Yale Graduate School of Arts and Sciences

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In the Company of Scholars:
A Geologist’s Perspective on Planet Earth

David Evans (BS 1992), professor of Geology and Geophysics, takes the long view of things. When he spoke to the Graduate School community in February, giving this year’s first ‘In the Company of Scholars’ lecture, his remarks spanned billions of years.

Evans began by tracing the history of planet Earth beginning with its formation some 4,600,000,000 years ago. In less than an hour, he brought the audience to the present day, predicted where humankind would be in 50,000 years, and suggested what the Earth itself might look like far in the future.

David Evans

David Evans

His talk, “Past and Future Movements of Continents Across Earth’s Surface, and a Geologist’s Perspective on the Ultimate Fate of Humankind,” was divided into three parts. First he gave a brief introduction to geology and the concept of “deep time”; next, he described the motion of continents across the Earth’s surface; and finally, he took a look into the future.

Our planet, he explained, is made up of about 15 tectonic plates that shift over time. Sometimes the edge of one plate slides under another; sometimes two plates collide and push up mountains. Around 300 million years ago, most of the plates collided, forming one super-continent called Pangaea. They started drifting apart 200 million years ago. A rift system opened up the Atlantic Ocean and pushed North America away from Africa. Over time, the plates drifted apart into the familiar land masses we see on maps. Current scientific evidence suggests that in the distant future, the plates may drift together again into a new configuration.

“In Connecticut, rifting has left behind igneous (volcanic) rocks that we know as West Rock, East Rock, and Sleeping Giant,” he said. These are the “direct result of the tearing apart of the crust 200 million years ago. They are embedded within a series of sedimentary rocks in a great rift valley. On either side of Connecticut’s central rift valley are the ‘highlands.’ In these areas, there is evidence of a great collision zone, a sort of stirring of the pot, with rocks organized into folds,” he explained. These folds are the Appalachians, which may have once been as impressive as the Himalayas, he said.

For Evans, the tectonic plates are like pieces of a jigsaw puzzle. Most of his research seeks to solve how the pieces fit in the past. What makes the puzzle difficult is that over millions of years, the edges have been frayed by geological events, so they don’t fit neatly together the way a commercial jigsaw puzzle does.

“We’re making progress by looking at the magnetization of rocks and realizing that Earth’s magnetic field sends off field lines that go through the Earth’s surface into space and back to the surface,” he said. “Qualitatively, if you were at the North Pole, the magnetic lines would run straight down. We take rock samples formed millions of years ago, measure the magnetic direction in those samples, and use this to draw maps of the Earth from different ages. We collect samples with a hammer or a portable rock drill and use an orientation device while the sample is still in place, so when we’re back in the lab, we’ll know where it was obtained and how it was oriented vertically and horizontally.” His research has taken him to Angola, Australia, Botswana, Brazil, Canada, China, Finland, Mexico, Mongolia, Namibia, Russia, and South Africa.

Provisional map of the world, 635 million years ago. North America appears turned on its side in tropical southerly latitudes, contiguous as a single continent with Greenland, Scotland, and Siberia. Red jagged lines indicate mid-ocean ridges that accommodated the breakup of Rodinia supercontinent, predecessor to Pangea. Green curves are subduction zones, where tectonic plates converge. Colored dots locate sedimentary accumulations in either deep water (dark blue), shallow water (light blue) or above sea level (earth tones). From Li, Evans, and Halverson, 2013, Sedimentary Geology v. 294, p. 219-232.

In Yale’s Paleomagnetic Facility, well-shielded from the ambient magnetic field, Evans and colleagues use a magnetometer to measure the magnetic direction of their samples and plot the data on a map to see where they originated and where they were found. Relative to the ancient position of the North-South Pole axis, assumed to be coincident with the ancient magnetic axis (and verified by numerous previous tests), Evans and colleagues are able to reconstruct the position of the continents over time.

“Geological science gives a unique perspective on humans’ evolving place within the natural world,” Evans said. That place is humbling. The oldest tangible signs of life on Earth are probably the conical rock structures known as stromatolites, found in the shallow waters of the Bahamas, western Australia, and South Africa, he said. These appeared 3.4 billion years ago, about a billion years after the formation of the Earth itself. If the whole of the planet’s history were compressed into one calendar year, our hominid ancestors showed up “late on New Year’s Eve,” he explained. In the last minutes of the year, they tamed fire, made music, and figured out how to brew alcohol — just in time to celebrate, he joked.

“Aside from the academic implications for the inner workings and changing surface environments of our planet,” his findings also provide “a framework for understanding patterns in the distribution of natural resources such as metal ore deposits and fossil fuels,” Evans said. After illustrating the long-term paleogeographic history of the planet, he concluded with speculations on how human society might fare “in the not-too-distant future, beyond our current age of unprecedented resource extraction which has enabled our high standard of living.”

Looking ahead 50,000 years, Evans asked the audience to consider six fates for humanity that have been proposed by scientists, religious groups or others: natural extinction caused by a meteor impact, pandemic, supernova explosion, or other catastrophe; manmade extinction from nuclear holocaust or other disaster; “End Times” religious apocalypse; conquest or annihilation by an invasion of extraterrestrials; survival, with human life confined to Earth or the inner solar system; and survival, with humans living in deep space. He said global warming is not a likely cause for human extinction, but allowed that it “might make us very uncomfortable.”

His own prediction is cautiously optimistic: people will manage to find solutions to the problems we face, and we’ll remain on Earth or nearby. He warned that the human population explosion that followed the Industrial Revolution has led to an unsustainable expenditure of “nature’s treasure trove – minerals and fossil fuels,” and that our era will one day be called the “Oil Age.” If the population doesn’t drop and if we don’t harness renewable energies, “We will outstrip the resources” of our planet. The ideal economic growth projection, taking the long view, he said, is zero percent.

Projecting millions of years into the future, Evans conjectured that, based on current trends in the movement of the tectonic plates, India will continue to collide with Asia; Australia might move north and crunch into East Asia; the Americas might change their current westward course and shift northward, and the west coast of South America might join up with the east coast of North America. “This is speculation, not science,” he noted.

In the Company of Scholars lectures are sponsored by Thomas D. Pollard, dean of the Graduate School of Arts & Sciences. Most recently in the series, there was a talk by Associate Professor of Political Science Susan Hyde, on March 25, who addressed the question, “Does Democracy Promotion Promote Democracy?” The final lecture this year will be on Wednesday, April 16, when Gary Tomlinson, the John Hay Whitney Professor of Music & the Humanities and director of the Whitney Humanities Center, presents “One Million Years of Music: The Emergence of Human Modernity.”