Doctor Rock,
How old are the oldest rocks in America, and where are they? How can I see them?
Jim Oldman,
Houston
Dear Jim,
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| An outcrop of Morton Gneiss in the Minnesota River valley |
The oldest dated rocks in the USA are in southwestern Minnesota and Michigan's Upper Peninsula. These rocks are called the 'Morton Gneiss' and are 3.6 billion years old, or 80% of the age of the planet (4.54 billion). The gneiss is a metamorphic rock, meaning it began as an even older sedimentary or igneous rock, but we don't know the age of those original rocks. The oldest rocks on earth are from an Eon called the Archean. During the Archean,
the cratons--the masses of rock that make up the basic, initial
structure of continents--formed. The crustal bodies that formed
were smaller than today's continents and are referred to as protocontinents. The oldest known anything on earth are zircon mineral grains 4.4 years old found in rocks in Australia.
Gneiss is a coarsely crystalline foliated
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| Archean rocks in North America |
metamorphic rock resulting from extremes of heat and pressure acting on even older rocks, which deform but do not quite melt. 'Foliated' means the mineral grains are aligned due to pressure. The Morton Gneiss was formed in the intense pressure beneath a long-gone mountain range that rose during the agonizingly slow-motion collision of continental plates. These collisions brought small land masses together to make up the interior of the North American plate.
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| This is classic geo-humor! |
Outcrops of gneiss in Minnesota can be found near Morton, Redwood Falls, Sacred Heart and Ortonville. To get to Morton, travel west from the Twin Cities on US 212 for about 100 miles, then south on US 71 for another 15 miles to the small town. Minnesota's own
EarthScienceGuy website gives more information on how to find outcrops of the rock near Morton.
But, you don't have to go to Morton to see the rock. The Morton Gneiss a widely-used building stone.
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Morton Gneiss on the exterior of Seattle's
Exchange Building, 821 Second Avenue. |
It is a stunning pink and black swirled rock, and very commonly used for head stones. Commercially known as ‘Rainbow granite', it is the oldest building stone you’ll find in the country, and perhaps the entire world. But, it isn't granite- many building stones carry inaccurate geologic terms. You can see Morton Gneiss in buildings all over the US: the Exchange Building in downtown Seattle, the West Publishing Building in St. Paul, the Banker's Life Insurance Building in Des Moines, the Adler Planetarium in Chicago, the Oklahoma Natural Gas Building in Tulsa, the Old State Library in Richmond, Virginia, Baltimore's Police Headquarters, and the AIG Building in New York. It is distinctive and once you are familiar with the rock's appearance, you may find it somewhere in your town. If you do, contact me via email and send a photo. tuckerd@geol.wwu.edu.
Learn more about the Morton Gneiss and see photos of cut rock used as building stone on my friend David Williams' website,
'Stories in Stone'. David is the author of
Stories in Stone- Travels through Urban Geology which describes building stones used in the US and their origins all over the world. And
here is another great website that describes and photographs the quarry operations.
