Volcanic rocks reveal Americas collided millions of years earlier than thought

A team of geoscientists led by Victor Piedrahita and J. Li has redrawn the timeline of one of the Americas' most consequential geological events: the collision between the South American Plate and Central America's continental mass. The evidence comes from an unexpected source—volcanic rocks in Colombia that preserve a magnetic record of what the earth was doing millions of years ago.

The researchers focused on rocks from the Combia Volcanic Province in central Colombia, material that crystallized between roughly 12 and 6 million years ago, during the late Miocene epoch. This was a critical window in geological time, when the two continental plates were actively interacting and reshaping the landscape. By studying the magnetic properties locked inside these rocks, Piedrahita's team could read the deformation history written into the stone itself.

The technique they employed is called magnetic fabric analysis. It works by examining how magnetic minerals within a rock are oriented—their internal geometry and alignment. When magma erupts and cools, or when volcanic debris settles, these minerals align in characteristic patterns that reflect the original volcanic process. But if tectonic forces later deform the rock, squeezing and twisting it under the immense pressure of colliding plates, those forces leave their own signature in the magnetic fabric, overprinting or erasing the original pattern. By comparing what they found to what they expected, the researchers could determine whether deformation happened before the rocks formed, during their formation, or long afterward.

What they discovered was striking: most of the volcanic rocks from the late Miocene still retained their primary magnetic fabrics—the original signatures of volcanic emplacement. This preservation meant the rocks had experienced little to no major tectonic deformation during the time they were forming. Some locations showed evidence of deformation, but it was limited in both intensity and geographic reach. The picture that emerged was one of relative tectonic quiet during the late Miocene in this region.

If the rocks were experiencing minimal deformation by 12 to 6 million years ago, that meant the most intense phase of the collision had already concluded. The strongest crustal shortening—the compression and thickening of the earth's crust that happens when two continental masses collide—had mostly finished before the late Miocene began. Piedrahita and Li concluded that the major collisional events between Central and South America occurred during the Oligocene and middle Miocene, roughly 34 to 12 million years ago. By the time the Combia volcanic rocks were forming, tectonic deformation had weakened considerably and become confined to smaller, more localized zones.

This revision matters because it changes how scientists understand the building of the Andes and the broader geological architecture of the Americas. The collision between these plates was not a single, instantaneous event but a prolonged process spanning millions of years. Knowing when the most intense phase occurred helps refine models of how mountain ranges grew, how the crust thickened, and how the landscape we see today took shape. It also demonstrates the power of magnetic techniques to interrogate ancient rocks and extract their hidden histories—a method that could be applied to volcanic regions elsewhere on Earth where direct evidence of past tectonic activity is scarce or ambiguous.

The work was funded by grants from China's National Natural Science Foundation, reflecting the international collaboration that increasingly characterizes modern earth science. As researchers continue to refine the timeline of the Americas' collision, these Colombian volcanic rocks will remain among the most eloquent witnesses to what happened beneath the surface, millions of years before humans walked the land.