Out in some states in the western US, places where sparse plants compete with rock and Star Trek villains for attention, there can be found a thin grey line between sedimentary layers. This line, an abrupt change in the composition of the sandwiching rock above and below, is a visible remnant of one of the most recent world-changing events, an actual border between geologic periods. It is, to be dramatic, the day the dinosaurs died.
With that lede out of the way, now let’s get down to more accurate stuff. Not all of the dinosaurs died then, and it sure as hell wasn’t a day – single days don’t leave distinctive sediment layers. But this demarcation in the rock is what used to be called the KT boundary, or the line between the Cretaceous and Tertiary periods, Tertiary being the one we presently live in (and ‘C’ already in use for the Carboniferous period, so geologists went the product advertising route, or perhaps simply liked Krispy Kreme donuts.) Now, however, the present period has been renamed the Paleogene, and it has now become the K-Pg boundary, and not somehow the Cr-Pg one – whatever.
For decades, this boundary was known, since the sauropod fossils largely stopped everywhere above it, in newer sediment. But it wasn’t known what caused this mass extinction. It was assumed to be some kind of major event, since mass extinction just doesn’t occur every day, and something pretty radical to take out so many species. But until the late 1970s, everything was speculative.
At that time, various geological surveys had revealed that the sediment layer was rather high in the element iridium. Iridium doesn’t appear often in the mineral record, and it has two primary sources: volcanic eruptions, and asteroids. This layer of iridium was being found in the same boundary layer the world over, indicating an event of catastrophic proportions; a volcanic plume that extended, with noticeable sedimentary deposits, across most of the planet is a hell of a big eruption, or series thereof. And the same can be said of an asteroid impact. One way or another, the planet was radically affected by the event(s), to be changed forever.
An eruption (or series thereof) that big, extending ash deposits across most of the world, would also be producing vast amounts of greenhouse gases, perhaps for very long periods of time – centuries to millennia. This would naturally slam the climate and thus the plant life, and untold other portions of the ecosystem – not hard to see how this could wipe out quite a few species. On the other hand, a major asteroid impact that produced that much particle matter in the atmosphere would also have a drastic affect, but from cloud cover reducing the warmth from sunlight, rather than trapping it as greenhouse gases do. There would also be the effects of the immediate impact crater and the tectonic activity triggered by it, including massive tsunamis and shock waves. The biggest difference between the two would be the length of time the effects lasted; the impact scenario is generally envisioned to be a shorter period of time than a lasting series of volcanic eruptions.
Pretty much everyone knows the asteroid theory is the favored one, and this is because we actually know the asteroid that remains the prime suspect. Geologic surveys revealed an ancient crater on the tip of the Yucatan peninsula in Mexico, near the town of Chicxulub (“CHEEK-shoe-lube,” a popular lubricant in kinky adult stores in Mexico. Okay, not really.) The crater, now determinable only by the circular gravitational anomalies, the shocked quartz, and the peculiar cenote caverns of the region, is massive – and more importantly, dates to that exact time period. “Exact,” in geologic terms, means within a few hundred thousand years. This is a small portion of the interesting controversy about the K-Pg terminator.
Because before this crater was found, there was another possible culprit: a region of intense volcanic deposition known as the Deccan Traps, in what is now India. They also date from that time period, and feature lava and ash deposits that indicate a vicious, long period of vulcanism, one considered capable of changing the atmosphere of the entire planet. This remains a competing theory, and to me, a curiously compelling one, since the volume of basaltic deposits from lava are enormous – over 512,000 cubic kilometers, or 123,000 cubic miles, and this is today’s figure, after 65 million years or so of erosion. The lava flows themselves, 2 kilometers (1.2 miles) thick, cover an area the size of Spain. The gases emitted into the atmosphere from these eruptions, over a period of perhaps 30,000 years, had to have been of a remarkable volume themselves.
A bit of perspective. The geologic feature thought to be responsible for these eruptions, something called a mantle plume, would have been positioned off the coast of Madagascar, near Reunion Island – that’s where the Indian tectonic plate was then. In the time since, India has traveled north and collided with the Asian plate, rather forcefully, since this collision formed (or is forming, to be accurate) the Himalayas. Seen on a relief globe or decent 3D mapping service, this crumple zone of Earth’s crust is eminently visible. It’s an interesting illustration of timescale.
While the consensus seems to lean towards the asteroid impact, we still don’t know for sure. One of the main reasons why this is not settled is that the extinction of species appears to take place over a period of 300,000 years, which seems unlikely with something as short-term as the after-effects of an asteroid. But we’ve never seen a major impact event, or even a relatively minor one – we’ve seen, at best, tiny little things, like the Chelyabinsk meteor which didn’t really impact (it shattered and produced fragments, no craters, much less one 180 kilometers across – that’s almost the distance between Philadelphia and DC.) Would there, for instance, be ripple effects among species, certain ones getting hit first, whose loss caused Change A in the ecosystem, leading to Species B eventually dying out, with Change B resulting, and so on? We really can’t tell – the fossil record isn’t that detailed, nor do we have more than a cursory knowledge of the foods or habits of the species extant at that time. Species also go extinct without the help of any such dramatic events; the real causes may be multiple and murky, or even coincidental.
Another thing that I suspect might temper the Deccan Traps idea is the long time frame over which they occurred, perhaps never dumping too much gas into the atmosphere at once. 30,000 years is a significant length of time – much longer than humans have been on the American continents, longer than we’ve been without Neanderthals or mammoths. Yet there’s also the fact that the Permian-Triassic (P-Tr) extinction, 252 million years ago, coincides with another major volcanic period, a suspicious detail that makes one think, at least.
There is no doubt, however, that both events had their effects on the time period. The Chicxulub impactor produced debris extending across most of the US and Mexico; the Deccan Traps are among the largest volcanic deposits known. The asteroid impact effects might have been too brief to cause so major an extinction; the Deccan eruptions might have been spread out enough not to change the climate radically. It is even possible that both are at fault, a coincidental one-two punch that prevented the potential recovery from either alone.
I have no images to illustrate even this thin sediment line, since I’ve never been out west – I have plans to go, since I’d really like to see this for myself. I’d also like a rock sample that contains that line, but (departing from the original topic a little) I also find this a little silly. Holding such a rock in my hands won’t impart anything more to me than research does, considerably less in fact, and to be honest, any such rock around here might have its own link to cataclysmic events – I just don’t know about it. Like meeting a celebrity or viewing a famous painting, the thrill is all in our heads, a strange emotional sense provoked by a perceived importance rather than any external stimuli. I suspect that the sense is actually confirmation; we’ve heard all the stories, but it takes the input of our eyes, ears, fingers, to tell us they were real. We may never know exactly what happened 65 million years ago to eradicate so many species, but right there sits a memorial, the barest hint of an abrupt turn for species on Earth. The rocks are the only remaining witness, but we, at least, can sometimes read their story.