The Jurassic-Cretaceous Extinction

Back to Contents

    In the middle of the Mesozoic Era, as the Jurassic Period faded into the Cretaceous Period, about 145 million years ago, life on Earth suffered a small but noticeable increase in the rate at which species went extinct, a minor mass extinction. Stegosauroids and some sauropods died out on land. In the sea some ammonoids went extinct. Among the marine reptiles, the ichthyosaurs and the plesiosaurs (mosasaurs didn't exist until the late Cretaceous), species went extinct at a greater than normal rate, decreasing diversity in those families. Diversity was also diminished among bivalves (essentially clams). There wasn't a lot of loss: as a mass extinction it's barely noticeable. But it did occur and we want an explanation for it.

    So far paleontologists have identified at least three major extinctions; the Permian-Triassic (251 million years ago), the Triassic-Jurassic (201 million years ago), and the Cretaceous-Tertiary (65 million years ago), that were caused by the effects of asteroids striking Earth, flood vulcanism being one of the major effects. In those events vast lava flows, covering hundreds of thousands of square kilometers, pouring out of Earth's mantle year after year, spewing huge quantities of carbon dioxide and sulfur oxides into the atmosphere. Those events, much more than the relatively local asteroidal impacts that caused them, extinguished whole genera, even families, of living beings. Sulfur oxide particles, condensing high in the atmosphere, formed a world-spanning haze that reduced the amount of solar heat reaching the ground and thus caused a rapid cooling of the planet, distressing life that was accustomed to a warmer world and changing the weather. At the same time it destroyed the ozone layer, allowing more of the deadlier frequencies of the sun's ultraviolet radiation to reach the ground. When the sulfur washed out of the atmosphere it acidified the rain, thereby adding more distress to an already distressed biosphere. Finally, carbon dioxide asserted its main effect, inhibiting the flow of thermal radiation from Earth's surface into space, thereby causing Earth to heat up until the increased rate of radiation from the ground restored the balance with solar radiation reaching the ground: that global warming altered the climate, making the weather display greater extremes of behavior. Those effects, taken together, imposed upon Earth's biosphere a distress that some life-forms could not endure. Thus, when we see evidence of a mass extinction in the fossil record, we expect to see evidence of flood vulcanism occurring at the same time.

    For the Jurassic-Cretaceous extinction, until recently, there was no indication of the massive lava flows that we would expect to find at the time of the extinction, nothing like the Siberian Traps (251 million years ago) or the Deccan Traps (65 million years ago). But now we have a candidate for a lava flow associated with the Morokweng impact (145.0 " 0.8 million years ago). It lies near the antipode of the impact and it occurred at the same time as the impact, within the error bars. The Morokweng crater, 70 kilometers in diameter, lies a little over 400 kilometers west of Johannesburg, at latitude 26˚ 28' South and longitude 23˚ 32' East, so we expect to find some kind of volcanic anomaly, something not due to normal geological processes, near 26.5˚ North, 156.5˚ West (or 203.5˚ East). Our candidate, dated at 144.6 " 0.8 million years ago, lies at latitude 33˚ North and longitude 158˚ East. The candidate is Tamu Massif, an extinct shield volcano or lava flow that lies about two kilometers beneath the surface of the Pacific Ocean.

    Covering an area roughly the size of New Mexico (over 260,000 square kilometers), the Tamu Massif lies atop the Shatsky Ridge, which itself may have been caused by the shock waves from the Morokweng impact. It lies currently about 7˚ north and 45˚ west of the Morokweng crater's antipode, but continental drift is sufficient to explain that discrepancy. It lies where three tectonic plates come together, perhaps the result of one plate being cracked by the force of the Morokweng impact. Some geologists believe that it exists as a result of hot spot activity, but there doesn't seem to be a good explanation for why a hot spot exists at that location.

    By way of contrast, consider Hawai'i. The Big Island itself lies atop a hot spot that provides lava for Moana Loa and Kilauea and also for the growth of a new island-to-be called Lo'ihi. That hot spot lies at the end of a chain of islands, atolls, and seamounts (the Emperor Seamounts) that extends across the Pacific Ocean from a point off northern Kamchatka where the Kuril-Kamchatka Trench meets the Aleutian Trench. We can see that the hot spot originated (a little over 85 million years ago) where two subduction zones come together, so we understand the existence of Hawai'i as representing a normal tectonic process, even if we don't fully understand how it works.

    Tamu Massif seems to have no explanation in terms of known tectonic processes. But we can see how the shock waves from the Morokweng impact propagated through Earth, were refocused at the impact's antipode, and generated sufficient force to burst open the crust and push lava up onto the sea floor. We don't know that this proposition stands true to history, but it gives us a good possibility.

    The Morokweng impact was spectacular, certainly, but what happened at its antipode was even better. Imagine an aerial view over the Pacific Ocean when the shock waves hit. Over an area tens of kilometers wide the ocean turned white with spray and humped up as if God Itself had dropped a depth charge onto the site, sending a super-tsunami surging out in all directions. Then the scene turned into something more impressive and terrifying. As incandescent liquid rock heaved up out of the mantle and surged across the ocean floor it superheated the overlying water. Under more than six hundred atmospheres of pressure (over six kilometers down) the water would not have become vapor. The hot water rose and when it reached a depth where the pressure was low enough, it blossomed into bubbles of steam, which rose rapidly, enabling the water under them to rise even faster. Over an area perhaps one hundred kilometers wide or wider the ocean exploded and boiled.

    As an immediate consequence, the atmosphere formed a self-assembling steam engine, a super-typhoon, that hovered over the site of the Tamu lava flow for years. Though proportionately smaller, that storm was analogous to Jupiter's Great Red Spot. But it was certainly big enough to alter Earth's climate and thereby contribute to the extinguishing of a number of species of life.

    Another factor in the disaster involved the release of vast amounts of carbon dioxide into the atmosphere. A large fraction came directly from the Tamu lava, but more came from heat driving dissolved carbon dioxide out of the seawater. The extra carbon dioxide necessitated that Earth warm up enough to force the trapped radiant heat through the atmosphere and out into space. That warm-up altered the climate in ways that extinguished creatures already on the verge of extinction and thus made the time one of transition from one geological period (the Jurassic) to another (the Cretaceous).

efefef

Back to Contents