The geology of Massachusetts includes numerous units of volcanic, intrusive igneous, metamorphic and sedimentary rocks formed within the last 1.2 billion years. The oldest formations are gneiss rocks in the Berkshires, which were metamorphosed from older rocks during the Proterozoic Grenville orogeny as the proto-North American continent Laurentia collided against proto-South America. Throughout the Paleozoic, overlapping the rapid diversification of multi-cellular life, a series of six island arcs collided with the Laurentian continental margin. Also termed continental terranes, these sections of continental rock typically formed offshore or onshore of the proto-African continent Gondwana and in many cases had experienced volcanic events and faulting before joining the Laurentian continent. These sequential collisions metamorphosed new rocks from sediments, created uplands and faults and resulted in widespread volcanic activity. Simultaneously, the collisions raised the Appalachian Mountains to the height of the current day Himalayas.
Warped and faulted basins formed in eastern Massachusetts and preserve coal from massive Paleozoic swamps as well as fossil plants and insects. The final phases of mountain building occurred as Laurentia completed its collision with Gondwana and Europe to form the supercontinent Pangea. Beginning around 200 million years ago, rifting broke apart Pangea. Erupting basalt lavas formed the new oceanic crust of the Atlantic Ocean, wedging apart Africa, Europe and North America along the approximate lines where the continents collided. The Connecticut River Valley and the Middleton Basin formed as failed rifts, filling with sediment that preserve dinosaur footprints. After the Jurassic, Massachusetts experienced long-running and continuous erosion with no more volcanic activity and only low-level seismic activity. The Appalachians shed sediments eastward, accumulating as the Atlantic coastal plain of southeastern Massachusetts.
A rapid cooling of the planet in the last 2.5 million years of the Quaternary resulted in one-mile thick ice sheets advancing southward and accelerating erosion. The Pleistocene glaciations rechanneled rivers and created large sediments deposits, including the terminal moraines of Cape Cod, Martha's Vineyard and Nantucket, as well as hundreds of smaller drumlins, eskers and kame deposits.
Today, universities in Massachusetts are hubs for worldwide geologic research while local professional and amateur geologists study landforms and geologic history, applying knowledge of regional geology to construction, environmental remediation, water resource management and quarrying.
Geological History Proterozoic (1.2 billion–539 million years ago) The early history of Massachusetts can be traced to around 1.2 to 1.1 billion years ago in the Mesoproterozoic Era of the Precambrian with some of the oldest rocks in the state situated in the Berkshires. What is now the western edge of the state was at the time the continental margin of the proto-North American continent Laurentia. The Grenville orogeny significantly altered many rocks through metamorphism as Laurentia collided with the Rio de la Plata Craton and Amazonian Craton to form the supercontinent Rodinia. In fact, the Berkshires are dominated by gneiss that metamorphosed during the Grenville orogeny.
The Rodinian continent broke up around 750 million years ago and rocks of proto-Massachusetts may have bordered the current west coast of South America in the west of the short-lived Pannotia supercontinent.
Africa, South America, Australia, India and Antarctica formed the new continent Gondwana while Laurentia and Baltica drifted away as separate continents throughout the rest of the Proterozoic.
In the late Proterozoic, a series of small island arcs began to form offshore of Gondwana while continental crust developed as part of the Gondwanan continent, before splitting off as microcontinents and other small continental terranes. For example, the Nashoba terrane formed as a mid-ocean island arc and is now wedged between the Merrimack and Avalon terranes, bounded by the Clinton-Newbury fault in the west. It extends south to Chester, Connecticut and Long Island Sound.
Beginning 550 million years ago, the microcontinent Avalonia, broke off of Gondwana and approached the eastern shore of Laurentia. Proposed as similar in size to Japan, Avalonia (also referred to as the Avalonia terrane) currently forms the basement rock of Connecticut, Rhode Island, eastern Massachusetts, eastern Maine, eastern New Brunswick, Nova Scotia, part of Newfoundland, southern England and Ireland, Belgium, the Netherlands and northern Germany (the European portions were separated during the subsequent break-up of the supercontinent Pangea 200 million years ago).
The quartzite and shale of the Westboro Formation appears to be the oldest Avalonian rock in Massachusetts. During the period when it was a part of the Gondwanan crust, the formation of the highly deformed mylonite of the Burlington mylonite zone began prior to 625 million years ago. Between 620 and 610 million years ago (rocks of these ages are southwest and north of Boston respectively) the Dedham granite intruded, contributing foliation in the Burlington mylonite and thin layers of feldspar and recrystallized quartz, while the Westwood granite developed at shallow depth and intruded the Mattapan volcanic rocks. Milford granite, which was widely used in building construction such as for the Boston Public Library formed 610 million years ago and is very similar to the Dedham granite, but with more granular quartz and feldspar and some mylonization, which appears at the surface in the Nobscot mylonite zone in Southboro and Framingham. A section of the batholith that domes slightly in the area of Milford is interpreted as the Milford antiform. Light gray granites of the Fall River batholith intruded at the same time. The Boston Basin formed as a faulted rift basin in the Avalonian microcontinent and extends an additional 10 miles eastward into Massachusetts Bay. The west-dipping Northern Border thrust fault is marked by a rocky slope near Massachusetts Route 60 and forms the northern bound of the basin, while the Blue Hills and Ponkapoag faults mark the southern boundary.
Beginning 596 million years ago, a period of volcanic activity erupted the fine-grained rhyolite and ashflow tuff, breccia and mudflows of the Mattapan and Lynn volcanic rocks. The Brighton volcanic rocks erupted later, 589 million years ago, producing layered basalt and quartz diorite dikes that were subsequently buried by sediments and hydrothermally altered.
The 2000-foot-thick Roxbury formation (including its 1550-foot-thick Dorchester member) deposited during this late Proterozoic time, with conglomerate and slate or poorly sorted Squantum member sediments. Although there is a lack of glacially-derived pebbles, some geologists have interpreted bedded siltstones as evidence of glacial deposition during the Snowball Earth global ice age.
Paleozoic (539–251 million years ago) Shelburne Falls and Bronson Hill island arcs The Iapetus Ocean opened between Laurentia and Gondwana beginning around 550 million, overlapping the beginning of the current Phanerozoic Eon in which multi-cellular life rapidly diversified. A series of island arc and continent-continent collisions built up the Appalachian Mountains throughout the Paleozoic forging many of the rocks in Massachusetts. The first collision between 485 and 440 million years ago began with the arrival of the Shelburne Falls island arc and resulting in the Taconic orogeny mountain building event. According to James Skehan, author of Roadside Geology of Massachusetts and a leading geology researcher at Boston College, Berkshire rocks were thrusted "like an accordion." As a result, many gneiss units were buried between three and 10 miles below the surface. The list of older Grenville orogeny-related gneiss units includes the Washington gneiss (the most widespread), Tyringham gneiss, Stamford gneiss or the Hoosac, Rowe and Moretown schists, which formed from continental shelf sediments. In places, areas of ultramafic oceanic crust rocks were "scraped" to the surface by the collision and commonly contain talc and soapstone.
A combination of radiometric dating and a graptolite fossil preserved in Taconic rocks indicates that the orogeny occurred in the Ordovician. In many cases, deep water sediments from offshore of the Laurentian continental margin were uplifted and metamorphosed, becoming the Greylock schist, Everett schist and Hoosac formation. Some preexisting landward sediments were also metamorphosed to become the muscovite, schist, and quartzite of the Dalton Formation and the Cheshire Quartzite (this latter quartzite is formed from heavily altered beach sands). Nearshore carbonates also experienced metamorphism, as in the case of the Stockbridge Marble, formed from limestone or the overlying Walloomsac Formation.
The uplands of central Massachusetts preserve widespread tectonic divergence. As the Bronson Hill island arc became the second terrane to come ashore, nappes and thrust faults were pushed in varying directions. The Hardwick and Coys Hill granites both intruded at this period and ultramafic rocks found with the Coys Hill granite may preserve the ancient oceanic crust of the Iaptetus Ocean. At the core of the Bronson Hill belt is the Monson gneiss, rich in microcline, plagioclase, amphibole schist, and eye-shaped augen gneiss. Ultramafic oceanic crust is preserved as peridotite. Much of the terrain of central Massachusetts is formed by domes that were created in the Cambrian and Ordovician. In fact, the Ammonousuc and Partridge formation mafic and felsic volcanic rocks ring uplifted domes, dating to the Ordovician. The Clough quartzite is younger, from the Silurian.
The Acadian orogeny, from 425 to 270 million years ago was the most extensive and long lasting Appalachian orogeny, as the microcontinent Avalonia (also referred to as the Avalon terrane) collided followed by a full collision between Europe, Gondwana (western Africa) and North America to form the supercontinent Pangea.
The Pelham dome—a large upland structure running north–south east of Amherst—appears to contain 600-million-year-old Avalon terrane rocks. These rocks were likely the subducting edge of the Avalon terrane which were later uplifted west of other intervening terranes sometime in the Devonian.
After the Bronson Hill island arc, one of the next terranes to collide was the Merrimack terrane, which trends west to east in Ware, Gardner, and Southbridge, as well as encompassing the rocks of Mount Wachusett. A large number of Merrimack terrane rocks are west-dipping and date to the Silurian, including the Oakdale and Eliot formations or the Paxton schist. The Ware Belt is part of the larger terrane, characterized by west-dipping schists, the four-mile wide Hardwick pluton and the Coys Hill granite. Further north, the Gardner Belt is an anticline with significant Silurian sandstone while Paxton schist is typically associated with the Southbridge Belt. Mount Wachusett is part of the 15-mile Mount Wachusett Belt: A Devonian-age nappe on top of Paxton schist, which was intruded by the Fitchburg granite 402 million years ago.
Terrane collisions: Nashoba, Avalon and Meguma