Geology of Massachusetts

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The geology of Massachusetts is the result of a process that began over a billion years ago. Massachusetts currently lies at an edge of a continent, and in the geological past has been the location of many continental collisions and collisions with other microcontinents and volcanic arcs, with their resulting mountain-building events. As a result, the state rock structure is squeezed together. Recently, multiple Ice Ages have shaped the modern geology of the state and have created notable landmarks such as Cape Cod.

Early history[edit]

A geological map of the state, showing how the rocks are squeezed together

The early history of Massachusetts can be traced to around 1.2 to 1.1 billion years ago in the Mesoproterozoic Era. Ancient areas of continental crust or cratons formed the nuclei of continents. The ancient continent Rodinia was formed as a byproduct of the collisions of earlier cratonic fragments. Mountains were formed during the Grenville orogeny. Some of the rocks of the Berkshire highlands are from this era.[1]:8[1]:12

Around 750 million years ago, Rodinia began to break up and the Laurentian craton was formed. Two hundred million years later, Gondwana was formed. Laurentia increased in size because of the addition of Japan-sized islands. It is possible that the Merrimack, Nashoba, Avalon, and Meguma terranes that make up eastern Massachusetts are remnants of these island arcs or microcontinents. It is highly likely that these terranes broke off from Gondwana at some point in their history and collided with Laurentia, between 550 and 370 million years ago. These resulting collisions were part of the Taconic orogeny. The Iapetus Ocean, separating the two continents began to open up when Rodinia broke apart. The size of the ocean eventually prevented migrations between the two landmasses.[1]:10[1]:12

Between 425 and 370 million years ago, the two continents began to collide, forming Pangea. During this time, the Acadian orogeny, which marked the collision of the two continents, occurred. Around 200 million years ago, Pangea began to break apart. This caused rift valleys to form within the interior of the continent. One of these rift valleys, the Connecticut River Valley is a failed rift of the continent. Basalt dikes present in the underlying rocks of the valleys are the result of volcanism.[1]:11

Major terranes[edit]

Laurentian[edit]

The rocks of the Berkshire Hills were once the edge of the ancient continent of Laurentia. The Laurentian Terrane is made up of Grenville gneisses of Proterozoic age that formed at depth within the continent, continental shelf deposits, and the remains of two volcanic island chains, which constitute the Shelburne Falls and Bronson Hill volcanic chains. The Shelburne Falls chain now lies in the Berkshire foothills, while the Bronson Hill chain lies just east of the present-day Connecticut River Valley. The Taconic Orogeny, which resulted from the collision of these volcano chains and the continent, helped to thrust, push, and metamorphose the group of Precambrian to early Paleozoic rocks between 485 and 440 million years ago. The terrane is also divided into six structural and stratigraphic divisions.[1]:13

Divisions[edit]

The divisions are:[1]:15

  • Taconic thrust sheet
  • Vermont-Stockbridge marble belt
  • Berkshire Massif, Hoosac thrust sheet and Row-Hadley belt
  • Shelburne Falls Volcanic Belt
    • Connecticut River Valley rock intrusion
  • Bronson Hill volcanic belt

Gondwanan[edit]

Mount Wachusett, which is part of the Gondwanan terrane

The rocks of the Gondwanan terrane are made up of four smaller terranes. These terranes were once part of other continents but have become fused onto the continent over the years. It is generally agreed that two of the terranes, the Meguma and Avalon were once part of Gondwana. Proof of Avalon being part of Gondwana exists because of trilobite fossils of the same species that have been found both in Massachusetts and Africa. This means that Avalon must have broken away from Gondwana during the end of the Precambrian, around 550 million years ago. At the very latest, it broke away by the Ordovician, around 490 million years ago. Fossils of the Meguma terrane also indicate a relation to the northwest Africa margin. The Avalon also originated as a volcanic island chain near the South Pole, on the fringe of the Gondwanan supercontinent. It is possible that late Precambrian quartzites of the eastern part of the state were deposited as beach sands on the continental shelf of Gondwana. These deposits were eventually caught in the Avalonian mountain building event, the same deformation which helped to form the Avalonian island chain.[1]:14

The Nashoba and Merrimack terranes, lying west of the Avalon, probably formed in the ocean near Gondwana between the Cambrian and Silurian time. The basement rocks which form the backbone of the formation dating from the Cambrian through Silurian time possibly broke away from Gondwana and headed towards Laurentia, ahead of the Avalon terrane. During the Cambrian, the Avalon, along with its companion terranes, began migrating from the South Pole. These terranes collided with each other over time, as recorded by granite plutons in the rocks.[1]:15

Divisions[edit]

From west to east:[1]:15

  • Merrimack Terrane
    • Ware belt
    • Gardner belt
    • Wachusett Mountain belt
    • Southbridge belt
    • Nashua belt
    • Rockingham belt
      • Clinton-Newbury fault zone
  • Nashoba Terrane
    • Bloody Bluff fault zone
  • Avalon Terrane
    • Dedham granite
    • Boston Basin
    • Milford granite (part of the Rhode Island batholith)
    • Narragansett Basin
    • Fall River batholith
      • Nauset fault
  • Meguma Terrane

Recent history[edit]

Glaciation[edit]

Expansion[edit]

Plymouth Rock, a glacial erratic left behind at the end of the last glaciation period

Around one million years ago, the continent began experiencing regular ice ages.[1]:20 During the Illinoian period, the Laurentide ice sheet covered the entire state in thousands of feet of ice. The Wisconsinan glaciation, thousands of years later, is the most recent glaciation. Estimates of the thickness of this ice have produced results where Mount Wachusett is covered with slightly more than four hundred feet of ice while Worcester was covered by anywhere between fifteen hundred and two thousand feet. By 23,000 to 22,000 years ago, New England was covered in ice. The ice was so thorough in its destruction of the region that the only thing left from the previous glaciation is Nomans Land, to the southwest of Marthas Vineyard.[1]:21

The ice sheet also contained five glacial lobes which covered the state, the Hudson Valley lobe, the Connecticut Valley-Worcester Plateau lobe, the Narragansett Bay-Buzzards Bay lobe, the Cape Cod Bay lobe, and the South Channel lobe. Where each lobe met, dimples of sediment formed where the meltwater naturally flowed off the glaciers. Terminal moraines also formed at the end of these lobes. The lobes that bisected the state helped to form Long Island, Block Island, the Elizabeth Islands, Marthas Vineyard, Cape Cod, and Nantucket.[1]:21

Retreat[edit]

A view of the retreating cliffs along Cape Cod Bay, made up of deposits from the last glaciation

Around 21,000 years ago, the ice began to retreat. Ice began retreating from Long Island between 21,000 and 19,500 years ago.[1]:21 Dating of postglacial materials on Marthas Vineyard indicates that ice melted from it earlier than 15,300 years ago. The Cape Cod Bay lobe was parked outside what is now Boston for over a thousand years, beginning 16,000 years ago and receded into southwest Maine by 14,500 years ago. The melting ice released vast amounts of water, resulting in glacial lakes dotted across the state. The largest, Lake Hitchcock, stretched from Connecticut to Vermont and New Hampshire. The crust also rebounded at a rate of 4.74 feet per mile, undercutting many of the glacial deposits,[clarification needed] especially in the Connecticut River Valley.[1]:22

Where the moraines met away from the end of the ice also helped deposits. This can be seen in the outer arm of the Cape, originally hills when the ice retreated, they have eroded and shifted over time to help produce the characteristic arm of the Cape.[1]:23

See also[edit]

Notes[edit]

  1. ^ a b c d e f g h i j k l m n o p Skehan, James W. (2001). Roadside Geology of Massachusetts (2001 ed.). Mountain Press Publishing Company. ISBN 978-0-87842-429-0.  - Total pages: 379

External links[edit]