Posted on April 02 2019
A little while back, the Geo-Joint went out into the Indian Ocean and had a look around at the Maldives. These islands, as well as Mauritius and Reunion islands to the southwest, and other islands and submarine ridges, were formed by a hotspot which pushed magma to the surface as the tectonic plates moved over it. Pretty impressive work, and the islands left behind are some very nice locales to inhabit or vacation on. The real masterwork of that magma plume, though, happened earlier, and a ways north of the Maldives. Nobody was around to see it 60 to 65 million years ago, but the evidence is gargantuan that when the hotspot was under what is now central western India, the magma flow just wouldn’t quit.
This area is called the Deccan Traps. No, they weren’t out there capturing lava with snares and deadfalls. More on the terminology later. The Deccan Traps stretch over an area of 200,000 square miles—about enough to cover Oregon and Washington. That’s some lava flow. And originally it was bigger. Before erosion wore its edges down, it may have covered 600,000 square miles. Now consider that the thickness of the basalt laid down is 6,500 feet—over a mile deep. Perhaps 122,750 cubic miles of basalt filled the basins. For comparison, the Mount St. Helens volcano coughed up less than a quarter of one cubic mile of material. Now, to be fair, the collection of events that built the Deccan Traps took an estimated 30,000 years, but the volume and extent of it is astounding. Displays of massive outpourings of such lava are called flood basalts, and are found at various places around the world. The Columbia River area in Washington, and Brazil, Antarctica, and Siberia also feature large flood basalts. They are not highly explosive eruptions, but flowing expulsions fed by hot spots, that fill huge basins. Think pahoehoe and a little a’a as found in Hawai’i over their hotspot, rather than Mt. Pinatubo or Krakatau. And why “traps?” Well, the landscape, after much erosion, developed a terraced, stair-step look. And the Swedish/Old Norse word for “stairs” is “trappa.” Emanuel Swedenborg, an 18th century Swedish scientist apparently initiated the usage of the term, which is applied to flood basalts worldwide.
The eruptions of the Deccan Traps are famous for what they might have caused. Until the Chicxulub Meteor became known, the downfall of the dinosaurs was blamed on the Deccan Traps’ effect on the atmosphere, and the climate. The timing was about right, and no other mechanism seemed evident. However, basaltic eruptions are not characterized by massive amounts of carbon dioxide release. Therefore the idea that those eruptions, as massive and long-lasting as they were, fueled enough climate change to doom the big lizards is not supported. Still, an atmosphere choking on an array of noxious chemical infusions going on for thousands of years probably didn’t help any, and by the time the meteor hit, the dinosaurs may not have been at the top of their game. A different extinction, however, may indeed have had its roots in another major basalt flow—the biggest ever on land, in what is now northern Russia. The Siberian Traps, produced as they were some 252 million years ago, may have generated as much as a million cubic miles of lava. The timing of the million-year-long event coincides with the late-Permian Era extinction, which saw the end of 70 percent of land-based species, and more than 90 percent of ocean life-forms. Recent studies explain that the Siberian basalts came up through rock rich in hydrocarbons, and the heating of those layers released the carbon dioxide requisite for a climate change event. So flood basalts may not be so innocuous, depending upon their location.
And those locations are truly worldwide. Besides the places mentioned earlier, sizeable flood basalts can be found in South Africa, Greenland, and notably in Iceland, where the largest basaltic flows in historic times produced close to 5 cubic miles of lava. The Eldgjá volcano, around 935 CE, was responsible for this particular eruption. Iceland had only been occupied by humans for a few decades at that point, and the nearly 50-mile-long fissure from which the lava spewed copiously must have awed the inhabitants. The eruptions led to heavy winters and unproductive summers as the ash dimmed the sun as far away as Europe and China. Of course, in Iceland, that cooled an already cold place and brought enough hardship to the island that settlement ceased for a time. Some have connected this event with a famous Icelandic poem written a decade or so later that described the fiery destruction of the Norse gods by eruption-like forces, and their replacement by a singular deity. It might have influenced the rise of Christianity in Iceland if indeed the population considered the destruction of Eldgjá the action of a wrathful god. So flood basalts may have altered minds as well as the earthly landscape, and in fact they also make their presence known outside of Earth itself. The smooth northern third of Mars has broad areas covered in such flows. The material is pretty much the same as we have here at home, but up there, basalt has different pressures, temperatures, and gravity to affect it, so a given flow will spread about six times farther on Mars than on Earth. Researchers who study the flood basalts of Earth hope to better understand the Martian surface when human feet eventually wander the distant red world. Maybe on that planet, they know how to make a better Mars Trap. Sorry, couldn’t resist.
Thinking of a little sojourn to India to see the Deccan Traps for yourself? This Nelles map of Western India covers most of the area, and includes a focus on Mumbai and Delhi. Available from Maps.com.
caption: The Deccan Traps (in purple here) cover a large part of India—long ago they were even more widespread, but erosion has curtailed their extent.
source: Wikimedia Commons: CamArchGrad (Public domain)
caption: Flood basalts (in purple again) occur worldwide. The Siberian Traps in Russia are distorted in size by the map projection, but they indeed produced the greatest volume of known land-based flows.
source: Wikimedia Commons: USGS (Public domain)
caption: The Western Ghats mountains of India—part of the vast and rugged Deccan Traps.
source: Wikimedia Commons: Nicolas (Nichalp) (CC by SA 2.5 Generic)
caption: The stair-stepping of the Deccan Traps can be seen here.
source: Wikimedia Commons: Cj. samson (CC by 3.0)
caption: Major stair-stepping in the Columbia River basalts, in Washington state. Flows there may have covered the most surface area of any flood basalt on land.
source: Wikimedia Commons: Williamborg (Public domain)
caption: We can’t be sure whether there is life on Mars, but flood basalts and trappa, yes!
source: NASA Earth Observatory (sepiatone) and NASA Mars Education at Arizona State University (black and white): NASA (sepiatone) and NASA/JPL—Caltech/Univ. of Arizona (black and white) (Public domain)