Parashant straddles the Basin and Range and Colorado Plateau. The western portion of the monument includes the giant Grand Wash Trough (Pakoon Basin). This is part of the Basin and Range transition zone. The eastern portion from the Grand Wash Cliffs to Mt. Trumbull is part of the southwest corner of the Colorado Plateau.
Key Points in Basin and Range Extension
A Deeper Dive into this Story
Parashant sits on the boundary of two surface features known as the Colorado Plateau and the Basin and Range Province of the west-central North American continent. This has been an area of incredible deformation over the last 525 million years. Geologists come from around the world to study its features. To see how the region has changed, check out these incredibly detailed landscape reconstructions by paleogeographer Ronald Blakey and his team. Two things to note on these maps. First, keep in mind that the United States as shown was located much closer to the equator at that time. It has since moved north. Second, the terms Ka = X thousand years ago (kilo annum) and Ma = X millions of years ago (mega annum).
The Colorado Plateau, famous for its dozens of colorful rock layers in places like the Grand Canyon, Zion, Arches, and Parashant, occupies western Colorado, most of Utah, northern New Mexico, and northern Arizona. It is made of colorful sedimentary and metamorphosed rocks that have been deposited over almost 2 billion years. The lowest levels, such as Vishnu Schist, were scraped off a subducting plate and scrunched onto (accreted) to North America from ocean plate subduction. The most colorful layers above were deposited in seas, rivers, and deserts that came and went. The Colorado Plateau is often described as a layer cake as it has many distinct and visually impressive rock layers stacked one on top of the next. It also includes a variety of volcanic features that worked their way up through the plateau much later.
The Colorado Plateau section of the North American continent is being pushed upwards by hot mantle rock. The reason the mantle rock can cause the Colorado Plateau to rise is due to heat. Hotter materials are less dense. At the molecular level, the mineral molecules are more energetic. This increases the space between them, making them lighter, thus buoyant. It is this hot material in the form of solid mantle rock (not liquid), that causes it to rise, lifting the Colorado Plateau that sits on top of it. The Colorado Plateau has been pushed up thousands of feet higher in elevation than it otherwise would be, buoyed from below. All this high elevation terrain created a steep gradient down to the ocean. Rainwater and melting snow cascaded off the plateau in the form of fast-moving streams and rivers, eroding into the plateau rapidly. This resulted in its sharp and scenic vertical topography. Many of the layers of the plateau are made of soft clays and sand, which are easily eroded. This has resulted in strange landforms, scenic landscapes, canyons, arches, and badlands. The most famous feature is the 277-mile long mile-deep Grand Canyon. The Grand Canyon was cut into the Colorado Plateau in less than 6 million years!
One area of research by Lavender, et al., using new seismic tomography technology from the groundbreaking US Transportable Array, seems to show that a large chunk of the bottom of the plateau (lower crust and upper lithosphere) may have detached from the layer cake rocks above, sinking into the mantle. Rising hot asthenosphere rock (which is still solid but moves like putty) may have rushed in to replace the cooler lithosphere and lower crust as they fell away. The layers that are delaminating may be due to rising plumes of liquid magma that stalled and refroze back into rock in the lithosphere. These magma plumes never made it to the surface. This increased the density of the rock at that level. The added weight may have caused these layers to pull away (delaminate) from the bottom of the plateau and sink down (a lithosphere ‘drip’) into the mantle. If you have ever had a shoe where the rubber sole came unglued in front and flopped down, this is basically what the bottom of the Colorado Plateau is doing as the bottom flap of the plateau is peeling off. As this happened, hotter mantle rock around it rushed in to replace the sinking slab. This put much hotter rock right below the plateau. This is just one of several hypotheses being researched by geologists. It is still controversial and heavily debated. Others propose a mantle plume welling up from the deep pushing the plateau higher. Seismic tomography is a new technology, but it is the best way scientists have to reveal processes deep in the earth where temperatures over 2,000 degrees Fahrenheit (1,100 Celsius) and pressures in the hundreds of thousands of pounds per square inch range (1-3 gigapascals) make exploration otherwise impossible. A final answer may be decades away that fully explains the process of Colorado Plateau rise.
Basin and Range Province
The Basin and Range is an arc of land that starts far south in the Mexican states of Chihuahua and Sonora and runs northwest through Big Bend National Park in West Texas, over to New Mexico and its Rio Grande Rift Valley, on to Arizona (including Parashant), western Utah, through all of Nevada, eastern California, and finally the high lava plains of southern Oregon and Idaho. It is characterized by a mix of rocks that were accreted through subduction of the Farallon plate under the western edge of the North American continent. Rock layers include seafloor sediments, ocean floor basalt (lava) rock, and ancient offshore islands arcs. The Basin and Range region also contains large bodies of magma that were injected up into the crust.
The Basin and Range is best known for being on the move. The B&R region is widening extensively as the ancient Nevadaplano/Mogollon Highland (the Andes-like mountain range in eastern California and Nevada) that collapsed starting 30 million years ago. It was further stretched by the northwestward motion of the San Andreas Fault. This stretching has resulting in a doubling of the surface distance from the state line of Arizona and Nevada out to the California coast in only 17 million years. See this animation to visualize crustal extension. The mountain ranges of Nevada and the rest of the Basin and Range are actually just massive slumped and tilted crustal blocks from extension. The base of the blocks slid down into the lithosphere toward the west. This tilted the tops of most slumping blocks 20-60 degrees, while some like the Virgin Mountains have been tilted almost 90 degrees from their original orientation. Other famous tilted blocks are Mt. Charleston west of Las Vegas, Wheeler Peak at Great Basin National Park, and Mt. Lemmon and Kitt Peak in southern Arizona. Some of these upended blocks expose very ancient crustal rocks that had been part of offshore island arcs accreted to North America. For instance, uplifted and exposed rock layers in the Virgin Mountains are up to 1.7 billion years old. As time has passed, erosion has filled the deep V-shaped valleys between the peaks, creating flat valley floors with shallow lakes called playas. The Bonneville Salt Flats is part of the largest of these playas. Hidden deep beneath the salt flats is a deep valley thousands of feet below but filled with sediments and salt. This crustal extension is even visible in the Grand Canyon. Most of the north-south side canyons are fractures caused by extension. Water, ice, and wind have eroded open these cracks into giant side canyons in the Grand Canyon.
Before the Basin and Range
Earth's crust may seem thick to humans, but in fact it is extremely thin relative to the thickness of earth's mantle and core. From the surface to the center of the earth is just shy of 4,000 miles. The crust here is 18 miles thick. Basically the crust is less than one percent of the diameter of earth, comparable to the shell of an egg. Meanwhile, the putty-like mantle is fully 84% of Earth's total volume. The mantle is the egg white, and the core is the yolk. Because Earth's crust is brittle, it is full of fractures, also known as faults. These are places where pieces of crust move against each other to relieve tension that builds up by mantle forces that push or pull on them. This has created a patchwork of giant plates that constantly jostle against each other as convecting rock circulates in the mantle.
While hard to believe, ocean waves once crashed on beaches at approximately the Utah/Nevada State line, just on Parashant's doorstep. The coastline moved west over time because of the subduction of Farallon Plate under North America's west coast. The subducting Farallon Plate was covered in deep layers of sediments and giant island arcs. Some of these islands were the size of Hawaii. One was as big as California! That one was scraped onto North America from central Oregon north into British Columbia. As the Farallon subducted, these islands, thick layers of metamorphosed basalt called serpentine, and deep layers of sediment on the seafloor were scraped off and added to the western edge of North America. Geologists call this conglomeration the North American Cordillera. It stretches from Mexico to Alaska. The materials that had been scraped off are what are known as accreted terranes. This added new land to North America which can be seen in the differences between these paleogeographic maps of the western US 245 Million Years Ago and 215 Million Years Ago when some of this material was being added.
Unlike most subduction zones where the oceanic crust dives steeply back into Earth's mantle and melts away, the Farallon did something unusual. It subducted but floated up against the bottom of the continent instead of diving at a sharp angle down into the mantle. This had a major impact on the landscapes of western North America. As the Farallon slid eastward, friction between the top of the Farallon and the bottom of the North American continent pushed up surface rocks from eastern California to central Colorado. This caused two massive mountain-building events, known as orogenies. The first was the Sevier, followed by the Laramide.
Almost unknown by the general public the Sevier Orogeny is named for the Sevier River of west-central Utah between Zion and Bryce Canyon National Parks. Unlike the Laramide Orogeny that came next and built the Rocky Mountains, the Sevier included two types of mountains. The first was a typical mountain range in what is today eastern California and Nevada. Called by some the Nevadaplano and others the Mogollon Highlands, it was a mountain range estimated to be as high perhaps as the Andes Mountains. Further east the friction between the Farallon and North American plates pushed brittle platy layers of surface sedimentary rocks into an overlapping pile called the Sevier Thrust Belt. This sat in Nevada and western Utah. The brittle sedimentary rocks in the thrust belt were broken into flat pieces that start to ride up and stack on top of each other, similar to how broken pieces of glass pushed by a broom start to stack on top of each other. The next mountain building event, the Laramide, happened further east, pushing up the Rocky Mountains, which reached their full height around 50 million years ago.
Here you can see the southwestern US 85 million years ago and again at 50 million years ago. Scroll down each image to see the events labeled. You can see how the Nevadaplano/Mogollon Highlands and Sevier Thrust Belt came up during the time the interior seaway was still in place 85Ma. By 50Ma the seaway was gone and the Rocky Mountains were uplifted.
Keep in mind that at the time the Sevier Orogeny happened, the surface distance between the ocean and the Utah/Arizona border with Nevada was much less than it is today due to crustal compression from the subducting Farallon plate. Easily seen features of the Nevadaplano/Mogollon Highland chain is hard to identify except by geologists due to crustal extension in the Basin and Range. Why it collapsed will be explained shortly. The total collapse and erosion of the Sevier event erased it from view and explains why it is so poorly known today. Meanwhile, the Laramide Orogeny that created the famous Rocky Mountains has left them high and photogenic to this day as they have not been impaced by extension.
The Sevier did leave clues for geologists to find in the slumped blocks of the Basin and Range. This includes debris from ancient rivers to prove it once existed. To understand how, we have to start with the basics of how a river creates deposits of rocks, gravels, and clay. The process is called imbrication. When a river flows off a mountain the water flows very fast down those steep slopes. Fast water has the kinetic energy required to tilt or roll rocks downstream. This includes everything from boulders to sand and silt. All the rocks in the riverbed roll downstream. As the flow slows down once the river reaches less steep slopes, the rocks that are too big to be rolled come to rest. If they are longer than round, the tip that sticks up is pushed over by the flowing water in a downstream direction. In effect, all rocks on a river bottom that don't sit flat tilt in the downstream direction. As the river reaches the bottom of the mountain, the slope, or gradient, levels out and the water velocity slows down. This slower water can no longer transport rocks, which then fall to the riverbottom. Once the gradient is nearly flat and the water slows, further the gravel drops to the riverbed. The slow and meandering river is only able to carry sand and silt. Eventually even the sand falls to the bottom too, creating sand bars. These sand bars contain crossbeds as they move downstream with the current, which also shows the current direction. At this point all that is left are silt particles suspended in the water. These end up in a lake or ocean bottom.
If the river that laid down sediments along its course moves somewhere else, those layers remain, and eventually can be buried by other sediments and preserved. Then if the river returns, or a new one forms, it can cut into these old riverbottom sediments and expose them. You can usually find this in mountainous terrain where a river has cut into a hill and exposed repeated layers of water-rounded rocks, gravels, and sand. These layers can cover hundreds of thousands of years of a river's flow direction and characteristics. This is like a puzzle for geologists to solve when they try to figure out what a river did, sometimes tens of millions of years ago such as with the rivers that once existed during the Sevier Orogeny. In addition, the types of river rocks in the layers can tell a geologist what part of a mountain range the river once drained. Geologists use their knowledge of the different rock types in a mountain range to establish ancient drainage patterns. Miners that explored Parashant in the 1800s did this very thing. They were seeking mineral riches so they studied the gravels coming out of canyons looking for the telltale green and blue gravels that told them that copper ore was just uphill from where the ‘color’ was found.
As for the disappeared Nevadaplano/Mogollon Highland, one clue to its existence were large rocks in ancient riverbed sediment layers where no high mountains currently exist. If you are in an area there aren’t mountains and find a sequence of large river-rounded rocks in one of these sediment beds, you can say that long ago there was a river in that place that ran fast and deep off a mountain that is now eroded away. Only a fast and deep river can transport large rocks to that spot. If all you see is gravel and sand, you can say that the river had a slower flow. If you just find sand, that is a very slow river. If you find layers of silt, that most likely was an ancient lake bottom because it can take many days for silt to settle to the bottom of a body of water that isn't moving.
The North American inland seaway invaded the center of the continent during the Age of Dinosaurs. West and east of that seaway the elevation was higher and ancient rivers of Nevada and western Utah flowed slowly to the east down into the interior seaway. One place to see this up close is the riverbed deposits of the Morrison Formation at Dinosaur National Monument. The Morrison is about 149 million years old. If you visit the famous fossil wall at that park, you will see that the bone wall is made of Morrison age riverbed sands and small gravels (as well as over 1,500 dinosaur fossils!). These dinosaurs were washed downstream and sank to the bottom of a large but slow moving river, similar perhaps to the Missouri River. (The only reason the fossil wall is almost vertical today is because it was tilted up almost 90 degrees by plate tectonics that bowed up Split Mountain millions of years ago.)
However, geologists started finding river deposits in other parts of the southwest like Lee's Ferry near Page, Arizona. Large, river-polished rocks there could only be deposited by a fast and eastward-flowing river. It was clear these rocks predated the Colorado River. This was the major clue that some large mountain range must have grown up in what is now eastern California, Nevada, and western Arizona and Utah. Geologists were finding more and more of these ancient sedimentary layers of water-tilted rocks showing eastward flow all over the southwestern quarter of the Colorado Plateau. They were left with no other possibility than there had to have once been a huge mountain range west of the Colorado Plateau. There is no such mountain range today. This unknown range had to be the source of these fast rivers. Meanwhile, the Colorado Plateau had to be much lower in elevation to give the raging rivers a place to flow down into it. It wasn't until much later that the Sevier Orogeny was understood. Once the Nevadaplano/Mogollon Highland and Sevier Thrust Belt were proven, the mysterious river sediments showing a steep eastward flow made sense.
So why does the Colorado River today flow to the west? It's simple, the topography has tilted the other way now due to the rise of the Colorado Plateau in the east and the collapse of the Nevadaplano/Mogollon Highland in the west!
What happened to the ancient Nevadaplano/Mogollon Highland then? And what does this have to do with Parashant?
Let's return to our subducting Farallon plate for that answer. Until around 30 million years ago all along the entire west coast from Mexico to British Columbia, the Farallon had been subducting under North America. Right around 30 million years ago the portion of the Farallon along California's coast finished subducting. You see, the Farallon Plate was on the east side of a seam in the ocean floor known as the East Pacific Rise spreading center. The spreading center, which runs north-south almost all the way from Canada to Antarctica on the ocean floor, produces new sea floor. The Pacific Plate was being formed on its west side, and the Farallon on its east side. The problem is that North America was moving west toward this spreading center. 30 million years ago North America overtook a section of the rise along the California Coast. This caused the spreading center itself to subduct under North America. The subduction zone was replaced by a transform fault known as the San Andreas fault complex. Meanwhile to the north there was still a lot of the Farallon to subduct from very northern California up to British Columbia, called the Juan de Fuca Plate, as seen here. The place where the San Andreas fault meets the still-subducting Farallon plate is at what is called the Cape Mendocino Triple Junction where three plates interact. South of the triple junction, the land on the west side of the San Andreas fault line is moving quickly to the northwest, while the east side of the San Andreas fault is moving relatively slowly to the southwest with the rest of North America. Here is where things get interesting and explain the Basin and Range landscape and what destroyed the Nevadaplano/Mogollon Highland.
The Death of the Nevadaplano/Mogollon Highland
There is a difference in speed between the San Andreas Fault where the Pacific Plate is moving northwest and the North American continent that is moving southwest. The San Andreas is moving up to twice as fast as North America. The Pacific Plate is moving northwest about 3-4 inches per year, while North America is moving southwest at 1.5-2 inches per year. In the area between them is essentially a complex broken zone of negative surface pressure.
The development of the northwest-moving San Andreas fault doomed the Nevadaplano/Mogollon Highland and the Sevier Thrust Belt. Geologists use a term called isostacy to explain how pieces of crust always seek elevation and density equilibrium with the rest of the surface of the earth. If something is heavy and at a high elevation, unless there are forces to keep it there, that high topography will sink until it is at equilibrium with everything around it. The Himalayas are a good modern example of this as they are being pushed up by the collision of the Indian subcontinent against the Asian Plate. Remove the pressure of the Indian subcontinent against Asia and the Himalayas will also collapse slowly.
To achieve equilibrium in the Basin and Range, something had to fill the growing low-pressure zone between the San Andreas fault and North America. The high Nevadaplano and Thrust Belt would do nicely. In a process called post-orogenic collapse, the Nevadaplano/Mogollon Highland collapsed and sank into this zone of low pressure. Blocks of crust that had been 20,000 feet above sea level dropped over ten thousand feet and spread out into the low pressure zone and forming the Basin and Range.
One aspect of the Basin and Range is why the elevation remains rather high to this day. Many of the valleys in Nevada are around 4,000 to 5,000 feet above sea level. It turns out that the extension and thinning of the Basin and Range crust didn't just cause the Nevadaplano to collapse, it also allowed hot lithospheric magma to get closer to the surface. Remember too that this includes pieces of the subducted Farallon plate. The hot lithosphere sits only 15-20 miles below the crust in western Parashant. Its heat makes the crust more buoyant, allowing the surface to be higher than if the crust was thicker and colder.
Exciting new research published in 2015 added yet more complexity to the picture. While still very controversial, this groundbreaking hypothesis proposed that it was perhaps the Yellowstone hot spot that may have actually supercharged northern Basin and Range extension 17 million years ago. At that time the hot spot was sitting under NE California and NW Nevada. The authors argue that the difference in speed between the northwestward moving San Andreas fault vs. southwestward movement of North America, as well as the gravity collapse of the Nevadaplano wasn't adequate to explain the huge crustal extension in the Basin and Range in Nevada which had doubled in width from what it had been prior to the start of crustal extension. They argue another force had to be involved, namely the Yellowstone hot spot. It would have provided the heat necessary to make the lithosphere and bottom of the crust less viscous (more fluidlike) and buoyant due to heat to the lithosphere. This relaxed the crust and helped the crust slump and widen out. The Yellowstone hot spot is also responsible for the vast volcanic plains of eastern Oregon and Washington, with stacks of lava flows up to 15,000 feet deep, and the mega eruptions in Oregon that were some of the largest eruptions known on earth.
As you can see, many forces contributed to the battered Basin and Range landscape. The Farallon is almost entirely subducted now. The remaining pieces of it will disappear under North America over the next 10-20 million years. This includes the Juan de Fuca, the Nazca, and the Cocos Plates. While still the subject of research, it appears much of the Farallon plate is still with us. This graphic from Berkeley University shows in red where pieces of the broken up Farallon Plate still sit close to the surface today. This will be important in the next section on magma melts.
To help readers understand how the giant cube-like blocks of Basin and Range crust can slump, rise, or tip to the side see the graphic at left. As movement of the San Andreas fault moves toward the northwest, this creates a zone of low pressure on the Basin and Range region. The crustal blocks of the ancient Nevadaplano and Sevier Thrust Belt are incredibly heavy and unevenly weighted. Once the pressure began to lessen 17 million years ago, these blocks started to drop and slump. Their upturned corners form the parallel tilted block mountain ranges of the Basin and Range.
Where blocks have tilted there should be deep valleys. What we see instead are broad flat basins. Ground-penetrating radar has determined that in fact the valleys are down there. We just do not see them because over millions of years the valleys filled up with sand, silt, and water from erosion of the uptilted blocks. This sediment is called 'basin fill.' One valley in Arizona is filled with 30,000 feet of sediment.
Basin and Range deformation continues to this day, but it isn't as rapid as it was between 17 and 15 million years ago when it first started. Still, these blocks continue to slowly jostle and grind against each other as they are acted on by the forces of plate tectonics hundreds of miles away and heat from the mantle just below the surface. The Basin and Range region is also alive with small earthquakes up to 5.0 magnitude. Scientists monitor the movements of mountain blocks that cause these quakes, including the stretching of the Pakoon Basin. This stretching is measured by lasers at permanent monitoring stations between the Grand Wash Cliffs and the Virgin Mountains. Right now the Pakoon Basin is widening about one centimeter each year.
So what will happen millions of years from now? Scientific investigations are ongoing to help us understand the forces at work in Parashant. This area will continue to change. As the San Andreas Fault continues its movement, it will set the stage for the Gulf of California to slowly move into the United States and creep toward Parashant. Eventually the ocean will again be on Parashant's doorstep!
When you think about plate tectonics and how the continents slide over, under, or past each other, have you ever wondered how they can do that? Flux melting is what greases plate tectonics all around the globe. Under great pressure supercritical water and carbon dioxide in the mantle create melts at plate boundaries. This makes the boundaries slippery so continents can move past each other. You may ask though what about those giant subduction zone earthquakes, like the one that hit Japan in 2011 and is predicted to hit off the Oregon and Washington coast in the next 100 years. The thing is, sometimes the plates are hung up at subduction zones. These areas are too close to Earth's surface where the rock is too cold and brittle to benefit from slippery flux melted magma. The tension is then released through a catastrophic megathrust earthquake which can last 5 minutes and may exceed magnitude 9.0. This creates catastrophic damage to buildings and has killed millions of people over time, but these quakes don't hurt Earth's plates as they move around.
At Pakoon and Tassi Springs air bubbles can be seen rising out with the water. These bubbles contain ancient gases. 1% of the gas content of the bubbles is helium. Helium is produced by the decay of uranium in Earth's mantle. It is this radioactive decay that keeps earth's core and mantle so hot. Basically, the presence of helium bubbles in Parashant's springs is a reminder that magma is relatively close to the surface.
The supercritical property of H2O and CO2 under great pressure is a challenge for geothermal electricity generating stations. There are several such plants in the southwest near volcanically active areas such as the Blundell geothermal plant near Milford, Utah. H2O and CO2 is under so much pressure that its super-critical nature actually can deteriorate the various pipes and drilling equipment deep underground. The geothermal industry is experimenting with different materials that can withstand the pressure and corrosive effects of super-critical water to take better advantage of the vast amount of energy available just a few miles below ground.
In Section 2 we will look at how magma causes the different kinds of eruptions we see around the world and why Parashant has the kind of eruptions it does.
Last updated: January 7, 2020