Geologists have long struggled to understand massive time gaps in Earth’s rock record, where millions—sometimes billions—of years are missing. Now, researchers at Utah State University have unveiled a groundbreaking forensic method using rusted iron minerals that may finally uncover when and how these puzzling intervals formed.
A New Forensic Clock Hidden in Rusted Minerals
A new study by Jordan Jensen and Alexis Ault, published in the March 4, 2025, issue of Geology, introduces a technique that uses iron-oxide minerals, particularly martite, to precisely date ancient oxidation reactions that occurred as rocks neared Earth’s surface.
These oxidation reactions, akin to the rusting of iron, act as natural time stamps that record when rocks were exposed to water and oxygen—conditions that only exist near the planet’s surface.
“A challenge for geoscientists is accurately constraining when rocks resided in the near-surface environment,” said Alexis Ault, associate professor at USU’s Department of Geosciences. “It’s tricky to pinpoint the timing of such processes, because the geologic evidence has often been erased.”
This new method offers a way to trace those elusive timelines with much greater confidence.
Unconformities: Earth’s Missing Geological Chapters
Unconformities are massive gaps in the geologic record, where ancient rocks directly underlie much younger strata—indicating that significant erosion events occurred in between, wiping away all trace of intermediate layers.
“Unconformities in the rock record are like missing chapters in the book of geologic time,” said Jordan Jensen, a USU Presidential Doctoral Research Fellow. “These gaps are the physical manifestation of past erosion events that removed evidence of past landscapes and environments.”
One of the most famous of these features is The Great Unconformity, which spans large areas across North America and separates billion-year-old igneous and metamorphic rocks from younger, often fossil-rich sedimentary layers. It’s visible in iconic locations like the Grand Canyon, but its origins remain hotly debated.
Martite: A Silent Witness to Deep Time
The study focuses on martite, a form of iron oxide that develops when magnetite transforms into hematite through oxidation. Though martite retains the outward appearance of magnetite, it contains microscopic hematite crystals that tell a very different story beneath the surface.
“Like diamond and its conversion to graphite, magnetite is not stable at Earth’s surface and slowly transforms to hematite in a process similar to how iron metals rust when exposed to air,” said Jensen. “Martite is often mistaken for magnetite, because its exterior still preserves the appearance of magnetite.”
Using advanced tools like scanning electron microscopes and (U-Th)/He thermochronometry, the team could detect these changes and precisely determine when oxidation occurred. That oxidation marks the moment the rock came close to Earth’s surface.
Dating the Great Unconformity Back to 1.4 Billion Years
Jensen and Ault applied their method to samples from 1.7-billion-year-old rocks located below a major unconformity in the Colorado Front Range, west of Denver. Their analyses yielded oxidation dates as old as 1.04 billion years, suggesting the unconformity could have formed as early as 1.4 billion years ago.
“When magnetite is oxidized, the geologic clock is reset, so to speak, revealing when these rocks were pushed to the near-surface of the Earth,” said Jensen.
This dating pushes back the likely formation of some parts of the Great Unconformity by hundreds of millions of years, challenging prior assumptions that tied its origin to a period of global glaciation known as Snowball Earth, which began around 635 million years ago.
A Broader Tool for Geological Time Travel
Because martite is widespread across different rock types and regions, the research team believes this method can be applied to many other sites globally to study weathering, erosion, and the development of critical mineral deposits over deep time.
“These tiny and resilient martite grains preserve the story of when these rocks were first exhumed to the Earth’s near surface, despite the many events like burial and mountain-building that could have destroyed the evidence,” Jensen explained.
In particular, a subset of their analyzed martite grains suggests that the erosion responsible for the Great Unconformity began much earlier than previously believed, predating the Snowball Earth glaciations by several hundred million years at their study site.
Rusted Rocks as Earth’s Ancient Timekeepers
The findings offer more than just a new data point—they provide a way to reconstruct lost epochs in Earth’s dynamic past. By tracking how and when rocks were exposed at the surface, geologists can piece together major shifts in tectonics, climate, and surface processes that shaped today’s continents.
“Martite is an iron-oxide and my research group is known for using iron-oxide textures and (U-Th)/He analyses to fingerprint earthquakes and slow slip events in seismically active faults,” noted Ault.
Their technique doesn’t just target Earth’s past—it could become a powerful tool for forecasting the future, especially in the context of mining, carbon cycling, and the long-term evolution of the continental crust.
