950 years ago the quiet forested landscape around Mt. Trumbull was interrupted by a dramatic volcanic event. A plume of magma had forced its way from deep below ground to the surface. Yet this was not an unusual event. The eruption, named Little Springs, was just the most recent in a long series of such eruptions in the Uinkaret Volcanic Field on the Shivwitz Plateau. As you can see in the aerial image above, there are several other older cinder cones in the same small area around this event. Looking at the entire UVF reveals over 100 more cinder cones. This volcanic field is north of the UVF vents that were the source of many of the lava flows that dammed the Colorado River.
Interestingly, around the time Little Springs erupted, this was a relatively busy time for eruptions in northern Arizona and southern Utah. Almost 1,000 years ago, a large extrusion of lava occurred just east of Cedar Breaks National Monument on the Markagunt Volcanic Field. Then, about 30 years after Little Springs, the Sunset Crater eruption took place near Flagstaff. Then things went quiet for 250 years. The most recent cinder cone and lava flow event in the region was in the Black Rock Desert volcanic field. It was called the Ice Springs eruption, near Fillmore, Utah. It happened a mere 660 years ago. The Ice Springs eruption is the second youngest lava flow in the continental United States. Since then the volcanic fields have been quiet. Magma has been tracked moving several miles below ground near Sunset Crater so perhaps another eruption is coming soon. The most recent lava flow in the continental United States happened 600 years ago near Mono Lake, CA in the Mono-Inyo Volcanic Field, on the western edge of the Basin and Range province.
Northwest Arizona and southwest Utah are littered with the derelict remnants of thousands of eruptions. Clearly this is an active volcanic area and continues to be subject to the larger forces of plate tectonics. It is inevitable that another eruption will breach the surface somewhere here again in the geologically near future. On the Shivwitz Plateau over 200 cinder cones and lava flows have been mapped on the surface, with many more buried beneath them. 154 eruptions have occurred in the last 1.5 million years. Right now it is hard to give intervals for eruptive events because only a few of the events have been dated by geologists. This is an area of research so in the coming years we will be able to better share what the eruptive trend is. However, dividing the know number of events over the last 1.5 million years, a very general average is one event every 9,700 years. However, eruptive events often come in pulses making predicting the next one close to impossible. Ultimately, the odds of another eruptive event on the Uinkaret Volcanic Field in the next 100 years is extremely low. It will likely be one of the other volcanic fields that erupts.
Will these eruptions be a hazard to people? The US Geological Survey, in their 2018 National Volcanic Threats Assessment, characterizes the eruptive threat from an eruption in the Uinkaret Volcanic field as Very Low Risk at position 153 (page 11). The biggest impact would be from lava erupting in the canyon. This would disrupt recreational visitors rafting the river. If a lava dam formed, that would be another issue as all downs would be impacted if the dam burst catastrophically as it has done in the past. The odds of this happening though are extremely low. The San Francisco Volcanic Field near Flagstaff was rated a Moderate Risk to people at position 80 as it is near the urban area of Flagstaff. The SFVF has had over 600 eruptions over its 6 million year life span. The Black Rock Desert Volcanic Field near Delta, UT also has a Moderate Risk but is at position 102. After examining the Santa Clara Volcanic Field by Zion National Park and St. George, Utah that was last active 32,000 years ago, the USGS did not give it an eruption threat rating as too much time has elapsed and there isn’t evidence of magma movement or melts in the region. That doesn’t mean, however, that this VF is extinct. It just has been dormant for a very long time. If it reactivates, we will have plenty of warning.
Even though the geologic record indicates that another eruption is extremely unlikely in the next century, it is worth exploring what we can expect if a new eruption is imminent in the Uinkaret or one of the other nearby volcanic fields. The first clue will be earthquakes. They will increase in frequency and strengthen over a period of months as the magma rises the surface. Different from a typical Basin and Range crustal extension earthquake readout on a seismograph, covered in Section 1, volcanic earthquakes have a different seismograph signature as you can see in the graph at right.
There are three types of volcanic earthquake. The first is a 'volcano-tectonic' earthquake from magma moving up and cracking bedrock apart. 'Long-period earthquakes' result from gases coming out of solution in the magma plume and rushing up to the surface. 'Harmonic earthquakes' are thought to come from magma pushing through passages below ground. When magma moves through these passages it creates friction against the rock it flows past. This in turn creates resonant vibrations.
Eruptions will not be a surprise as they will give months worth of clues we can detect by both NPS and USGS instruments. This will give us time to study, make eruption predictions, and prepare. USGS volcanologists will analyze these first data and send a team to do comprehensive monitoring as quakes grow stronger and more frequent while magma rises closer to the surface. People will eventually begin to feel the small quakes. In the final days leading up to the eruption audible rumbles like distant artillery or strange thunder will fill the air as rising magma widens the fissures and fractures bedrock near the surface. Another visual clue near the likely eruption site will be an increase in the number of boulders that will break off cliff faces as the ground shakes.
Once the magma rises close enough to the surface, one of the first causalities will be trees. Heat, hydrogen sulfide (H2S), sulfur dioxide (S2O) and carbon dioxide (CO2) escaping from the rising magma will poison the soil. A sulfur stink like rotten eggs will be in the air. Larger animals, alarmed by the rumbling ground and strange smells that irritate their nose and throat will begin to migrate away from the area, aware that something life-threatening is about to happen. Water temperatures at springs will also increase and become acidic as they become infused with CO2, killing aquatic organisms.
Hydrogen sulfide, sulfur dioxide, and carbon dioxide escaping from the ground may collect in low spots like on the surface of ponds, in meadows, and in animal burrows. Invisible to the animals, after only a few breaths they will lose consciousness and quickly die. This will especially be true of rodents and small birds with their faster metabolisms. At Mt. Rainier by Longmire Village there is a large meadow where CO2 comes out of the soil and collects in little depressions in the ground. Not dangerous to humans as the gas is only at ground level, heavy CO2 displaces lighter gases in the holes. Small birds have been observed landing in these little pockets in winter. The pockets are warmer and snow free, enticing birds to land there. Unfortunately they quickly asphyxiate. In a volcanically active area of Africa a similar phenomenon has happened in deeper depressions called mazukus, occasionally killing large animals like water buffalo. This likely will not be an issue with the type of volcanics we have here.
The most obvious visual sign of the impending eruption will be the opening of a giant crack in the ground. Our local magmas rise from depths around 15-55 miles below the surface in the lithosphere. Magma follows faults, wedging the hot magma into these cracks and widening them into dikes. As the rising magma reaches an impermeable layer, it moves sideways, creating a sill until it finds another weak vertical fault which it then forces its way into. Around a half mile below the surface where pressures are still in the range of tens of thousands of pounds per square inch, a high pressure foamy gas head forms at the top of the plume. This gas begins to expand faster and faster, increasing both the rise of the magma, and also exponentially increasing the magma's ability to move rocks out of its way. Sometimes this displaces and bows the ground up into a surface dome like a blister. This opens stress fractures in the brittle surface rock. As the fractures propagate up to the surface, the weight of the overlying rocks will no longer be able to slow the rising magma.
The first thing to erupt from the vent are the high-pressure gases and the lava foam at the top of the rising magma plume. As gases force open the vent, a high-pitched hiss and then a crackling roar of escaping gases would be heard for miles, similar to a jet engine. Escaping gases have been compressed at such high pressures they blast rocks into the sky. Shortly after gases erupt, a series of loud bangs and fluid-like sputters announce the arrival of the bubbly fountain of liquid magma. A dramatic fountain of hot glowing red rocks will begin to shoot into the sky.
So what powers a lava fountain to shoot cinders, also known as scoria, so high in the sky? All magma contains gases in solution; especially at the top of a rising plume. This is primarily water and carbon dioxide, as well as hydrogen sulfide. Other gases are found in small amounts. These are known as volatiles. Deep underground these volatiles are kept under intense pressures. As explained in Section 2, this puts them in a supercritical state, similar in volume to their liquid state.
Let's talk about these volatiles. For the purposes of the explanation in this paragraph, the term ‘volatiles’ refers to what we see with our own eyes at sea level where H20 is primarily in its liquid state while CO2 and H2S are in their gas state. To really understand what drives the explosiveness of an eruption and how an eruption can throw lava bombs weighing thousands of pounds through the air, we need to understand the power of volatiles exsolving out of magma.
The first thing to point out is that water as a vapor at sea level takes up 1,600 times more volume than it does in its liquid state. If you took 1 liter of water and boiled it, it would fill 1,600 one-liter bottles with water vapor at sea level. 1:1,600 is water's expansion ratio.
Likewise, CO2 gas takes up 550 times more volume than when it is in its liquid state. The thing is, CO2 has to be at least at 60 pounds per square inch and relatively cold to be in its liquid state so we can only see that in a laboratory. To give you an example using carbonated soda, if you collected all the CO2 gas in a bottle of soda pop, you would have about 3.4-5 times the volume of the bottle in gas as there is soda pop. How is that possible? At 60psi CO2 turns into a liquid if not too hot. When a soda is carbonated at the bottling plant with the water and syrup, liquid CO2 is injected in its liquid state into soda at up to 1,200psi. At the top of an unopened bottle of soda you can see a small head of pressurized gas. This is pure CO2 put in by the factory that traps the dissolved CO2 in the soda in its liquid state. An unopened bottle of soda is at least 60psi. When you open the bottle and hear the psssshhhh noise, you have let out that head of CO2. However, now the soda pop is at ambient air pressure around 12-15psi. This allows CO2 to start coming out of solution, or 'exsolve' and escape the soda in the bottle. For those who wonder why CO2 doesn't all escape the soda at once, this is because of the intermolecular forces (positive/negative attraction) of the water that is surrounding the individual liquid molecules of CO2. It takes time for CO2 molecules to find each other and form bubbles big enough to break the molecular forces of water to rise up and escape. This is why soda makes people burp. They literally are drinking liquid CO2 still trapped in the soda water. This soda is now inside our body, which is much warmer than in the bottle. Heat excites the molecules in the soda, which helps CO2 find other CO2 in our stomachs. Heat speeds up the exsolving process. This is also why a soda will explode in a hot car. Inside the bottle so much CO2 is trying to exsolve it puts more pressure on the bottle than the bottle can handle, typically at pressures over 120psi. Meanwhile, if the soda is kept cold after being opened, the CO2 will take hours instead of minutes to exsolve. This is because colder water molecules don't vibrate as strongly as they do when heated. The stronger the vibration in a solvent like water, the easier it is for gases (the solutes) to escape the molecular bonds that are holding on to them. Lastly, the rate of exsolving CO2 reduces over time as more and more CO2 escapes, leaving less CO2 to find its friends to escape the H20. Chemists admit that it is almost impossible to remove every last molecule of C02 from water.
Magma is similar to the soda example, but the amount of available gas to drive an eruption is truly unbelievable. Take a sample magma body that is 1 cubic kilometer. That is 1 billion cubic meters. Now take the fact that a sample magma in the San Francisco Volcanic Field was shown to contain about 1% water and 0.5% carbon dioxide by weight. 1.5% of 1 billion is 15 million cubic meters of H2O and CO2. If the water tries to expand in volume 1,600 times to its gas state, and CO2 increased about 550 times, that is 16 cubic kilometers worth of water vapor and almost 3 cubic kilometers of CO2 gas in 1 cubic kilometer of magma! It is the expansion of these two gases from their supercritical state to their gas state that can throw those lava bombs that weigh thousands of pounds out of a volcanic vent.
When a basaltic (low silica) Hawaiian type of magma finally reaches the surface here in Utah or Arizona, the pressure of exsolving H2O and CO2 blasts the magma up and out of the ground. However, because our local magmas are usually so fluid, they can release their gases more gradually, resulting in pressures that aren’t nearly as high as a higher silica sticky magma eruption like Mt. St. Helens where pressures at the top of the magma plume in the neck of the volcano were as high as 500MPa/72,000psi.
Research at Mt. St. Helens indicates that prior to its eruption its magma contained an astonishing 5% water. This means that for the 1 cubic kilometer of magma that erupted, 50,000,000 cubic meters of it was water. Multiply that by 1,600 and it shows that Mt. St. Helens produced an astonishing 80 cubic kilometers of water vapor. Mt. St. Helen's sticky magma had held almost all those dissolved gases so tight that the gases had to pressure detonate the magma to get out. This fragmented the magma into an uncountable number of microscopic particles that created that pyroclastic cloud that reached 30,000 meters (100,000 feet) in the sky.
The next time you look at a photo of a lava fountain or pyroclastic cloud, just think about those invisible high-pressure gases that are blasting volcanic material high into the sky.
Let’s return to our local low silica/low water magma and the the lava fountain eruption phase to see what the lava itself is doing. As lava particles are blasted into the sky and fall to the ground as cinders, the lava has experienced a major pressure change. Underground it was under tens of thousands of pounds per square inch. As it shoots from the vent into earth's atmosphere, the ejected material is now only experiencing about 12-15psi, or just about 1 Atmosphere. (12psi is the air pressure at Little Springs at 6,000 feet above sea level, sea level is 14.7psi). This rapid pressure change allows all the dissolved gases in the liquid lava droplets to come out of solution. This puffs up the cinders into rock foam (basically like popcorn).
The ejection of cinders determines the construction of the cinder cone. If they are still semi-melted as they fall, they will melt (weld) together to make a strong cinder cone. If the weather is especially cold, rainy, or the cinders fall a long way, they will basically freeze in air, creating a less stable cinder cone made of loose scoria.
As the cinders blast from the fissure, a hot, noxious, hazy cloud of deadly gasses will waft away from the cone. Even at a distance, it can irritate the skin, nose, throat, and lungs. These toxic gases are known as vog, or volcanic fog. The more moisture there is in the air, such as if it is raining, the worse the vog will be as some of the gases turn the water droplets acidic.
Cinders keep piling up around the vent, creating a growing cone of loose, glassy, sponge-like stone. The cinders can be black, grey, or even reddish orange, depending on the chemistry of the magma plume and if they get wet and rust and turn reddish orange. If you visit Zion National Park, look closely at the red roads there made of orange-red cinders.
Scientists who examined the Little Springs eruption site determined that a true cinder cone didn’t form there. They categorized it as a 'spatter rampart.' This means that the lava blasting out of the vent was more gloppy, made of larger less gassy lava bombs and irregular chunks that piled up instead of more typical fine pieces of scoria as you find at Vulcans Throne or Sunset Crater. While unusual compared to a more typical cinder cones found in this region’s volcanic fields, spatter ramparts are not too unusual and often created by Hawaiian-type (mafic) eruption events like Little Springs.
The first phase of a Parashant eruption is the most violent as high-pressure gas blasts out liquid lava in a fountain. Over time the height of the fountain subsided as the majority of the volatile gases had escaped. Material ejected from the vent sometimes transition to larger blobs of molten rock, which was the majority of the fountain phase at Little Springs rather than fine cinders. After a few months of this kind of activity, the lava fountain at Little Springs had created a spatter rampart 350 feet high.
While the most explosive gas-releasing eruptive activity at the Little Springs cinder cone has stopped, the magma body was not done. The majority of the magma plume still needed to escape the confines of the earth. The magma chamber was still being pushed on from all sides by the surface rocks around it. At this point the magma had lost the gases to push out against the country rock that was pushing in on it. Imagine holding an open bottle of honey and squeezing it. The compressing force of your hand is just like the force exerted on the magma from the country rocks on all sides of it. This began the lava flow phase of the eruption.
The magma body at this point now has very little gas left in it once the lava fountain phase ended. At this point the magma is almost entirely liquid rock. This makes lava far heavier than the light spongy cinders that make up the cinder cone. Frequently the lava flow phase of an eruption damages at least part of the cone that built up. Lava is so dense and the cinder cone so light that the lava won't rise up through the center of a cinder cone. Instead, the lava pushes its way under the bottom of the cinder cone on the downhill side. This floats part of the cone. The shearing force from the moving lava causes part of the cone to collapse onto the top of the lava flow. That part of the cone is then carried away on top of the lava flow, like a package on a conveyor belt. That is why many cinder cones have an opening on one side rather than a perfectly symmetrical cone. Some cinder cones are able to survive intact because the lava phase either was weak, the lava was highly fluid and didn't lift the cone as far, or the cinder cone was cemented enough to hold together as the lava flowed under it.
The lava flows that emerge in Parashant are the more viscous 'spiny' version, called an a’a flow. Unlike the Hawaiian lavas that come out very liquid and move quite fast, Parashant's lava comes out as liquid but transformed quickly into a jumble of semi-molten boulders due to higher silica content. This blocky type of lava is why tree casts have not been found here. Tree casts require fast flowing low-silica lava called pahoehoe. That liquid lava can flow around a tree without pushing it over. After the lava cools what is left of the cooked tree decays, leaving a vertical tube. The Little Springs lavas were more like bulldozers through the forest knocking everything over.
The rough A’a lava forms a rough rocky surface due to higher silica percentages. As explained in Section 2, the more silica there is in a magma, the more bonds form from the increased silica. This is what creates the cracks and blocks of an A’a flow. Instead of stretching smoothly, the surface loses elasticity and rips, twists, shears, and fractures, forming the jagged and sharp surface. Volcanologists call these A'a rocks 'clinkers' in part because when walked over, they make a high-pitched clinking sound. They are still so sharp after 1,000 years they tear up leather gloves and boot soles. It is not safe to walk on a lava flow because of this without proper protective gear.
Once the last of the magma had been squeezed from the ground, the lava flows ground to a halt. The loud crashing sounds of the wall of boulders being pushed along the ground eventually ceased. For months afterwards, sizzling water and occasional blasts of steam emanated from the lava flows after a rain. Yet they too eventually cooled. Quiet and stillness again took over the plateau after perhaps a year or more of volcanic chaos. Animals slowly returned to the area. In time dust fell from the sky and centuries of leaves and pine needles began to build up a soil layer at the edge of the flows. Trees, grasses, and lichen began to colonize the flow from the outside in. Interestingly it is the west side of the lava flow that has the most soil and new plant growth. Prevailing winds come from the west thus more dust was deposited on that side, helping the forest recolonize the west side of the flow. However, it will be tens of thousands years before the Little Springs lava flows become totally covered with soil and plants in this semi-desert environment.
A piece of pottery was found in the flow showing that people were at Mt. Trumbull at the time of the eruption and interacted with the lava. Using carbon dating, charred plant matter burned by the lava established that the eruption happened in about the year 1050CE (common era).
Because this is a volcanically active area, volcanologists monitor earthquakes in the region to look for signs of the next eruption. Based on the geologic record we can confidently say that beneath our feet, here in the Parashant, or perhaps up by Cedar Breaks, or over in Fillmore, a magma plume could start rising any time.
A few years ago seismometers picked up minor earthquakes just north of Sunset Crater near Flagstaff. The pattern was the signature of magma on the move rather than a fault zone earthquake. The magma body, 3-4 miles below the surface, has since stopped moving, but may be getting ready to be the next eruption in the continental United States.
So what would the government do if earthquakes indicated that an eruption was likely? First, the USGS Volcano Hazards Program staff would meet with state and federal officials. They would set up a network of seismic and GPS monitoring stations to take real time measurements of gases and ground deformation to better understand how much magma is coming up and how big the eruption is estimated to be. These scientists would pass the results of their analysis on to public safety officials who would establish a safety exclusion zone. If this happened at Parashant, since the monument is remote and over 1 million acres in size, this would likely not cause a closure of the whole monument.
During monitoring, volcanologists would use equipment to study the volcanic earthquakes and determine when magma would reach the surface and where the vent will open. Then, depending on calculations for size, they would recommend a closure area of a few square miles in diameter around the likely eruptive zone. People would be evacuated. Cattle ranchers would have time to come and get their livestock.
Once the actual eruption started, the goal would be to keep people a safe distance from the red hot falling cinders and lava bombs that would be shooting out from fissures. As the cinder fountain fall zone is a relatively small danger zone, a viewing area could be set up perhaps a mile or so uphill from the fissure. That would give enough space to protect spectators from lava bombs and toxic volcanic gases. Another factor to consider is that if the eruption happened between April and October, forest fires would be a risk because falling cinders will set all dry plant material on fire near the erupting fissure. Once the cinders phase had been exhausted and the lava flow began, it would likely be so slow moving most people could observe it from a relatively close distance.
What if the cinder cone erupted at the edge of the Grand Canyon? First, all raft trips down the Colorado River would have to be stopped and rafters evacuated by helicopter. As lava cascaded into the river, the interaction of water and lava would create dangerous gases. In addition, the lava would create a reservoir behind the growing lava dam that might last years. On top of that the riverbed would be dry downstream and Lake Mead may drain to a critically low level.
To close out this amazing story, just think, the next time you hear on the news that there was a local series of earthquakes it could be magma movement. But…the odds of that are extremely low in the next century. Earthquakes here in Utah and Arizona will mostly be from the mostly harmless jostling of slumping crustal blocks in the Basin and Range and movement of the San Andreas fault. When it comes to volcanism, the area of greatest risk in the continental United States is the Cascade Mountains or the Mammoth Mountain area of Southern California.
Still, there is always a chance we may witness one of the most interesting geologic shows on Earth, right here in the American Southwest, in our lifetime!
Last updated: January 7, 2020