Submitted by cakedayCountdown t3_10evzud in askscience
When I was in school (90s), scientists weren’t sure how the dinosaurs died. Asteroid, comet, volcanoes, etc. Now, they seem pretty confident about an asteroid in the Yucatán Peninsula. What convinced them, since they already knew that the Peninsula was formed by a giant rock that kicked around so much debris 65-66 million years ago?
> Finally once the crater was found, dated and confirmed it was accepted more or less.
This ignores a pretty active literature stream that has persisted since the impact hypothesis was proposed (and which continues to this day) that questions whether this was the cause (e.g., McLean, 1985, Courtillo & Cisowski, 1987, Pope, 2002, Keller et al., 2004, Fastovsky & Sheehan, 2005, Keller et al., 2020, etc.).
> some claim the impact caused the volcanic eruption, the shock waves converging on the far side of the planet where India would have been at the time
This is generally not what is argued for. What has been suggested is that the impact may have triggered a large pulse of Deccan Traps volcanism, but the timing of the start of Deccan Traps volcanism is demonstrably before the impact (e.g., Renne et al., 2013, Schoene et al., 2014, Renne et al., 2015) but timing of the main eruptive pulse remains controversial, i.e., it may have occurred sufficiently after the impact to be unrelated (e.g., Sprain et al., 2019).
> But less evidence for that
This is debatable, viable kill mechanisms tied to either event are pervasive in the literature (as are people pointing out issues with the alternative kill mechanism(s)). Arguably, the idea that neither the Deccan Traps nor the Chicxulub impact alone would have caused the extinction, but that the occurrence of both in short succession was enough to start the cascade is becoming closer to a consensus view (e.g., Petersen et al., 2016, but also the Renne et al., 2013 and Schoene et al., 2019 papers cited earlier). Similarly, there are suggestions that the K-Pg extinction was relatively protracted, perhaps occurred in pulses, and started before the impact, but with a pulse in extinction linked to the impact (e.g., Tobin, 2017)
Thank you for the citations
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This is great. Thank you so much.
You're very welcome. Other people have also given some good ideas as well as more detailed explanations.
This and other answers speak to something much broader about the sciences and really, most branches of academia.
It's one thing for a potentially game-changing fact or new data to be identified or noted. The actual conclusions based on said data can take decades, or maybe reach no resolution at all, or the conclusions may change based on new analyses, etc.
The impact hypothesis was controversial for many years, even after the Alvarezes published their findings in 1980 about iridium deposits that were almost impossible to explain any way other than an asteroid impact. Some scientists still favored existing hypotheses, including volcanism, sea level changes, and even chronic constipation. (See Keith M. Parsons' "The Great Dinosaur Controversy" published in 2003.)
Many scientists believed that major collisions between celestial bodies ceased long before the dinosaurs went extinct. It wasn't until 1994, when Shoemaker-Levy 9 crashed into Jupiter, that we witnessed such an event occuring. 100% undeniable proof that collisions still occured, which meant they definitely could have occured 65 million years ago.
There was also the problem of the impact crater. The original 1980 paper was based on iridium deposits around the world, but there was no known impact crater of sufficient size that could be dated to the right time. The Chixulub crater was discovered in the 1970s, but it wasn't identified as an impact crater until 1990. (Previously, it was thought it may have been a volcanic caldera.)
All this is to say, evidence has trickled in to support the impact hypothesis, and while the pieces were there in the 90s, it has taken some time to put them all together.
And the discussion isn't over either! While many experts got together and concluded in 2010 that an asteroid impact was the main cause of the K-Pg (formerly K-T) extinction event, there is still significant evidence that volcanism at least played some part. The dinosaurs may well have been on the decline for a long time, and the impact simply sealed their fate. Anyone who says "the asteroid killed the dinosaurs" is probably oversimplifying a complex issue, but it would be a huge mistake the ignore the asteroid impact altogether.
Source: A 2015 essay I wrote as an assignment covering a controversy in science during my undergraduate studies.
What does volcanism mean in this context?
Obviously it doesnt mean that a massive bunch of dinosaurs were too close to active volcano and melted.
Large explosive volcanic eruptions can pump enough particulate matter into the high atmosphere to interrupt weather patterns. One of the largest in recent history, the eruption of Mount Tambora in 1815, for example, caused the "year without summer", and a number of similar eruptions happened earlier in recorded history where we did not, at the time, know the source, but have since discovered the locations. We know from geological evidence, far far larger volcanic eruptions and events have taken place, and an event on such a scale could cause sufficient particulate matter to enter the atmosphere that it could create a period of several years of insufficient sunlight reaching the surface of the earth to massively disrupt ecosystems and create a mass extinction event. There is geological evidence that one such event, in the Deccan Traps of modern day India took place at the right sort of time (on a geological scale) and was large enough that it is a plausible candidate for causing the extinction event that ended the dinosaurs.
> an event on such a scale could cause sufficient particulate matter to enter the atmosphere that it could create a period of several years of insufficient sunlight reaching the surface of the earth to massively disrupt ecosystems and create a mass extinction event.
So the potential cooling effects of large explosive volcanic eruptions (e.g., events like the 1815 Mount Tambora eruption) are not disputed, but this is not actually relevant for Deccan Traps volcanism or the suggested kill mechanisms related to them. For the Deccan Traps as the cause of the K-Pg extinction, the kill mechanism may have been global warming from pulses of greenhouse gases released by the volcanism (e.g., Tobin et al., 2012) or a combination of this along with ocean acidification and ocean warming (e.g., Keller et al., 2020). I.e., flood basalt eruption and its effects do not equal large caldera eruption and its effects.
So you got me curious about multiple factors relating to the current and previous extinction events.
O-S was climate and atmospheric composition, greenhouse effect of volcanos, and loss of oxygen.
The Devonian event(s) were ecological, but I can't find much on it.
P-T was ocean acidification, oxygen loss, greenhouse caused by volcanism
T-J was again, acidification, climate changed, and oxygen loss.
K-G, again, acidification, climate change, but I can't find atmospheric composition.
Regardless, it appears that the extinctions occur once those worldwide changes start, regardless of the cause. What I'm curious about now is if there is any evidence at how inhospitable the planet was by the end of the extinction events.
In this specific case they are referring to Flood Volcanism. The Deccan Traps (I probably spelled that wrong) are a large igneous province in India where, about the same time as the Dino asteroid (geologically speaking), large cracks opened in the crust and absolutely immense volumes of basaltic lava poured out in flows hundreds of feet thick covering thousands of square miles one on top of the other. The cumulative depth of these flows are measured in miles.
Over what time span did these flows spread?
Would an individual dinosaur be overtaken or could they run away?
Or were they pressured by being pushed out of their environments?
Some of the individual flows happened within 50,000 years of each other. I believe the total time of the flows was 750,000 to a million years or so.
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Could an asteroid impact of that scale cause volcanic action or is it still too tiny?
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This is covered in one of our FAQs that no one ever reads.
Yes but my question was about what changed between the Alvarez (1980) findings but hadn’t been accepted in the 90s
What changed was the release of drilling data the reveled shocked quartz and other types of damage to multiple types of rock that doesn't happen naturally with earth's forced existed over a very wide area and to incredible depth. This data was the smoking gun that showed a huge impact crater did indeed exist, when previously we had suspicion but no proof of an impact.
And it gained even more traction when they did some more drilling to pinpoint the central peak, early 2010's I think.
Prior to the drilling data release, all they knew was there was ejecta all over the peninsula.
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It's been a while since I was a geology major but I learned that by the time of the K/T boundary, a lot of non -Avian dinosaurs were already extinct so it isn't like the impact killed the dinosaurs as much as it was the final blow.
Is that still accurate?
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The impact crater was discovered in 1991. After that, it took several years to convince most scientists it was the cause. People had to write papers showing how the effects of the impact could cause extinction.
The volcano hypothesis remained around because of the Deccan traps which happened slightly before. However, many studies have shown that the climate change caused by the volcanism was small (loss of 2°C which is negligible compared to the variation of temperature that happened during the hundreds of millions of years the dinosaurs lasted) and that the dinosaurs survived during the time between the start of the Deccan trap volcanism and the impact.
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horsetuna t1_j4ts8uz wrote
I don't know much about why they thought the peninsula formed. The current crater from the chixulub impact is half under the land and half under the sea, and does not seem to follow the coastline as it is today.
Mostly what convinced people was the timing and size. Before the Alvarez team (father and son) found the iridium in the KT boundary, there wasn't any evidence that there was a meteoric strike at the right time of the right size. After they found the iridium, they looked for other records from mining/gas companies, as people wanted the smoking gun .. the crater itself.
They calculated how big a bolide would be needed to coat the earth in such a way with this amount of iridium and then calculated the size of the crater, as well as the age.
The crater had actually been known for a while but the company that did the surveys wasn't keen on sharing their info due to competition concerns (not specifically about the crater iirc)
Finally once the crater was found, dated and confirmed it was accepted more or less. Better climate modelling showing the extent of the conditions also helped the case
Many think it wasn't the ONLY factor though. But a contributing one. The last straw that broke the camels back so to speak.
For instance the Deccan traps in India is the remains of a massive flood basalt that occured around the same time and likely contributed to the situation with the bolide (some claim the impact caused the volcanic eruption, the shock waves converging on the far side of the planet where India would have been at the time. But less evidence for that).