Monday, December 6, 2010

A Billion Years Ago in New Jersey

How old are the oldest rocks in New Jersey and where are they located?

Geologists know that the rocks in the mountains of North Jersey’s Highlands are the remnants of ancient Appalachian Mountains that at one time rivaled the Rocky Mountains.

It has been accepted that they are the oldest in the state at about a billion years old.

But the actual dating wasn't known precisely until the New Jersey Geological Survey, within the Department of Environmental Protection, teamed up with the U.S. Geological Survey and the Australian National University in a project funded by private grants to provide the most precise dating ever of New Jersey’s oldest rocks.

Rocks collected from the Highlands were analyzed using a highly sophisticated dating technique at the Australian National University. The technique allowed researchers to date the rocks to within nine million years of certainty, a degree of specificity never attained before.

The NJ Highlands at the Wanaque Reservoir
They found that the rocks are actually quite a bit older than the generalized billion-year-old estimate ascribed to them. Most of the rocks fell in a range of 1.02 billion to 1.25 billion years old, but a narrow belt stretching from Wanaque to Ringwood was dated at 1.37 billion years old, making these the oldest rocks in New Jersey.

“Unraveling the geologic history of the New Jersey Highlands from the age of bedrock obviously is interesting to scientists,” said State Geologist Karl Muessig. “But it also has practical applications for environmental risk assessment. For example, potassium-rich granites of a certain age in the Highlands contain higher concentrations of radioactive elements than most other granites and are likely to produce higher radon levels in soil and water. More precise mapping of these granites will help better identify areas that may pose greater public health risks from radon.”

Geologists have long understood that the mountains of the Highlands were formed during a mountain-building period known as the Grenville Orogeny, which occurred about a billion years ago.

Amazingly, this was a time when land that is now part of South America was adjacent to what is today New Jersey.

“Rocks of the New Jersey Highlands form the roots of the ancestral Appalachian Mountains that were formed during a collision of continental land masses about one billion years ago,” Volkert said. “The result of this mountain-building event uplifted the earth’s crust in eastern North America, including the Highlands, to heights rivaling the present-day Rocky Mountains.”

Hundreds of millions of years of weathering have left erosion-resistant granite and gneiss that form the rugged ridges and steep-sided hills that are characteristic of the Highlands region. But the wearing-away occurred unevenly, meaning the rocks that you see jutting from a hillside or at a valley floor - or even just a short distance from each other - are likely to be of differing ages.

When you consider that it has only been 65 million years since the extinction of the dinosaurs, and 200 million years since continental drift and plate tectonics opened up the Atlantic Ocean, the age of the rocks is awe inspiring.


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