What is a supervolcano?

Supervolcano is not a meaningful geological term, it is a media term. What most of us think of as a "normal" volcanic eruption is caused by ash and lava being vented from a magma chamber, a miles wide bubble of molten rock that has risen through the crust to a point just a few miles below the surface. Because it is hot, the magma wants to expand. As it gets close to the surface, the magma is able to create cracks above it that allow some of the molten material to escape. The surface manifestation of these pressure valves are volcanos.

For the last decade or so the term Supervolcano has been popular for describing caldera collapses. These happen when the the entire surface above the magma chamber gives way at once. If a normal eruption is a pressure cooker letting off a little steam through a safety valve, a caldera collapse is a pressure cooker without a safety valve blowing its lid. These produce enormous amounts of ash and gas. They produce sudden cooling events around the world; if other conditions are right they can tip the Earth into an ice age. And, of course, they bury and kill lots of things downwind from the eruption, preserving lots of high quality fossils. Think "giant Pompeii". The key points here are that these are singular eruptions lasting only a few days and producing mostly ash.

Lately, supervolcano has been being used in the press to describe flood basalts. These come from a different type of event that is not a familiar type eruption and does not form a volcanic cone. Flood basalts form when molten rock comes to the surface through long cracks and flows across the land covering hundreds--even thousands--of square miles of land. The flows can last for years and new flows can appear in the same area repeatedly over millions of years, burying the land under thousands of feet of rock. The Columbia Plateau of Eastern Washington and Oregon is the result of over two hundred seperate flows that occured over about a seven million year period of time (the dating of the flows is extremely contentious). These do not produce very many fossils because lava flows slowly enough for most animals to simply walk away and most plants get burned up. Even so, there are some fossils produced under special conditions. The key points here are that these are very slow events that produce mostly rock.

I'm complaining about this distinction because of news articles like this:

Scientists Explore Origins of 'Supervolcanoes' on the Sea Floor: Ancient Goliaths Blamed for Multiple Mass Extinctions

"Supervolcanoes" have been blamed for multiple mass extinctions in Earth's history, but the cause of their massive eruptions is unknown.

Despite their global impact, the eruptions' origin and triggering mechanisms have remained unexplained. New data obtained during a recent Integrated Ocean Drilling Program (IODP) expedition in the Pacific Ocean may provide clues to unlocking this mystery.

The article does not make clear that they are talking about flood basalts and not caldera collapses until paragraph twelve.

It appears that "supervolcanoes" is being used by journalists, and scientists who talk to jouranlists, to mean "big ass volcanoish happenings." They have managed to take vague term and make it completely useless.