Geologic time scale /

How old is the earth? The majority of scientists agree that the earth was formed about 4.5 billion years ago. Just how long ago was that? To really get to the bottom of that concept, you have to think in terms of geologic time.

Humans have difficulty wrapping our heads around the concepts of geologic or “deep” time. If we’re lucky our lives play out over 80-100 years. This seems like a very, very long time to most of us, but in terms of geologic time, it’s as insignificant as a grain of sand on a beach.

One of the easiest ways to understand the enormity of deep time and the relatively short time humans have been around is to compare the entire four and a half billion year history of the Earth to one calendar year. In this comparison, one day in our example year would represent 12.3 million years in the evolution of the earth.

Our earth’s existence as a planet dawns at sunrise on New Years Day in our analogy year. The oldest rocks round outcropping on the earth today are ~3.9 billion years old metamorphic rocks in Greenland, which would have formed in late February in our example year. Before that, the earth was all molten rock. By the end of March, water started to fill the oceans, an atmosphere took shape, and life appeared on earth. Animals with skeletons didn’t show up in the seas of earth until 540 million years ago, or November18th in our example year. The first plants evolved from algae around 400 million years ago, or on November 30th.

Stromatolites are ancient fossil rocks that preserve cyanobaterial colonies thought to have lived around 2.7 billion years ago, or near the end of May in our example year. /

Think about that for a second – that means that if you think about the history of our planet in terms of one calendar year, there wasn’t even anything alive even remotely resembling an animal until the end of November.

Dinosaurs roamed the earth en masse from the late Triassic period, about 230 million years ago, for 160 million years until they were mostly wiped out by a giant comet that smashed into the earth near the present day Yucatan Peninsula in Mexico 65,000,000 years ago at the boundary between the Cretaceous and Tertiary periods. In our example year, the dinosaurs would have appeared on Earth December 13th, and all traces of them would have vanished by December 27th.

An artist’s rendition of a comet smashing into the earth, like the one thought to have caused the extinction of the dinosaurs some 65 million years ago. /

Some of the events I discussed in the last few weeks’ columns are interesting to consider in terms of our calendar year analogy. The Appalachian Mountains towered as high as today’s Himalayas at around the 4th of December. The granite that would later be uplifted to form the Sierra Nevada mountains of Yosemite cooled underneath ancient volcanoes 100 million years ago, or on Christmas Eve. The Long Valley Caldera near Mammoth Lakes erupted 760,000 years ago, or about 10:30 pm on December 31st. The Sierra Nevada were eroded by glaciers that carved the lines we rip at Squaw in the Pleistocene epoch, 2.5 million years ago, or two and a half hours before midnight on New Year’s Eve.

Humans appeared on Earth about 200,000 years ago, or 24 minutes before midnight on December 31st in our example year. Our species evolved to the point of being physiologically equivalent to our moderns selves at around 50,000 years ago, or only 6 minutes before midnight. Jesus was born 2011 years ago, or 14 seconds before midnight. The 1960 winter Olympics at Squaw were held 51 years ago, or 0.35 seconds before midnight.

Hopefully all of the above examples provide you with a better way to visualize the enormity of geologic time. When it comes down to it, we humans really haven’t been around on earth for very long at all. In the context of the history of our planet, we have done incredible amounts of potentially irrevocable change to our natural environment in no time at all. With climate change playing out around us right now, it’s somewhat disconcerting to think about what other changes might lie around the corner and how quickly they are taking place.

Further reading:

The “calendar year” analogy of deep time is outlined in great detail by the geologist and author John McPhee in his book “Annals of the Former World,” which would be a fantastic read for anyone casually interested in learning about geology, plate tectonics, paleontology, and even climate change.

Next week:

Next week Dr. Kaye is taking a week off from writing this column because he is in New York City to compete in the Red Bull Creation contest creating the future. So look for a fresh Ask Dr. Kaye column the following week on July 17th. Post your questions in the comments and he will pick a good one to answer.


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