Read my LIP
By Larry Larason
LIP is a geological term. What it describes is not the kind of lip anyone would kiss. It’s an acronym for large igneous province. Now, that may sound like a region with a lot of volcanoes, but that’s not quite right. A LIP is defined as an area of at least 100,000 square kilometers [more than 38,500 square miles] covered with lava. This lava is emplaced in a short time, by geological standards, of a few million years, or less. So, it’s a special kind of volcanism.
The lava in a LIP is also rather special. It’s called flood basalt. It seldom forms volcanic cones, but instead gushes out of the ground in humongous amounts and inundates the landscape. LIPs are only partially understood, but they seem to be caused by mantle plumes – magma rising from the mantle, pushing upward until it breaks through the crust, still very hot, and liquid enough to be capable of covering tens of thousands of square miles in a matter of days or weeks.
LIPs are found around the world. Some are associated with continental rifting; others appear to have popped up almost anywhere, including on the sea floor. This is because the mantle hot spots are far deeper than the continental plates and not related to what is happening on the surface.
Let’s look at one LIP in the U.S. We call it the Columbia River Basalts. Beginning about 17 million years ago magma rose beneath the surface. The crust was bowed up and fractured by pressure from underneath. As the pressure continued, the cracks became fissures, and then lava poured out. It moved fast; one flow that was studied in detail over its entire 300-mile length was found to have been emplaced in less than a week. Over three million years flow followed flow, averaging about one every 75 years, creating a stack of basalt nearly 6000 feet deep in places. The flows originated around the corner of Oregon, Washington, and Idaho, but in some cases the lava ran as far as the coast of the Pacific Ocean almost 500 miles away from the erupting fissures. The hot spot that created this large LIP is thought by some geologists to now lie beneath Yellowstone, where there have been catastrophic eruptions in the past, as recently as 640,000 years ago.
Two of the more famous LIPs are the Siberian and Deccan Traps. Traprock is an old name for basalt. It comes from a Scandinavian word for stair steps, which refers to the ledgy, terraced look of the edges of the flows after erosion.
Siberian Traps: While all the Earth’s land masses were scrunched up in the supercontinent Pangaea, what is probably the largest volcanic episode ever in the history of the Earth erupted about 250 million years ago at the end of the Permian Period in what is now Siberia. Flood basalts covered 800,000 square miles – an area the size of Europe. Even after all those years of erosion the basalt is still about a mile thick in places.
The Deccan Traps erupted about 65 million years ago at the end of the Cretaceous Period in central India. This LIP covers almost 200,000 square miles, that is after erosion, with lavas up to 6,500 feet deep. The eruptions were quite sudden; the basaltic plateau may have formed over only 30,000 years. The Deccan Plateau houses the famous Ajanta Caves, which were carved out of basalt cliffs by Buddhists beginning in the second century BCE.
At the end of the Triassic Period, 201 million years ago, the Central Atlantic Magmatic Province [CAMP] erupted in four pulses over about 600,000 years. At that time the super-continent of Pangaea was breaking up. The CAMP’s lava was laid down at the rift opening up where the Atlantic Ocean is now. As the continents split apart, the basalt flows were also split, and remnants of this lava are now found on four continents: along the eastern seaboard of North America from the Carolinas to Nova Scotia, including the palisades of New York and New Jersey; in Portugal; Morocco and parts of South America.
No human being ever witnessed the formation of a LIP. This is probably good because LIPs are implicated in mass extinctions.
You may have noticed that two of the LIPs I mentioned occurred at the times of major mass extinctions: Permian and Cretaceous. Next month I’ll write about the possible role of LIPs in the big die-offs.