explosives (pictured at right) and listening for its echo with a seismograph. Roman Motyka is pictured above stringing out geophone cable prior to the explosion. Photos were taken by Matt Nolan.