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Wednesday, October 5, 2022
Dr. Brown: Geological and universe time very different than human time

Five billion years ago when our solar system took form, the early sun was surrounded by a vast swirling disc of stardust left over from the collapse and explosions of earlier stars in the neighbourhood.

From that disc, through a process of progressive accretion, the Earth formed more than 4.5 billion years ago.

It would be several hundred million years before Earth settled enough for the earliest signs of life to appear and a further two billion years before photosynthetic cyanobacteria harnessing light energy were able to generate enough oxygen in the seas and atmosphere to support the evolution of high energy dependent mammals and eventually highly complex intelligent life like humans.

That’s one of many breathtaking stories recounted by master geologist turned storyteller, Andrew H. Knoll, a Harvard professor in his 2021 book, “A Brief History Of Earth.” He traces the more than 4.5-billion year dynamic history of how Earth came to be the world we recognize and offers glimpses of where Earth might be headed in the future.

Knoll reminds us it wasn’t so long ago that our ancestors believed that coastlines, mountains and valleys were permanent fixtures of Earth.

That was until findings such as fossilized shark’s teeth jutting out of stones in the Tuscan hills, marine fossils buried in limestone at the top of Mount Everest or the Alps, or the close fit between the coastlines of the Americas and Europe and Africa, combined with a growing body of other evidence, suggested a far more changeable and mobile Earth than earlier generations of humans might have imagined.

The fact the eastern seaboard of the Americas seems to fit snugly into the western seaboard on the other side of the Atlantic suggested to a few globe watchers that at one time they might have touched one another. Evidence came in the Second World War when sonar revealed mountains and trenches in the Atlantic seabed and later, in the 1950s, a prominent north-south ridge at the bottom of the Atlantic was found.

Then came evidence that the magnetic polarity of samples flipped several times in samples taken at successively farther westward or eastward intervals from the ridge.

These natural polarity reversals were found to occur at intervals of several hundred thousand years and strongly suggested the Atlantic sea bottom, and hence the Earth’s crust, was growing, beginning at the ridge line, and expanding in ribbons of alternating polarity toward the Americas west of the ridge and Eurafrica on the other side.

Later studies showed the Earth’s crust near the ridge was much younger than successively older samples taken farther away from the ridge. On the scale of human lifetimes, the expansion of the Atlantic is glacially slow – roughly an inch a year. But played out over millions of years, the story is starkly different. The Earth’s continents, with the exception of Antarctica, formed a single continent 180 million years ago.

Moreover, the evidence suggests there have been at least five such separations and contractions of continental tectonic plates in the past and they are slated to continue far into the future for as long as the magma beneath the Earth’s crust remains hot enough for convection currents to provide the engine for moving continental plates around.

If Atlantic tectonic plates are growing, somewhere else tectonic plates must be shortening, usually by a process called subduction, whereby one plate is forced beneath an adjacent plate.

That’s precisely what’s happening in the Pacific, where subduction is taking place beneath the western edge of the Americas and in a ring, which reaches from Alaska, through the Aleutian chain of islands and down the western side of the Pacific to Indonesia. That subduction ring is responsible for a string of volcanoes and the development of mountain ranges, including the Andes in South America.

How can we determine the age of the Earth? Serendipity helps.

Dating from its infancy, the Earth’s crust contained small silica grains, called zircons, some of which contained tiny bits of Uranium 238, which decays to lead with a half-life of 4.47 billion years.

Importantly, there wasn’t enough room when the crystals formed for lead to be incorporated. That means any lead found later in those crystals must have come from the decay of Uranium 238. 

Using this Uranium 238 clock, geologists were able to date some rocks in riverbeds in Australia containing early zircon crystals as far back as 4.38 billion years. Impressive but probably not the earliest.

That's a brief sample of what’s in store for you should you read Knoll’s book. For my part, I will view some of those geological wonders with a different eye now and be reminded once more that geological and universe time are very different than human time.

Dr. William Brown is a professor of neurology at McMaster University and co-founder of the InfoHealth series at the Niagara-on-the-Lake Public Library.