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October 10, 2021

What will happen to Yellowstone once its rhyolite magma system shuts down? To understand the future, geologists look to the past—in this case, to Yellowstone Caldera’s older but smaller sibling, Henrys Fork Caldera!

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Mark Stelten, research geologist with the U.S. Geological Survey.

Digital elevation model of Yellowstone National Park and vicinity
Shaded relief map of Henrys Fork Caldera and vicinity. The margin of Henrys Fork Caldera is shown in blue. Note the smooth, low-relief topography within the caldera compared to the steep and dynamic topography associated with Yellowstone Caldera (at the right side of the image).

When will Yellowstone erupt again? This is one of the most common questions people have when they think of Yellowstone, and it is an important topic for volcanologists. Yellowstone has had three caldera-forming eruptions in the past 2.1 million years, the most recent of which occurred ~631 thousand years ago and resulted in the formation of Yellowstone Caldera, so it is possible that Yellowstone may produce another large eruption of rhyolite in the future. However, the occurrence of another large eruption at Yellowstone represents only one possible future. It is also important to understand what will happen to Yellowstone if the upper-crustal magmatic system feeding its eruptions and powering its hydrothermal features finally shuts off, as will eventually occur, and as did occur in all of the older caldera centers that define the Yellowstone-Snake River Plain hotspot track.

To get a glimpse into this possible future, we can look to Yellowstone Caldera’s older sibling, Henrys Fork Caldera, which formed as a result of a smaller caldera-forming eruption approximately 1.3 million years ago. Henrys Fork Caldera is located southwest of Yellowstone National Park, between Island Park, ID, and Ashton, ID, and can easily be viewed along Highway 20. When compared to the present-day Yellowstone Caldera, there is a stark contrast in topography. The landscape within Yellowstone Caldera is a reflection of explosive eruptions that produced the overall basin, as well as viscous rhyolite lava flows that formed broad plateaus and steep domes. The result is a dynamic landscape full of relief. In contrast, Henrys Fork Caldera is generally flat and lacks significant relief, except for its caldera margins and where the Henrys Fork river has cut through the caldera.

The reason for this difference is that Henrys Fork Caldera has been filled in with basalt lava flows that erupted after its upper-crustal magma system shut off. During Yellowstone’s second volcanic cycle, Henrys Fork Caldera was probably underlain by an upper-crustal magma chamber much like that beneath present-day Yellowstone Caldera, where a rhyolite magma body (mostly solid right now) resides at a depth of 5 to 17 km (about 3 to 10 miles). This upper crustal magma chamber prevents denser basaltic magma, which is stored below the rhyolite magma chamber, from rising to erupt within the caldera. However, once this upper-crustal magma chamber cools and solidifies, basalt magmas can penetrate the upper crust and erupt.  This is what happened within Henrys Fork Caldera—after the rhyolite magma chamber cooled and solidified, basaltic magma was able to ascend and erupt. These basalts, similar to those that erupt in Hawaiʻi, are less viscous than rhyolite lava flows, so they fill in any topographic lows. At Henrys Fork Caldera today, only the tops of rhyolite domes can be found.  The rest of the rhyolite volcanic rocks have been buried under basalt, which smoothed out the caldera’s formerly impressive topographic highs and lows.

The same process also occurred in other calderas that dot southern Idaho and mark the path of the Yellowstone hotspot.  We know that the calderas exist because we can see the ash layers that are evidence of past eruptions.  But basaltic lavas that erupted after the calderas went silent, combined with erosion, flattened out the topographic highs and lows of the region, resulting in the Snake River Plain that we know today.

This scenario—of basaltic eruptions that follow the end of rhyolite eruptions—may also occur at Yellowstone Caldera after its upper crustal magmatic system shuts off. When that happens, the amazing topographic highs and lows of the Yellowstone region will be erased—all part of the life cycle of a caldera within the Yellowstone system!  

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