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TITLE: United Plates of America


[Photo]
North America’s saga began over four billion years ago, when fragments of continental crust first formed. Some of these fragments coalesced into microcontinents, which later collided to assemble the craton — the heart of North America. Plate tectonics has since enlarged this craton, and sediment has blanketed much of it, obscuring its eventful origins.

 





Cutaway to the Craton

Throughout much of the central United States, younger sedimentary rocks conceal the craton. But in Michigan, Wisconsin, and Minnesota — as well as throughout much of Canada — it is well-exposed.

  [Photo:]

2 Billion Years Ago – A Continent Is Born

Six microcontinents — some formed from Earth's earliest continental crust — collided and stuck together to build the North American craton. The craton later became a part of three supercontinents: Laurussia, Pangaea, and Laurasia. Now it underlies 70 percent of North America. Parts of the craton contain rocks that range in age from 2.5 to 4 billion years, including Earth's oldest-known rock. Most were highly deformed and metamorphosed as the craton was assembled.

  Photo: Earth's Oldest-known Rock Photo: Granitic Gneiss  
  Earth's Oldest-
known Rock
Granitic
Gneiss
 
  [Photo: Craton]

1.8-1.65 Billion Years Ago – The Continent Grows

Less than 300 million years after North America's craton was assembled, vast tracts of rock were added to its margins as continental crust continued to form. A particularly large piece of new crust joined the craton's southern and southwestern edges between 1.8 and 1.65 billion years ago.

The piece of crust added 1.8-1.65 billion years ago now underlies a region that extends from Arizona to Missouri.

Photo: Gneiss
Gneiss

 

  [Photo: Big Addition]

1.1 Billion Years Ago – A Near Rift

North America narrowly escaped a fate that has befallen other continents: splitting apart, or rifting. About 1.1 billion years ago, from what is now Kansas to the shores of Lake Superior, the craton began to split. Had this rifting continued, North America would have become two continents separated by a new ocean basin. But after 20 million years, the rifting ceased. The failed rift is 2,000-km (1,200-mi) long. Huge volumes of basaltic magma welled up, creating a stack of igneous rocks 25-km (15.5-mi) thick. Their strong magnetic signals enabled scientists to determine the extent of the rift.

  Photo: Metamorphosed Basalt Photo: Native Copper-bearing Conglomerate  
  Metamorphosed
Basalt
Native Copper-
bearing Conglomerate
 

 

  [Photo: Failed Rift]

1.1 Billion Years Ago – Collision!

Soon after North America's craton began to split, another continent collided with its eastern edge raising a towering mountain belt. Surface rocks were buried to depths of 26 km (16 mi) and heated to temperatures of 700-800 °C (1,300-1,475 °F). Vestiges of this collision of continents are preserved in the metamorphic rocks to the right.

Eroded away long ago, the ancient mountain belt extended from present-day Labrador to Mexico and was 300-600 km (190-380 mi) wide.

  Photo: Photo:  
  Blue Quartz
Veins in Gneiss
Wollastonite
Skarn
 

 

  [Photo: Continental Collision]

700 Million Years Ago – Sediment Blanket

Starting about 700 million years ago, the North American craton was periodically flooded by shallow seas. They left behind widespread layers of sediment, which formed a veneer of sedimentary rocks. The rock layers are little disturbed — an indication that the underlying craton has been relatively quiet for the past 500-700 million years.

This map of North America shows how much of the craton is blanketed by sedimentary rocks.

 

  [Photo: Exposed craton and Sedimentary blanket]

These three specimens from southern Nevada include the craton and part of its sediment veneer. Cratonic schist was covered by near-shore sandstone, then deeper-water limestone, as a shallow sea encroached.

  Photo: Biotite Schist Photo: Granitic Gneiss Photo: Gneiss  
  Biotite Schist

Arkosic
Sandstone

Dolomitic
Limestone
 

 

  [Photo: Callville Formation, Tapeats Sandstone, and Vishnu Schist(craton)]

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Smithsonian National Museum of Natural History Department of Mineral Sciences website Credits