The geology of Yellowstone National Park, and its precursors in Oregon and Idaho

Yellowstone has the distinction of being the first national park in the U.S., and probably in the world. President Ulysses S. Grant signed the bill to protect more than one million acres—mostly in Wyoming—in 1872. Yellowstone was set aside because of its hydrothermal displays—the park contains more than 10,000 features, including the world’s greatest concentration of geysers, hot springs, mud pots, and steam vents.

Visitors are attracted to Yellowstone for the great variety of hydrothermal features and wildlife. Here elk amble next to an area of fumaroles (steam vents).
This map of Yellowstone National Park shows the types of fluids emitted from hydrothermal features, which are the surface expression of hot magma that is deep below the surface. The water in hydrothermal systems comes from rain and snow melt that seep into the ground, where it is heated and chemically altered during its circulation through hot rocks overlying the magma chamber. The hydrothermal fluids buoyantly rise to the surface where they mix with colder groundwater, causing precipitation of the siliceous sinter or calcareous travertine that form impressive cones, mounds, and terraces in thermal basins. Note the “hydrothermal explosion craters”; these features occur when pressure release causes hot water to turn to steam. These are violent and dramatic explosions of boiling water, steam, mud, and rock fragments that can reach heights of 2 km (1.2 miles) and create craters up to more than 2 km (1.2 mi) in diameter. They occur, on average, every 700 years. Map from USGS site:

During recent decades, as the “super-volcano” idea has been popularized, visitors have become more aware of the volcanic origin of Yellowstone Park. A volcano is characterized as a super volcano if the Volcano Explosivity Index (VEI) is greater than 8, meaning the volume of the measured eruptive deposits are >1,000 cubic km (240 cubic miles).

Of the three main eruptions at Yellowstone—Huckleberry Ridge (2450 cubic km / 588 cubic miles), Mesa Falls (280 cubic km / 67 cubic miles), and Lava Creek (1000 cubic km / 240 cubic miles)—two qualify Yellowstone as a super volcano. To get some sense of this volume, consider the Grand Canyon in Arizona, whose volume is estimated as 4170 cubic km / 1000 cubic miles. So…the amount of tephra emitted during the Huckleberry Ridge eruption would have filled more than half the entire Grand Canyon! Diagram from USGS site: [Note: the Toba eruption was in Indonesia.]
Of course, the ash did not all enter into the Grand Canyon, but some probably entered it, as the tephra spread over vast areas of the U.S. As at Crater Lake National Park, the Yellowstone eruptions likely started with a huge blast that created a tephra column extending upward tens of miles. These columns put immense amounts of ash into the atmosphere and likely caused global cooling for a few years after the eruption. Map from USGS web site:

Most of Yellowstone’s eruptions have been rhyolite—silica-rich magma often containing gases that make the eruptions highly explosive. The three extraordinarily large explosive eruptions in the past 2.1 million years (illustrated above) each created a giant caldera within or west of Yellowstone National Park (see map below). During these eruptions, enormous volumes of hot ash, pumice, and other rock fragments spread outward as pyroclastic flows over vast areas where they were welded together to form extensive sheets of hard lava-like rock, called ash-flow tuffs or ignimbrites, that in places are more than 400 m (1300 ft) thick!

The largest eruptions, which created the Huckleberry Ridge, Mesa Falls, and Lava Creek Tuffs, account for more than half the material erupted from Yellowstone in the past 2.1 million years. Because such enormous amounts of magma were erupted during each explosive event, the roof of the magma chamber collapsed, and the ground above subsided by many hundreds of meters to form the calderas.

This map shows the extent of the three calderas within and adjacent to the park. The youngest caldera, called the Yellowstone caldera, was formed during the Lava Creek eruption 640,000 years ago. This map also shows volcanic rocks that have erupted since the last caldera was formed, and earthquakes that have occurred on faults that cut through the area. Map (produced by USGS) from:
This photograph of the Grand Canyon of the Yellowstone shows ash-flow tuffs that are reddish in color because they have been altered by the action of hydrothermal activity. Hot fluids flow through the rocks and change their chemistry. The upper and lower falls (falls at the upper part of the canyon in this photo) form where more resistant rock yields to rock that is less resistant because it has been so altered. This canyon is clearly formed by a river, since it has a V shape. Although ice covered the park during past glacial periods, most evidence has been removed, and this canyon probably formed within the past 10,000 years. The Grand Canyon is located near Canyon Village (see blue arrow on the map at the top of this post).

But why have these vast volumes of lava and pyroclastic flows erupted at a location that is in the middle of the North American plate? Most volcanic action occurs at plate boundaries. For example, the volcanoes of the Cascade Range are a result of the Cascadia subduction zone, a convergent type of plate boundary (see post on October 16, 2020). In contrast, Yellowstone is a volcano that has formed over a “hot spot” fed by plumes of hot material rising from deep in the Earth, perhaps from as deep as the outer core. As a tectonic plate moves over a hot spot, a chain of volcanoes is produced, with volcanoes younging in the direction from which the plate is moving.

For example, let’s first look at the planet’s most famous hot spot, which has produced the Hawaiian Islands. They are located in the middle of the Pacific plate, which is moving northwest at a rate of ~10 cm/yr (4 in/yr). The age of Kauai tells us that the Pacific plate was located over the hot spot around 5 million years ago. The islands get progressively younger to the southeast, to the “Big Island” that is currently sitting on top of the hot spot. Imagine taking a candle (hot spot) and moving a piece of cardboard (tectonic plate) over the candle. Geologists are fortunate to have these hot spots that appear to remain stationary for long periods of time; it is one of the ways that we can calculate the rate that plates are moving. Because the Hawaiian plume rises through oceanic crust, the produced magma has a silica-poor composition (mostly basalt) compared the silica-rich magmas (mostly rhyolite) produced when a plume rises through continental crust. Diagram from TASA Graphics.
In a similar way, the Yellowstone hot spot has created a chain of volcanoes parallel to the direction of the North American plate, which is moving toward the southwest (as shown by blue arrow) at a rate of about 2.3 cm/yr (1 in/yr). At least seven major volcanic fields have been identified, starting with McDermitt in southeast Oregon, which began ~16.5 million years ago, and extending to Yellowstone, which started ~2 million years ago. These volcanoes have all had explosive, caldera-forming eruptions because of the silica-rich nature of the magma.

You will notice that the hot spot track goes mainly through Idaho, along the Snake River Plain. An interesting place to visit is Craters of the Moon National Monument. Here, the explosive volcanoes associated with the hot spot have given way to more silica-poor lavas (basalt). Although the area has moved off the hot spot, extension associated with the Basin & Range Province has caused volcanism to continue in an area that was already weakened by extensive eruptions. For more information about this site, see my July 18, 2020 blog post. Map from USGS site:

But what’s the vast grey area toward the northwest of the hot spot track that’s labeled Columbia River Basalts? Surprisingly, these volcanic flows are also a result of the hot spot, but they went flowing off in a direction away from the hot spot! The reasons for this are too complicated to explain in this post, but suffice it to say that the continent changes dramatically at the eastern edge of Oregon. Most of Oregon consists of disparate terranes (crustal pieces with diverse origins) that have been “glued” together over millions of years. So most of Oregon has continental rock that is relatively thin and weak. In contrast, the continental rock to the east is older, thicker, and more consolidated. As the continent was moving to the southwest over the hot spot, the top of the plume was sheared off at the discontinuity. This caused a lot of melting along the boundary between weaker and stronger parts of the continent.

Here is a more detailed map of the area covered by the Columbia River Basalts (CRB). As the Yellowstone-like (silica-rich) eruptions were beginning, first at what is now Steen’s Mountain, and then at McDermitt, more silica-poor magmas were also produced, causing eruptions of basaltic lava that flowed north to where they were captured by the Columbia River and carried downhill all the way to the coast. The total volume of lava erupted was enormous—approximately 175,000 cubic km (42,000 cubic miles)—which is enough to cover the entire U.S. (including Alaska and Hawaii) in a layer of lava 17 m (58 ft) thick! Most of the basalt’s volume erupted in about 1 million years (~16.5–15.5 million years ago), but smaller eruptions continued until about 5.5 million years ago. These types of vast outpourings, which typically occur at hot spots, are called flood basalts. They are able to flow so far away from the hot spot because they are basaltic lavas that are silica poor and very fluid, and because they are emitted from fractures at an extremely fast rate. Map from USGS web site:
As shown on the map above, there are many places to see the CRB in Oregon and southeastern Washington. This photo is from the John Day Fossil Beds National Monument in central Oregon. It shows older, brightly-colored sedimentary and volcanic rocks below and dark colors of the Columbia River Basalts on the skyline. Imagine how the landscape must have looked about 15 million years ago—a vast area of black basaltic lava rock that had been very fluid and had filled in any low parts of the landscape.
Silver Falls State Park, located about 80 km (50 miles) southeast of Portland, is another place to find Columbia River Basalt. Because the lava rock is relatively resistant to erosion, water falls are often created at the edge of the basaltic plateau.
What is truly stunning is that the basalt flows made it all the way to Oregon’s Pacific coast, and even a bit to the south. Seal Rock, between Newport and Waldport, is the southernmost occurrence of the CRB along the coast. The vertical structure is cooling columns—they occur as the lava shrinks during cooling to form vertical cracks. Basalt columns can be found many places in Oregon, and they are often “harvested” for placement in garden landscapes.

Why did two different types of materials erupt from the same hot spot: silica-rich, caldera-forming, mostly pyroclastic flows and air falls to the northeast, and silica-poor, highly fluid lavas to the northwest? Mainly, this is because the magma under volcanoes to the northeast of the hot spot were traveling upward through continental crust and picking up more silica along the way. In contrast, the magma under fissures to the north of the hot spot were traveling upward through crust that was thinner and also less silica rich.

This diagram illustrates the classification of igneous rocks. On the left side are felsic rocks with over 60% silica. These rocks are called granite when they cool deep underground (plutonic or intrusive) and rhyolite when they cool above ground after being erupted (volcanic or extrusive). The high silica content makes them highly viscous (i.e., “sticky”) and the gases these magmas often contain makes the eruptions highly explosive.

Mafic rocks, on the other hand, have <50% silica; they are called gabbro when cooled underground and basalt when erupted onto Earth’s surface. The lower silica content makes mafic magma much less viscous (i.e., very “runny”). Oceanic crust is made of mafic rocks, which are made of much denser minerals than those found in the felsic rocks that make up continental crust. This is why oceanic plates always sink beneath continental plates when they converge with each other. Diagram from:

Finally, what is the likelihood that Yellowstone will erupt again? Despite the sensationalism associated with a “super volcano”, the likelihood of Yellowstone erupting with a vast, caldera-forming explosion is highly unlikely in the context of our human time scale. The most likely eruptions would be a hydrothermal explosion or a lava flow within the pre-existing caldera. The USGS’s Yellowstone Volcanic Observatory has a rigorous monitoring program and will let us know if trouble is brewing!

Important references

U.S. Geological Survey Yellowstone Volcano Observatory web site:

National Park Service web site about the geology of Yellowstone:

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