America's Volcanic Past -
Massachusetts

Volcanic Highlights and Features:
[This list is just a sample of various Massachusetts volcanic features or events and is by no means inclusive.]

• Massachusetts
• Massachusetts Regions
• Berkshire Mountains
• Boston
• Connecticut Valley
• Diabase Dikes
• Greenfield
• Holyoke Range
• Little Brewster Island
• Newton
• Pine Hill
• Poets Seat Tower
• Sharpners Pond

Berkshire Mountains:⁴
The Berkshire Mountains are primarily gneissic rocks (mostly granite biotite gneiss) with small areas of quartzitic rocks (mostly quartzite, quartzite conglomerate, and feldspathic quartzite).

Boston's Geology hundreds of millions of years ago:⁵
The softest Boston rocks are argillite (related to slate and shale but harder) and volcanic ash. The argillite was originally deposited as clay in either a lake or marine embayment; the volcanic ash was blown out of the many volcanoes that were active in the area during the time the clay or mud was begin deposited. Gravel was interlayered with the clay and cemented into a hard rock called "conglomerate", locally called "puddingstone". This rock crops out widely in Roxbury, Dorchester, and Brookline. Also interlayered with these sediments were volcanic flows, ashes, cinders, and the great variety of deposits formed by volcanoes. These deposits -- now hard rock -- are best seen in nearby Mattapan, Hyde park, Milton, Lynn, and Saugus. Hundreds of millions of years have elapsed since these deposits were laid down, and the ash and mud not only has been compressed, cemented, and solidified into rock, it has also been deeply buried by later deposits and drawn deeper into the Earth's interior, where it was squeezed, folded, broken, and turned up on end. Today, these badly distorted rocks are at the surface again only because of the erosion of the many thousands of feet of rock that once overlay them.

Boston's Landscape hundreds of millions of years ago:⁵
In all probability, the region was a broad lowland surrounded by hills or mountains of granite. A large body of water, either a large lake or perhaps an arm of the sea, occupied part of the lowland. Small volcanoes and at least one large cone that rose a mile or more in height dotted the plain and surrounding uplands. The ash from these volcanoes blanketed the plain and was carried by the rivers to the lake or sea where it mixed with silt and clay that was being carried by rivers from the upland. The rivers descending onto the plain from the surrounding highlands also carried gravel, which was deposited in flood plains and which makes up much of the Roxbury "puddingstone". This landscape of remote Paleozoic time can be matched with landscapes existing today. One can see similarities in the Puget Sound lowland and the nearby Cascades in the State of Washington; the Sacramento Valley, with Marysville Buttes and Mounts Shasta and Lassen in the distance; the Imperial valley, at the head of the Gulf of California depression, where volcanic centers existed until very recent time.

Boston's Bedrock Surface:⁵
The erosion that has sculpted Boston's surface during the last million or so years has worn down these rocks, but at different rates, depending principally on their hardness. Most of this erosion is the result of glacial ice. Harder volcanic rocks and conglomerates stand out as hills and knobs, whereas the softer argillites and volcanic ashes form valleys and are mostly buried beneath deposits of clay, sand, and gravel.

Connecticut Valley

Connecticut Valley Lava Flows:¹
The lava flow, now seen as prominent ridgelines overlooking the valley lowlands, formed as basalt oozed out of faults associated with the Eastern Border Fault. There are two major flows totaling 150 feet thick in Greenfield and Deerfield, but they look like one layer since they occurred one after the other. Just to the south is the Holyoke Range, a 600 foot series of flows that form a prominent ridge that can be traced over 50 miles. The basalt has some radioactive minerals that can be dated due to their decay rate, we know these lavas solidified about 194 million years ago in the early Jurassic Period of the Mesozoic. This date is very useful in studying Connecticut Valley rocks since all the sedimentary layers below (such as found in Greenfield and Northampton) are older than this, and all the layers found above the lava (such as in Montague and Amherst) are younger. Thus the Connecticut Valley sequence is determined to be early Mesozoic -- from late Triassic through early Jurassic Periods.

Diabase Dikes:³
Two Jurassic diabase dikes, 3-to-5-meters-thick, dark-gray to black, aphanitic diabases, cut the Newburyport Complex and Sharpners Pond Diorite. The dikes strike northeast and dip steeply. In both cases, the dikes are subparallel to the northeast trending joints in the bedrock and are cut by northwest trending joints. Similar dikes occur throughout Massachusetts and have been assigned a Jurassic age on the State map (Zen, 1983), but may also be Triassic (Wones and Goldsmith, 1991) or even Devonian to Carboniferous (Ross, 1984). The ages of similar dikes range throughout the Mesozoic elsewhere in New England (Foland and Faul, 1977). [See Sharpners Pond Diorite below]

Greenfield:¹
Greenfield, bounded on the east by New England's largest river, the Connecticut, and on the west by the highlands of the Berkshire Hills, is one of the best places in the world to study geology. All three rock types (igneous, sedimentary, and metamorphic) are easily visible.

Greenfield's Jurassic-age Lava:¹
Topographically, Greenfield's main residential and business area is about 250 feet above sea level, on the flat lake bottom plain of Lake Hitchcock, a glacial lake that drained about 12,500 years ago. To the west early Paleozoic Era schist rocks of the Berkshire Hills reach elevations of 1,100 feet, while several miles away at the east end of town a prominent, scenic ridge, site of Poets Seat Tower, rises 250 feet abruptly above the flat valley lowlands. It's composed of early Jurassic age lava, 194 million years old, that was tilted by faulting as the great supercontinent of Pangea broke up. The more gentle eastern side of the lava ridge slopes to the shoreline of the Connecticut River which has a scenic 40 foot waterfall (Turners Falls) just before entering Greenfield, and flows along a valley with abundant outcrops of red sandstone sedimentary layers, which, sometimes, display dinosaur footprints.

Holyoke Range:¹
The Connecticut Valley lava flow, now seen as prominent ridgelines overlooking the valley lowlands, formed as basalt oozed out of faults associated with the Eastern Border Fault. There are two major flows totaling 150 feet thick in Greenfield and Deerfield, but they look like one layer since they occurred one after the other. Just to the south is the Holyoke Range, a 600 foot series of flows that form a prominent ridge that can be traced over 50 miles. The basalt has some radioactive minerals that can be dated due to their decay rate, we know these lavas solidified about 194 million years ago in the early Jurassic Period of the Mesozoic. This date is very useful in studying Connecticut Valley rocks since all the sedimentary layers below (such as found in Greenfield and Northampton) are older than this, and all the layers found above the lava (such as in Montague and Amherst) are younger. Thus the Connecticut Valley sequence is determined to be early Mesozoic -- from late Triassic through early Jurassic Periods.

Little Brewster Island:⁵
The entrance to Boston from the sea is via a broad bay dotted with many islands. Some of these islands are barren bedrock outcrops, but most are drumlins. All the drumlin islands of the harbor were well wooded at the time of the first settlement, but they were soon cleared and converted to farmland. The approach to Boston by large vessels was hazardous because of these islands and shoals. Nevertheless, it was not until 1716 that the community built a lighthouse on one of the rocky outermost islands, Little Brewster Island (Lighthouse Island). Little Brewster Island is a narrow elongated outcrop of rock consisting of both argillite (the rock that underlies much of the harbor, Boston, and Cambridge) and diabase, a hard, resistant igneous rock. It is precisely because of the outcropping of the more resistant diabase that the island is there. A new stone lighthouse was built on the same site in 1783, and this, still flashing, is the oldest active lighthouse in the United States.

Newton and the Boston Basin:⁶
The rocks of Newton, Massachusetts, belong essentially to a single sequence of volcanic and sedimentary deposits formed during Devonian time or about 350 to 400 million years ago. The sedimentary rocks consist of the Roxbury Conglomerate or "Puddingstone" and the Cambridge Argillite or Slate. The Brighton Volcanics are distributed throughout the approximately 5,000 foot thick sequence of sediments and represent a variety of volcanic products. The youngest rocks of the bedrock sequence are dike rocks, which are earthquake fissure fillings which may be as young as 150 million years. Elsewhere in the Boston Basin these rocks rest upon the Mattapan Volcanics. Just outside the Boston Basin region there are rocks as old as approximately 600 million years, the Dedham Granodiorite. This igneous rock has not been encountered within the Boston Basin. It is likely however that the sedimentary and volcanic rocks just referred to have been built upon this older foundation. Deep drill holes in Newton therefore would be likely to penetrate the Mattapan Volcanics and the Pre-Cambrian foundation of Dedham Granodiorite.

350 to 400 Million Years Ago:⁶
We may visualize the deposits of the Boston area some 350 to 400 million years ago as consisting of gravels, sands and muds which were being deposited beneath the ocean just east of a chain of high mountains which then occupied the New England area. The Roxbury Conglomerate was deposited at the foot of high mountains by streams which washed sediment down the mountains and out to sea. Generally speaking heavier cobbles and pebbles rapidly settled out of the water and came to rest relatively close to the mountains of east central Massachusetts. While the sediments were being deposited, volcanic activity was taking place chiefly in that part of the basin of deposition where the conglomerate was being deposited. Also volcanic ash deposits are found in the slate of Cambridge and Somerville.

Folding and Faulting:⁶ The folding and faulting of the Boston Basin rocks resulted from mountain building forces, which pushed the alpine mountains of east-central Massachusetts in an easterly direction. This thrusting produced faults such as the Boston Border Fault and folds such as the Shawmut anticline of Newton. These structures were developed when blocks of the earth's crust were squeezed together.

Pine Hill

Pine Hill (Boston):⁵
Outcrop of rock where a thick biotite-bearing diabase dike is exposed and where volcanic rocks in contact with older quartzite and colorful porphyritic granite can be seen.

Jurassic-age Lava:¹
Topographically, Greenfield's main residential and business area is about 250 feet above sea level, on the flat lake bottom plain of Lake Hitchcock, a glacial lake that drained about 12,500 years ago. To the west early Paleozoic Era schist rocks of the Berkshire Hills reach elevations of 1,100 feet, while several miles away at the east end of town a prominent, scenic ridge, site of Poets Seat Tower, rises 250 feet abruptly above the flat valley lowlands. It's composed of early Jurassic age lava, 194 million years old, that was tilted by faulting as the great supercontinent of Pangea broke up.

Sharpners Pond Diorite

Sharpners Pond Diorite:³
The Sharpners Pond Diorite is a medium-to dark-tray, fine-grained biotite-hornblende diorite that contains inclusions of amphibolite and feldspathic biotite gneiss. The rocks of the Sharpners Pond Diorite are exposed south of the Clinton-Newbury fault. An U/Pb zircon age of 430 +/- 5 million years (Zartman and Marvin, 1991) established a Silurian age for the Sharpners Pond Diorite.

Excerpts from:
1) Richard D. Little, Geologic History of the Connecticut River Valley near Greenfield, Massachusetts, Connecticut River Homepage Website, 2001
2) USGS/NPS Geology in the Parks Website, 2001
3) Gregory J. Walsh, 2001, Bedrock Geology in the Vicinity of the Knowles and Andreas Well Sites, West Newbury, Massachusetts: USGS Open-File Report 01-353.
4) Gardner C. Bent, 1998, Streamflow, Base Flow, and Ground-Water Recharge in the Housatonic River Basin, Western Massachusetts and Parts of Eastern New York and Northwestern Connecticut: USGS Water Resources Investigations Report 98-4232
5) Kaye, C.A., 1976, The Geology and Early History of the Boston Area of Massachusetts, a Bicentennial Approach: USGS Bulletin 1476, 78p.
6) Skehan, J.W., Barton, S.J., and Barton, C.W., The Geology of Newton: City of Newton, Massachusetts, Website, 2004.

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