Geological History of Jamestown, Rhode Island
First Four Billion Years
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.
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 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 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)
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.
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