Dust flux, Vostok ice core

Dust flux, Vostok ice core
Two dimensional phase space reconstruction of dust flux from the Vostok core over the period 186-4 ka using the time derivative method. Dust flux on the x-axis, rate of change is on the y-axis. From Gipp (2001).

Monday, November 28, 2011

Geological Hazards n+1: Caldera eruptions

Sorry about the title, bandwidth is too slow to go back and check what number I'm on.

Spent a few days at Axim, on a hotel on a hill where several years ago another geologist and I were digging a hole through a pyroclastic deposit full of doubly terminated quartz. The hotel was just being built at the time--only a few of the huts were in place. The workers pointed out a man dressed in rags just dismounting from a hammock slung between two coconut trees. "There's Rastaman," they said. A certain distinctive odor commonly associated with Jamaica (and Canada for that matter) wafted up from the shore. I thought the man a vagrant, but it turned out that he was the hotel owner.

Our view as we toiled in the sun.

We panned through the pyroclastics looking for a hint of diamonds. Diamonds in Ghana can be associated with pyroclastic flows, even though this model is somewhat at odds with the more typical kimberlite model.

Anyway, this association with volcanic flows is a good starting place for today's topic.

At the beginning of last year I gave a brief geological lesson to the grade four classes at my daughter's school. The first topic I decided to discuss was the age of the Earth. So I asked the class how old they thought the Earth was.

Most of them knew it was very old, but had no idea of the number. But to these kids, even ten thousand years seemed tremendously old. To be fair, most adults can't really conceive the concept of a billion years except as a one followed by nine zeroes.

One girl boldly stated, "According to my calculations, the Earth is exactly 699,998 years old." Well, I thought that was a fascinating answer, both for its length and its precision. Naturally I was curious as to the nature of her calculations.

She said that she knew the Earth was destroyed every 700 thousand years, and since it was going to end in two years (2012), it was 699,998 years old.

Well, that wasn't as involved a calculation as I had hoped, but that business of the Earth being destroyed every 700 thousand years was very interesting. Very interesting indeed.

Seven hundred thousand years is roughly the recurrence interval of supereruptions of the Yellowstone Caldera.

A caldera is a massive volcano, larger than anything seen in our lifetimes. Beneath the suface is a magma chamber that is so large that after it empties itself, the ground over the chamber collapses into it, leaving an enormous crater. Krakatau and Tambora (1815) are recent examples of calderas.

Caldera eruptions are so large they have the potential to destroy civilizations. The eruption at Santorini at about 1628 BCE devastated Minoan civilization. It also left a very picturesque crater which I hope to visit before the next one.

The Tambora eruption was even larger than that at Santorini, and had a devastating effect on local agriculture. It also affected global climate. Crops failed throughout the northern hemsiphere, leading to the worst famine in the 19th century.

While Santorini, Krakatau, and Tambora were all impressive, Yellowstone is on a completely different scale. The supereruptions spewed out about ten times as much magma as the Tambora eruption.

 Yellowstone is a particularly big one, which erupts leaving a crater tens of km in diameter, spewing out thousands of cubic kilometers of debris.

USGS map of Yellowstone caldera craters (click here to enlarge).

Ash from these supereruptions has covered up to half of North America to a depth of a metre. Imagine an event of that magnitude happening now. It would be a civilization-ending event.

Not to alarm you, but there has been a lot of recent activity, suggesting that the magma chamber is once again being filled. But one should remember that the majority of eruptions from the Yellowstone hotspot are smaller than the supereruptions, so the recent activity may be a precursor for an eruption but not necessarily a supereruption.


  1. I've heard the 700k-year average statistic for the CO caldera before, but I've never heard a dispersion statistic, confidence interval, or the like.
    Do we think that is because we have too few observations to make this type of datum meaningful?

  2. There is a problem with lack of observations. The 700,000 year recurrence interval is only based on the last three major events. Prior to that, the recurrence interval appears to be longer. Either there has been a change in dynamics, or the geological record becomes spottier the farther back we go, as larger, younger events destroy the evidence of older events.