NPS photo Zion has several volcanoes erupt inside its boundary, but these erupted long before Zion became a park. Many others, from 1-3 million years ago, erupted just north of the park, and many others have erupted much more recently. Zion is in an active volcanic field called the Santa Clara. Eruptions in this field are typically on the order of one every few 10,000 years. The last one was 32,000 years ago in Snow Canyon State Park. However, just north of the park in the Markagunt volcanic field near Cedar Breaks and Panguitch Lake, there was an eruption only 1,000 years ago, with several others in the last 10,000 years.
See this volcanism website from Grand Canyon-Parashant National Monument, located just south of Zion on the Arizona Strip, for an extensive explanation for the volcanism of region, including Zion and the other nearby volcanic fields. They all share the same story of Basin and Range crustal extension and the melting of the basement layers of the Colorado Plateau that have created thousands of eruptions over the last few million years in Utah, Arizona, New Mexico, and Colorado. 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:
Summary of the Zion Volcanism Story: The eruptions of Southwest Utah were a real enigma. After decades of research, especially after plate tectonics was understood, the picture became clear. Volcanism in the American southwest and northern Mexico have intrigued volcanologists from around the world. There isn't a subduction zone here like the one in Washington and Oregon that created volcanoes like Mt. St. Helens or Mt. Rainier. This region also doesn't have a hot spot that feeds eruptions in Hawaii, Iceland, or the Galapagos Islands. 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 and northwest Mexico, allowing Earth's mantle to swell closer to the surface. The stage was set for Basin and Range extension 30 million years ago when the North American continent moved southwest and overtopped the East Pacific Rise west of California. The East Pacific Rise prior to this had been making new seafloor that was subducting under California. This is what created the ancient volcanoes of Yosemite and Kings Canyon. To the north off Washington and Oregon a piece of the East Pacific Rise still sits offshore. Call the Juan de Fuca plate, this small bit of the ancient Farallon plate still subducts. However, subduction zones put pressure on the content they dive under. This caused the western US to compress and thicken. When the East Pacific Rise subducted under California, the subduction zone died. The subduction zone put so much pressure on the southwest, it created a mountain chain with peaks over 20,000 feet high. It was called the Sevier-Mogollon Highlands and was like the modern Andes Mountains. This range stretched hundreds of miles through Arizona, Utah, Nevada, and California. When the East Pacific Rise subducted under California, the eastward pressure on the crust evaporated and the mountain range collapsed of its own weight. The pieces of crust in this zone all began to settle, extending the surface distance westward. This action doubled the width of Nevada. This also thinned the crust. Earth's mantle also responded to this thinning by swelling up into the part of the crust abandoned by the thinning crust. 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. What follows is a new and active area of research. What is known is that an arc of volcanoes can clearly be traced from Delta, UT to Flagstaff, AZ, over to El Malpais National Monument near Albuquerque, north to Capulin Volcano, and into southern Colorado. There is more volcanism here than can be attributed strictly to Basin and Range extension and melting of the depressurized melting of the crust. As the Colorado Plateau uplifts, the base of the plateau seems to be melting for some reason. Using seismic tomography, which uses earthquake waves to map the inside of the earth, geologists have found massive structures larger than mountain ranges that seem to be falling away below the plateau and diving into Earth's mantle. As these sink, hotter mantle rock from the asthenosphere rises around the falling structures and replaces them. This puts very hot rock right below the crust, which melts in the lower pressure. This provides the magma to explain the extensive and frequent volcanism along the southern edge of the Colorado Plateau. Almost all the eruptions in this region are considered by geologists to be mafic (low in silica), which produces basalt. The most well-known basalt is produced by the volcanoes of Hawaii. These magmas have low levels of silica, between 45% and 53%. This allows the magma to flow like syrup. However, volcanism in the Basin and Range and around the Colorado Plateau is called 'bi-modal.' This means it can be low silica and flow like syrup, which is the predominant type of eruption, or the magmas in rare instances, stall underground for decades or centuries. While they sit below the surface, they melt additional silica-rich rock into the magma. This thickens the magma making it more like a putty. Silica molecules basically link together in long chains, clotting the magma. This is known as silica polymerization. These magma types are known as andesite, dacite, or the stickiest, rhyolite at over 70% silica. The stickier the lava, the more explosive it is as trapped water and carbon dioxide can't easily escape. Local magmas typically have up to 0.5% carbon dioxide and between 0.5% to 1.5% water. They are mixed in with the rock of the mantle, often put there at subduction zones. In magma, water and carbon dioxide are known as volatiles. They want to expand into their gas state but are under so much pressure they can't until the magma reaches the surface and low pressure. Once the magma breaches the surface, the two volatiles explode out of the magma the same way that a shaken soda pop blasts out when the cap is removed. A fluid magma will let a lot of gas out more gently over time. The stickier the magma, the more violent the degassing explosion will be because very little of the gas was able to escape before the magma reaches the surface, when basically the whole magma body pressure detonates in a short period of time. For instance, a typical low silica eruption may last months or years, but a sticky magma eruption like Mt. St. Helens, lasted only 9 hours because the whole magma plume detonated because of overpressure. In addition, these high silica magmas can be very high in water. Mt. St. Helens was about 4.8% water, but some subduction zone magmas can be as high as 7-8% water. When a stale magma body becomes high in silica but doesn't have much gas, it will erupt slowly, creating a lava dome. One of the nearest is Mt. Elden in Flagstaff, famous for the melted wax lava flows coming off its flank. However, if a high silica sticky magma is high in water and carbon dioxide, it will erupt explosively, creating vast pyroclastic ash clouds like Mt. St. Helens. A rain of pumice may also fall as larger blobs of sticky magma expand and cool, full of millions of air pockets. The San Francisco Peaks in Flagstaff is basically Arizona's version of Mt. St. Helens. The other nearby volcanic field that can feature high silica eruptions is northwest of Zion. Called the Black Rock Desert Volcanic Field, it is just south of Delta, UT. That volcanic field has also seen lava flows of obsidian. Obsidian is formed from high silica magmas with almost no water or CO2 that cooled so fast they formed volcanic glass. The magma that rises in the region uses faults to complete their journey to the surface once it reaches the brittle zone of Earth's crust. The brittle zone is approximately the top seven miles of the crust. Once it reaches this zone, water and carbon dioxide (under incredible pressures) are injected into microscopic cracks ahead of the rising magma plume. This breaks them open and the magma follows. Scientists look for the telltale earthquake signals to differentiate between earthquake types. When magma is breaking rocks ahead of an eruption it creates a specific earthquake type which is different from the frequent regional earthquakes where giant crustal blocks are sliding against each other. This sliding is due to Basin and Range extension that continues to this day. So how does the magma get to the surface? Here in the southwest, with only a few exceptions, each magma body follows a unique path to the surface. Unlike Mt. St. Helens and other subduction zone volcanoes with well-established plumbing systems that magma uses time and time again, Zion and the St. George Utah area are in a volcanic field. Volcanic fields have eruptions at various locations. Each eruption is one and done, never to erupt again. However, another magma plume will rise and erupt somewhere else in the field. When magma begins to rise 15-55 miles below the surface in this area, it pushes up through putty-like soft rock like the blobs of fluid rising in a lava lamp. About 7 miles below the surface the magma encounters the brittle zone of hard rock. Magma must find the easiest path through this. Water and CO2 that starts to fizz out of the rising magma and are injected into microscopic cracks. This break bedrock apart. Basically, magma fracks using what is known as hydrothermal fluid to clear a path for it to rise. In a haphazard fashion the magma will rise through a network of these cracks. It may move horizontally, then vertically, then horizontally again until it breaches the surface. However, when the next magma plume rises from somewhere in the volcanic field, it will find a different path to the surface. Earth's crust is always jostling and shifting, especially in the Basin and Range and on the Colorado Plateau. Volcanic fields typically last millions of years and then go extinct. A good example of this are the Hopi Buttes near Petrified Forest National Park. It was active from about 9-6 million years ago and then went extinct. About the time that volcanic field died, the San Francisco volcanic field started erupting a hundred miles to the west. The Santa Clara Volcanic field that Zion is a part of became active about 3.5 million years ago, about the same time as several other volcanic fields in the region from the Black Rock Desert to Uinkaret also activated. Because of Basin and Range crustal stretching and the rise of the Colorado Plateau, the ground in this region is so fractured that magma has many paths to rise through. These cracks, called joints, can easily be seen in a satellite view of Zion. The roughly north-south cracks are great paths for magma to use. So, when people ask if, say, Crater Hill will erupt again, the answer is no. It will never erupt again but somewhere else there will be an eruption near Zion and St. George. In the Uinkaret volcanic field on the Arizona Strip just south of Zion (which shares the same volcanic story as the Santa Clara volcanic field) one volcano erupted in the very bottom of the Grand Canyon in the Colorado River. All that is left of this eruption is the famous volcanic neck that river rafters know as Vulcans Anvil. The next volcano that could erupt in Zion may erupt in a very unusual place, like the Narrows! The most recent eruptions near Zion were witnessed by people from the Southern Paiute and Hopi tribes. This includes one at Panguitch Lake near Cedar Breaks National Monument 1,000 years ago, another in Parashant National Monument 950 years ago, another at Sunset Crater 920 years ago. The most recent eruption was near Fillmore, UT, that erupted only 660 years ago. Zoom in on a satellite image of Fillmore and the shocking black of that lava flow will be clearly visible. While another eruption could happen at any time, it may also be thousands of years before the next one. A few years ago, in the San Francisco volcanic field around Flagstaff, AZ, earthquakes from magma movement were detected near Sunset Crater. There is a liquid magma body there just a few miles below the surface that may one day rise. Of all the volcanic fields along that southern arc of the Colorado Plateau, the San Francisco volcanic field is thought by geologists to be the most likely one to have an eruption in the next few centuries. So what is one of these eruptions like? Well, in the modern age, geologists will be the first to know. Earthquakes from rocks broken by rising magma will be detected by seismometers. These first earthquakes will be too faint to be noticed by people. But 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. A person won't go to bed thinking everything is fine and wake up to a new volcano in their back yard. 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' or fizzing, out of the magma and effuse out of the ground. Rodents may suffocate on these gases as well as small birds that land in depressions in the ground that collect heavy carbon dioxide. This creates a super high-pressure foam head on the top of the rising magma plume. This foam is injected into the rocks above, cracking it apart. As the magma is just below the surface the land bows up slightly. Stress cracks form on the surface of the ground. This is followed by the deafening roar of gases. Water vapor and carbon dioxide, at thousands of pounds of pressure per square inch, begin to jet out of the ground. When water and carbon dioxide in the magma can escape, they start in their liquid/supercritical molecular size. They are under incredible pressure and are around 2,000 degrees F (1,200 degrees C). They have a lot of energy but are still confined. What makes a volcanic eruption so violent is the expansion ratio of both volatiles the microsecond they can break free of the magma at the site of the eruption. Carbon dioxide expands about 550 times in volume from its liquid state to its gas state. (Yes, CO2 can be a liquid, but only at over 60 pounds per square inch). Water, however, has 3 times the expansion ratio of CO2. The expansion ratio of water is what makes it so useful for power generation and steam locomotives. It has an expansion ratio of 1,600 times its liquid volume. That means 1 liter of water expands to 1,600 liters of water vapor. So even though an average local magma has only 0.5% CO2 and, say, 1% H20 in a 1 cubic kilometer magma body, that is a huge amount of volatiles. The 1% water in a cubic kilometer is still 10,000,000 cubic meters of water! And if 10 million cubic meters of water expands 1,600 times, that means there is about 16 cubic kilometers of water vapor hidden in 1 cubic kilometer of magma. The violence of all volcanic eruptions is due to the water and CO2 blasting out of the vent, flinging lava, boulders, and ash clouds into the sky. The magma itself isn't particularly dramatic. Once one of these eruptions begins, it may take a few months, or a year or two, to expend the gases in the magma. During the first few months the gas pressure is high creating a lava fountain. As the lava shoots into the air it shreds apart in the air, cools, and falls to the ground as cinders, also known as scoria. This piles up into a cinder cone. Once the last of the gases vents out, the lava fountain will peter out, perhaps going through a phase where it spatters out blobs of lava, lava bombs, etc. Eventually the eruptive power of the gas is gone. Now there is just a body of liquid magma in the ground. It is being squeezed on all sides by Earth's crust. No longer able to push back by gas pressure, the 2nd phase begins. Like squirting toothpaste from a tube, the lava flow phase begins. Lava effuses from the ground slowly. However, it does not come out of the top of the cinder cone. The cinder cone, made of that rock popcorn called scoria, is so light the lava lifts the base of the cone on the downhill side and flows out from underneath. Sometimes the friction from the lava rips apart part of the cone, which flows away on top of the lava. The lava flow phase could take months or years as well. Once most of the lava is squeezed out of the ground, the eruption is over, and the lava slowly cools. Some lava flows, such as the ones that flowed into the Virgin River and the Colorado River, form lava dams. At least two blocked the Virgin River between Virgin and Rockville, making lakes that stretched into the park. The ones in the Grand Canyon at Toroweap were some of the highest dams. One was 1,400 feet high. These created lakes tens of miles long, one of which was likely over 100 miles long. Imagine those first weeks or months of the lava dam. These lava flows were so big and hot they evaporated the river away. This dried up the river downstream, devastating the ecosystem that relied on that precious desert water. One day the river is there. The next it is gone. It would have taken months or even a few years, but eventually the lake that formed would overtop the lava dam and restore the river. However, lava makes a terrible dam because as it cools it cracks into boulders. Once the river overtopped the dam, water could erode it on the downhill side, allowing the waterfall to recede upstream eating through the lava dam until it reached the lake itself. At this point there would be a blowout, sending a huge flood 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. 
NPS photo/Adrienne Fitzgerald Columnar joints, like these in the Crater Hill basalt flow, form as the lava cools and contracts, and are oriented perpendicular to the surface of the flow.
Return to main Rock Layers page
Paleogeographic map courtesy of Ron Blakey, Colorado Plateau Geosystems, Inc. |
Last updated: July 29, 2020