November 11. Cretaceous flood basalts and amethyst

Two great episodes of volcanism took place during the Cretaceous, one early in the period, and one at the end. We’ll talk more about the second one near the end of the month, but today’s topic is the first one. 

In South America about 128 to 138 million years ago, thick basaltic flows poured into the Parana Basin. The Parana Basin was a persistent low-lying zone on the South American continent, located in southwestern Brazil and adjacent areas, where the Parana River flows today. In some ways it was similar to the intracratonic basins of North America, such as the Williston, in that they were initiated early in the Paleozoic, probably during Ordovician time. But the Parana Basalts definitely make it a different beast from the sedimentary Williston Basin. 

Present-day South Atlantic showing features related to Tristan Hotspot and South Atlantic Ocean opening.

The volcanism was most likely related to rifting that marked the onset of active sea-floor spreading as the South Atlantic Ocean began to form. One reason we suspect this is that there is a conjugate pile of volcanic rocks on the African side, in Namibia and Angola, where it is called the Etendeka Province. Furthermore, there are two lines of seamounts, underwater volcanoes, that extend from the coasts of both South America and Africa to the Mid-Atlantic Ridge. Those seamounts, called the Walvis Ridge on the African side of the Atlantic and the Rio Grande Rise on the South American side, trace back essentially to the Mid-Atlantic Ridge at Tristan da Cunha, a volcanic island whose magma source is thought to be the Tristan Hotspot.

Tristan da Cunha is pretty much just like Iceland, which also straddles the Mid-Atlantic Ridge and a hotspot. The difference between tiny Tristan da Cunha and massive Iceland is that the Tristan hotspot has been active since the Cretaceous whereas the Iceland one is quite recent. The two ridges that extend away from the Mid-Atlantic Ridge, and the volcanic piles on land in the Parana and Entedeka Provinces, reflect the episodic volcanism from the Tristan hotspot as the Atlantic Ocean opened.

Some estimates put the Parana-Entedeka’s biggest eruptions, about 132 million years ago, as some of the largest volcanic events in earth history.

The Parana flood basalts in Rio Grande del Sul province, Brazil, are famous for their beautiful amethyst-lined tubes and geodes. Amethyst is purple quartz whose color is related to trace amounts of iron in the silica crystal framework, together with some degree of natural radiation. The occurrences in the Parana are unusual in their intense color and uniformity. The geode formation was probably a two-stage process, with the openings forming initially in the original molten rock because of blobs of material that was unable to be incorporated into the basalt magma. Later, the cavities were the sites of deposition of quartz tainted by iron and other elements, including some rare earth elements. The entire process of deposition of the amethyst probably took place over a time span on the order of 40 million years in the early to mid-Cretaceous.

Today, these amethysts make Brazil the leading producer of gem-quality amethyst in the world, and of course the huge amethyst-lined pipes are famous around the world too.

—Richard I. Gibson

Largest volcanic eruptions 

Amethyst tubes

Map (public domain) from NOAA (annotated by Gibson)

August 30. Siberian basalts

Original extent of Siberian traps (see below for credit)

At just about the end of the Permian Period, one of the most voluminous volcanic eruptions of the past 500 million years occurred, in what is now Siberia. The rocks are largely basalts, relatively iron-rich, dark, fine-grained igneous rocks that erupted from many vents and perhaps some fissures scattered through the area. The eruptions spanned about a million years, 251 to 250 million years ago, and ultimately covered as much as 7 million square kilometers, about 2,700,000 square miles – about the area of the conterminous United States excluding Texas and California. Probably less than a third of the original extent is still preserved; the rest has been eroded away.

Flow after flow was erupted, along with explosive eruption of ash and other volcanic products. The flows are usually called “flood basalts” because they flooded out of the vents and fissures to cover vast areas. And the whole package is often referred to as the Siberian Traps – the word “trap” in geology comes from a Swedish word meaning “step,” because the piles of flows typically create a stair-step landscape.

There have been several huge eruptions of flood basalts through earth history, including 11 of various sizes in the past 250 million years. The Columbia Plateau in Washington and Oregon, the Deccan in India, and the Parana basalts in Brazil, are all huge, but the Siberian traps were the largest volcanic flows in the past half billion years.

What caused it? The ultimate cause of all the flood basalts is not completely certain. Ideas include active rifting, pulling apart of the crust, under special circumstances such as unusually thin crust. A mantle plume, a hot spot like those beneath Yellowstone and Iceland today, could have made it happen, especially if associated with rifting. It’s been suggested that an impact might have blasted the crust to the point that vast volumes of magma would erupt through the crater, but there isn’t much support for that idea.

The age of the eruptions has been pinned down quite accurately. The eruptions do have a moderately wide range, taken together, from about 256 to 246 million years ago, but the age dates really do strongly cluster at 251 to 250 million years ago. This is right at the Permian-Triassic boundary, the end of the Permian, when the greatest mass extinction in the history of life on earth occurred. It’s pretty much inescapable, given the extremely close correlation in time between the vast Siberian eruptions and the mass extinction event, that there was probably a connection between the two. But there’s more to it than a simple one-for-one correlation, and we’ll explore that in more depth tomorrow.

* * *

Today is the birthday of William Stephens Twenhofel, August 30, 1918, in Madison, Wisconsin. His father, William Henry Twenhofel, has been called the patriarch of sedimentary geology: he pioneered the study of sedimentation as a subdivision of geologic science. The son, William Stephens, was a geologist with the U.S. Geological Survey who worked on ore deposits in Alaska, among many other things.

—Richard I. Gibson

Links and references:
Siberian Traps (Cowan)  
Siberian Traps ages 
Siberian Flood Basalts 
Multiple eruptions – not one
Extinction connection

Map by Jo Weber, under Creative Commons license.