upright=1.4|Laurentia, also called the North American craton
Laurentia or the North American Craton is a large continental craton
that forms the ancient geological core
of North America
. Many times in its past, Laurentia has been a separate continent
, as it is now in the form of North America, although originally it also included the cratonic areas of Greenland
and also the northwestern part of Scotland
, known as the Hebridean Terrane
. During other times in its past, Laurentia has been part of larger continents and supercontinent
s and itself consists of many smaller terrane
s assembled on a network of Early Proterozoic orogenic
belts. Small microcontinent
s and oceanic islands collided with and sutured onto the ever-growing Laurentia, and together formed the stable Precambrian
craton seen today.
The craton is named after the Laurentian Shield
, through the Laurentian Mountains
, which received their name from the Saint Lawrence River
, named after Lawrence of Rome
In eastern and central Canada, much of the stable craton is exposed at the surface as the Canadian Shield
; when subsurface extensions are considered, the wider term Laurentian Shield
is more common, not least because large parts of the structure extend outside Canada. In the United States, the craton bedrock is covered with sedimentary rocks on the broad interior platform in the Midwest
and Great Plains
regions and is exposed only in northern Minnesota, Wisconsin, the New York Adirondacks
, and the Upper Peninsula of Michigan
. The sequence of rocks varies from about 1,000 m to in excess of 6,100 m (3,500–20,000 ft) in thickness. The cratonic rocks are metamorphic
with the overlying sedimentary layer
s composed mostly of limestone
s, and shale
s. These sedimentary rocks were largely deposited from 650 to 290 million years ago.
The oldest bedrock, the Archean provinces Slave
, and Nain
, are located in the northern two thirds of Laurentia. During the Early Proterozoic they were reactivated and covered by sediments, most of which has now been eroded away.
The metamorphic and igneous rocks of the "basement complex
" of Laurentia were formed 1.5 to 1.0 billion years ago in a tectonically active setting. The younger sedimentary rocks that were deposited on top of this basement complex were formed in a setting of quiet marine and river waters. During much of Mississippian
time, the craton was the site of an extensive marine carbonate platform on which mainly limestones and some dolomite
s and evaporites were deposited. This platform extended from either the present Appalachian Mountains
or Mississippi Valley
to the present Great Basin
. The craton was covered by shallow, warm, tropical epicontinental or epicratonic sea
(meaning literally "on the craton") that had maximum depths of only about 60 m (200 ft) at the shelf edge. During Cretaceous
times, such a sea, the Western Interior Seaway
, ran from the Gulf of Mexico
to the Arctic Ocean
, dividing North America into eastern and western land masses. Sometimes, land masses or mountain
chains rose up on the distant edges of the craton and then eroded down, shedding their sand across the landscape. Subduction
of the continent towards the Northwest, that lasted approximately 1.4 to 1.2 billion years, likely caused enrichment of the lithospheric
mantle beneath the orogenic belts of the Grenville Province
. This enrichment is thought to have contributed to the formation of the major supercontinent Rodinia
The southwestern portion of Laurentia consists of Precambrian basement rocks deformed by continental collisions (violet area of the image above). This area has been subjected to considerable rifting
as the Basin and Range Province
and has been stretched up to 100% of its original width. The area contains numerous large volcanic eruptions
The position of the equator during the Late Ordovician epoch
( million years ago) on Laurentia has been determined via expansive shell bed records.
Flooding of the continent that occurred during the Ordovician provided the shallow warm waters for the success of sea life and therefore a spike in the carbonate shells of shellfish. Today the beds are composed of fossilized shells or massive-bedded ''Thalassinoides'' facies (MBTF) and loose shells or nonamalgamated brachiopod
shell beds (NABS).
These beds imply the presence of an equatorial climate belt that was hurricane free which lay inside 10° of the equator at 22.1°S ± 13.5°.
This ecological conclusion matches the previous paleomagnetic findings which confirms this equatorial location.
Several climate events occurred in Laurentia during the Phanerozoic
eon. During the late Cambrian
through the Ordovician
, sea level fluctuated with ice cap melt. Nine macro scale fluctuations of "Global hyper warming", or high intensity greenhouse gas conditions, occurred.
Due to sea level fluctuation, these intervals led to mudstone deposits on Laurentia that act as a record of events.
The late Ordovician
brought a cooling period, although the extent of this cooling is still debated.
More than 100 million years later, in the Permian
, an overall warming trend occurred.
As indicated by fossilized invertebrates, the western margin of Laurentia was affected by a lasting southward bound cool current. This current contrasted with waters warming in the Texas region.
This opposition suggests that, during Permian
global warm period, northern and northwestern Pangea
(western Laurentia) remained relatively cool.
* Around 4.03 to 3.58 Ga
, the oldest intact rock formation on the planet, the Acasta Gneiss
, was formed in what is now Northwest Territories
(older individual mineral grains are known, but not whole rocks).
* Around 2.565 Ga, Arctica
formed as an independent continent.
* Around 2.72 to 2.45 Ga, Arctica was part of the supercontinent Kenorland
* Around 2.1 to 1.84 Ga, when Kenorland broke apart, the Arctican craton was part of the landmass Nena
along with Baltica
and Eastern Antarctica
* Around 1.82 Ga, Laurentia was part of the supercontinent Columbia
* Around 1.35–1.3 Ga, Laurentia was an independent continent.
* Around 1.3 Ga, Laurentia was part of the landmass Protorodinia
* Around 1.07 Ga, Laurentia was part of the supercontinent Rodinia
* Around 750 Ma, Laurentia was part of the landmass Protolaurasia
. Laurentia nearly rifted apart.
* In the Ediacaran
(635 to 541 ±0.3 Ma), Laurentia was part of the supercontinent Pannotia
* In the Cambrian
(541 ±0.3 to 485.4 ±1.7 Ma), Laurentia was an independent continent.
* In the Ordovician
(485.4 ± 1.7 to 443.8 ±1.5 Ma), Laurentia was shrinking and Baltica was expanding.
* In the Devonian
(419.2 ± 2.8 to 358.9 ±2.5 Ma), Laurentia collided against Baltica, forming the landmass Euramerica
* In the Permian
(298.9 ± 0.8 to 252.17 ±0.4 Ma), all major continents collided against each other, forming the supercontinent Pangaea
* In the Jurassic
(201.3 ± 0.6 to 145 ±4 Ma), Pangaea rifted into two landmasses: Laurasia
. Laurentia was part of the landmass Laurasia.
* In the Cretaceous
(145 ± 4 to 66 Ma), Laurentia was an independent continent called North America.
* In the Neogene
(23.03 ± 0.05 Ma until today or ending 2.588 Ma), Laurentia, in the form of North America, collided with South America
, forming the landmass America.
External linksPaleogeography of the Southwestern US
paleogeographic history of southwestern Laurentia, goes back to 1.7 billion years ago.
– Paleogeographic history of western Laurentia, goes back to the Permian period.
from The Smithsonian National Museum of Natural HistoryMap of Laurentia
Category:Geology of North America
Category:Precambrian North America
Category:Natural history of North America