Magma Melts and Eruption Types

Volcanic plumbing system
This diagram shows that rising magma follows zones of weakness, either horizontal, forming a sill, or vertical, forming a dike. Uinkaret VF magma comes from about 15 to 55 miles below the surface.
The first section on the Colorado Plateau and the Basin and Range, as well as the Geology section of the website, provided a solid foundation to understand Basin and Range crustal extension. This page explains how magma is injected in the crust, why there is such a range in eruption types from lava fountains and lava flows to explosive pyroclastic events. It is worth mentioning that it is highly unlikely we will see an eruption in the next century. If we do, it will not likely be a catastrophic Mt. St. Helens-style eruption, it will be like what was seen in at Kilauea in Hawaii in 2018. The volcanic fields in this region are actively monitored and only the one near Flagstaff has shown any unrest.

There are two main reasons why there is so much volcanism in the area. The first is Basin and Range extension. As the region west of the Colorado Plateau thinned and fractured crust, magma can rise through these fractures and erupt on the surface. Geologists say that "magma follows tectonics." As the crust was stretched, this created a zone of relatively low pressure in the lithosphere. This caused the mantle to swell outward to replace the zone that once was occupied by the formerly thick crust. The mantle rock is now in a zone of lower pressure as it is closer to the surface of the planet. This lower pressure causes a 'decompression melt' that creates pockets of magma. This magma is buoyant and begins to rise. Once magma reaches the brittle outer zone of the crust (the outermost 7 miles of crust) gases begin to form at the top of the magma body. The gases start to pry apart faults, as well as 'joints,' which are cracks in the crust. These cracks can be microscopic. Through the pressure of gases (water and carbon dioxide) coming out of solution from the magma, these volatile gases pry open cracks. The magma then follows. This forms a network of dikes and sills as the magma follows the path of least resistance.

The second reason there is so much volcanism in the region comes from new research using earthquake waves called seismic tomography. These studies revealed that perhaps most, if not all, of the magma erupting is coming from the melting of the base of the Colorado Plateau! Looking at a map of the volcanic fields in the western United States shows that all along the outer edge of the Colorado Plateau, especially the southern half, is an arc of VFs. It goes from Delta, Utah south the St. George, then southwest through Parashant to the Flagstaff area, then east through Hopi Buttes volcanic field (VF) near Petrified Forest National Park, then through El Malpais VF near Albuquerque and finally north through Santa Fe to Valles Caldera and Capulin Volcano National Monuments. As explained in Section 1, research shows that volcanism in this area is being supercharged by melting of the base of the Colorado Plateau by hot asthenospheric rock. That magma then flows out to find weak zones like the active faults in Parashant, where the magma can rise and erupt.

Volcanic activity does shift from place to place over time as the dynamics of the faults change. Parashant has one active volcanic field, called the Uinkaret VF, which is centered around Mt. Trumbull. There are also three extinct ones in Parashant. The first is in the Black Rock Mountain area (named for the black volcanic rock there), the Shivwitz Plateau VF, and the Grand Wash Trough VF. These are much older than the Uinkaret VF and are extinct now. The oldest is the Shivwitz VF, active about 9-6 Ma. It includes landmarks like Mt. Dellenbaugh, which last erupted 6.8 million years ago. The next oldest is the the Grand Wash VF about 6-4Ma in the Pakoon Basin. It's most prominent features are the volcanic neck at Pakoon Springs and Olaf Knolls, both about 4.5 million years old. The Black Rock Mountain VF erupted from about 5 to 2.5 million years ago. Other than Black Rock Mountain, other prominent eruptions were Wolf Hole Mountain (about 3.5 million years old) and Seegmiller Mountain (2.5 million years old).

The Uinkaret is Parashant's only active volcanic field. It started erupting ~3.6 million years ago creating the lava flow cap on top of the Mt. Trumbull mesa. There have been hundreds of eruptions from this field. Over 200 of them have been mapped on the surface. There are likely many more buried beneath those or eroded away. One that is almost totally gone is Vulcans Anvil in the Colorado River. It is the small remnant of the volcanic neck that the river will soon erase from the landscape. And yes, that means this volcano erupted in the bottom of the Grand Canyon up through the Colorado River. Other remnant volcanic necks can be seen in the cliff walls of the Grand Canyon. Two huge faults in Parashant and the Grand Canyon helped the magma rise in this volcanic field. This will be explained further in the next section, but they are the Hurricane and Toroweap faults, the two most active faults in Arizona.

Parashant is also very close to several volcanic fields near St. George, UT. One is called the Santa Clara VF. The SCVF has been very active over the last few million years. Its cinder cones and lava flows dot the landscape all around St. George. The Santa Clara volcanic field got its name from the historic town of Santa Clara. Its most recent eruption happened in what is now Snow Canyon State Park 32,000 years ago, leaving behind two cinder cones and a nine mile long lava flow. The next oldest eruption was just below the West Temple cliff of Zion NP. Called Crater Hill, it erupted about 120,000 years ago. The likelihood of an eruption in the St. George area is extremely low. Most of its activity was between 700,000 and 100,000 years ago.

There are several nearby volcanic fields to St. George. One of them, the Markagunt volcanic field is also likely to erupt again in the next thousand years. It includes parts of Cedar Breaks National Monument and Zion National Parks. It last erupted about about 1,000 years ago near Panguitch Lake. In the last 10,000 years it has erupted several times. The Black Rock Desert VF is just south of Delta, UT. It created several cinder cones and lava flows including the Pahvant Butte and Ice Spring eruptions. Ice Spring is the most recent event in Utah, erupting about 660-720 years ago. It also features a rhyolite dome about 400,000 years old. The USGS said it has a Moderate Risk to human activities if it erupts again. This field does have a history of explosive high-silica eruptions and pyroclastic ash explosions, similar to Mt. St. Helens. This is due to its magma sometimes sitting for extended periods of time and melting additional silica into the magma before erupting. This chemistry will be explain later, but it basically makes the magma very sticky and explosive, unlike the typical Hawaiian-style eruptions in the region. The San Francisco Peaks VF is found just east of Flagstaff and it too is of Moderate Risk. It's two most recent eruptions were Sunset Crater, about 930 years ago, and SP Crater, about 4,500 years ago. Volcanologists give it the highest odds of erupting in the near future compared to the other volcanic fields. It shares the same explosive risk as the Black Rock Desert VF as it also has featured a few pyroclastic eruptions. Still though, most of its 600+ eruptions were the less destructive glowing orange lava fountains and lava flows type. A few years ago, volcanologists tracked an active magma body moving below the SFVF by looking at earthquakes caused by magma flow several years ago. However, that magma body has since stopped moving.
pinatubo eruption
Is this sort of eruption in our future? Probably not. This is a high water, high viscosity dacitic magma eruption of Mt. Pinatubo. Local magmas that have risen around the Colorado Plateau for the last few million years have almost always been low water and low viscosity lava flow events.


What Kind of Eruption Will Happen Here?

So you may wonder, are these volcanic events anything like the explosive eruptions of Mt. St. Helens or Mt. Pinatubo? Or is what happens around here more like Kilauea and its lava fountains and lava flows? There are six types of volcanic eruptions. Mt. St. Helens and Mt. Pinatubo featured the most dangerous kind of eruptions, which are 'Plinian' style eruptions, characterized by explosive pyroclastic events where massive grey clouds shoot ash up to 100,000 feet into the sky. Kilauea, meanwhile, sits over a hot spot in the mantle and features the least explosive eruptions. Since Hawaii is one of the worlds best-known volcanoes, this eruption type is now called a 'Hawaiian-style' eruption. Parashant, meanwhile, sits at the edge of the melting Colorado Plateau, which is the source of magma. This magma is low in silica, like that found in Hawaii, making it flow like syrup rather than be explosive. The next time we have an eruption in Parashant, people will most likely see a calm (volcanically calm, that is) Hawaiian-style eruption.

Before we dive indepth into the volcanism at Parashant, we'll first look at those devastating subduction zone pyroclastic eruptions like Mt. St. Helens and Mt. Pinatubo to see what sets them apart from what happens here. Explosive eruptions from Cascade Mountain volcanoes like Mt. St. Helens, Mt. Rainier, Mt. Shasta, Lassen Peak, and others, are the result of something surprising...water in the magma! Water is a pretty big deal when it comes to volcanic eruptions. To understand why this is, we first need to look at plate tectonics and the movement of continental and oceanic plates around the globe and how plate tectonics puts water into some magmas.

We will start our journey out in the ocean on the seafloor. Sections of the seafloor are really huge oceanic plates. Oceanic plates start their life at what is called a spreading center, rise, or ocean ridge. This is where magma emerges from a long linear seam on the ocean floor. This seam is opened by upwelling magma from the convective circulation of Earth's mantle. The magma emerges from the seam and cools into black basalt. The new ocean floor moves away from its spreading center as yet more new magma comes out. This pushes the seafloor towards continents like a conveyor belt very slowly, usually a few centimeters each year. These ocean ridges are thousands of miles long. If you look at a map of the world's ocean floors, y
ou can see the East Pacific Rise and the Mid-Atlantic Ridge run north and south almost from pole to pole. Seafloor spreading in the Atlantic, for instance, is what separated Africa from South America tens of millions of years ago and created the Atlantic Ocean. Only in Iceland can you see the Mid-Atlantic Ridge without getting wet.

After millions of years of slow travel, ocean crust that was created at one of these spreading centers finally reaches a continent. Because ocean crust is heavier (more dense) than continental crust, it sinks below the lighter continental rock. This is happening right now off the coasts of Oregon and Washington State where what is left of the Farallon Plate (known today as the Juan de Fuca and Gorda Plates) is subducting.

So how does water gets into subduction zone magmas? First, the lava rock on the seafloor has metamorphosed into serpentinite. This means it now has water as part of its mineral structure. In addition are particles of clay that settled on the seafloor. These clay particles also contain water. They are considered hydrate minerals. As the serpentinite and clays reach the subduction zone, they are subducted. As they dive into the mantle, they get incredibly hot. This causes the subducting material to change into different minerals and frees the water. One may ask why the water doesn't escape as steam. Quite simply it can't. Water is trapped by rock on all sides. The water ends up under intense pressure and heat as it is dragged further and further into the mantle. At a certain point it effuses into the nearby mantle rock with other volatile gases like carbon dioxide and hydrogen sulfide. Due to pressure these volatiles are not in their gas state, they are spread out in the mantle.

Basically the water is now stuck in Earth's own pressure cooker and exists in a state of equilibrium with everything else. These subduction zones have been operating for billions of years recycling water into the mantle. Earth’s mantle has a zone very enriched by subducted water where it is mixed with minerals. You may have heard the factoid that there is perhaps 3-5 times more water in the mantle than in all the oceans of the world. What that means is that the water and other volatiles are thoroughly mixed with the other mantle rock. There aren’t giant ocean-sized pockets of water underground. But, the mantle is rich in water and carbon dioxide. This means that a magma melt that rises toward the surface is also rich in these ‘volatiles.’

Scientists use measurements like 'atmospheres' to measure pressure, or 'pascals.' Most people in the United States are more familiar with pounds per square inch, so that is what we will use here. At sea level, air pressure is 14.7 pounds per square inch. This translates to 1 Atmosphere of pressure, or 101,000 pascals of pressure. In Earth's upper mantle, pressures can exceed an unbelievable 500,000psi or 3.4 gigapascals! This is an incredible amount of pressure. Keep in mind that is just the pressure in the upper mantle. The rest of the mantle and Earth's core are under many times more pressure! At the same time, temperatures exceed 2,000 degrees Fahrenheit/1,100 degrees Celsius in the upper mantle. At these temperatures and pressures, the rocks in the lithosphere act like putty rather than brittle hard rock.

So what does all this pressure and heat do to the water (H2O) that was dragged down? Atoms and their molecular bonds are tough, so they aren't damaged, it just causes them to act a different way than we see. They become a form of matter that humans can only see when created in a laboratory. At these pressure and high temperatures, H2O, as well as carbon dioxide (CO2) from the subduction zone goes through a change into a different state of matter. It isn't liquid water or water vapor anymore. It becomes 'supercritical.’ Videos like this show how carbon dioxide can be made into liquid and then go supercritical. H2O and CO2 go from a clear liquid into a weird cloud-like supercritical fluid-gas. At the molecular level, supercritical water can move through very hot semi-molten rock, almost like a ghost moves through walls in a fiction movie. In this state they literally change the melting point of silica. Pure silica has a melting point of about 2,800 degrees F, but different types of silicates have different melting points because of the other types of atoms in the molecule. However, when supercritical H2O and CO2 are infused into silica, the melting point drops to around 2,100 degrees F. Keep in mind this melting temperature is not a fixed point as the intense confining pressure from the rocks that encircle the magma chamber also plays a major role determining the exact melting point.

The next thing to know is that the amount of melted silica in a magma determines how that magma is classified. Silica percentage is critical because this is what determines a magma's viscosity. Viscosity is a measure of a liquid's fluidity. The more viscous a fluid, the less easily it flows but the more gas it can trap in solution, making high-silica magma more explosive. In any magma chamber, silica is the primary ingredient, and the amount of silica determines how an eruption will behave. The silica mineral that makes up magma is silicone dioxide, or SO2. SO2 forms little pyramid-shaped molecular networks called tetrahedras. If a magma body is low in silica, these tetrahedra float around loosely or form very short chains. This results in a low viscosity magma that flows easily as seen in this footage from Kilauea. There isn't enough silica to clot the magma and make it more viscous. However, if more silica gets melted and mixed with the magma, those tetrahedra start connecting with each due to intermolecular forces. Think of intermolecular forces like magnets on the molecules, pulling them together. The molecular attraction creates long chains, lattices, or other larger structures. This is called polymerization. The more of these structures there are, the more the lava increases in viscosity as these structures basically get twisted up with each other, preventing the magma from flowing smoothly. Newly made magma or magma right from Earth's mantle is usually low in silica. As the amount of liquid silica increases as melts move through the crust, silica starts forming longer SO2 chains. The longer the magma takes to rise, the more silica it incorporates and the more viscous it gets.
Subduction zone graphic
Subduction zone off Washington and Oregon showing how the seafloor subducts under the continent, dragging water into the mantle and causing melts.


ductile rock, metamorphosed
This used to be a banded iron formation with flat layers of black, white, and orange rock. It was stretched and folded like putty while buried in the crust and held at high temperatures for a very long time. Magma could push through this.

Colorado Geological Survey

Melting Magma

Earth's mantle is not a liquid body. It is a solid. However, it is under so much pressure and so hot that it, along with the lower part of the crust, acts more like putty. 'Ductile' is the geologic term for this plastic property of hot rock under high pressure. The mantle is always slowly moving, even though it is solid, because of this putty-like state. Earth's heat, which comes from the decay of uranium, creates convective currents. It is very slow, moving about the same speed as your fingernail grows. To get a volcanic eruption, the first step is for some of this putty-like mantle to melt.

Melting of magma is a function of temperature and pressure. In the mantle, temperatures exceed 2,000 degrees Fahrenheit and pressures can be hundreds of thousands of pounds on every square inch. Boiling water is a good analogy. If you boil water at sea level where air pressure is highest, it boils at 212 degrees Fahrenheit. Go up into the mountains to 7,000 feet elevation. Air pressure is lower so water will boil at 200 degrees F. It is both pressure and temperature that determine when something goes through a state change from solid to liquid to gas.

One way that magma will melt if temperature stays the same is from a reduction in pressure. This happens when the mantle swells outward into areas formerly occupied by the crust as it does in the Basin and Range. When the crust began to stretch out and thin, the surface of the Earth in that area was no longer in equilibrium in terms of how weight is distributed at the surface. To remedy this imbalance, and because the mantle is putty-like, it responded by swelling and rising into the zone that was formerly occupied by the thinning crust. This put the top of the mantle into an area of lower pressure than it had been in before. It crossed the melting point because of this. This is called 'decompression melting,' or a dry melt. Another kind of dry melt is when new hot magma injected up into old lukewarm magmas. The new magma adds intense new heat to the system, causing old cooler magma to remelt.

What happens along the edge of the Colorado Plateau is a third kind of dry melt. As explained earlier, a giant slab of the base of the plateau has broken off and is falling into the mantle. As it falls, much hotter asthenospheric mantle flows up around the falling slab, replacing the old cold base of the plateau with very hot mantle rock. This then melts pockets of the very outer mantle and lower crust. This new magma works its way through the crust until it reaches a fault zone where it can rise to the surface most easily such as in Parashant.
xenolith in basalt
This charismatic lava rock in Whitmore Canyon contains a pair of xenoliths that almost look like eyes. These were picked up by rising magma and brought to the surface. They are either the 1.7 billion year old Vishnu or Rama schist, which are the Grand Canyon basement rocks.

NPS - J. Axel

Let’s revisit the water and carbon dioxide that were dragged into the mantle in a subduction zone. These two molecules get compressed and heated into a supercritical state. At a subduction zone they induce what is called a 'wet melt.' Supercritical H2O and CO2 take on a special property in their supercritical state. They effuse (move) through the crystal mush of the mantle and chemically lower silica's melting point by several hundred degrees. This is known as flux melting and is why water is so important in determining how explosive an eruption is. Once a wet melt has started, the pockets of liquid magma start to incorporate more and more silica from the surrounding magma. This makes the magma more and more sticky. As can be expected, magma in a subduction zone that started as low silica mafic basalt becomes higher in silica, creating andesitic, dacitic, or even felsic magma. The more time that H2O and CO2 has to melt silica into the magma will determine how sticky the magma gets as it rises.

It should be pointed out that magma is a mix of different minerals. Some melt at lower temperatures than others. This is known as partial melting. Other minerals may stay solid but get caught up in the melt as hard rocks within a liquid magma, like peanuts in melted ice cream. Those unmelted minerals may melt later as the magma plume rises to a level where lower pressure crosses their melting point threshold. If they don't melt, they remain as xenoliths in erupted material. Other xenoliths in magma were broken off in the upper crust as the magma rose violently (see photo). As various minerals melt into the growing blob of liquid magma, this changes the magma's overall mineral makeup. Geologists look at volcanic rocks from eruptions and develop a mineral profile. Each eruption has its own distinct chemical mix of minerals because of what melted into the magma ahead of the eruption.

Sometimes a mineral doesn't melt and retains its original form. These xenoliths are basically rocks inside of rocks. Curiously, the Little Springs eruptive event contains a high number of xenoliths. The geologists who studied that eruption determined that the magma rose very quickly and carried unmelted rocks up from the asthenosphere with the magma. This indicates a very fast ascent with no intermediate stopping point. Usually magmas rise in stages, which allows these unmelted rocks to drop out of the magma. One xenolith from the Volcans Throne eruption at Toroweap was the size of a basketball, which is also highly unusual.

Magma also contains quite a few molecules we know of as gases on Earth's surface, primarily water(H2O) and carbon dioxide (CO2). These are known as ‘volatiles’ because they determine how violent an eruption will be. In the case of local magmas in the Uinkaret and San Francisco volcanic fields, the content of CO2 in magma is about 0.5%, and the H20 between 0.5-1.0%. Under all the pressure in the upper mantle, these volatiles are forced to exist in their supercritical state in equilibrium with the minerals around them. As such, they take up only a tiny portion of the overall volume in the magma. However, when H2O and CO2 are erupted in Earth's atmosphere, their "expansion ratio" is significant. CO2 expands in volume about 550 times. H2O, meanwhile, expands 1,600 times in volume! The difference in volume between the liquid and gas state of CO2 or H20 is huge.

Let's do some basic math with a rising magma body that is 1 cubic kilometer in volume, or 1 billion cubic meters. It has 1.0% water and 0.5% carbon dioxide. Doing the math tells us there is 10 million cubic meters of supercritical water and 5 million cubic meters of supercritical carbon dioxide in that magma. So how much gas will that make? Based on these gases' expansion ratios, that makes an astonishing 16 cubic kilometers of water vapor and 2.75 cubic kilometers of CO2. That makes almost 19 cubic kilometers of gas inside only 1 cubic kilometer of magma. When it comes to subduction zone volcanoes like Mt. St. Helens, things can be much more dramatic. Those andesitic or dacitic magmas can have over 5% water, creating 80 cubic kilometers of water vapor that is available to explode out. This is why a subduction zone eruption like Mt. St. Helens is so devastating.

Volatiles other than H2O and CO2 include poisonous hydrogen sulfide (H2S) or its oxidized form called sulfur dioxide (SO2), carbonyl sulfide (COS), carbon disulfide (CS2), hydrogen chloride (HCl), hydrogen (H2), methane (CH4), hydrogen fluoride (HF), boron, hydrogen bromine (HBr), mercury (Hg) vapor, Helium (He), organic compounds, and even gold! Volcanologists analyze the gases that are emitted by a rising magma plume to understand more about a pending eruption.
igneous dike rising through a cinder cone
This vein of mafic magma forced its way up through the scoria layers of the Whitmore Sink cinder cone. This magma rose during the later, low gas, stage of the eruption after the cone was formed. The boulder in the foreground is a large piece of the vein (dike) that broke off and fell into the wash

NPS - J. Axel

Magma Types

Let's take a look at the kinds of magma formed in a melt and how viscosity is measured for different lavas depending on silica content. Viscosity for very dense liquids like magma is measured in Pascal seconds, or "Pa s." Pa s values below assume a fixed temperature and gas content.

1) Magma that contains 38-45% silica is known as ultramafic. This is rarely seen on earth's surface as it is primarily a mantle rock that comes out on the ocean floor at those spreading centers. This type of magma is very hot, around 2,000 - 3,100 degrees Fahrenheit. It's viscosity is less than 100 Pa s.

2) Magma that contains 45-53% silica is categorized as basaltic, or highly mafic. Mafic magma usually comes from a mid ocean ridge spreading center, over a hot spot like Hawaii, or like the magma that erupted in Parashant from the melting of the lower crust. This magma contains very little water. Due to its low silica percentage it is very syrupy when melted. When mafic magma (think Hawaiian lava flows) cools, it is usually black, brown, grey, or even greenish. Dissolved gases can easily exsolve (bubble out of solution) from mafic magmas. This keeps gas pressure low. The fluid lava lets gases escape when pressure builds up. Eruptions are not very explosive because so much of the gas can escape. One of the only times that mafic magma eruptions are violent is when rising magma encounters ground water. It heats this ground water beyond the boiling point. Trapped under surface rock, the pressure on the water at the molecular level prevents the water from evaporating until the water gets so hot that its potential energy ruptures the rock above that traps it. This causes what is known as phreatic eruption, resulting in a hole in the ground called a maar. This is different from a sticky magma that causes a pyroclastic eruption that will be explained shortly. The viscosity of mafic magma is between 100 for pahoehoe lava up to 10,000 Pa s for a'a lava. For comparison, honey at room temperature is about 10 Pa s, so 2000 degree F mafic magma is two to three orders of magnitude more viscous than room temperature honey. Both Kilauea and Parashant lavas are basaltic. This type of magma is also quite hot, around 1,600 - 2,100 degrees Fahrenheit.

3) Magma that is 53-63% silica is andesitic. Andesite rocks are usually medium grey in color when cool. This lava is fairly viscous and sticky, increasing its explosive potential. However, if its gas level is low, it can form a slow-moving lava flow that usually doesn't go very far from its source. These flows can be quite tall because the lava is so sticky. At Panguitch Lake, Utah, is a 1,000 year old lava flow that is basaltic-andesitic in composition. It is around 100 feet high. The more sticky/putty-like the magma becomes, the harder it is for dissolved gases (including water) to escape from the silica molecular lattice while the magma is still rising to the surface. Since this sticky magma makes it much harder for volatiles to escape, when they finally do they are much more explosive than mafic magma eruptions. Its viscosity is between 1,000,000 and 10,000,000 Pa s. This type of magma is around 1,150 - 1,600 degrees Fahrenheit.

4) Magma that is 63-69% silica type is 'dacitic.' Mt. St. Helen's most recent eruption was categorized as dacitic. Lava and ash from dacitic eruptions is medium to light grey or even an off-white. Its viscosity is between 10,000,000 and 1,000,000,000 Pa s. Andesitic and dacitic lavas are the most common magmas that erupt above a subduction zone. This type of magma is 1,150 - 1,600 degrees Fahrenheit.

5) Magma that is 69%-77% silica is rhyolitic, or highly felsic, which means it is very viscous and super sticky. These magmas are usually found where magma has been allowed to sit for great periods of time, such as on continents above a hot spot, like Yellowstone where it is considered 'stale' or ‘mature.’ In this situation, mafic minerals are heavier and sink to the bottom of the magma chamber. This leaves the sticky and lighter felsic magma molecules at the top of the chamber where they are the most likely to rise and erupt. There is a wide range in color of rhyolitic rocks. Their lavas can be pink to light purple in color and the ash grey to off-white. Obsidian, known as volcanic glass, is also formed by felsic magma that contains almost no water and cools very quickly. It can be black, orange, red, brown, gray, dark green, and semitransparent. Felsic magma has an unbelievable viscosity of 1,000,000,000 - 100,000,000,000 Pa s. Interestingly, felsic magmas are much less dense than mafic magmas but many orders of magnitude more viscous because of those interlocking silica molecular chains. This magma barely flows. It is in liquid form at the lowest temperature, around 1,100 degrees F. Rhyolitic eruptions have happened repeatedly in the San Francisco volcanic field by Flagstaff, AZ, including eruptions of pumice and obsidian.

The two well-known types of felsic volcanic rock are pumice and obsidian. Neither occur naturally in Parashant.

Pumice (volcanic glass foam) is a very lightweight and off-white spongy high-silica rock. Pumice can only be formed by very sticky magmas. When dacitic or rhyolitic lava is erupted, it is full of those dissolved volatiles. As soon as those molecules are erupted into Earth's low-pressure atmosphere, the volatiles violently come out of solution as miniature pockets of gas in the sticky blobs of ejected felsic lava. In each blob gas pockets form in a microsecond. Pumice is the volcanic equivalent of popcorn. Because this kind of lava is so sticky, the gas pockets that form inside the blobs of ejected magma can't rupture. In mid-air the pumice pieces puff up dramatically in size, and then cool and harden, trapping most of the gases in millions or billions of little sealed pockets called vesicles. It does all this in mid-air and then falls to the ground. These sealed air pockets are why pumice floats on water. Felsic eruptions on the seafloor also create vast amounts of pumice that rise to the surface and form rafts of pumice that can stretch for miles. See this boat moving through a pumice raft.

People often confuse pumice with vesicular basalt (black volcanic rock full of air pockets) that you find in Parashant. Only pumice floats in water. Vesicular basalt can’t float. Vesicular basalt was so fluid when still molten that when bubbles pockets formed, they exploded and collapsed before the lava hardened. Or they created really large bubble pockets which merged with other bubble pockets forming very large vesicles that broke open. This lets water in and they can't float. Plus, mafic magma is much more dense than the felsic magma, making it heavier. Pumice is considered a volcanic glass (amorphous solid), meaning the molecules did not have time to organize into crystals before cooling. The nearest deposits of pumice are near Flagstaff, Arizona in the San Francisco Volcanic Field from the 200,000 year old Sugarloaf eruption.

Obsidian (solid volcanic glass) is another type of high-silica eruptive rock. It is extremely felsic, at over 70%+ silica. It is usually one of the last things to erupt from a high-silica magma body and contains almost no gas. It cools so fast that the minerals in the lava can't crystalize. Like pumice, it is an amorphous (disorganized) solid. Since it has no organized crystal structure, when it is chipped it breaks irregularly, often in conchoidal shapes. Obsidian is similar to regular glass which is made from beach sand high in silica. This is a 1,300 year old obsidian flow at Newberry Crater in Oregon. The black in the photo is the obsidian while the light grey is pumice/rhyolite lava. Obsidian deposits don't always erupt on the surface. Instead they might inject themselves into weak zones between rock layers around the volcano. They are later exposed by erosion. The nearest source of obsidian is the Black Rock Desert near Milford, Utah, 150 miles from Parashant. High silica eruptions happened there 2.5 million years ago. Native peoples brought the obsidian down to Parashant to use as arrowheads, spear tips, and cutting tools. Obsidian's glass edges are sharper than metal thanks to its amorphous molecular structure. This is why it is still used by surgeons today as a specialty scalpel.

The lowest temperature magma that erupts on Earth is a high calcium 'carbonatite' magma that erupts from the Ol Doinyo Lengai volcano in
Tanzania that has lava that is only 900 degrees F!
Shiprock volcanic neck New Mexico
Shiprock, in New Mexico, is an old volcanic neck millions of years old. Two feeder dikes are seen here. Where they joined the greatest amount of magma came up. Thousands of feet of surface erosion exposed this volcanic neck and the two source dikes.

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You may wonder how it is that magma can push its way up through solid rock. Earth's heat makes the country rock around the magma body more ductile (putty-like), allowing the magma to push up through those rocks like a very slow motion version of a lava lamp. It takes decades or centuries for magma to rise. Essentially, because of the physics of 'isostasy' of the mantle and crust, a liquid that is less dense than the material above it must rise to the surface while the heavy solid crust spreads out to accommodate the rising magma in the more ductile zone of the crust. Once it reaches the brittle outer zone of the crust, those volatiles in the magma start coming out and force their way into microscopic cracks and pries them open. The first volatile to do this is carbon dioxide about 7 miles below the surface. As the magma nears the surface, water takes over as the primary rock breaker.

Regardless of whether it is mafic or felsic, a magma plume contains so much gas that a frothy head of gases start to form on the top of the magma plume near the surface. As the magma climbs, the downward pressure from gravity and the rocks above is reduced more and more. This allows more gas to come out of solution in the magma as pressure drops, which causes the gases to put more and more force against the remaining rocks above. It is this gas pressure that is the primary bulldozing force that powers the last stage of a rising magma plume. As the magma body intrudes in the cracks in brittle cold crust near the surface, so much pressure has built up that the gas head on the magma plume now has the power to shatter bedrock or push whole parts of the crust up and out of the its way. This bedrock shattering creates a particular earthquake signature that volcanologists look for to tell them that magma is on the move.

Now let's look at an eruption of a silica-rich magma body like what feeds a typical Mt. St. Helens event. As the magma plume gets closer to the surface, remember that this kind of magma is high-silica from flux melting in the subduction zone, so it is extremely sticky and not much water or CO2 has come out of solution. This leaves an incredible amount of potential energy in the magma that hasn't been released. This kind of magma is just waiting to explode like a bottle of soda pop left in the sun in a hot car. One little crack in the bottle will set it off. For the magma, the trigger for the runaway eruption might be a landslide like what happened at Mt. St. Helens that suddenly removed pressure from the magma. It could be a steam explosion of superheated water in the top of a volcano that removes just enough rock for the magma to start exploding. Whatever the cause, as soon as high-pressure felsic magma is exposed to Earth's very low-pressure atmosphere, in an instant a vast networks of micro fractures propagate through the molecules at the top of the magma body. The molecular bonds holding the magma together are shattered as volatiles finally have the freedom to expand to their gas state. These gases that had been trapped for millions of years under unimaginable pressure suddenly break free of their magma prison and create a cataclysmic explosion!

Remember that the confining pressure in the neck of a stratovolcano like Mt. St. Helens can still be tens of thousand pounds per square inch while atmospheric pressure at the volcanic vent is a mere 10-15psi. Atmospheric pressure is essentially nothing compared to the pressure being pushed out by the volatiles trapped in the high-pressure magma. Gases at the weakest point near or at the very top of a magma plume blow out first. The water molecules expand to 1,600 times their former volume. Think of the magma in the neck of one of these stratovolcanoes as a string of hundreds of firecrackers down the neck of the volcano. The top of the magma plume is the first firecracker to go off, then the next one, then the next. When you look at one of those clouds of billowing ash coming out of one of these eruptions, the vast majority of what you are actually seeing is the gas exhaust from the sequence of mini-eruptions going down the neck of the volcano. This happens because as the pyroclastic debris created by first explosion leaves the volcano, the fresh 'top' of the magma body explodes next, exposing more of the magma, which then explodes, and so on in a propagating chain reaction down the neck of the volcano. It can take many hours for this sequence to play out until last of the felsic magma has depressurized and detonated. At this point, residual magma may expand up the neck of the volcano and seal the vent, or continue to feed a growing dome. This can be seen today. There is a growing dome in the crater of Mt. St. Helens. If it keeps up its current rate of growth, it could fill the crater, returning Mt. St. Helens back to a volcanic cone once again. Believe it or not, the cone we know of as Mt. St. Helens is only 4,000 years old, built up by a myriad of eruptions.

It should be noted that a 'typical' eruption should only be looked at in a general sense when it comes to volcanic systems. When looking at specific volcanic events we quickly find that each eruptive event is unique in its chemical/mineral makeup and each eruption has its own unique set of behaviors. Deep in the melt zone are a variety of rock types. Sometimes a lot of silica gets melted, sometimes not. There are many factors that influence an eruption, so the information here is only meant as a general guide of typical behavior. For example, even though seawater gets into subduction zone volcanoes, not all eruptive events at subduction zone volcanoes are explosive. Sometimes subduction zone volcanoes have basaltic/mafic eruptions. Even Mt. St. Helens had a Hawaiian eruption as you can see at Ape Cave where a pahoehoe lava flow poured out of Mt. St. Helens 1,900 years ago. Around Crater Lake (Mt. Mazama) in Oregon are several shield volcanoes and lava flows like you find in Hawaii or Parashant, while Crater Lake itself is a caldera from a high silica eruption. Oregon features the full range of volcanic eruption types such as the obsidian, pumice, and rhyolites at Newberry Crater near Bend, OR. In addition, some volcanic fields feature a variety of magmas. Utah's Black Rock Desert Volcanic Field features everything from basaltic cinder cones and lava flow to andesitic and dacitic ash, as well as rhyolite domes. The magma that came up in the Black Rock Desert area got its silica by melting through ancient North American continental rocks. Sometimes it didn't pick up much silica and stayed mafic. Other times magma bodies picked up extra silica turning them felsic/rhyolitic.

Some volcanoes and volcanic fields in the Basin and Range are known as 'bi-modal' where they will erupt either basaltic or rhyolitic lavas over time. This is due to the conditions in the magma chamber during the melt.

Continue to Section 3 which dives into Parashant's ancient volcanism and lava dams on the Colorado River.

For those who are interested, see this section of an open-source volcanism textbook for more technical detail than is covered here.

Last updated: April 5, 2024

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