Owl Acres stands on a specific if tiny place on the earth’s surface. It can be measured in longitude, latitude and altitude coordinates and thus pinpointed precisely. With the help of satellites, my phone can give me those exact coordinates, and they would be unique to this very spot on the entire globe. But Owl Acres has been leading a double life for much of its history. The rock below a certain depth—about 150 miles or so, has carried on its melting and solidifying, flowing, compressing, expanding, and, deep down, conducting radioactive processes. In other words, the mantle and the core beneath present-day owl Acres, are, generally speaking, the same rocks that have been there since the beginning of time. The top layers, the brittle lithosphere and crust, have a different story.
Imagine you have a rubber ball with a layer of glass fused to the outside. If you drop that ball on cement, the glass will crack and break but will stick to the rubber. That’s kind of like what happened to the earth, except that it was stresses from within and perhaps an asteroid collision or two over a couple billion years that caused the earth’s crust to crack and break apart into separate chunks. Those chunks sat on a hot, malleable layer called the asthenosphere, which allowed them to move about on its surface.
The piece of crust that we know today as North America, (A.K.A. the Laurentian Craton) formed some 2 billion years ago. (A craton, according to the Oxford Dictionary, is “a large stable block of the earth’s crust forming the nucleus of a continent. The Laurentian Craton forms the nucleus of the North American continent and is the oldest such craton on earth. It extends today from northern Canada to Texas and from the Atlantic to the Rockies. Thus, the ancient rock of this craton lies directly beneath Owl Acres.
Two billion years ago, the earth was still organizing itself. It was a violent time when volcanoes and earthquakes were common. These upheavals broke the brittle outer layer of the earth into pieces. Some of those pieces, like the Laurentian Craton, were as big as continents. Over hundreds of millions of years, those huge chunks of the earth’s crust wandered around the globe, bumping into other pieces, sticking with them or breaking away. At least four times the Laurentian Craton, the chunk of earth that would eventually form the majority of the North American continent, found itself in the neighborhood of other continent-sized chunks, and decided to stay for a hundred million years or so, forming what scientists call supercontinents. Each time, our craton got tired of its neighbors and moved away, looking for a better neighborhood. By the beginning of the Cambrian period, 541 million years ago, it had located on the equator. It spent millions of years there as it began to build itself toward what we know today. But the earth does not stay still. Big plates of rock shift and move, forcing other chunks of rock, like the Laurentian Craton, to move and change. It happens on a time scale of millions and billions of years, and it leaves its evidence in each layer of rock and time. Our craton’s last supercontinent was called Pangea. After some 60 million years as part of the Pangea supercontinent, (about 180 million years ago), our craton declared its independence once more and moved away from the Pangea neighborhood, eventually ending up where it is today.
By studying the layers of rock, especially the most ancient rock that can still be seen in some places in Canada and the U.S., we can measure the age and length of time these processes occurred. These layers of time help us understand just what has happened beneath our feet. There are no exposed rocks on Owl Acres, unless you count the rocks we brought in for the driveway. But some 30 miles down, ancient bedrock occurs. In the top 30 miles, records of half a billion years of activity are preserved in layers that tell a more detailed story of the past.
Photo : USGS animation