Castastrophes and mass extinctions – are they periodic?

            From T.S. Elliot’s poem “The Hollow Men” (conclusion):

This is the way the world ends

This is the way the world ends

This is the way the world ends

Not with a bang but a whimper.

            Actually, it will probably be a slow bang.

            At the Ask-a-Geologist desk, we have received quite a number of end-of-the-world queries. These could be consolidated into a single sentence with two parts:

            Q: (1) Are mass extinctions real, and (2) will another one happen soon?

            A: There has been accumulating evidence over the past century that animal life on Earth has been decimated repeatedly. The biggest documented extinction events (though not all) are:

  • ~440 million years ago (the demise of the Bryozoa, among other fossil species, marking the end of the Ordovician period),
  • 251 million years ago (the “Great Permian Extinction” or “Great Dying” that saw the disappearance of over 95% of all genera living at the time including the Trilobites),
  • 219 million years ago (the end of the Carnian stage in the late Triassic period, coincident with the appearance of the huge, ~85-km Manicougan craters in Quebec, Canada),
  • 65 million years ago (the end of the Cretaceous period and with it most of the dinosaurs), sometimes called the Chicxulub event, named for a village in northern Yucatan, Mexico.
  • 33 million years ago (The demise of the Cassidaria family of mollusks near the end of the Eocene, after ancestors of the horse first appeared),
  • 1 million years ago (the boundary between the Pliocene and the Pleistocene epochs).
  • 40,000 and…
  • 12,000 years ago.

            What could possibly cause all these extinctions?

            In the past century geologists have come to realize that the Earth’s crust doesn’t change gradually, but instead it apparently evolves episodically – something usually called “punctuated equilibrium.” The “punctuations” generally take one of two general forms: asteroid or comet impacts, and episodic convulsions of the Earth’s deep interior.

            Let’s consider the first possible reason for extinctions: asteroids or comets.

Positions of Near-Earth Objects (NEOs) in the inner Solar System (Image: NASA/JPL).

            For some time now, astronomers have known about a 26-to-30-million-year cycle of our Solar System – that it oscillates in and out of the plane of the galaxy as it revolves around a supermassive black hole at the galactic core (Sagittarius-A* in the center of the Milky Way). There is a very rough (in other words, very arguable) periodicity in asteroid impacts mapped in the Earth’s crust. Some scientists put this periodicity at about 26 million years.

            The thinking goes something like this: as the Solar System passes through the plane of the Galaxy, there are more close approaches by other stars, which disturb the previously-stable orbits of Oort belt objects. These are icy planetesimals orbiting far beyond Kuiper Belt objects like Pluto and Sedna, reaching out to 50,000 astronomical units from the Sun (up to a light year away). The Oort belt is where most of the comets come from. Thus, a disturbance out at this distance could send one or more into the inner Solar System. These may directly impact the Earth, or may disturb or deflect one or more asteroids orbiting between Mars and Jupiter. Asteroids are far more common in the mid-to-inner Solar System, but comets generally have a much high relative velocity with respect to Earth. Since kinetic energy goes as the mass times the velocity squared, a comet could potentially do quite a bit more damage for the same size/mass if it impacted the Earth.

            For more than a century now, scientists have been aware of these extinctions in the paleontological record. The cause of the great Permian Extinction of 251 million years ago is still not fully understood, but may be related to huge seafloor craters now known to exist off the northwest coast of Australia (Bedoubt) or the Falkland Islands east of Argentina. The extinction of the dinosaurs 65 million years ago actually has a ‘smoking gun’: a huge, 150-to-180-km crater now lying beneath the northern edge of the modern Yucatan Peninsula of Mexico. There is other evidence: ginormous tsunami deposits elsewhere in the Caribbean including Haiti, a tektite strewn field throughout the American southeast, and distinctive fragments found in Montana, and in eastern Pacific Ocean deep-sea drill cores thought to be from this impact.

            Keep in mind that the Earth’s crust is a very dynamic place; while we see thousands of craters on the Moon, we see few on the Earth. Careful mapping has identified only 184 asteroid-impact craters on the Earth, even counting the tiny recent ones like Wabar in Saudi Arabia, and Henbury in Australia. The Earth’s crust is evolving constantly because of plate tectonics and weathering, so any evidence of impacts is steadily being erased.

            The second possible reason for mass extinctions: gargantuan volcanic eruptions.

            An article in EOS, the Transactions of the American Geophysical Union (Rampino, et al., 2013), points out that there have been roughly cyclic episodes of large igneous provinces (LIP’s). The Deccan Traps, making up much of western India, is just one of these provinces: kilometers-thick, near-continent-sized basalt flows all erupted over a fairly short window of time (geologically speaking). A vast basalt province in Siberia called the Siberian Traps, and the huge Columbia River basalts are among the others. These are thought to be the result of large upwelling mantle plumes; for scale imagine the US east of the Mississippi River being covered by miles-thick flows of basaltic lava.

            The geologic record shows these LIP’s to have occurred around

  • 390 million years ago
  • 295 million years ago
  • 251-250 million years ago (the Siberian Traps)
  • 200 million years ago
  • 185 million years ago
  • 135 million years ago
  • 100 million years ago
  • 65 million years ago (the Deccan Traps occurred close in time to the Chicxulub impact, causing some confusion about a possible connection between the two events)
  • 30 million years ago
  • 17-14 million years ago (the Columbia River Basalt province).

            From these ages, frequency-domain filtering (and your eye if you plotted them out) suggests an apparent rough cyclicity of 28-to-35 million years, especially prominent starting 135 million years ago.

            When volcanic centers this size erupt, there is a huge degassing process associated with it: sulfur dioxide and vast amounts of carbon dioxide are released. When Mount Pinatubo erupted in 1992, it sent a proportionally smaller cloud of SO2 into the stratosphere – and the Earth’s average temperature cooled for two years afterwards. And that’s just from what happens in the stratosphere, from a single point volcano.

            Could there be a third reason for mass extinctions?

            Around 40,000 years ago, most of the large animals of Australia abruptly disappeared. These included the rhino-sized, wombat-like marsupials called Diprotodons, giant 200-kg kangaroos, a goanna bigger than the modern Komodo dragon, a giant goose-like bird twice the size of the emu, and many others. These animals had survived at least two episodes of climate change prior to 40,000 years ago. In North America about 12,000 years ago, most of the large “charismatic megafauna” of North America (mammoths, giant sloths, camels, cave bears, saber-tooth tigers, etc.) suddenly disappeared. In both cases, these mass extinction events (and a more recent event on Madagascar that is still very much on-going) correlate closely with the arrival of the human species in these regions. The implication of overhunting is hard to miss here. As the human population surges past 7 billion today, the largest mass extinction in the past 65 million years is fully underway, and the Passenger Pigeon is just one of the first and best-known victims (Kolbert, 2014). Habitat loss, overhunting, and accelerating climate change are the proximate mechanisms for this current and stunningly rapid mass extinction event.

            The End of Things As We Know It

            There are Near Earth Objects (NEO’s) out there that NASA and the US Air Force are monitoring (the number keeps growing, but at least the search process is now automated). Based on their known sizes (we can generally only see the big ones) and what happened at Chicxulub 66 million years ago, many of these could wipe out human civilization as we currently know it… not if, but when, one hits us.

            If Yellowstone (just one of several known supervolcanoes) unzipped tomorrow, it would cover the eastern two thirds of the United States with a vast blanket of ash, suffocating most living things within the first several hundred kilometers in all directions. The gas-release and the peripheral consequences would devastate the entire planet. To put things in perspective, the last eruption 640,000 years ago left an off-white layer 20 meters (65 feet) thick called the Pearlette Ash Formation near Colorado Springs… 800 miles away. Nothing survived under this ash blanket. I have personally pulled a camel’s tooth from the bottom of this formation.

            However, the real problem may be even more imminent. The greatest mass extinction in the history of the planet is underway right now (Kolbert, 2014). The levels of carbon dioxide in the atmosphere crossed over 400 ppm in 2013 (Showstack, 2013), and the rise is accelerating.

            It reached 415 ppm in 2019 ( ).

The Keeling Curve, showing atmospheric carbon dioxide in the atmosphere (Image: Peter Dockrill, Science Alert, 2019).

  As Pogo said, “We has met the enemy, and it is us.”