Small lava eruptions in the Yellowstone volcano caldera are “more dramatic” than was thought and occur in clusters — making then, while not catastrophic, “still a big deal”.
This is the conclusion of a new study by researchers who used radiometric dating to refine the timeline of volcanic activity in the iconic national park.
The Yellowstone caldera — a cauldron-shaped depression in the ground produced when a magma chamber empties and collapses — formed some 631,000 years ago.
The supervolcano eruption released some 240 cubic miles of rhyolitic (thick, silica-rich) magma and deposited ash over a large swathe of the United States.
These dramatic events are rare, however — only three are thought to have occurred in the last 2.1 million years — with smaller-scale eruptions being much more common.
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These small episodes of volcanic activity are known as “intracaldera” eruptions for how they take place via vents within the caldera.
They typically take the form of lava flows, swelling lava domes or, in rare cases, small explosive eruptions.
While far smaller in volume than caldera-forming eruptions — typically only 0.1–17 cubic miles — the total amount of rock erupted by the intracaldera events is on a par with their larger counterparts.
The lava released in these episodes has “filled in” much of the Yellowstone caldera, which is partly why visitors to the national park do not see an obvious depression in the landscape.
Geologists believe that since the Yellowstone caldera was formed 631,000 years ago more than 86 cubic miles of material have erupted from at least 28 rhyolitic intracaldera eruptions, which can be grouped into two stages.
Between 580,000–250,000 years ago, at least six eruptions were deposited in the caldera. These are known as the Upper Basin Member rhyolites.
More recently — between some 160,000–70,000 years ago — there have been 22 eruptions that make up the Central Plateau Member rhyolites.
In their study, geologist Dr Mark Stelten and his colleagues at the US Geological Survey.set out to precisely constrain the timing of the Central Plateau Member rhyolites.
To do this, the team used a radiometric dating technique called ³⁹Ar/⁴⁰Ar geochronology, which involved measuring the decay of potassium into argon in a mineral called sanidine which is found in Yellowstone’s rhyolitic lavas.
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The researchers determined that the 22 eruptions that produced the Central Plateau Member rhyolites occurred in five brief episodes — at 160,000, 150,000, 111,000, 104,000 and 71,000.
These episodes saw two–nine rhyolites erupted from volcanic vents spaced out over a few to several miles over the course of 400 years or less.
Each saw between 2.5–31 cubic miles of magma erupted. Stelten said: “For comparison, the Mount St. Helens eruption in 1980 erupted about 0.25 cubic kilometers [0.06 cubic miles] of magma.”
This means, he explained, that intracaldera eruptions are more dramatic events that previously appreciated — often involving multiple eruptions occurring in different parts of the caldera at the same time.
By extension, if each of the five eruption episodes are actually one single volcanic event, this means that the long-term eruption rate at Yellowstone is even lower than thought.
Stelten concluded: “The new geochronology results show that intracaldera rhyolite eruptions are more dramatic but less frequent than previously appreciated.
“While they may typically lack the huge explosions that characterize caldera-forming eruptions, they can happen in groups where multiple rhyolite eruptions occur over a short duration.
“Don’t let Yellowstone’s smaller rhyolite eruptions fool you. They are still a big deal!”
The full findings of the study are published in the journal Bulletin of Volcanology.
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