Volcanoes of Zion and Southwest Utah

Basalt lava flows at Lava Point

NPS photo

Although no eruptions have occurred in Zion in recent time, there were some in the surrounding area. The most recent eruptions near Zion were witnessed by the Southern Paiute and Hopi tribes, including events at Panguitch Lake (1,000 years ago), Parashant National Monument (950 years ago), and Sunset Crater Volcano (920 years ago). The latest eruption occurred near Fillmore, UT, just 660 years ago, with the lava flow still visible on satellite images. While another eruption could happen at any time, it might also be thousands of years away. Recently, magma movement was detected near Sunset Crater in the San Francisco volcanic field, which geologists consider the most likely to erupt within the next few centuries among the volcanic fields of the southern Colorado Plateau.

Visit the Interactive Geologic Map of Utah from the Utah Geological Survey to click on the different volcanic eruptions in the state over time. As you will see Zion is surrounded by volcanic eruptions. The map also identifies other surface rock types and their ages including the layers of the Colorado Plateau.

Zion Volcanic Eruption Timeline:

• 1.1 million years ago a very large outpouring of magma occurred on the Kolob Plateau near Lava Point. This lava flow traveled all the way down to the Virgin River, likely damming it and creating a temporary lake.
• 310,000 to 220,000 years ago, several eruptions occurred in the Grapevine Wash area on the way up to the Kolob Terrace. The cinder cones from these eruptions include Firepit, Spendlove, and Lamb Knolls. The Kolob Terrace road winds past these cinder cones and their lava flows.
• 120,000 years ago, Crater Hill erupted just below the West Temple between Rockville and Virgin. Its lava flow dammed the Virgin River creating a lake that stretched into Zion Canyon perhaps as far as Canyon Junction.

Summary of the Zion Volcanism Story:
Volcanism Causes: The volcanoes in this region are due to not one, but TWO reasons. The first is Basin and Range crustal extension (collapse and stretching of the crust to the west) that has been going on for 17 million years. The second is Colorado Plateau uplift and melting of its base the last 6 million years.

Basin and Range extension has stretched the crust thin in the American southwest, allowing Earth's mantle to swell closer to the surface. When the East Pacific Rise subducted under California, the eastward pressure on the crust evaporated, and the mountain range collapsed of its own weight. This action thinned the crust, allowing the mantle to rise. The mantle is only 15-18 miles below Zion today. This reduction in pressure lowered the melting point of mantle rocks, allowing them to liquify into magma. This was the first source of magma for the local volcanoes.

Uplift of the Colorado Plateau is the other major reason for local volcanism along the southern half of the Colorado Plateau's boundary. An arc of volcanoes can be traced from Delta, UT, to Flagstaff, AZ, and beyond. As the Colorado Plateau uplifts, the base seems to be melting. Seismic tomography has revealed massive structures sinking below the plateau, allowing hotter mantle rock to rise and melt in the lower pressure, providing magma for the frequent volcanism along the southern edge of the Colorado Plateau.

Magma Composition: Almost all the eruptions in this region are considered mafic (low in silica), producing basalt. However, volcanism around the Colorado Plateau is classified as 'bi-modal,' meaning it can be low in silica or, in rare cases, high in silica due to additional melting of silica-rich rock underground. High-silica magmas can be very sticky and explosive, trapping water and carbon dioxide that expand violently upon reaching the surface.

Local magmas typically contain up to 0.5% carbon dioxide and between 0.5% to 1.5% water. This combination can lead to explosive eruptions, similar to Mt. St. Helens, or slower lava dome formations, as seen at Mt. Elden in Flagstaff.

The magma rising in this region utilizes faults to reach the surface. Once it reaches the brittle zone of the crust, water and carbon dioxide are injected into microscopic cracks, allowing the magma to follow. Scientists monitor earthquake signals to differentiate between those caused by rising magma and regional tectonic activity from ongoing Basin and Range extension.

Magma Pathways: Magma in the southwest, including Zion and St. George, follows unique paths to the surface due to the area's volcanic field structure. Each eruption occurs at different locations and is typically a one-time event, with no repeat eruptions from the same site. As magma rises from 15-55 miles below, it pushes through soft rock and encounters the brittle zone around 7 miles down. Here, water and CO₂ are injected into microscopic cracks, allowing the magma to frack its way upward through a network of pathways.

The region's fractured ground provides numerous routes for magma. These joints are visible in satellite images of Zion, particularly in the north-south orientation. While specific sites like Crater Hill (which can be seen from the Coal Pits Wash trail) will not erupt again, new eruptions may occur nearby. The Santa Clara Volcanic Field, where Zion is located, became active about 3.5 million years ago, and the next volcanic activity could happen in unexpected locations, such as the Narrows.

Eruption Dynamics: So, what is one of these eruptions like? In the modern age, geologists will be the first to know. Earthquakes from rocks broken by rising magma will be detected by seismometers. As the magma rises near the surface, small earthquakes will be felt, and sounds like faint thunder will be in the air. People will not be surprised by one of these events; there will be at least weeks, if not months, of advance warning before an eruption. As the magma nears the surface, carbon dioxide and water will start 'exsolving' from the magma, effusing out of the ground and creating a super high-pressure foam that cracks the rocks above. Stress cracks will form on the surface, followed by the roar of gases.

The expansion of volatiles is what makes a volcanic eruption violent. Carbon dioxide expands about 550 times in volume when it transitions from liquid to gas, while water has an expansion ratio of 1,600 times. This means a cubic kilometer of magma with even a small number of volatiles can create a massive volume of explosive gas, leading to the eruption.

Once an eruption begins, it may take a few months or longer to expend the gases. Initially, a lava fountain will shoot cinders into the air, forming a cinder cone. Eventually, as gas pressure decreases, lava will flow slowly from beneath the cone, which can take months or years.

Lava flows can create dams, temporarily blocking rivers like the Virgin River. At least two such dams blocked the Virgin River between Virgin and Rockville, forming lakes that stretched into Zion. These flows were so hot they evaporated the river, devastating the ecosystem dependent on that water. Eventually, the lakes would overtop the lava dam, leading to dramatic floods downstream.

There is no way to know when the next eruption will be in this region, but there will be a next time. It could be 5 years from now, or 10,000 years from now. Still, scientists have placed seismometers throughout the region looking for those first characteristic earthquakes as magma fights its way to the surface. Perhaps we'll see one of the most amazing shows on Earth here in southwest Utah.

 

Two cinder cones seen on Kolob Terrace Road. Firepit Knoll on left, Spendlove Knoll on right.

NPS photo/Michael McDevitt

These volcanic features can be seen along State Route 9 near the town of Virgin, and at several places along the Kolob-Terrace Road, including Lava Point.

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