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You'd think the impact of an asteroid 10 kilometers across which exploded with the energy of 25 million one-megaton nuclear warheads and which carved a crater 20 kilometers deep and 300 km across would create enough devastation to wipe out the dinosaurs, wouldn't you?
But, it turns out, it had help.
The asteroid in question slammed into the Earth about 66 million years ago just off the coast of the present-day Yucatan Peninsula, centered near a town called Chicxulub (pronounced cheek-shoo-lube), so we call this the Chicxulub impact. The reign of terror started by this event was global and catastrophic: The direct impact was scary enough, but it set off a chain of other events equally devastating, including wiping out the ozone layer, raining down fiery tektites across the planet, and more. For a long time this impact hypothesis was pretty controversial, but it's now widely accepted as being the main cause behind the mass extinction event that ended the Cretaceous Era, and 75% of all species of life on Earth with it.
However, evidence is mounting that the impact alone, as apocalyptic as it was, didn't wipe out the dinosaurs all by itself. At least, not directly. It's looking more like the ridiculously huge impact triggered seismic activity all over the world, including volcanism on a terrifying scale. This was a one-two punch that doomed a huge number of plants and animals across the planet.
Scientists have long looked toward India's Deccan Traps for evidence of this. This is a vast area of volcanism, covering over half a million square kilometers to depths of up to two kilometers. The traps actually predate the time of the impact, but right around that time, 66 or so million years ago, the output of the Traps underwent a serious uptick. The idea is that the seismic waves screaming through the Earth opened up the valves, so to speak, allowing the eruptions' rate to increase. Studies of bivalves in the oceans support this idea, too, showing massive climate change on geologically rapid timescales. Together, these effects wiped out the non-avian dinosaurs and a lot of their cousins.
A new and very interesting study has just come out supporting this claim. It starts on the assumption that if the impact greased the wheels for eruptions nearly all the way around the planet at the Deccan Traps, then there may have been other seismic events triggered as well.
The study focuses on mid-ocean ridges. These are long chains of undersea mountains created by magma upwelling from the mantle as continental plates drift apart. As the plates separate magma seeps into the crack between them, cools, and creates new seafloor. The mid-Atlantic ridge is probably the most well-known of these, but they exist literally all over the world.
This new work looked at the amount of magma extruded over time in these ridges, and found that right around the time of the Chicxulub impact something like 200,000 to six million cubic kilometers of magma was extruded above the usual rate. Correlation isn't causation, but the timing makes it look very much like this was related to the impact.
Just as cool is how they found it: Using satellites that measure the sea level height! You might think the height of the ocean is constant, forming the surface of a sphere, but it's not. If there is denser than normal material under one spot in the ocean, then it has a little bit more gravity than the stuff around it. That draws water to it, creating a bulge in the sea level height. This is a subtle effect, and has to be teased out from other effects like changes in water temperature (warmer water expands) and flowing currents, but a lot of satellite data over a long time is now available, making model gravity maps of the sea floor possible.
Using these models, the scientists found the excess magma and were able to get the ages of it by looking at how rapidly a given ridge is spreading; if for example each plate moves away from the upwelling lava at, say, 35 millimeters/year, then after a million years it will have moved 35 million millimeters, or 35 kilometers.
Together, these data show that there was a pretty decent spike in the rate of magma seeping out of the Earth's mantle 66 million years ago. It may have been caused by the impact the same way the Deccan Traps get ramped up: The vast energy of the seismic waves created by the force of the impact shook the crust of the entire planet, making it easier for the magma to well up.
This is incredible to me. When I was a kid the extinction of the dinosaurs was the biggest mystery in science, at least as far as the public was concerned. When the asteroid impact idea came around there were a lot of arguments — pretty heated ones — but over time the evidence grew and a consensus started to settle. Arguments became more detailed and narrowly focused as the broader implications of the hypothesis were accepted.
For example, it seemed that the amount of devastation was hard for an impact to accomplish, especially over such a huge length of time (hundreds of thousands of years or more for some lingering effects), and it looked more and more like something else must have happened.
And now it seems we have another nearly literal smoking gun: tectonics amplified by the impact.
This is all still pretty new, and over the next few years we'll learn more, and zero in on the causes and effects of this enormous event that changed the course of life on Earth. It's worth studying. These events are incredibly rare — Chicxulub was the last such big impact, tens of millions of years ago — but it's clear they've had a profound, well, impact on our planet. And we owe our existence to this last collision. Without it the non-avian dinosaurs wouldn't have been wiped out, birds wouldn't have evolved the way they did, and mammals wouldn't have been able to exploit the huge opening left behind by the larger animals when they died.
Studying this is literally studying ourselves. And this is an endeavor we do on the surface of our planet, from high above it, and even deep beneath its waters.
The lesson: Study everything. It all fits together. And it tells us where we are, and how we got here.