The Cassia Hills are a relatively quiet part of the state of Idaho today, but once upon a time the region was home to a series of cataclysmic volcanic eruptions. Over the course of several million years, they burned a track towards the infamous Yellowstone supervolcano up in Wyoming, like a fuse leading to a rather considerable-sized bomb.
Painstaking geological fieldwork, published in the journal GSA Bulletin, has uncovered evidence of at least 12 enormously explosive eruptions. Their combined destructive behavior during the last 16 million years caused Earth’s crust to sink (or “subside”) by more than 3 kilometers (roughly 1.9 miles), leaving behind a deep volcanic basin. One of the eruptions 8.1 million years ago released more than 760 million Olympic-sized swimming pools of lava.
This region is known for a series of particularly violent and prolonged effusions of a type of lava known as rhyolite. Rhyolite, unlike the far more common basalt, is a particularly viscous (“gloopy”) magma that tends to be relatively “cool” for molten rock – namely, at temperatures of around 650 to 800°C (1,200 to 1,470°F). It traps a lot of gas, meaning that its eventual eruptions tend to be fairly explosive.
A map tracing out the volcanic path of destruction towards Yellowstone (Y). Thomas R. Knott et al./GSA Bulletin
The rhyolite that erupted at Cassia Hills and along the nearby Snake River Plain was far hotter, sometimes soaring to temperatures of 1,050°C (1,920°F), and scientists think this is due to a plume of incredibly high-temperature molten rock shooting up from the mantle beneath the western United States. These plumes are also known as “hotspots,” and they fuel extremely hot volcanism all across the planet, from Iceland to Hawaii.
Generally speaking, they make basaltic magma, but if the plume interacts with descending (“subducting”) plates of continental rock, or with pre-existing magma chambers, the magma can become “contaminated.” Different mineral compositions and different cooling rates change the composition of the magma, and in the case of the Cassia Hills region, this can sometimes produce the more explosive-prone rhyolitic magma.
Volcanic activity in Iceland is also driven by a hotspot. Gardar Olafsson/Shutterstock
The entire region is known to volcanologists as the Yellowstone-Snake River Plain Volcanic Province, and it is one of the youngest areas of hotspot volcanism on Earth. The hotspot is currently beneath Yellowstone, battling its way up through a subducting slab of ancient rock, but it was once further to the west.
As it fired up towards the surface, the hotspot caused several enormous eruptions – 12 of which are recorded in this new study. The hotspot has remained stationary over time, whereas continental drift has meant that the surface above it has been constantly moving westwards. This means that the surface volcanism has been traced eastwards, towards present-day Yellowstone.
Yellowstone is known to erupt catastrophically every 650,000 years, and it is currently recharging its basaltic magma chamber, which some estimate to be roughly 600 cubic kilometers (145 cubic miles) in volume. This may sound pretty big – and to be fair, it is – but it’s still just a third of the size of the magma source that produced the supereruption at Cassia Hills 8.1 million years ago.
Yellowstone may have a violent future ahead of it, but the entire region had a rather spectacularly destructive past too.