PULLMAN - Stephen Self never paid much attention to basalt until people started saying it might have killed the dinosaurs.

Prior to that, there didn't seem to be any point. Basalt was a drab, monotonous, common rock. It represented the extrusive phase of volcanic eruptions, unlike the pyroclastic rocks he'd been studying since the 1960s.

Pyroclastics - the bombs, ash and super-heated rocks that spew out during explosive eruptions like Mount St. Helens - could tell you things about conditions in the magma chamber or about what goes on in the initial stages of an eruption. They were interesting.

Basalt could put you to sleep. It was more likely to puddle than explode.

"I found basalt extremely boring," said Self, 65, a visiting professor with The Open University in England, who gave a recent talk at Washington State University.

What changed his mind was the debate over how and why dinosaurs died off 65 million years ago.

In 1980, Walter and Luis Alvarez proposed that an asteroid impact caused the mass extinction. By the time Self joined the University of Hawaii faculty in the early 1990s, an alternative hypothesis had been proposed: Enormous eruptions called flood basalts not only killed the dinosaurs but caused several other mass extinctions as well.

"Flood basalts are the largest volcanic eruptions on Earth," Self said. "They often appear to coincide with mass extinctions or major environmental disturbances. Over the last 300 million years or so, there have been about six coincidences."

Up until the dinosaur debate, very little was known about how flood basalts actually worked, he said. The provinces were made up of hundreds of individual lava flows, each one representing a separate eruption - but how long did an eruption last? How much material did it contain? How did it compare to present-day volcanism?

Answers found close to home

As Self and others began investigating these questions, many of the answers came from the Columbia River flood basalts, which form the tiered, brown cliffs along the Snake and Clearwater rivers near Lewiston.

Covering roughly 70,000 square miles in Washington, Oregon and Idaho, the 16 million-year-old Columbia River province is the youngest and smallest flood basalt on Earth. It's also the best-studied, Self said, thanks in part to the Ice Age Lake Missoula floods.

"The great floods carved canyons through the plateau and provided a level of exposure here that's unique," he said. "If you were to go to the Parana (flood basalt province in South America), you might drive a thousand miles and see two outcrops."

Another reason the province is so well known, Self said, is because of the enormous amount of geochemical work that's been done here.

Basalt all looks very similar, he said, so the only way to track specific lava flows across hundreds of miles is through their subtle chemical differences.

"To understand the eruptive mechanism, you need to know what one eruption produced," Self said. "This is the only province where the chemistry is known well enough to track individual eruptive units."

The research that's been done to date highlights some similarities and astonishing differences between flood basalt magmatism and the volcanic activity that can be seen in places like Hawaii or Iceland today.

The major difference is the sheer volume of magma, Self said. The Columbia River province contains at least 175,000 cubic kilometers of basalt. Other provinces have 2 million or more - enough lava to bury the western United States. Individual eruptions generated as much as 5,000 cubic kilometers.

Nothing in recorded history comes even close, he said. The ongoing eruption at Mount Kilauea in Hawaii, for example, has produced about 1.5 cubic kilometers of basalt lava since it began in 1983. The 1783 Laki eruption in Iceland, the largest eruption ever described by witnesses, generated about 15 cubic kilometers, most of it in the first two months.

Close look finds the familiar

Yet when Self and his students took a closer look, they found the same type of lava and same type of structures in flood basalt provinces as they did in modern-day lava fields.

"We don't have to appeal to anything really out of the ordinary to explain these large fields," Self said. "They're all formed of what's called inflated pahoehoe sheet lobes."

During a typical eruption in Hawaii, a magma lake or fire fountain may form near the vent or fissure. The pahoehoe lava then collects into channels or tubes and runs downhill. At the far end of the flow it spreads into slow-moving sheet lobes - amoeba-like blobs with intertwining fingers of lava spilling out in all directions.

As each lobe cools, Self said, it forms an upper and lower crust, helping to insulate the molten lava in the middle. The lobe inflates as more lava flows in, creating something like an expanding pipe that enables liquid rock to move farther and farther down field. Eventually the whole system stagnates and the lava solidifies.

Modern sheet lobes may grow to be several feet thick. In flood basalt provinces, they're typically 50 to 150 feet thick - and every flood basalt eruption is made up of thousands of lobes, perhaps tens of thousands.

"It isn't a single sheet of lava flowing for hundreds of kilometers," Self said. "Sheet lobes are the basic building blocks of flood basalt provinces. They're like tiles in a big mosaic."

The debate continues

Why flood basalt eruptions are so voluminous is still a point of contention. Some geologists believe they're the surface expression of deep-seated mantle plumes - massive tubes of heated rock that rise hundreds of miles through the Earth's mantle. Others think they're shallower melts related to variations in the thickness of the lithosphere, the rigid shell that rests atop the mantle.

Whatever their source, flood basalts were floods in volume only, not in terms of the speed at which the magma flowed.

Based on work that linked the thickness of a sheet lobe crust to the amount of time it took to grow, Self estimates individual flood basalt eruptions lasted for decades or centuries, rather than for weeks as researchers initially thought.

"You could walk away from them," he said. "My prediction is that lava moved across the system at a rate of a meter per second or less," or about 2 mph.

Much more work still needs to be done, though, to fully understand how lava moved between the feeder dikes - where magma first reached the surface - and the thousands of sheet lobes where it spread across the landscape.

"I have a sketchy idea about the flow pattern, but we really don't know the details of what the connection was like," Self said. In fact, the Columbia River province is the only one where a system of feeder dikes has even been identified and described.

Better age dating is also needed, he said, so researchers can say with greater certainty how long an eruption lasted and whether it coincided with biological events like a mass extinction.

Finally, more research is needed to determine if flood basalt eruptions really can cause mass extinctions.

"It's difficult to see how an eruption can impact the atmosphere to that extent," Self said. "We need to understand that better."

One thing is clear, though: Basalt has proven to be much more interesting than he first thought.

Source: Associated Press