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A walk around on the surviving root of the ancient volcano By BILL CHAISSON editor@eagletimes.com Mount Ascutney syenite Buy Now Ascutney syenite (above), an igneous rock, with inclusions of darker, older “country rock.” BILL CHAISSON WINDSOR, Vt. — As I stood on the summit of Mount Ascutney looking down at the town of Windsor, I stood corrected. During the recent attempt by Robert Hutchins of Hartland to get the state of Vermont to rename this iconic moutain Cas-Cad-Nac, he said that a local geologist told him Ascutney had been the site of an ancient volcano. Because the mountain is made almost entirely of a plutonic, intruded igneous rock, I doubted that this was so. Further reading, including the excellent blog called geologyuppervalley.com, revealed that some geologists indeed find evidence for the existence of volcano, which has now entirely eroded away. Most of Mount Ascutney is made of syenite, which is a lot like granite, except that it has little or no quartz in it. Instead it is predominantly plagioclase feldspar (quite white), orthoclase feldspar (pinkish), with flecks of dark minerals like biotite mica and hornblende scattered through it. All that feldspar makes syenite a very light colored rock, but at certain junctures along the hiking trails in the state park you find bedrock and boulders of syenite with dark rocks apparently floating in it like fruit in a Jello mold. The Upper Valley geology blog is written by retired physician Howard Frankel and Dartmouth geology professor Brian Dade. It seems to have been posted about 15 years ago. The evidence for the volcanic origin of Ascutney appears to come, at least in part, from papers from 1989 and 1991 by Jill S. Schneiderman, a geology professor at Vassar College. As Frankel and Dade related the evidence, the dark rocks floating in the syenite “like raisins in oatmeal” are pieces of the bedrock into which the molten magma intruded 118 million years. Apparently when a volcano experiences an explosive eruption it more or less destroys its own plumbing. The walls of the chambers surrounding the magma collapse and pieces of the walls fall into the molten mass. Igneous rocks are divided into extrusive or volcanic rocks (a reference to Roman god Volcanus) that spill out onto the Earth’s surface, where the magma is then called lava, and intrusive or plutonic rocks (a reference to the Roman god Pluto). The volcanic rocks generally weather and erode away more easily than plutonic rocks, and that is ostensibly what happened to the extrusive rocks of Ascutney. In the 118 million years since the igneous activity produced the rocks as part of the opening of the modern Atlantic Ocean, the volcanic rocks have been carried off first by wind and rain and more recently by successive glaciations over the last 2 million years. In the Ossipee Mountains in eastern New Hampshire (see “A volcano in New Hampshire, June 16 Eagle Times) there are still volcanic rocks — basalt and rhyolite — to see; they have not all been eroded away. There is a joke about getting directions in New England, the locals tend to give you landmarks that are no longer there. “Go up to the crossroads where Hulbert’s farm stand used to be and take a right ...” The absurd part of the joke, of course, is that if you knew where Hulbert’s used to be, you probably wouldn’t need directions. But the philosophically interesting part of it is that the local population carries an invisible landscape in their memory and it is real because it is shared and completely inaccessible to anyone who doesn’t share the memory. At Ascutney there is only the proverbial memory of the volcano; all trace of it is gone. The memory takes the form of the dark country rocks floating in the syenite, the purported record of an event for which there is perhaps no direct evidence. But as they say “the absence of evidence is not evidence of absence.” If you have no direct evidence, maybe you are not looking in the right place. For some reason I presumed there was no volcano because there were no volcanic rocks around. Now I have a lot of reading to do ... • • • Another fascinating aspect of Mount Ascutney is the vertical zonation of its plant communities. The village of Ascutney is only 400 feet above sea level, but the peak of Mount Ascutney is over 3,800 feet above sea level. While forests at the foot of the mountain are deciduous northern hardwood communities, the upper reaches of the mountain are dominated by a boreal coniferous forest of red spruce and balsam fir. Air temperature decreases 6 degrees Celsius (~11 degrees Fahrenheit) with every 1,000 meters (3,281 feet) of altitude. In other words, you can expect it to be about 10 degrees cooler on top of Ascutney compared to the base of it. This is roughly the same as driving (or walking) 500 miles north at the same altitude. In other words, the plant communities at the top of Mount Ascutney are the same ones you would find at 400 feet above sea level in the middle of Quebec. One of the plants that is plentiful at the base of the mountain and becomes progressively less common and then is essentially absent above 3,000 feet is the hobblebush (Viburnum lantanoides). This shrub ranges down the Applachians all the way to Georgia, but is confined to lower and lower altitudes further and further north in its range (which does extend up into Quebec). This time of year the showy flowers of this viburnum are long past and the leaves have begun to turn purple with the approach of autumn. Next year’s leaves look like two tiny hands clasped in prayer at the end of the higher branches. Unlike a lot of shrubs there are no protective scales over the leaf buds; the following year’s leaves are exposed to the winter weather and they open in the spring. As the tallest mountain around for many miles, Ascutney feels especially rewarding to hike repeatedly, a world unto itself, revealing new secrets every time.
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