Jake Lowenstern: I'm Jake Lowenstern, I work for the U.S.
Geological Survey and I'm the Scientist-in-Charge of the
Yellowstone Volcano Observatory. Today, we're going to
talk about some of the eruptions that occur at Yellowstone,
have occurred in the past, how big they get, how we
know when they might be coming, what we need to worry
about, what we don't need to worry about.
[Scoring Text: When was the last supereruption at Yellowstone?]
The last one of these mammoth eruptions, so called
super eruptions, was 640,000 years ago.
And that created this green deposit, the Lava Creek Tuff.
There were three of these really large eruptions.
There's the Lava Creek Tuff, and then there's the
Huckleberry Ridge Tuff that's 2.1 million years ago.
And then you can just make out down here, this is the
Mesa Falls Tuff and it erupted 1.3 million years ago.
It was a bit smaller than the other two eruptions.
[Scoring Text: Have any eruptions occurred since
the last supereruption?]
Since the eruption of the Lava Creek Tuff 640,000 years ago,
we've had a lot of eruptions within the Caldera,
probably 80 different eruptions, both within the Caldera
and outside the Caldera. Many of them are these really
large lava flows. The most recent lava flow was down here
on the Pitchstone Plateau 70,000 years ago, and it's a
really enormous eruptive unit. These are big eruptions
but they're just within Yellowstone and they don't
appear to be highly explosive.
Besides that we have a number of smaller eruptions that
are similar in composition. These are rhyolite lava
flows, they're really sticky and gooey and don't move
very far from their source, and these are in pink.
We also have basalt lavas that are more similar to
what gets erupted in Hawaii. It's more rapid moving
material. And they're found out here, outside the
Caldera and then out in the area to the east as well
(Text: Jake meant to say "...to the west as well"). One of
the reasons that we know that Yellowstone still has an
active magma system other than the fact that there's
so much heat coming out, is the fact that we never see
these basalt lavas actually making it out of the Caldera.
And that's because it's denser than the rhyolite and it
can't penetrate through this very viscous, sticky and
less dense material. And as a result, it just ponds
beneath the rhyolite magma chamber and loses its heat
to the rhyolite. When you're outside the Caldera,
the rocks are cooler and there isn't any rhyolite,
and the basalt can just create cracks and come up
and make it to the surface.
[Scoring Text: Is Yellowstone overdue for an eruption?]
When you see people claiming it's overdue, usually the
numbers they come up with they say that the last eruption
was 640,000 ago, but that it erupts every 600,000 years.
And so therefore, it's 40,000 years overdue.
But in fact if you average the eruption intervals,
there's 2.1 million to 1.3 million.
(Text: Interval 1 = 800,000 yrs. Interval 2 = 660,000 yrs.
Average is 730,000 yrs) And then there was another
eruption 640,000 years ago, and you average those numbers,
you come up with something that's over 700,000 years.
And so in reality, even if you tried to make this argument,
it wouldn't be overdue for another 70,000 years or so.
The other thing that is important to realize is that
when anybody does statistics based on two eruptive
intervals, they're just kind of playing games because
we don't know. There's no clock down there.
The magma is going to erupt when it wants to erupt.
There's been a lot of things that have happened over
the last 600,000 years that might indicate there's less
likely an eruption. For example, about 500 or 600 cubic
kilometers or a couple hundred cubic miles of magma
was erupted just in the last 150,000 years.
So you might think that that would slow the clock
down in terms of the next eruption.
[Scoring Text: What does the magma below indicate
about a possible eruption?]
The last piece of evidence we have in terms of the
next eruption at Yellowstone is that when scientists
or seismologists do tomography, like a CT scan on a
human being, you do a tomographic image using seismic waves.
And when they try to image the magma chamber, generally
they find that the magma that's down there is not wholly
molten. There maybe molten bodies down there, they're
on the order of 100 or 200 cubic kilometers, but they
don't see this massive magma chamber that's all molten
material. So we don't expect that there's enough
liquid magma down there to produce one of these immense
eruptions as it happened in the geologic past at Yellowstone.
[Scoring Text: What else is possible?]
Having said that, we don't know everything.
It's possible it will erupt. It's possible it will have
small lava flows. It's possible, but not very likely,
that it'll have another massive eruption.
Ultimately, this is stuff that's happening four or five
miles beneath us but everything we know seems to
indicate that nothing right now is truly unusual.
The kinds of activity we see at Yellowstone is the
kind that's been going on for the last 100 years.
And from our geologic evidence, has gone on for
thousands of years before then.
[Scoring Text: Why didn't you think the recent
earthquake swarm would lead to an eruption?]
Occasionally we get swarms of earthquakes and some of
these swarms can be quite intense.
We had one here in 2009, in the end of 2008 that was
almost 1,000 earthquakes. And there were many of them
that were above magnitude three. Some of them were
felt in the northern part of the lake, but that's
pretty much all that happened. We had no evidence that
there were any explosions that were occurring.
The Caldera had been moving up for a period of years,
for about four years, and the Caldera had moved up
about this distance, over a very broad area.
But we didn't see any change that was occurring when
the swarm was happening. And so we didn't really see
any of the other indicators that would make us really
think that something bigger was about to happen in
terms of a volcanic activity.