Quick answer: New Jersey’s Watchung Mountains are three long ridges made from three separate basalt lava flows that erupted about 200 million years ago as the supercontinent Pangaea began to split apart. Gas bubbles and cracks in those flows later filled with zeolites, prehnite, calcite, and quartz, which is why quarries around Paterson are world-famous for collector minerals.
New Jersey’s famous trap-rock mineral finds, the stilbite, prehnite, and datolite that made Paterson-area quarries legendary among collectors, all come from the same geological feature: three stacked basalt flows known as the Watchung Mountains, each one a separate eruption event millions of years apart.
Three flows, not one
The Watchung Mountains, First, Second, and Third Watchung Mountain in local naming, correspond to three distinct lava flows, geologically named the Orange Mountain, Preakness, and Hook Mountain Basalt, erupted roughly 200 million years ago as the supercontinent Pangaea began rifting apart and the Atlantic Ocean started to open. Each flow cooled with its own set of gas cavities, and each was later invaded by its own generation of hydrothermal fluid depositing zeolite minerals, which means the specific basalt flow a quarry cuts into genuinely affects which minerals and crystal habits turn up there.
Rifting apart, mineral by mineral
The rifting itself is what made the whole system possible: continental crust stretching and thinning created exactly the kind of extensive fracturing and volcanic activity that later hosted the hydrothermal mineral deposition collectors prize today, the same rifting that, on a much larger timescale, eventually produced the Atlantic itself. This stilbite lot and the material behind our general Paterson piece both trace back to this same basalt sequence, just extracted from quarries cutting through different points in the three-flow stack.
It’s a genuinely rare situation where a century and a half of serious mineral collecting owes its entire existence to three specific lava flows and the continental rifting that produced them, a fairly narrow geological event with an outsized legacy in mineral cabinets worldwide.
Deeper dive: the lava that ended the Triassic
The Watchung basalts are not a local curiosity. They are a piece of the Central Atlantic Magmatic Province (CAMP), one of the largest volcanic outpourings in Earth’s history, exceeding 2.5 million cubic kilometres of lava, erupted as Pangaea tore open to form the Atlantic Ocean (Wikipedia, Hickory Grove Basalt).
- The middle Watchung flow, the Preakness Basalt, has the same chemistry as the Hickory Grove Basalt farther south and erupted at the same time, within a few hundred thousand years of 201.5 million years ago, based on zircon uranium-lead dating.
- Mercury and carbon-isotope records place that flow sequence within the main pulse of CAMP volcanism, which coincides with the end-Triassic mass extinction, dated to about 201.4 million years ago (Wikipedia).
Put another way: the ridge you drive over on Route 78 or Route 280 is made of the same eruptions blamed for one of the five great mass extinctions, the event that cleared the way for dinosaurs to dominate the Jurassic. The stilbite, prehnite, and calcite in a New Jersey specimen grew later, in the gas pockets and cracks those flows left behind.
Source: Wikipedia, Hickory Grove Basalt (citing zircon U-Pb geochronology and mercury/carbon-isotope studies).
