“Trap” means “step” when talking about huge basalt lava piles like this one. Erosion shapes those old flows into a series of giant staircases.
The common meaning — “gotcha!” — might also apply to the Deccan Traps.
This enormous basalt flood happened some 66 million years ago and, along with the Chicxulub impact on the other side of the planet, may have set a deadly trap for almost half of all life on Earth, including the nonavian dinosaurs.
Continental flood basalts
Don’t panic, but down through geologic time, in various parts of the world, the ground has occasionally gaped open in fissures that flooded the land with tens of thousands to millions of cubic kilometers of molten basalt.
Like this, but supersized, lasting much longer than this 0.8 km3 Kilauea eruption in Hawaii did in 2018, and with lava possibly fountaining up to the stratosphere from more than one multi-mile long fissure.
These high-volume eruptions of iron-rich lava pulse on and off for at least several hundred thousand years before winding down.
Eventually whatever lighted the fire goes out, and an enormous heap of basalt — the size of France and over a mile thick, in the Deccan Traps case — is then left to cool into black rock plateaus that erode into a step-like shape.
That’s what a continental flood basalt is — relatively quick volcanism on a scale that dwarfs anything a single fire mountain, even Toba, can do.
Frozen in time, these eruptions show us one of the rare but awesome landforms known as a large igneous province (LIP).
What causes basalt flooding like the Deccan Traps?
Geophysical and chemical studies show that the process involves movements of extra-hot rock many miles below our feet, in the stony mantle that separates and protects us from Earth’s fiery core.
What boffins can’t agree on is whether this high-temperature rock rises from deep within the mantle as a convecting plume or develops in the upper mantle through interactions with the overlying continental crust. (Hooper et al.)
Unfortunately, the Earth isn’t a transparent beaker and conditions at the core-mantle boundary can’t be duplicated in the lab. But you get the general idea. The second proposal — interactions with the overlying continental crust — adds a thin solid layer to this fluid and pokes things down in it, including but not necessarily limited to the downgoing tectonic plates in a subduction zone. As you might imagine, the picture is MUCH more complex in this scenario.
Whatever sets one off, continental flood basalts have never happened on Homo sapiens’ watch.
We’re lucky. This stretch of geological peace and quiet probably is a major reason why we’ve been so successful.
When that much red lava erupts, the only thing any living being can do is get out of the way, if possible, and hunker down in hopes of surviving the global consequences of a huge injection of volcanic gases and toxic chemicals.
Whatever those consequences might be.
They’re not well understood yet. There isn’t much useful information about the environmental repercussions of a continental flood basalt in the geological record, compared to the data volcanologists routinely collect during eruptions today.
Too, it’s not easy to model such complex and violent events, even with modern computers.
Nevertheless, several researchers have tried to envision the environmental effects of a LIP eruption.
Their work suggests that basalt flooding, in addition to spreading around nasty stuff like fluorine, could cause drastic temperature swings and global dimming from sulfur pollution, as well as drought in some regions, flooding in others, changes in the ocean’s chemical composition (really bad news on a water planet like Earth), and/or runaway greenhouse effects from an overdose of carbon dioxide.
Not surprisingly, many flood basalt episodes are associated with major mass extinctions in the geologic record.
But let’s also not forget the famous science rule: “Correlation does not equal causation.”
Or, in plain English:
- Basalt floods have happened without a die-off. The last one, for example, in the Pacific Northwest some 16 million years ago, might have warmed up the planet slightly, but it didn’t affect the background global extinction rate very much. (Courtillot)
- There have been major mass extinctions, like those that closed the Ordovician period hundreds of millions of years ago, without any basalt flooding. (Bond and Wignall)
But only one basalt flood event is connected with both a global extinction AND a massive bolide impact.
The Deccan Traps
Containing over 1 million km3 of erupted material, this continental flood basalt is second in size only to the 250-million-year-old Siberian Traps.
To geographers, it’s known as the Western Ghats mountain range. (Of note, “ghat,” in peninsular India, has the general meaning of “steps.”)
Besides covering much of northwestern India, Deccan Traps lava has also been found in the Seychelles Islands and on some of the submarine ridges beneath the Indian Ocean that stretch between the subcontinent and Réunion Island.
Deccan Traps basalt traveled far, but not all the way to Réunion Island. Plate tectonics reconstruction shows that, at the start of flooding, India sat where Réunion Island is now.
While the Deccan Traps erupted, the Seychelles rifted away and India tracked northward and rammed into what’s now Asia.
Mantle-plume supporters believe that ongoing volcanism on Réunion marks the hot-spot “tail end” of the Deccan Traps plume:
Piton de la Fournaise (“Furnace Peak”) on Réunion Island, February 19, 2019: The last gasps of a dying mantle plume?
Earth scientists used to think that it took millions of years to pile up the Traps. Sure, the “steps” are huge, indicating lots of lava, but there could have been long pauses between each basalt flood.
Indeed, lower lava fields in the Deccan Traps (the older “steps”) are sometimes topped off by ancient soils that have preserved fossils, including dinosaurs.
This was taken to mean that the lava cooled and hardened before the next flow covered it up.
In other words, the old idea was that enough time passed for life to recover between individual Deccan Traps fissure eruptions.
And dinosaurs on the subcontinent were just as awesome as those now sleeping in the world’s best-exposed end-Cretaceous fossil beds, in North America.
Some of the upper “steps” (the later ones) also have soil layers, but no dinosaur fossils have been found in them.
Clearly the end-Cretaceous extinction happened somewhere between the older (lower) and younger (upper) “steps,” though the exact K/Pg boundary position hasn’t been located there yet.
All of this could have unfolded over millions of year.
But that picture of a gradual Deccan Traps eruption changed in the 1980s.
Around this time, an earth science community that had long rejected the notion of catastrophes like Noah’s Flood shaping Earth processes was coming to grips with irrefutable new evidence of a giant impact around end-Cretaceous boundary time.
Debates about this were sometimes polarized enough to shed more heat than light on what might have caused the end-Cretaceous extinction.
Meanwhile, in India’s Western Ghats, a few field geologists were trying to work out details of the subcontinent’s tectonic collision with Asia. Instead of learning about that, they made a new discovery.
The paleomagnetic data they collected showed that the whole Deccan Traps LIP probably erupted in just a few hundred thousand years — a geological blink of the eye.
An area the size of France and over a mile thick.
In its own way, this was just as spectacular an idea as the dino-killing space impact hypothesis: huge volumes of red lava spewing out of the ground in intense eruptions unlike anything human beings have ever witnessed.
Indeed, lava from one Deccan Trap eruption– reportedly, the longest lava flow on Earth — traveled over 930 miles east to the Bay of Bengal!
But this scenario meant that the pauses between fissure eruptions were probably too brief for life in the seas and on land to fully recover.
Repetitive “hammering” of Earth’s environment each time there was a basalt flood would eventually drive many forms of life worldwide to the wall, even without the Chicxulub impact. (See Renne et al. in the source list at the end of this post for an interesting but still controversial hypothesis that links the Deccan Traps and impact together.)
It was a sobering conclusion to reach at a time when the halls of Geoscience were already ringing with controversy.
Suddenly, unwittingly, we had become actors in the debate over the KT boundary: and we were not in the “impactist” camp.
— Vincent Courtillot, one of those paleomagneticians, in “Evolutionary Catastrophes” (see source list)
Well, over the last few decades earth scientists have calmed down a bit. Their natural human reactions to world-shifting paradigms were overcome by sheer curiosity.
The second largest known continental flood basalt; an asteroid or comet impact; and one of the “Big Five” mass extinctions, all happening within a span of time less than 200,000 years long — the challenge of unravelling this unique 66-million-year-old concatenation of circumstances is right up Science Alley, no matter which “camp” one might be in.
We laypeople aren’t seeing as many headlines now about the complex events at the end of the Cretaceous period, but cutting-edge scientific research and painstaking field work are ongoing.
However, our post must close without resolution.
Many daunting tasks await those who really, really want to know what happened to the dinosaurs and how our era, called the Cenozoic, began.
Today, parts of the western Deccan Traps are UNESCO World Heritage Sites.
This LIP hosts one of the world’s Top Ten biodiversity hot spots, as well as at least one risky but incredibly beautiful tourist ride. From a feline perspective, both the world’s largest and smallest cats (whom we’ll meet on Friday) now live there.
So it goes on Earth.
In the midst of life comes death, but then life finds a new way and prospers for a while, until the next turn of the wheel.
Featured image: Nicholas via Wikimedia, CC BY-SA 2.5
Courtillot, V. 1999. Evolutionary Catastrophes. (English translation with revisions) Cambridge: Cambride University Press.
Encyclopædia Britannica. 2019. India: The Deccan. https://www.britannica.com/place/India/The-Deccan Last accessed June 11, 2019.
Glišović, P., and Forte, A. M. 2017. On the deep-mantle origin of the Deccan Traps. Science, 355(6325): 613-616.
Hance, J. 2012. India’s Western Ghats rainforest declared UNESCO World Heritage site. https://news.mongabay.com/2012/07/indias-western-ghats-rainforest-declared-unesco-world-heritage-site/
Hooper, P.; Widdowson, M.; and Kelley, S. 2010. Tectonic setting and timing of the final Deccan flood basalt eruptions. Geology, 38(9): 839-842.
Oppenheimer, C. 2011. Eruptions That Shook the World. Cambridge: Cambridge University Press.
Prothero, D. R. 2006. After the Dinosaurs: The Age of Mammals. Bloomington and Indianapolis: Indiana University Press. Retrieved from https://play.google.com/store/books/details?id=Qh82IW-HHWAC
___. 2011. Catastrophes! Baltimore: The Johns Hopkins University Press.
Renne, P. R.; Sprain, C. J.; Richards, M. A.; Self, S.; and others. 2015. State shift in Deccan volcanism at the Cretaceous-Paleogene boundary, possibly induced by impact. Science, 350(6256): 76-78.
Schoene, B.; Samperton, K. M.; Eddy, M. P.; Keller, G.; and others. 2015. U-Pb geochronology of the Deccan Traps and relation to the end-Cretaceous mass extinction. Science, 347(6218): 182-184.
US Geological Survey. 2019. Overview of Kilauea Volcano’s 2018 Lower East Rift Zone eruption and summit collapse. https://volcanoes.usgs.gov/vsc/file_mngr/file-224/OVERVIEW_Kil2018_LERZ-Summit_June%202019.pdf Last accessed June 18, 2019.
Wikipedia. 2019. Deccan Traps. https://en.m.wikipedia.org/wiki/Deccan_Traps Last accessed June 11, 2019.
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