Time & Deep Time

Banded Iron Formation, BIF, Lake Michigan's Left Coast, lakemichigansleftcoast.com, Elizabeth G Fagan

The Geologic Timeline

Put aside the drumbeat of your life. Clear your mind. Gather an image in your head of planet Earth spinning in tandem with Mercury, Venus, Mars, and the giant planets around the Sun, an average star in the Milky Way galaxy. How old do you reckon Earth is? The Banded Iron Formation (BIF) shown here is 2.7 billion years old—that’s billion with a B. It’s among the oldest rocks on Earth’s surface. But it doesn’t reflect the entire span of our planet’s lifetime. Read on for an amazing analogy.

To get a sense of the scale of Earth history, imagine walking back in time, 100 years per step—every pace equal to more than three human generations. A mile takes you 175,000 years into the past. [Twenty miles], a hard day’s walk to be sure, correspond to more than 3 million years.

But to make even a small dent in Earth history, you would have to keep walking at that rate for many weeks. Twenty days of effort at twenty miles a day and 100 years per step would take you back 70 million years, to just before the mass death of dinosaurs. Five months of twenty-mile walks would correspond to more than 530 million years, the time of the Cambrian “explosion”—the near-simultaneous emergence of myriad hard-shelled animals.

But at 100 years per footstep, you’d have to walk for almost three years to reach the dawn of life, and almost four years to arrive at Earth’s beginnings.

—Robert M. Hazen, The Story of Earth, p. 27

Eon Era Period Epoch Began Years Ago
Hadean* 4.6 billion
Archean 4.0 billion
Proterozoic 2.5 billion
Phanerozoic Paleozoic Cambrian 541.0 million
Ordovician 485.4 million
Silurian 443.8 million
Devonian 419.2 million
Carboniferous 358.9 million
Permian 298.9 million
Mesozoic Triassic 252.17 million
Jurassic 201.3 million
Cretaceous 145.0 million
Cenozoic Paleogene 66.0 million
Neogene 23.03 million
Quaternary Pleistocene 2.58 million
Holocene 11.7 thousand
Anthropocene** 10.0 thousand

* The Precambrian Supereon comprises the Hadean, Archean, and Proterozoic eons. The Precambrian begins with Earth’s formation 4.6 billion years ago and covers nearly 90 percent of our planet’s history. In the Precambrian, Earth was a place of intense chemical and geological processes in which life was virtually impossible. It ends with the Cambrian Period, 541.0 million years ago, when hard-shelled creatures appear in the fossil record. Because geological processes continuously recycle Earth’s crust, little evidence of the Precambrian exists.

** The Anthropocene is a proposed new epoch either after or within the Holocene. The word combines the root “anthropo”, meaning “human” with the root “-cene”, the standard suffix for “epoch” in geologic time. It is so named based on overwhelming global evidence that Homo sapiens has altered atmospheric, geologic, hydrologic, biospheric and other Earth processes. The start of the Anthropocene is being debated but is generally agreed to begin approximately 10,000 years ago, about BC 8000, with the end of the last glacial period.

Elizabeth G Fagan
Elizabeth G Fagan

about the author

The 2012 superstorm Hurricane Sandy is mainly remembered for devastating New York City. Sandy was so massive she caused 25-foot waves on Lake Michigan in Chicago, some 900 miles to the west. Millions of stones washed up onto Montrose Dog Beach, the place where Fagan regularly walked her German Shepherd Rosie. A childhood interest was renewed as she collected rocks and fossils on that beach. She continues rockhounding on Lake Michigan’s Left Coast with her second-generation German Shepherd, Ruby.
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