The record in the rocks: Yngvar Isachsen and Adirondack geology

TICONDEROGA | For researchers focused on understanding the complexities of Adirondack geology, the name of the late Dr. Yngvar Isachsen is almost up there with other far-seeing geologists like Louis Agassiz (discoverer of the Ice Age) and Alfred Wegener (discoverer of continental drift). In 1975, Isachsen, a friendly, lanky New York Geological Survey geologist who often liked to wear Western-style bolo ties, was the first scientist to propose that while the Adirondack Mountains are composed of some of Earth's oldest igneous and metamorphic rocks, the range is, in fact, quite young in geologic terms. No other mountains on North America's east coast are as rugged or as precipitous. There's something going on below the surface that is, over millions of years, slowly pushing the Adirondacks ever skyward. A frequent visitor to the Adirondacks, Isachsen liked to visit the Ticonderoga area on his many field trips to the range. He enjoyed local dining before heading out, with rock hammer in hand and sample bag slung across his shoulder, to sample the local ridges and High Peaks. In between his Adirondack discoveries, Isachsen also discovered the Panther Mountain Impact Structure, an eroded, six-mile-wide 375-million-year-old asteroid impact crater in the Catskills, located just west of the New York Thruway, near Woodstock. (The Panther Mountain impact site of the Catskills should not be confused with New York's other Panther Mountain located near Tupper Lake.) Born in 1921, Isachsen's death at age 81 was a shock to his family, friends, and professional colleagues. It seemed that the gentle scientist, a life-long lover of New York's myriad rocks and fossils, was never going to stop. But just like the rocks and minerals he spent a lifetime studying, Isachsen, too, would return to the good earth as a mixture of various chemical compounds. The date of his passing was Oct. 13, 2001. While geoscience and Adirondack studies have marched on since Isachsen's death right after the 911 terrorist attacks in New York City, this state geologist made a lasting mark by helping to start unraveling the mysteries of Adirondack origins. Old rocks, new mountains While Yngvar Isachsen wasn't the first person to notice the circular shape of the Adirondacks, and its curious central-radiating drainage pattern — as imaged by earth-orbiting satellites — he was the first to propose that activity deep in the mantle might be pushing up the region. "The Adirondack Mountains constitute an anomalously large, domical uplift on the Appalachian foreland," he wrote in 1975. "The dome has a north by northeast and south by southwest axis of about 118 miles long, and an east-west dimension of about 87 miles. It has a structural relief of nearly a mile, and local topographic relief of up to .74 miles. First-order leveling in 1955, and again in 1973 along a north-south line at the eastern margin of the Adirondacks, showed an uplift rate of 2.2 mm (.08 inch) per year at the latitude of the center of the dome and a subsidence rate of 2.8 mm (.11 inches) per year at the northern end of the line near the Canadian border." Maybe you would like to have Isachsen's technical language told in layman's terms? Simply stated, the Adirondacks are on the rise at all four cardinal points. According to Associate Professor of Geology Dr. Will Amidon of Middlebury College in neighboring Vermont, Isachsen was a scientific visionary of sorts. "Isachsen's ideas seemed ridiculous for many years," Amidon told The Sun, "until the last decade... From data collected over roughly three years, seismologists created 3D models of seismic velocity in the Earth's lithosphere (including the crust). These show areas of very warm rock suggesting a plume of warm mantle material is sitting beneath (upstate) New York and pushing the land surface upward... The strongest evidence for recent uplift is right around the Ti-Lake George-Saratoga area. Also due westward to Sacandaga and Hudson Gorge and southern High Peaks areas. If modern uplift and faulting is happening, that is likely where it is has been happening. I am actually in the process of writing a paper about this..." The big mystery that faced Isachsen during the 1970s was what was behind the uplift of these very old rocks? What did it mean to our understanding of the evolution of the North American continent? And at least one geologist is pondering the question: could a Yellowstone-type supervolcano ever erupt here in the far distant future? Continued. Special thanks to the Middlebury College Department of Geology's Drs. Peter Ryan and Will Amidon for their assistance with this report.

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The record in the rocks: Yngvar Isachsen and Adirondack geology

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