Humans in Geologic Time

by Marli Miller

You don’t see many rocks where I live in western Oregon. Lush forests or savannah-like areas dotted with Oak, yes, but the bedrock mostly lies beneath the vegetation and a thick mantle of soil. Just south of town though, the highway cuts through a hillside to expose rock that reminds us of our geologic setting – and taken in context, points to Earth’s incomprehensibly long history.

I’m standing in a soft rain at the foot of the road cut, looking upwards at some fifty feet of strata. The rocks are sedimentary, having been deposited in lakes and rivers, but there’s also a thin layer of light-gray ash near the top of the cut and a much thicker one near the bottom – and slashing vertically through everything are two narrow gray, almost black, bodies of igneous rock. Called dikes, they worked their way upwards as molten rock along cracks in the older sedimentary rock before cooling and crystallizing where they are now.

Highway Roadcut showing sedimentary rock of the Oligocene Fisher Formation cut by mafic dikes, Oregon.

The road cut described in the essay showing the lower ash as well as the vertical dikes.  Photo: Marli Miller

Evidence of ancient volcanic activity abounds: ash, dikes, even the sedimentary rocks are made mostly of volcanic particles. And on clear days, I can walk to the south side of the road cut and look eastward to forested ridges of the Western Cascades that give way in some fifty miles to the snow-capped peaks of the High Cascades. They’re all volcanic. The High Cascades volcanoes are young and active whereas those of the western Cascades are long extinct and deeply eroded. Combined, the two parts of this range formed over a period of about forty million years and produced miles and miles of lavas, ash and debris flows, and volcanic-rich sedimentary rocks.
Trying to imagine this forty million-year history is like watching the stars, filling me with wonder and leaving me humbled, subdued, and exhilarated, all at the same time. And by geologic standards, forty million years isn’t extremely long. Death Valley, California, one of my favorite haunts, showcases some 30,000 feet of sedimentary rock that was deposited over a period of 400 million years, between about 700 and 300 million years ago – and beneath them lies rock that records conditions on Earth’s surface over a billion years ago. Anyone can see these rocks, touch them, and imagine the incomprehensible.

Basement rock in Death Valley National Park.   Photo: Marli Miller

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When I was in college, I encountered a textbook passage that helped me visualize geologic time. The author, Don Eicher of the University of Colorado, imagined Earth’s entire 4.54 billion year history compressed into a single year. He pointed out that, in this calendar, Earth’s first organisms would have lived sometime in late March, the first creatures that produced shelly fossils appeared November 17, and the first land plants around November 23. He included many noteworthy events, but they didn’t include the Cascade Range. By my calculations, the forty million years of Cascade volcanic activity existed for about the last three days of this calendar year. The first humans? They appeared about 35 minutes before midnight.
It boggles the mind to think about how much humans have done in those thirty-five minutes, so much so that we’re considering naming a geologic period of time after ourselves: the Anthropocene. It’s likely that we’d designate it an “epoch” so it would naturally come after our present epoch, the Holocene, which followed the Pleistocene (Ice Age), which followed the Pliocene, and so on. Of course, any formal designation would require people to agree on exactly when it began, and for that there is little consensus because humans caused major changes on Earth at different places during different times. The one time most researchers agree affected everywhere at once was the widespread atomic weapons testing in the late 1940s and 1950s, which spread radionuclides across the planet and are now an identifiable part of the sedimentary record.
Without question, humans are an indomitable geologic force, especially since the early 1950’s. It’s no wonder many researchers call that time “The Great Acceleration,” when our technology and energy use increased exponentially. And less than forty years later during the 1990s, we recognized that our activities moved more material each year than any other natural force such as rivers or landslides or glaciers. Since then, our “bioturbation” has only increased. We have colonized every habitable nook and cranny on this planet and are precipitating environmental catastrophes that we’re only beginning to comprehend.
But to call this human time period a geologic epoch seems to imply a certain longevity to both the period and to its trademark origin: Homo sapiens – and since 1950, the earth has aged barely a half second in its year-long calendar. I wonder how much longer we can last. To me, the concept of the Anthropocene as an epoch seems contingent on us surviving our environmental crises and continuing for thousands of years. After all, the Holocene Epoch, which it would replace, lasted for 10,000 years, and all the epochs before lasted millions. Instead of a new epoch, we may instead be in a transitional period from the Holocene to something that postdates Homo sapiens. If that’s the case, then we’re witnessing more of dramatic event: mass extinctions and environmental changes that will herald an altogether different ecosystem.
I reach down at my feet and untangle a wet plastic bag from some rocks that have fallen from the road cut. So different, yet both are pieces of sediment just the same. Both may find their way into the rock record. I wonder how our part of the record will appear long after our time has passed. I imagine we’ll be a readily identifiable layer, thick in some places and thin in others, and everywhere representing the same moment in time. It won’t be an epoch, but a fleeting second or two in Earth’s year of ages.

Marli Miller, PhD, of the University of Oregon, teaches, researches, and writes about geology and contributes geology photographs to other instructors and researchers through her website Among her books are Roadside Geology of Oregon (2014), Roadside Geology of Washington (2017) and Oregon Rocks (2021).