The history of the Yellowstone hotspot
This photo comes from an area of the United States known as the Snake River Plain. The rocks you see behind the river are part of a large series of igneous rocks that scrape across the western United States from Oregon through Idaho, finally ending at Yellowstone National Park. This area really is a plain – the land is fairly flat, filled with rolling hills and gentler river slopes, but it stands out because it’s surrounded by large, steep mountain ranges on all sides.
The Snake River Plain is believed to have been sculpted by the same forces that today produce Yellowstone; giant volcanoes. For the last 15 million years or more, a hotspot has been sitting beneath the western U.S., supplying heat that has driven large volcanic eruptions.
These eruptions form calderas – large craters like Yellowstone produced when huge amounts of magma are erupted to the surface, leaving a large gap in the earth at the top of the magma chamber and causing the rocks above to collapse downward. In a sense, the Snake River Plain was formed by the Earth devouring the mountain ranges that used to be there.
Some parts of the history of this hotspot are known, some parts are less well known. It’s believed there are many calderas along the Snake River Plain, but how many eruptions have taken place is not well known because many of the older calderas have been buried by rocks from the younger eruptions. We know pretty well when the Yellowstone caldera eruptions have taken place (the most recent eruption was 640,000 years ago), but the older calderas aren’t that well constrained. Understanding the behavior of previous eruptions will give us insights into how the Yellowstone volcano might work in the future.
New research from the lab of Dr. Bindeman at the University of Oregon tells the story of one of these calderas, the Picabo caldera, located in eastern Idaho not far from where this photo was taken (close to the city of Blackfoot, for reference). The caldera itself is buried under up to 2 kilometers of younger rocks, so the remnants of most of the eruptions were hidden. However, there are a series of recent drill cores through the SRP that sample these rocks, and using those cores they tell the story of this caldera’s activity.
Based on the drilled rocks, they identify 8 distinct eruptive units that could represent eruptions from this caldera. All took place between 6 and 9 million years ago. The sizes are difficult to estimate but several were likely on the scale of the Yellowstone eruptions.
The authors also piece together how the magma chambers that gave rise to the eruptions formed. The mineral zircon is formed in magma chambers and generally does a good job of recording the chemistry of the magma that formed it.
By measuring zircon chemistries, the authors find that the first eruptions at a caldera are produced by the assembly of a series of smaller magma chambers, each with its own chemistry. These distinct magma chambers lead to the formation of distinct zircon compositions which survive until they are erupted, allowing them to be measured today.
After the first eruption(s), there is a transition. The first magmas have a variety of zircon compositions, but these are followed by eruptions with nearly-homogeneous zircon compositions.
This work therefore suggests that the mechanism for forming a caldera like Yellowstone involves a series of smaller magma chambers and eruptions that eventually join together to form the giant volcanoes. Those large calderas are then capable of multiple eruptions until they finally quiet as the magma supply moves away.
Several final details are also worth noting. First, the eruptions at this site took place roughly 500,000 years apart, similar to the age differences between eruptions recorded at Yellowstone (8 eruptions over about 4 million years). That result suggests the timing between eruptions at Yellowstone, of just over 500,000 years, is consistent through a large part of the Snake River Plain.
Finally, the timing of this caldera’s eruptions overlaps with the eruptions of two neighboring calderas. This result we’re less familiar with; only Yellowstone has erupted within the last 2 million years, but at this time, there were up to three Calderas erupting during the same time interval. This state could be one that the Yellowstone hotspot returns to in the future as it continues to migrate across the western U.S.
Both this research project and the drilling operations funded in part by the National Science Foundation._ _
