Geological History of Jamestown, Rhode Island

Earth's First Four Billion Years
Formation of Proto North America

Many types of detailed evidence has enabled reconstruction of the development and movement of the world's land masses, as well as the evolution of life.

Continents and continental fragments have been sliding about on the earth's molten core, crashing into each other and then moving apart, for as long as there have been masses of land on the earth.

Forming the First Continents

Earth is about 4.5 billion years old. The oldest rocks are about 3.9 billion years old. Microfossils have been found in 3.5 billion year old rocks. At that time the earth's atmosphere was mostly CO2.

The development of anaerobic bacterial photosynthesis and blue-green algae lowered the level of CO2 in the atmosphere and released O2. This had a" reverse greenhouse effect" on the earth's energy balance, leading to widespread glaciation between 2.5 and 2.0 billion years ago. Oxygen levels stabilized below current level around 1.6 billion years ago, then resumed a slow rise - paving the way for new life forms dependent upon oxygen.

The initial land masses were hundreds of micro continents and island arcs. The first continental grouping is thought to have appeared about 3 billion years ago. Called Ur, it consisted of relatively small pieces of present day Africa, India, Australia and Antarctica.

By 2.5 billion years ago about 25% of the current continental crust had formed. These most ancient land masses, or shields, are shown in green on this sketch. (In their current locations, not where they formed.)

These lands moved continuously as ocean beds formed and spread. When larger bodies of land impacted with smaller bits, they were added to their margins. Collisions with larger land masses raised mountains and unleashed flows of magma which welded the colliding pieces together.

The early continent Artica (North America and Siberia) emerged about 2.5 billion years ago.

Around 2 billion years ago, Atlantica formed.  It consisted of parts of eastern South America and Western Africa. And, about the same time, Baltica (northern Europe) formed.

Approximately 1.9 billion years ago, Artica and Baltica collided, raising mountains on both continents. By about 1.5 billion years ago, 80% of the earth's continental crust had been formed.

Forming Proto North America (Laurentia)

The core of "Proto North America" was built by agglomeration of many relatively small land masses between 2.5 and 1.5 billion years ago. This agglomeration is known as Laurentia.

Laurentia was made up of six former microcontinents. Named "provinces", they are Slave, Nain, Superior, Wyoming, Hearne and Rae.

The oldest known rocks on earth have been found in the western part of the Slave Province, in Alberta, Canada. They are more than 4 billion years old.

Compare the purple areas of this sketch with the green areas in the sketch above.

Sediment built up along the continental margin of Laurentia. Between 1.3 to 1.0 billion years ago collisions with multiple continents thrust large amounts of that sediment onto the eastern seaboard of Laurentia, creating an enormous chain of extraordinarily high mountains that we now call the Grenvilles.  

The Grenvilles (colored red in the map above) formed in several stages as the result of multiple continental collisions. The initial collision occured around 1.3 or 1.2 billion years ago. Others followed over the next 200 to 300 million years; ending around 1.0 billion years ago.  

The Grenville mountains stretched from northern Mexico, through Texas, through the U.S. midwest and eastward to the eastern edge of Pennsylvania, into southeastern Ontario, Quebec, Labrador, and continued into southern Sweden. Grenville rocks are usually sedimentary rock with igneous intrusions that have been folded and metamorphosed.

(The Grenvilles have been eroded, buried and modified by later collisions. In most areas they are not visible at the surface; and where they are, what is visible is but a small remnant of their original mass.).

The Supercontinent Rodinia (circa 1 Billion Years Ago)

In this picture proto North America is labeled Laurentia, refering to the shield region plus subsequent "permanent" additions. Present-day Canada and Greenland are toward the east. Northern Mexico is on the Equator, to the west.

The Grenville collisions and contemporaneous collisions on the (present) western (then northern) side of the North American shield resulted in formation of a supercontinent, Rodinia. Rodinia straddled the equator and reached most of the way from pole to pole.

Rodinia was in existance for about 350 million years - from about 1.2 billion years ago to about 750 million years ago. Obviously, the configuration varied during its existance.The sketch shown below is consistent with most data and theories. All of the continental masses shown below have become incorporated into still larger continents and have moved about considerably over the past billion years.

There is ongoing research into the exact configuration of Rodinia and the detailed timing of events leading to its creation and breakup. These events happened before there were complex organisms to leave helpful clues in the rocks laid down at the time, there were many pieces to this puzzle, the events took place over hundreds of millions of years and these ancient shields and the newer rocks that were created along their margins are now, for the most part, deeply buried and distorted by subsequent events.

Click on the picture to see a larger version with additional notes and attributions.
 
When Rodinia was in existance, Laurentia (or proto North America) was sandwiched between two large blocks of what are now Southern Hemisphere continents.

To the north was the original continent of Ur, very much expanded after 2 billion years of growth and often referred to East Gondwana. To the southwest are pieces of the old continent of Atlantica (Congo, West Africa and Amazonia) - often referred to as West Gondwana. Baltica and Siberia, are nearby, to the southeast and northeast of Laurentia, respectively.

This artist’s conception of the supercontinent Rodinia as it began to fragment is by Tomo Narashima.

Rodinia broke into two pieces about 750 to 725 million years ago. The lands to the north of Laurentia (East Gondwana) drifted north, while rotating counter-clockwise, beginning the formation of the Panthassa (later Pacific) Ocean. Laurentia (along with Siberia and Baltica) and the components of West Gondwana moved southward, rotating clockwise. Laurentia approached the south pole, then moved northward again. (Greenland appears to have passed over the south pole around 600 million years ago).

Gondwanda / Pannotia (circa 550 Million Years Ago)

About 600 to 550 million years ago, most of the world's large land masses came together again, this time in the southern hemisphere. A new supercontinent, Gondwana (shown at right) was formed. North is to the right in this graphic - south to the left.  (The South Pole was near the "A" in "Africa").

A volcanic arc formed offshore of both Africa and South America. Called Avalonia, this string of islands would eventually contribute land to New England, other east coast US states, Canada, Ireland, England, Wales, Morocco, Portual, Spain, France, Belgium, Holland and Germany.

The J (near the left side of the drawing) indicates the approximate place where Jamestown first formed.

When Gondwana formed, Laurentia and Baltica were nearby, and some authorities have suggested that they may have been united with Gondwanda in an even larger supercontinent, Pannotia.

Collisions associated with this event are sometimes called the Pan-African Orogeny, which united most of the pieces of present-day Africa and South America and raised a ring of enormous mountains around the West African craton or shield as a result of the multiple collisions.

There is some question whether all of the proposed Pannotia components were fully and simultaneously connected or, if so, whether they remained connected very long. Whatever the exact truth may be, the approximate relative configuration of continents about 550 million years ago is generally agreed.

This period is known as the Vendian, a transitional period in the development of life. Many forms of multi-celled life appeared, some of which became precursors to later forms.

Ice House Earth

In general, the Earth has experienced periodic swings in average global temperature. Ice age conditions tend to be prevalent during times of continental collisions and supercontinent formation, especially when massive amounts of land are located near the poles. This was certainly true in the Late Precambrian. As large land masses moved toward polar areas, the global heat circulation system was disrupted and the earth's climate went from hot to cooler. The Grenville and newly-formed Pan-African mountain ranges both contributed to creating Ice House conditions where the South Polar Ice Cap covered much of Africa and South America.    The climate in North America, Europe, Siberia and the eastern part of Gondwana was, in contrast, warm and sunny. 

The Grenville Mountains Erode

Erosion and weathering wore the Grenville mountains down to their innermost cores which were covered by younger rocks that began forming approximately 600 million years ago.

Today, only a small portion of the remnants of the Grenville mountains can be seen at the surface; mostly in areas that were swept clean of overlying deposits of softer rock by glacial action or which have been subject to recent (and ongoing) uplift (the Adirondack dome).

Exposed Grenvilles can be seen in parts of Canada, the Adirondacks, the Hudson and Jersey Highlands, Manhattan, Westchester NY, the Reading Prong, the Green Mountains and the Berkshire Hills of western Massachusetts. (They are not found further east. New England and the Canadian Maritime provinces had not yet formed.)


Click this link to the next installment of this narrative

Avalonia

More than 550 million years ago, Rhode Island was part of a micro-continent named Avalonia, which formed off of Africa, south of the equator.
Avalonia drifted north and west, toward Proto North America.

Or, go directly to any Geological History page:

Introduction and Summary: 565 Million Years of Jamestown's Geological History
Prelude: The Earth's first 4 billion years - forming Proto North America, Rodinia, Gondwana
Avalonia: Rhode Island was once part of a micro-continent called Avalonia
Acadian Orogeny: Avalonia collides with the mainland of Proto North America (Laurentia)
Alleghenian Orogeny: North America collides with Africa, forming Pangaea
The Atlantic Forms: Pangaea breaks up, the Atlantic forms, the Appalachians erode
Glaciation: Glaciers form and rework the land
The Holocene Epoch: Post-glacial Rhode Island - rising seas - the time of modern man
Building the Northern Appalachians: Significant event summaries with links to more information
Guide to Bedrock in and around Jamestown and Narragansett Bay
Additional Information and References
 

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