Michigan lays claim to oldest rocks in US. Where you can find 3.6 billion-year-old stone

Michigan, Minnesota and Wisconsin all feature ancient rocks known as gneisses — metamorphic rocks banded with various minerals — but an analysis determined that the gneiss in northern Michigan is the oldest. Experts traced the age of the respective gneisses by dating zircon, a mineral in the rocks.

A March-April 2025 report of the Geological Society of America estimates the state's Watersmeet gneiss to be more than 3.6 billion years old, beating Minnesota's Morton gneiss, now considered 3.5 billion years old, for most ancient.

"(B)ased on our analyses, we propose that the Watersmeet gneiss wins the prize for the oldest rock, at >3.6 Ga," the report stated. "Taken together, the Archean rocks of the Wyoming-Minnesota-Michigan block represent the oldest continental crust in the United States, the nucleus around which the younger rocks of the nation were assembled."

Nawadaha Falls, a waterfall on the Presque Isle River Michigan's Upper Peninsula in Porcupine Mountains Wilderness State Park.

Here's what to know.

What is now the oldest rock in the US?

Michigan's Watersmeet gneiss is now estimated to be the oldest rock in the country at more than 3.6 billion years old, researchers Carol D. Frost, Paul A. Mueller, Marion E. Bickford and Robert J. Stern said in the GSA report.

How was the determination made?

The researchers studied the ages of comparative gneiss rocks in Michigan, Minnesota and Wyoming using newer techniques. The scientists dated the rocks using zircon, a mineral in gneiss, the GSA said.

"Dating zircon grains using ... techniques is now celebrated as the optimal method for determining when igneous rocks formed," the report said. The technique involves measuring the decay of various isotopes within a rock sample.

What is gneiss?

"Our candidates for the oldest U.S. rock are ancient gneisses with complex histories. As one might expect, these candidates do not exist in isolation but are parts of larger entities commonly referred to as age provinces, gneiss complexes, terranes, or cratons. The oldest rocks in the United States are located in the north-central part, where our three candidates are found, in the Archean Wyoming Province, the Minnesota River valley subprovince of the Superior Province of the Canadian Shield; and the Watersmeet gneisses of the Upper Peninsula of Michigan. Each of these candidates is a gneiss that was originally an igneous rock," the report explains.

Where is Watersmeet gneiss in Michigan?

What was the previous oldest rock in the US?

The Morton gneiss rock in Minnesota was previously known to be the oldest rock in the U.S. at more than 3.8 billion years old, ScienceNews reported. However, the new analysis dates the rock to about 3.5 billion years old, the GSA report said.

"The Morton gneiss — no longer the oldest rock in the world, or in the United States — nevertheless serves as an outstanding example of how U-Pb zircon data can be used to unravel complex Archean histories from a single sample," the GSA report stated.

Contact Jenna Prestininzi: [email protected].

This article originally appeared on Detroit Free Press: Michigan lays claim to oldest rocks in US. Where you can find 3.6 billion-year-old stone