Watersmeet Gneiss
This earthcache brings you to the Watersmeet Gneiss, recently identified as the oldest rock formation in the United States. The Watersmeet is part of a mantled gneiss dome whose core dates to the Achean, preserving evidence of Earth's earliest continental crust. This earthcache discusses the age and formation of these ancient rocks and introduces cachers to basic concepts of zircon geochronology. Please note the terrain and difficulty ratings - the earthcache requires a bit of a hike across a portion of the Ottawa National Forest. The ground is soft and uneven and can be pretty wet at times.
Note: Geocachers accessing this earthcache must park at the designated parking locations or a more distant approved parking area and hike to the outcrop. It is not legal to park on the shoulder of US45 for anything other than emergency purposes. Doing so to access this cache is dangerous and could result in your vehicle being ticketed or towed.
Watersmeet Gneiss Outcrop
Questions
1) Describe in your own words how such ancient rocks could survive in this part of North America.
2) Based upon the information provided below, how old is the Watersmeet Gneiss?
3) Observe the outcrop at the posted coordinates. Do the rocks lay horizontal and flat or do they exhibit evidence of uplift and folding? What evidence do you observe that leads to this conclusion?
4) Observe the banding in the gneiss. Which minerals are more abundant - the lighter feldspars and quartz or the darker biotites? Based on the information provided below, which mineral is the most abundant in the gneiss?
5) Do you see examples of augen mineral grains in the rocks at the posted coordinates? If so, are these minerals common or rare in the rock?
6) Post a picture of yourself or an item of your choice at the outcrop located at the posted coordinates.
Cratons and Early Continents
The western portion of Michigan's Upper Peninsula is located within the Canadian Shield, a belt of exposed Precambrian (older than 541 million years) rock that forms the core of the North American craton. A craton is a vast region of tectonically stable continental crust. Such crustal rocks were highly deformed during the formation of cratons billions of years ago, but since then have remained largely unmodified except at their edges, where they collide with other plates, or in regions where underlying volcanism intrudes into them.
The reason cratons are stable has much to do with their formation billions of years ago. The Earth's surface is divided into two kinds of tectonic plates - silica-rich continental crust and denser iron and magnesium-rich oceanic crust. When these two kinds of crust collide with one another, the denser oceanic rocks will subduct under the lighter continental rocks. This process continues to the present day in places like the Pacific Northwest, resulting in mountainous volcanic landscapes like the Rocky Mountains and Mount St. Helens. As the subducting rocks descend into the mantle, these rocks lose some of their lighter minerals due to differential melting. These minerals return to the surface as lava, adding additional silica-rich rocks to the continental crust.
Many geologists believe that melting of mantle rock located below the oceanic crust was more severe during the early formation of continents than in the present day because these portions of the upper mantle had not yet become depleted of 'hot' radioactive isotopes. As a result, they became less dense than the underlying mantle or the overlying oceanic crust and peeled off, adhering to the basement of continental rocks that were less dense still. In the present day, these cool, relatively isotopically inert zones of rock form mantle keels of intermediate density that continental cratons ride upon, further preventing subduction and recycling into the mantle.
Archean Geochronology
After billions of years of stability, these ancient cratonic rocks may become exposed at the surface, where they can be sampled by geochronologists and dated. Dating ancient rocks is not straightforward, however, as these rocks often existed for billions of years before the continents 'chilled out' and became tectonically stable. As a result, they typically preserve several different volcanic and metamorphic events, not just their period of initial formation.
Geochronologists date rocks by sampling minerals within the rocks that contain radioactive isotopes within their chemical structures. One of the most common minerals used for dating is zircon (ZrSiO4), which will incorporate radioactive uranium. Geochronologists can measure the decay of uranium into lead within zircon grains and obtain an estimate of the age of the zircon. However, the decay clock gets reset any time a rock is subjected to high heat or pressure during volcanic and metamorphic events. This means that the same geologic formation will often yield multiple ages depending on where the formation is sampled and the specific geologic history of that part of the rock. Additionally, older zircon grains can become incorporated into younger rocks through sedimentary processes or differential melting, further complicating the process.
So, how do geochronologists sort through this array of ages to identify those that reliabilty date the rocks? The short answer - statistics. They plot the ages and standard deviations graphically and identify clusters of dates that are in concordance, meaning that they are analytically identical to one another. Greater concordance within a group of dates means greater reliability, while discordance indicates uncertainty. Plotting all dates from a given rock on a concordia diagram allows geochronologists to identify all major volcanic and metamorphic events represented, which can be correlated with identical-aged events in other dated rocks. They can also identify the oldest cluster of dates, which likely represent the age that the rock originally formed.
Formation of the Watersmeet Dome
You are standing within the Watersmeet Dome, a 25 x 8 km mantled gneiss dome that is a part of the Canadian Shield. Like all cratonic rocks, those of the Watersmeet Dome have a complex history. Throughout the Precambrian, mountain building tectonic processes folded younger Proterozoic rocks into older Archean rocks, resulting in several major deformation, partial melting, and metamporphic episodes, each of which are recorded in the zircons found within these rocks. The most recent event is the Penokean orogeny, dated to ca. 1.87 to 1.83 billion years ago, after which the dome was deeply buried, only to be exposed again in the present day after hundreds of millions of years of erosion.
The Watersmeet Gniess
The exposure located at the posted coordinates is of Watersmeet Gneiss. This rock is located at the center of the Watersmeet Dome. Geologists refer to the Watersmeet as a 'tonalitic augen gneiss', which means that it is a coarse-grained, layered metamorphic rock that is rich in feldspar. Mineralogical analysis of the Watersmeet indicates that it comprises approximately 48% plagioclase, 30% quartz, 8% K-feldspar, and 13% biotite grains and likely originates from the metamorphism of feldspar-rich igneous rock. The augen structure of the Watersmeet refers to the presence of light-colored, 'eye-shaped' plagioclase grains within the felsic lenses of the layered gneiss.
Example of Large Augen Structures (image courtesy USGS)
Geochronologists have obtained three major clusters of concordant dates from the Watersmeet Gneiss. The oldest cluster dates the original formation of the rock at 3.62 billion years. A second cluster at 2.59 billion years dates a major volcanic event that intruded the gneiss. Finally, a cluster of dates between 1.75 and 1.8 billion years can be attributed to the Penokean orogeny. Interestingly, discordant dates ranging up to 3.8 billion years indicates the presence of even older zircons that likely became incorporated into the gneiss due to melting of earlier continental rocks.
So, we currently believe the Watersmeet Gneiss to be the oldest rock in the United States, at 3.62 billion years old, but older zircons preserved within the rock indicate that even older continental crust existed nearby. This means that future research may reveal previously undiscovered 3.8 billion year old rocks in the United States. This would still place us behind Canada, which boasts the oldest known rocks in the world - the Acasta Gneiss, dated at 4.03 billion years.
Sources: 1) Frost et al. 2025. "USA's Oldest Rock? A Simple Question with a Complex Answer." GSA Today 35(3-4):4-10; 2) Mindat.org - Watersmeet Dome, Gogebic County, Michigan, USA; 3) "How Continental Keels and Cratons May Have Formed." Earth-logs.org October 2020; 4) "Canadian Shield." The Canadian Encyclopedia (thecanadianencyclopedia.ca); 5) "America's Oldest Rock" PlanetGeo Podcast; 6) "Granite Augen Gneiss." USGS.gov