Yellowstone National Park is a place rich in recent and ancient human, evolutionary, and geological history. Its spectacular features are shaped by the same processes happening almost everywhere else on the globe, and one spectacular process that rarely happens on Earth.

Layers Upon Layers

We'll approach Yellowstone geology the way we approach most geology, by describing the layers of rock that form it. The oldest rock in Yellowstone National Park is Precambrian in age, over 500 million years old. You can find some granites, but thanks to the multiple episodes of mountain building that begat the Rocky Mountains, a lot of these have been metamorphosed into gneisses and schists¹.

During the Palaeozoic Era (500 - 235 million years ago), Yellowstone National Park and most of the rest of the Western United States lay beneath or on the edge of a shallow sea. During this time, many of the sedimentary rocks of Yellowstone were deposited, most of them limestone, sandstone, and shale. The Mesozoic Era (235-65 million years ago) generally brought about more terrestrial environments, with sandstone and shale bearing river features. Some of these formations in the park have been found to contain dinosaur fossils. At the end of the late Cretaceous Period, 65million years ago, the first signs of Yellowstone's volatile volcanic nature can be found, which continues to manifest itself today.

Grand Canyons

Yellowstone has also been shaped by rivers and streams. The Grand Canyon of Yellowstone², immortalised in paintings, photos and engravings, was cut solely by the Yellowstone River. It is roughly 20 miles long, and has two major waterfalls. Water plunges 109 feet over the Upper Falls, but thunders 308 feet over the Lower Falls. Rivers and streams also bring mineral-rich sediment down from the mountain, creating good soils on the floodplains. The Continental Divide, separating the Mississippi drainage basin from the Western United States, also runs through Yellowstone.

Rivers of Ice

Glaciers have been intermittently shaping Yellowstone National Park for the past two million years. The most recent glaciation began about 50,000 years ago, when snow piled up northeast of the Park on the high mountains of the Absaroka, in the Beartooth Wilderness area. Once the glaciers started moving, they ploughed through the park to converge over Yellowstone Lake, completely covering the Park and surrounding area in over a thousand feet of ice. The glaciers left their scars at the peak 25,000 years ago on the top of all but the highest mountains in the area, Mount Washburn and the highest ridges of the Absaroka Range.

When the deep-freeze finally broke 15,000 years ago, the ice had transformed Yellowstone. At the very end of the Ice Age, much of the glaciers' meltwater was trapped behind a giant ice dam, leaving the fertile lake deposits we see in Hayden Valley when it finally ceased to exist. Rivers of ice widened and scoured existing river valleys, and left behind fields dotted with granite boulders from faraway mountains³. Glacial ponds, striated hillsides⁴, chiselled peaks, and polished mountain faces all bear witness to the geological force of ice⁵.

Glacial deposits are often affected by thermal features, particularly in the Norris Geyser Basin. Hot water from geysers and hot springs percolates through the deposits and alters them, forming minerals we don't expect to see at the surface. However, collecting is strictly forbidden without a permit in Yellowstone National Park. All mineral collectors must keep their hands to themselves.

Did you just feel the ground move?

Yellowstone National Park is home to a number of active fault systems, a fact with which an unlucky group of campers became acquainted on 17 August, 1959 at 11.37pm. At that moment, a 7.8 magnitude earthquake struck in West Yellowstone along the Hebgen Lake Fault. It shattered the surrounding rock with three major faults, displaced the stream Red Creek as much as 20 feet, and altered thermal features in other parts of the Park. It shook the reservoir above Madison Canyon so badly it sent a 20-feet-high wall of water into what was, at that point, a campground. Further down, 44 million cubic yards of solid rock, half of a 7600-feet-high mountain, fell into a valley at 174 miles per hour, smashing house-sized boulders into tents as campers slept, and sending hurricane force winds up the narrow valley. Stories abound of entire families being killed as they slept, and of children being orphaned because a house-sized boulder fell on their parents' tent six feet away. Luckily, the fallen mountain dammed the canyon, creating Earthquake Lake before the flood could reach further down the valley. 28 people are thought to have died in the quake and following landslide and flood, and are still buried beneath the rubble, so it is unlikely that the true death toll will ever be known.

Thermal Features

Geology is the main reason why Yellowstone was made into the world's first National Park. In particular, it earned that honour because of the thermal features that lie within its borders.

Yellowstone is literally a hotbed of geologic activity. The increased temperatures only a few miles beneath the surface superheat groundwater found naturally in the rock. This has the same effect as boiling a sealed bottle. The superheated liquid in the bottle has to find somewhere to escape, so it often blows the top off. In Yellowstone, the water escapes to the surface through existing cracks in the rock, giving the park the honour of having 90% of the world's thermal features.

Several things can happen to the superheated water at this point, depending on the shape of the crack and how much water is in it. If the crack opens all the way to the surface with no obstructions, the water will form a hot spring. The heat is released at the surface, and the water is allowed to recirculate back underground.

Thanks to the miles of rock the water has travelled through, hot springs are rich in minerals such as sulphur, which give them a distinct odour and technicolour hues that are nearly impossible to capture with a camera. These same minerals also support some of the most bizarre and primitive communities of microbes on Earth. Ancestors of these organisms that survive in boiling water left their traces in rocks over 3.5billion years old! They thrive in giant microbial mats in 70°C (160°F) water. When the water cools sufficiently, hot springs can form step-like rock features called terraces⁶, the most spectacular of which are the Terraces at Mammoth Hot Springs. Other notable hot springs in the park are Prismatic Spring (the largest), and Morning Glory Pool, which was sadly vandalised in the early days of the park, robbing it of the most spectacular of its colours. Visitors can still see debris in the pool.

Sometimes a hot spring provides a home for sulphide-reducing bacteria. These micro-organisms use the hydrogen sulphide, which gives the hot springs their characteristic odour, for energy, with the waste product being sulphuric acid. This eats into the rock of the hot spring, turning it to clay. Eventually, this clogs the plumbing, limiting the hot spring's water supply. However, the hot spring doesn't die, instead it becomes a mud pot. Mud pots are bubbling cauldrons of acidic mud. Gases that would normally boil away with the water of a hot spring escape through the viscous gunk, with a sound and a smell that reminds many of flatulence. The most famous mud pot is Artists' Paintpots, a colourful mass of boiling mud that reminds one of a mad artist that has eaten too many beans.

When a hot spring has a plumbing problem deeper in the rock, things get interesting. A constriction in the crack leading to the surface partially blocks the water, causing pressure to build, until it blows past the obstruction and rushes out into the atmosphere. This is known as a geyser. Geysers can erupt regularly, like Old Faithful, or sporadically, like the spectacular Steamboat Geyser which reaches 300-400 feet. Some have months or years between eruptions, while other erupt nearly every minute! Sometimes, the water supply for a geyser will run out. The geyser will disappear, leaving a calcium carbonate geyser cone as a tombstone.

The fourth major thermal feature in Yellowstone is the most common and the most puzzling to uninitiated tourists. Fumaroles, or steam vents, are essentially geysers that do not have enough water to erupt. Instead, they belch steam from a hole in the ground, smelling like sulphur and making a noise that the guidebook describes as thunder, but sounds to some like a giant toilet flushing. Steam vents are the hottest thermal features in Yellowstone, and are very easy to find.

Yellowstone's thermal features can be dangerously unpredictable. Thanks to thermal feature surprises, the National Park Service has had to move parking lots and tourist boardwalks. A new geyser or hot spring can open up at any moment, especially on the thin crispy crust known as sinter around existing thermal features. Hundreds of people have been injured or killed when they stepped off the boardwalk for a better picture of a thermal feature, only to have one open up under their feet.

Yellowstone Volcanism

Most visitors to the Park know there are different forces at work in Yellowstone the instant they see steam shooting from the ground, but exactly what is going on remains a mystery to many. Many leave with the understanding that Yellowstone is a volcanic caldera which is still quite active but tame, its heat powering the geysers. As usual, there is much, much more to the story.

Around 640,000 years ago, a few thousand miles away in what would become Ashfall, Nebraska, lay an environment similar to the African savannah. Elephants, rhinos, three-toed horses, and antelope-like sabre-tooth deer shared a waterhole, and were preyed upon by dire wolves. That went on until a cloud of volcanic dust buried the area in eight feet of ash. In the 1970s, scientists traced the composition of the ash back to the world's most famous National Park. This ash was from what is called a supervolcano. This brings up the image of a giant, cone-shaped mountain belching gooey lava and smoke for many people. However, a mountain large enough to cough out that much ash and leave a crater the size of Yellowstone would have had to have been several hundred miles in diameter. With all that we know about geology and physics, we know that individual mountains simply cannot get that large. However, as with most problems, a good hike sheds some light on some things, in particular, a 1,400-feet climb up Mount Washburn.

The view from Mount Washburn looking south reveals the bulk of Yellowstone National Park is a relatively flat, rolling plain edged by mountains. Large mountains, like the Grand Tetons to the south, the Absaroka Mountains to the east, and of course, Mount Washburn. What we know about geology tells us that mountains don't form naturally with a 37-mile gap in between them. This begs the question, what happened to the mountains that we should have seen there?

In short, they blew up, then collapsed within a matter of weeks, slumping into Yellowstone, spewing out 240 cubic miles of debris and leaving a 30km by 50km volcanic caldera. Compare this to the Minoan Eruption of Santorini, or Tambora, one of the most massive volcanic events in recorded history.

This is not the first time this has happened. There have been at least three supervolcano eruptions recorded in Yellowstone's geologic record. The most recent blast 640,000 years ago is known as the Lava Creek event. The Mesa Falls eruption occurred roughly 1.3 million years ago, preceded by massive lava flows that you can still see on the edges of the park. The Huckleberry Ridge eruption 2.1 million years ago was possibly the most massive volcanic event in the history of the Earth for which there is rock record, based on the extent of the ashfall all over the world.

The average time between eruptions? 600,000 years.

Hot Spot

What caused the volcano to form in the first place? Yellowstone is nowhere near any tectonic plate boundaries, so it shouldn't even be there, surely?

Not exactly. Yellowstone is right above what is termed a mantle hot spot. Scientists theorise that 25 million years ago, the molten portion of the Earth's core ejected a mass of hot material directly below Yellowstone. Ten million years ago, that plume reached the surface of the earth, and spread out to form a shape much like a martini glass. These are not entirely uncommon as there are approximately 40 active hotspots on Earth today, nearly all of them in oceans⁷. The Yellowstone Hotspot had the power to burn through an entire continent. Molten rock exists under the Park today anywhere from three to eight miles below the surface by geophysical estimates. Compared to the rest of the planet, parkgoers are truly skating on thin ice.

The End of The World

The last time Yellowstone erupted, there was very little warning. Mountain ranges bulged, and the earth shook with the pressure from the rising molten rock, until the pressure finally got too high for the mountains to withstand. When they finally gave up the ghost, thousands of tons of burning hot gases, molten rock and ash were sprayed into the atmosphere like fountains and carried around the world. The world's climate experienced several years of volcanic winter when the ash blotted out the sun. Ash covered North America from Saskatchewan to the Gulf of Mexico, and from Iowa to off the coast of California. As John Good and Kenneth Pierce so aptly put in their text, 'Interpreting the Landscape: Recent and Ongoing Geology of Grand Teton and Yellowstone National Parks':

Nearer the vents, fiery clouds of dense ash, fluidised by the expanding gas, boiled over crater rims and rushed across the countryside at speeds of over 100 miles an hour, vaporising forests, animals, birds, and streams into multi-coloured puffs of steam.

Of course, the last eruption did not relieve all the pressure. It is still there, still building just beneath the surface, powering the thermal features all the tourists gawk at. Another eruption is not only possible, it is very likely, and will most probably be the end of civilisation as we know it. Such an eruption would bury the grain-producing farm fields of the US Great Plains, and potentially put enough ash and debris in the atmosphere to achieve a 'nuclear winter' effect.

Does this mean you should put off your trip to Yellowstone? Of course not. Yellowstone was originally preserved for its astounding geological features. From the canyons, to the lakes, to the frothing geysers and hot springs, it is spectacular in every sense of the word. And its preservation means that even today, one can walk on the boardwalks around the Artists Paintpots, contemplating art, flatulence, and the end of the world.

More Information

Our Dynamic Earth is an exhibit in the William Younger Centre, Edinburgh, Scotland, that explains the processes that shape our planet.

USGS Yellowstone Volcano Observatory A real-time monitoring center for earthquakes and volcanic activity within Yellowstone.

US National Park Service Official Yellowstone National Park Site contains links to all kinds of information, from historical sites to the best places to see wildlife.

Our thanks to Tav's Dad for the main photograph.