Plate Tectonics I: Yellowstone
Yellowstone is Shaking - Real-world Applications: Yellowstone and Earthquakes
For many people, the name "Yellowstone" is synonymous with national parks. Here is a little background material about Yellowstone, and then an introduction to earthquakes, which are common in the Yellowstone area.
Yellowstone was the first national park, for a while it was the biggest (at approximately 60 miles by 60 miles—100 km by 100 km—it is still a big one), and it is widely considered to be the best. You could probably identify a dozen features or more in Yellowstone that, separately, would merit protection by a national park. The Grand Canyon of the Yellowstone is a great chasm cut through volcanic rocks that have been “cooked” by hot water and steam circulating through them, in the process gaining the yellow color that gave the park its name. The Canyon contains two large waterfalls (rather unimaginatively called the Upper and Lower Falls), where the river plunges over more resistant rocks. Mammoth Hot Springs is a mountain being turned inside-out, with acidic hot-spring waters dissolving caves underground, bringing the dissolved limestone to the surface, and depositing some of it as gleaming terraces. Specimen Ridge is home to at least 20 petrified forests complete with petrified roots, standing one on top of the other, from roughly 40 million years ago. Volcanic ash and debris flows buried the standing trees, and chemical reactions caused the silica in the ash to move into the wood, replacing it (a subject for much later in the course). A new forest grew and was buried, and this was repeated 20 or more times.
Yellowstone is especially famous for its thermal features. Various lines of evidence indicate that there is a body of melted rock (magma) under the park, now centered beneath the northeast side of the park. The rocks under most of the park are anomalously hot at shallow depths. In addition, the park receives abundant rainfall and snowfall. The water from rain and melted snow circulates deeply through rocks broken by numerous earthquakes, and the water is heated from below. In some places, the water is heated all the way to steam, which emerges from holes known as fumaroles. In other places, hot water bubbles to the surface in beautiful springs. If the bubbling action mixes in enough mud, then paint pots, mud pots or mud volcanoes develop.
Sometimes, cold water on top holds hot water down, with the pressure preventing the boiling of the hot water in a pressure-cooker effect. Eventually, a little boiling manages to expel a little of the water above, reducing the pressure, and allowing more boiling, and a geyser erupts. Geysers require heat, water, and a tight, tough plumbing system to hold the hot water and withstand the high pressures. The volcanic rocks of Yellowstone are rich in silica, which is dissolved and re-precipitated by the hot waters to seal cracks in the rocks, helping produce geysers. Perhaps half of the world’s geysers are in Yellowstone, including the largest and most spectacular ones. Yellowstone also is noted for many waterfalls besides those in the Canyon, for several other interesting features, and abundant wildlife, which we'll visit later in the course.
Yellowstone itself is centered on the Yellowstone Caldera, a collapse feature related to three great volcanic eruptions, or periods of eruptions. The caldera, roughly 50 x 30 miles (80 x 50 km), includes Yellowstone Lake but extends well beyond it. (No lake in the nation is both higher and larger than Yellowstone Lake, yet it is only a piece of the caldera.) The eruptions occurred roughly 1.8, 1.2, and 0.6 million years ago. Each of these eruptions moved roughly 1000 times more material than was moved by the Mt. St. Helens eruption of 1980 we will discuss soon; thick deposits of ash that were erupted from Yellowstone are found in the Badlands region of South Dakota. The erupted material that spread across South Dakota was removed from a magma chamber, and after removal, the “top fell in” to create the large depression that is the caldera. (Note that it is easy to take the timings of the eruptions...1.8 million years ago, 1.2 million, 0.6 million...and think that it is about to explode again. There is no evidence that an eruption is imminent, though. Dr. Alley recalls a worried tourist asking a ranger when the volcano would erupt, with the ranger replying that as long as he was not running away, the tourist didn’t need to worry!)
Yellowstone has many lessons to teach us. (It would be fun to have a course on the geology of Yellowstone alone, and we certainly could fill a semester.) The size of the Yellowstone eruptions is of considerable interest, especially considering the likelihood that they will recur sometime. Here, we wish to use Yellowstone to introduce earthquakes.
European exploration of the Yellowstone region probably began with “mountain man” John Colter during his return from the Lewis and Clark expedition in 1806, although Native Americans had used the region for thousands of years before. Colter brought back fantastic tales of the region, which were largely dismissed because they seemed impossible. Other travelers, and especially Jim Bridger in the 1850s, returned with similar tales, which also were discounted, in part because Bridger was a bit of a tall-tale teller. He is credited with stories of petrified birds sitting on petrified trees singing petrified songs (an exaggerated description of Specimen Ridge), of rivers that ran so fast they became hot on the bottom (the Firehole River, which has hot springs on the bottom), of trying to shoot an elk and missing because a mountain of glass was in the way (Obsidian Cliff, where rapidly-cooled volcanic rocks have made a glass called obsidian, which was mined, shaped into tools and decorations, and traded by the native Americans), and more.
To separate fact from fancy, the Washburn expedition from Montana (Washburn was surveyor-general of the Montana Territory) visited the region in 1870, and first developed the idea of a national park. The government-sponsored Hayden expedition of 1871 provided scientific documentation of the wonders of Yellowstone, supported by the artwork of Thomas Moran and photography by W.H. Jackson, which convinced Congress to found the park in 1872.
While in the park, the Washburn party felt earthquake activity. Breaks in recent stream and glacier deposits showed the geologists of the party that faulting had occurred recently, and those geologists already knew that motion on faults can produce earthquakes. Since then, modern monitoring equipment has detected numerous quakes in the area. (The United States Geological Survey identifies about 2000 earthquakes per year in Yellowstone, although with much year-to-year variability, and many smaller ones occur that the USGS doesn’t highlight.)
On August 17, 1959, a Richter-magnitude 7.5 quake occurred, centered near the northwestern boundary of the park. Many of the geysers were changed, and at least 289 springs suddenly erupted as geysers, including 160 with no prior record of eruption. The ground over the quake (at the epicenter—the place above the center of the quake) was broken, with one side dropping roughly 6 feet (2 meters) relative to the other side, and with a little twisting and turning causing even larger drops in some places. A large landslide was triggered, burying a campground, damming the Madison River to form Quake Lake, and burying many highways. 28 people were killed. Some survivors had their clothes torn off by the immense blast of wind pushed out of the way by the huge landslide. The Old Faithful Inn was evacuated, and the west entrance to Yellowstone closed. The University of Utah’s Seismograph Station has a nice summary of the press reports. You may find it interesting to search for and read the report from the Billings Gazette that a beauty pageant was going on in the historic Inn with 800 people watching, and that “Moments after the queen had been crowned and she was walking down the aisle to the plaudits of the crowd, the first, mighty shock hit. Everyone in the place dashed for the door.”
Take a Tour of West Yellowstone, Earthquake Lake
Take a quick virtual tour with Dr. Alley through the U.S. Forest Service Madison River Canyon Earthquake Area.
Want to see more?
Here is an optional virtual tour you might also want to explore. (No, it won't be on the quiz!)
The Original Iceland Geyser - Yellowstone has roughly half of the world's geysers, but there are geysers in New Zealand, Iceland, and elsewhere. The original "geyser" is Geysir, in Iceland. Here is a short film clip of Geysir erupting, just for fun. Filmed by Vicki Miller.