Chapter 6: Mt. Ascutney.
Mt. Ascutney stands just west of the Connecticut River and the town of Windsor, VT. The mountain dominates the skyline viewed from hilltops to the north and south, and is accessible from I91 exits 8 and 9. It was a source of inspiration for Augustus St. Gaudens, the premier nineteenth century American sculptor, and for the Cornish Art Colony. The mountain stands over 3,000 feet [1 km] above the surrounding countryside. There are four hiking trails, as well as a paved road open during fair-weather months, to the peak. A hike to the top from base level is a breathtaking experience.
Mt. Ascutney from the top of Gile Mountain tower. [October 2006]
The rocks lying beneath Mt Ascutney are of two types—igneous and metamorphic. The central and upper portions of the mountain are underlain by igneous rocks emplaced during multiple episodes of molten intrusion occurring over several millions of years starting about 118 million years ago. These igneous rocks are classified by the relative amounts of quartz, feldspar, and iron-rich minerals present in the rock. Syenite is light-colored, quartz-poor and potassium feldspar rich. Granite is light-colored, quartz and feldspar rich. Gabbrodiorite is darker-colored, and contains sodium feldspar, quartz and iron-rich minerals. These rocks are similar in composition and geologic history the igneous rocks underlying the White Mountains and the Ossipee Mountains of New Hampshire.
Underlying the lower elevations and flanks of Mt Ascutney are the early Paleozoic-age metamorphic rocks that are commonly found throughout east central Vermont, and are the same as (or very similar to) the phyllitic schists seen in outcrops on Gile Mountain and described in earlier chapters. These rocks were initially deposited as muds and sands in the proto-Atlantic Ocean more than 500 million years ago. Subsequently, at least two major mountain-building episodes associated with tectonic plate collisions and the closing of the ancient Atlantic turned the mudstones and sandstones to metamorphic schist. All this occurred over the course of 100 million years beginning about 485 million years ago. The sediments were deeply buried and caught in a tectonic ‘vise’ during the episodes of mountain building and component minerals were re-crystallized, and the rocks folded and fractured by the intense pressure and heat. One of the more abundant metamorphic minerals that developed in these rocks during metamorphosis is sheet-like mica, which contributes to the distinctive shiny appearance and tendency of the Gile Mountain Formation to cleave in plates. These older, metamorphic rocks are the regional ‘country rock’ into which the younger igneous rocks intruded 118 million years ago.
Among the many geologically interesting aspects of Mt Ascutney are i) the different composition of its underlying intrusive rocks—granite, syenite and gabbro. ii) Ascutney’s geological significance as one of other, similarly-aged intrusive volcanic mountains in the region, including the White Mountains and the Ossipee Mountains of New Hampshire, and iii) the distinctive fractures seen in the granite and syenite. We briefly touch on each of these topics.
Syenite, which has little or no quartz, is the most abundant rock-type underlying Mt Ascutney. Quartz, however, composed as it is of the two most common elements (oxygen and silicon) in the Earth’s crust, is the most common mineral on the surface of the Earth. How could syenite not end up with some quartz in its structure? Similarly, how could mafic gabbro be emplaced with compositionally contrasting felsic rocks?
Consider how intrusive igneous rock forms. A body of deeply buried liquid magma, say in a chamber underlying a volcano, slowly cools over geologic time. As the magma cools, a mineral forms when the temperature of the magma drops below the solidification point of that specific mineral. The process is not unlike ice forming on a pond in early winter as the temperature drops below the freezing point. One significant difference, however, is that after the first minerals solidify, the elements that combined to form those minerals are gone from the still-molten magma mix; the chemical make up of the magma is thereby changed. As further cooling occurs, other minerals solidify, further changing the residual melt. The actual temperature-dependent sequence of minerals that comes out of the melt reflects the original composition of the magma and the temperature and pressure in the chamber. Greater depth in the crust means greater pressure, and greater pressure affects the mineral composition. This process of sequential mineral formation from melt was extensively studied and described by N. Bowen over a hundred years ago, and so is referred to as Bowen’s Reaction Series.
Briefly stated, Bowen informs us that darker, mafic minerals crystallize from magma at the highest temperatures and, thus, first in a magma cooling over time. Felsic minerals, such as orthoclase feldspar and the micas, crystallize at somewhat lower temperatures, and thus later in a cooling magma. Quartz crystallizes from a melt at the lowest temperature, and so is the last mineral formed. Usually, when quartz forms, only oxygen and silicon are left in the magma because the formation of the other minerals has used all the other elements that were in the original magma mix. Quartz is made of oxygen and silicon only, so quartz formation completes the solidification of the magma. If the formation of the minerals that precedes the formation of quartz uses up all the material in the magma chamber while the chamber stays above the solidification point of quartz, then no quartz can form, Hence syenite.
The presence of gabbro at scattered locations within Mt Ascutney reflects material crystallized at relatively high temperatures, and so early during the cooling of an intrusive melt. The predominant syenite composition of Mt Ascutney, on the other hand, reflects either i) precise temperature and pressure conditions allowing feldspar and mica, but not quartz, to crystallize from the magma melt, or ii) an original magma composition that for some reason, was anomalously poor in silicon and oxygen.
The range of cooling ages and predominantly felsic compositions of the intrusive rocks underlying Mt Ascutney, the White Mountains, the Ossipee Mountains, and other related granitic rocks suggest that they were emplaced during the splitting apart of the super-continent Pangaea and the progressive opening of the modern Atlantic Ocean beginning some 200 mya.
Along I91 in Massachusetts and Connecticut, there is corroborating evidence in the accumulation of reddish sandstones and mudstones. The composition and fossil contents of these formations indicate that they were deposited in freshwater lakes and river floodplains in desert-like settings. Also basalt lava flows developed in the stretched and thinned crust during this time. Thus, the entire region was characterized during early Mesozoic time by extensive volcanic activity and arid, desert-like conditions. A reasonably analogous modern environment is the East African Rift Valley.
Mt Ascutney is but one entity in a chain of Mesozoic-age intrusive mountains extending from the Monteregian Hills near Montreal and continuing southward through the White Mountains, and related ranges, of New Hampshire and further as a series of sea mounts extending into the Atlantic. Early on, geologists speculated that this geographical alignment might be associated with progressively younger ages of the intrusive mountains to the south, and might thus be indicative of the passage of the North American continent over a region, fixed in position with respect to the moving, overlying tectonic plate, of an upwelling mantle plume of magma. Such phenomena are called ‘mantle hot spots’, and are something of a hot topic among some geologists.
The Hawaiian Islands and the Galapagos Islands are examples of Hot Spot Volcanoes. What are particularly compelling about the Hawaiian Islands are the progressively older ages of extrusive igneous rocks from modern volcanism on Big Island to the northwest all the way to the Kure and Midway atolls. The trend of increasing age further extends northwest along a chain of seamounts (the Emperor Seamounts), which are the eroded remnants of island volcanoes now submerged.
As intriguing as the ‘hotspot’ origin of the Ascutney and White Mountains seems, it is unlikely to be the case. Recently painstakingly detailed studies of the ages of these rocks do not indicate a systematic pattern of younging to the south, from Montreal to Neddick Point in Maine. The volcanoes that once dominated our landscape during the Mesozoic 100-to-200 mya are now thought by most geologists to have instead arisen randomly in time and space during early stages of continent rifting, and not the result of continental plate passage over a mantle hot spot. This change in perspective regarding details of the region’s geologic history is an excellent example of how scientific understanding, in general, improves with ongoing study and debate but with very little, if any, doubt about the big picture of how the natural world works.
In any event, the Mt Ascutney we see today is but a shadow of its former self--it is essentially the root of a former volcano that stood above the regional landscape some 118-125 mya. The conical shape of the modern mountain simply reflects the shape of the now-exhumed intrusive rock body, and thus the shape of the original magma chamber underlying the volcano. Almost all of the once-overlying extrusive igneous rock, as well as some of original intrusive rock, has been eroded. Slow, but relentless weathering and erosion of the landscape has yielded debris, carried southward by the Connecticut River and, during the Pleistocene, by glaciers.
One rock exposure cited by geologists as strong evidence of Mt Ascutney’s volcanic origins can be found at Crystal Cascade along the Weathersfield Trail on the south flank of the mountain. The stream at this beautiful locale surges over a vertical falls of about 80 feet. There is much to see here, but practice care, as the rocks are steep and slippery. Walking uphill, the Weathersfield trail approaches the stream from the east and then proceeds along the stream’s eastern bank. If you cross the stream (with care!), there is an unmarked trail that descends along the west side of the cascade for a look at the impressive face of the falls.
Crystal cascade from the bottom. Note the massive, vertical wall of syenite and the schist at the bottom on the left as you face upstream.
The top of the falls. Note the absence of xenoliths in the rock rim of the falls.
Further upstream the syenite is filled with xenoliths and looks like leopard skin.
More xenoliths in the syenite.
Further upstream is more schist showing contact metamorphism to gneiss where the syenite intruded.
In the bedrock floor of the stream, a visually striking type of rock is exposed. It has the appearance to some of ‘raisins in oatmeal’. Essentially what you see is the light-colored Ascutney syenite with darker ‘strange rocks’ or xenoliths embedded within. The xenoliths are fragments of the Paleozoic country rock into which the Ascutney syenite intruded. The xenoliths are not just on the surface, of course, but scattered all through the rock like, well, raisins in oatmeal. In this context, this rock is called a diatreme breccia. The term breccia refers to any rock made up of distinctively angular fragments bound together by a matrix. So it might even apply to, say, a landslide deposit that had solidified. In this case, the syenite matrix suggests a volcanic origin, represented by the term diatreme. A diatreme eruption is an especially explosive eruption with rapid rise of magma rushing to the surface and a rapid expansion of gases contained in the magma accounting for the destructive violence. The breccia is thought by geologists to reflect the catastrophic collapse, and choking by the debris, of a volcanic pipe and mountain that vented molten magma from depth to the Earth’s surface.
Such explosive phenomena are associated with the partial destruction of the volcano itself. A modern example of such a volcano is Mt Saint Helens in Washington State, and a prominent and well-known landform of similar origin is Crater Lake in the Cascade Range of Oregon. Crater Lake fills a caldera, volcanic crater, created by a destructively explosive eruption that is about 5 miles [9 km] in diameter. The caldera, in this case, is all that is left of ancient Mt Mazama, originally 12,000 feet [4 km] tall. Wizard Island, in the center of Crater Lake, is the remnants of a volcanic vent choked with magma and debris--a diatreme.
If one clambers down the west side of the falls and inspects the rock of the face, one will see that there are no xenoliths in it. This rock is thought to have been formed from upwelling magma sometime after the explosive eruption that resulted in the diatreme breccia. After the breccia formed, it cooled and contracted. Contraction resulted in the development of joints and fractures between the central magmatic mass and the surrounding Paleozoic country rock. Subsequently, new magma welled up into region and either melted or pushed its way into the fractures that ‘ring’ the existing magma chamber. This type of intrusion is known as a ring dike and is seen elsewhere in the region, most notably in the Ossipee Mountains. Sometimes a ring dike is the only remnant of an exhumed, weathered and eroded ancient volcano and, because of its concentric geometry, has been misinterpreted by some as a meteor impact crater.
The work of the weathering during exhumation can be seen everywhere on and in the mountain. The cracks and fractures that pervade the granite and syenite are called joints. There are three sets of joints, each distinguished by orientation and each is perpendicular to the other two. Radial joints, as the name suggests, are oriented like spokes of a wheel, are vertical and run down the mountain. Tangential, or strike, joints are also vertical and are parallel with the circumference of the mountain. Sheeting joints are more-or-less parallel with the mountain surface, and thus perpendicular to both the radial and sheeting joints. Although these joints are pervasive, they are nowhere continuous through the mountain. Instead, they are understood to reflect the direction of stresses that developed in the rock during cooling after original deposition and opened later during exhumation.
Quarrymen understand and utilize rock joints in their work. Some long-abandoned quarrying equipment and digs are present at four sites on the mountain. The most impressive of these is the Norcross quarry on the Brownsville trail climbing the north flank of Mt Ascutney. Rock extracted from this quarry made it as far as the Columbia University Library, and the Bank of Montreal. However, commercial success of these endeavors was relatively short-lived. Ascutney syenite is rich in iron-bearing minerals, which, upon exposure in a building face, tend to rust and bleed, thus rendering the structure less visually appealing.
References Mt. Ascutney:
1. Frank H. Clark. Glimpses of Ascutney. The Opinion Press,
Bradford, Vermont, 1905.
2. Reginald A. Daly [1903] Geology of Ascutney Mountain, Vermont, U. S. Geol Survey Bull 209: 1-122.
3. G. Nelson Eby, J. Gregory McHone [1997] Plutonic and Hypabyssal intrusions of the Early Cretaceous Cuttingsville Complex, Vermont, In Crover, T. W., Mango, H. N. and Hasenohr, E. J. [eds] Guidebook to Field Trips in Vermont and Adjacent New Hampshire and New York. New England Intercollegiate Geological Conference, Castleton, Vermont, p B2/1-B2/17.
4. K. A. Foland, Henry Faul [1997] Ages of the White Mountain Intrusives—New Hampshire, Vermont, and Maine, USA, Am J Sci 277: 888-904.
5. Sharon Harkay [ed]. Mount Ascutney Guide, 6th Edition. Ascutney Trails Association, Windsor, Vermont.
6. Jill S. Schneiderman [1989] The Ascutney Mountain Breccia: Field and Petrologic evidence for an overlapping relationship between Vermont Sequence and New Hampshire Sequence rocks, Am J Sci 289: 771-811.
7. Jill S. Schneiderman [1991] Petrology and mineral chemistry of the Ascutney Mountain igneous complex,Am Mineral 76: 218-229.
