Steamboat Geyser eruption seen through the trees in Yellowstone National Park on September 17, 2018. (Photo by Jacob W. Frank/ National Park Service)
The Yellowstone region today is wonderland. It is full of spectacular geysers and colorful hot springs, home to lakes and meadows and mountains and valleys, and covered by forests and grasslands. What we see today is a result of volcanic activity that has shaped the landscape over the past 2 million years.
But what did Yellowstone look like before volcanic activity blew several large holes in the region and covered huge swaths of land with thick lava and ash flows? What was Yellowstone like before it became wonderland?
To understand the answer to this question, geologists have looked at the characteristics of the areas bordering the Yellowstone region — at the mountain ranges, rock types, and faults that make up areas like the Tetons and Jackson Hole, and like the Gallatins and Paradise Valley.
During about 4–7 million years ago, the Yellowstone hotspot was located under southeastern Idaho, feeding eruptions occurring from the Heise volcanic field. That sequence included multiple large calderas that formed via major explosions, spreading ash across the landscape — including Jackson Hole and the area that is now Yellowstone.
The pre-volcanic Yellowstone landscape was mostly made of high-elevation areas — there was no basin present like there is today. Instead, mountain ranges that ran mostly north-northwest to south-southeast extended across the region. Today’s Gallatin and Madison ranges in the north were probably connected to the Tetons and other mountains to the south, forming sets of continuous ranges that were all bounded by large faults. Fault-bounded ranges like these are common throughout the western USA today — part of the Basin and Range province, which extends from eastern California to western Wyoming and Montana.
We can see the evidence for these formerly continuous mountain ranges in patterns of earthquakes and eruptive vents. Seismicity maps show several north-northwest to south-southeast bands of earthquakes beneath Yellowstone Caldera, possibly delineating the still-existing faults that controlled the mountain ranges that were blown apart when large explosive eruptions began in the Yellowstone region. There are also several roughly north-northwest to south-southeast alignments of vents for rhyolite lava flows that erupted after Yellowstone Caldera formed, especially during about 160,000 to 70,000 years ago. Just like patterns of earthquakes, the vent alignments might also have been controlled by the preexisting faults associated with the destroyed mountain ranges.
And because there were mountains throughout the Yellowstone region before the big explosions, erosion was an important process. The high mountain ranges were gradually being ground down, and sediments eroded from these peaks accumulated in valleys at the bases of the ranges. Some of these sediments still exist today, capped by thick blankets of ash from caldera-forming eruptions of the Yellowstone system.
The first volcanic eruptions from the Yellowstone region began at least 2.2 million years ago, and the first of three great caldera-forming eruptions — that which deposited the Huckleberry Ridge Tuff — occurred 2.08 million years ago, spreading thick ash over an area larger than the state of Connecticut and dramatically altering the landscape.
Today, many visitors to Yellowstone National Park approach the area from the north, south or west. In driving through the mountains and valleys that lead to wonderland, take a moment to appreciate the landscape you are traversing. Those areas today exemplify what Yellowstone used to look like a few million years ago.
Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution is from Michael Poland, geophysicist with the U.S. Geological Survey and Scientist-in-Charge of the Yellowstone Volcano Observatory.