TRENTON — How old are the oldest rocks in New Jersey and where are they located? Perhaps these questions haven’t exactly kept you up at night, but geologists have been wondering about them for a long time.
They know that the rocks in the mountains of North Jersey’s Highlands, remnants of ancient Appalachian Mountains that at one time rivaled the Rockies in might, are the oldest in New Jersey. They also accept that these rocks are about a billion years old. But they never knew precisely how old — until now.
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.
Rich Volkert, a geologist with the New Jersey Geological Survey, and a colleague from the U.S. Geological Survey in Denver, collected rocks from the Highlands, which were then analyzed using a highly sophisticated dating technique at the Australian National University.
The researchers were able to date the rocks to within nine million years of certainty, a degree of specificity never attained before. 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 — 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.
Volkert was joined by John Aleinikoff of the U.S. Geological Survey in looking for a comprehensive cross-section of rocks that would give them their best chances of finding the fine degree of age differences they were seeking.
The rocks were shipped to the Australian National University in Canberra, where scientists used an intense beam of energy to measure the half-life of radioactive isotopes in zircon particles that contain lead and uranium. This method is known as the uranium-lead method of geochronology using the Sensitive High-Resolution Ion Microprobe (SHRIMP) technique.
“This is the state-of-the-art method for determining the precise age of ancient rocks such as those that have been metamorphosed under conditions of high temperature and pressure as is the case with the bedrock in the New Jersey Highlands,” Volkert said.
The rocks that the researchers dated are as old as those found in the Adirondacks and parts of southeastern Canada. The belt of oldest rocks is some 350 million years older than previously known. That’s quite a difference, even considering the long spans of time geologists work with.
“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,” Volkert said. “This is why we are so excited about the new level of understanding we now have about the age of these mountains.”
“This type of study is a geologist’s dream come true,” Muessig added. “Over the years, the New Jersey Geologic Survey has produced significant insights into these ancient and complex rocks, but the analytical methods used in this study have opened the door on the true antiquity of the Highlands. Through a more precise scientific understanding, comes an even deeper respect for these venerable mountains.”
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