My next post about the Geology of the National Parks Through Pictures is from our move across the country from Utah to New York. Along the way we visited 13 National Parks as well as some other sites. This was the 3rd National Park along the way.
You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.
-----------------------------------------------------------------------------
 |
| Southern entrance sign |
I would consider Yellowstone to be one of the premier geological sites on the planet. Even though this type of volcano, a hotspot volcano, is not unique to Yellowstone, the conditions in which this hotspot is located and the preservation of the landscape within the National Park make it a rather exceptional geological destination. We'll start with some background on what Yellowstone is.
 |
| Yellowstone's magma plume below the surface of the Earth. Image courtesy of National Geographic. |
While it does not look like a "typical" volcano, Yellowstone is one of the largest volcanoes on the planet, however most of that volcanic mass is "hidden" below ground. Yellowstone is what is known as a "hotspot" volcano. This means that magma rises from the mantle towards the surface from one location.
 |
| Movement of the North American plate across the Yellowstone Hotspot. Image courtesy of NPS.gov. |
This hotspot is essentially fixed in place, however the plates on the surface of the Earth continue to move across it. The movement of the plate across the hotspot creates a string of volcanoes, where the volcano furthest away on the string is the oldest. It also means most of the volcanoes along the string are likely non-active, with only the ones currently over the hotspot having any form of volcanic activity. Another well known hotspot volcano is Hawaii, where you can easily see the string of volcanoes over time with the current hot spot being located under the Big Island. In the image above you can see the string of former locations where the North American plate used to reside over the Yellowstone Hotspot as the plate moved towards the southwest over the last 16 million years. The previously talked about Craters of the Moon National Monument and Preserve, is another National Park that is a result of the Yellowstone Hotspot.
 |
| The most recent Yellowstone calderas, within and surrounding the current boundaries to Yellowstone National Park. Image courtesy of NPS.gov. |
Within the Yellowstone National Park boundaries, there are the remnants of two out of the last three eruptions. These eruptions produced calderas, the large craters within which the volcano erupted from leaving a bowl shaped depression. The calderas within the Yellowstone NP boundaries are dated at 2.1 million years old and 631,000 years old. While the last major eruption was over 600,000 years ago, Yellowstone is far from a dormant volcano, with the volcanic features that litter the landscape as evidence of the ongoing volcanic activity within the park.
 |
| Yellowstone Lake |
The large lake seen in the caldera map above is Yellowstone Lake and it occupies a large portion of the most recent 631,00 year old caldera, known as the Yellowstone Caldera. Each of the eruptions also have a name that they are known by, with this most recent eruption known as the Lava Creek Eruption.
 |
| Grand Canyon of the Yellowstone |
With all of the fairly recent geological features, erosion by water creates some of the most stunning attractions within the park. One of these features is known as the Grand Canyon of the Yellowstone along the Yellowstone River. There are a few factors going into the creation of this canyon. One of the factors is that all of the very hot magma beneath the park lifts the entire region upwards. This force pushing the land up is very similar to that seen at the Grand Canyon, where as the ground moved upwards, the rivers within the landscape erode downwards at a quicker pace. This can be seen here as the Yellowstone River eroded downwards as the land surface pushed its way upwards. However, the rate of erosion is also fairly high, even for this phenomena, and that is because within this portion of the park, the Yellowstone River follows a fracture zone of the Yellowstone Caldera. Here hot water and steam rise up from deeper within the Yellowstone system and it alters the overlying rocks. These volcanic rocks, a type of rock known as rhyolite which had been erupted during the last major volcanic eruption, were weakened by this alteration, causing the rocks to break down easier and the river to rapidly erode down through rock layers.
 |
| The 110,000 year old West Yellowstone lava flow |
Across the Yellowstone landscape are remnants of ancient lava flows from past eruptions. The vast majority of these are rhyolitic in nature, far different than the basaltic lava flows of Hawaii. The difference is the silica (quartz) content of the lava. Since the Hawaiian hotspot erupts within the middle of the oceanic plate, a plate made primarily of basalt, and therefore has a low silica content, the lava itself is generally basalt and has a low silica content. The lower the silica content the lower the viscosity of the magma, and the more likely you'll have quick flowing lava. Since the Yellowstone hotspot is over continental crust though, the magma is more silica rich, and therefore the lava that is erupted more often produces a high silica volcanic rock known as rhyolite, such as that seen in the West Yellowstone lava flow above. You'll note that this lava flow is significantly younger than the youngest caldera date of 631,000 years old at 110,000 years old. While this location along the edge of the Firehole River is within the Yellowstone caldera, even though the major eruption was 631,000 years ago, there had been numerous other smaller eruptions since. Rhyolitic lava, as seen here, is a much thicker, viscous lava that is able to hold in the volcanic gases within the lava much better. This results in a rock that often has a lot of gas pockets as can be seen here. Since rhyolitic lava is much thick and holds in the gas much better than basaltic magma, this means that rhyolitic volcanoes also tend to be much, much more explosive in nature.
 |
| Lewis Falls |
The multiple lava flows that had erupted over time also produce lavas that differ remarkably in composition, and hardness. This leads to many waterfalls forming across the area such as the above Lewis Falls near the southern entrance to the park, which is a waterfall that flows over the ~72,000 year old Aster Creek lava flow. Here the upper rhyolitic lava is far stronger than the lower lava, creating a shelf from which the waterfall forms over.
 |
| Gibbon Falls |
Towards the central portion of the park is Gibbon Falls which sits upon the 640,000 year old Lava Creek Tuff and has eroded its way upstream from the edge of the Yellowstone Caldera.
 |
| Rustic Falls |
And moving even further north is Rustic Falls, near Mammoth Hot Springs in the northern reaches of the park. Here Glen Creek flows over the far older Huckleberry Ridge Tuff from the 2.1 million year old caldera.
 |
| Old Faithful Inn building blocks of rhyolite |
Rhyolite is so prevalent within the park, and so sturdy (as can be seen since it is the resistant rock forming the waterfalls listed above), and therefore had been employed as a building stone for many of the buildings within the park as well, especially the historic Old Faithful Inn.
 |
| View of my father back in 1997 and me in 2010 during an Old Faithful eruption. |
But by far the most notable feature of Yellowstone are the hydrothermal features. These range from geysers to hot springs across the park. And of these, Old Faithful is the most well known of these.
 |
| Example of the fracture system below ground under a geyser. Image courtesy of yellowstonetreasures.com |
As I am sure you are aware, Old Faithful is a geyser. One amongst many geysers in the park and one of the densest concentration of geysers in the world. Geysers work when rain and snow percolate into the ground, creating ground water. This groundwater is heated up by the presence of a heat source, the Yellowstone magma chamber in this instance. This heated water then rises through cracks and fissures in the ground. As it heats up and rises it slowly dissolves the surrounding silica within the rhyolite rocks.
 |
| Old Faithful erupting facing north. |
The majority of the fractures below Old Faithful lie within glacial sands and gravels. The dissolved silica within the super heated waters starts to precipitate out of the hydrothermal fluids, stabilizing, and slowly constricting the cracks and fissures that make up the network.
 |
| Plumbing beneath Old Faithful. Image courtesy of Smithsonian Magazine. |
As the water is heated up, it also expands. However, since the cracks keep the heated water contained, the water is not allowed to expand, resulting in water that has become "super heated" (a phenomenon where water can surpass the boiling point but remain as water and not turn into steam). As it moves upwards eventually the water reaches near the surface where there is no more overriding pressure from the surrounding rocks and the water is allowed to expand. Since it is super heated, the expansion immediately causes the water to turn to steam. It is this sudden expansion and steam production that produces the semi-regular to regular geyser eruptions. The regularity of the eruptions is due to the complexity of the fracture network, the regularity of the ground water inflow, and how many external vents there are. The more vents connected to a system the less regular the system is likely to be. Old Faithful's independent plumbing network is what likely led to the regularity of eruptions.
 |
| Old Faithful erupting facing south towards the Old Faithful Inn. |
Within the Old Faithful system, the cracks and fissure plumbing network expands over 650 feet across and holds more than 79 million gallons of water leading to ~8,000 gallons of water released per eruption.
 |
| Beehive Geyser on Geyser Hill near Old Faithful |
The dissolved silica within the hydrothermal fluids can crystalize within the fracture network of the geyser, and it can also crystallize around the base of the geyser as it erupts. This materials is known as sinter, and can produce a column of silica like a funnel for the geyser such as at Beehive Geyser above.
 |
| Steamboat Geyser |
Around Old Faithful isn't the only location that geysers can be found within Yellowstone as well. Steamboat Geyser, a bit to the north of Geyser Hill, in the Norris Geyser Basin. Steamboat Geyser is known as the world’s tallest active geyser.
 |
| The Norris Geyser Basin |
Around the various geyser basins, there are also numerous hot springs found. Hot springs are produced when you don't have the constricting plumbing that makes a geyser a geyser.
 |
| Anatomy of a Hot Spring. Courtesy of the NPS. |
The water within a hot spring can exceed the boiling point of water and often produce some of the most vivid colors within the park. The pH within each of the hot springs is also extremely variable. While most hot springs have a tendency to be more basic (greater than a pH of 7), there are also some hot springs with a pH of 1 to 2 (the same acidic content of battery acid).
 |
| Hot spring within Geyser Hill near Old Faithful |
While, as the image above describes, blue is the most typical color of the hot springs, the myriad of other colors are typically the result of thermophiles, microorganisms like bacteria, that thrive in the intense heat.
 |
| A hot spring within Geyser Hill near Old Faithful |
Interestingly enough, different thermophiles produce different colors, and each one lives within a narrow range of temperatures so the gradation of colors is due to the temperature gradient within the pools.
 |
| Emerald Spring |
There are also other factors to the hot spring colors like minerals dissolved within the water. At Emerald Spring above, the orange color that can be seen around the outer edge of the pool is made up of sulfur deposits crystalizing. Lighting effects caused by the blue of the water and the yellow of the sulfur to mixing makes the water appear as if it is an emerald green.
.JPG) |
| Terraced travertine deposits near Mammoth Hot Spring |
Similar to the hot spring deposits, flowstones that form up in the Mammoth Hot Springs part of the park are created as the thermal waters flow out of the ground and deposit their dissolved minerals. In this instance it is typically calcite (calcium carbonate), forming what is known as travertine. This is similar to deposits that can be seen within cave systems.
 |
| Orange Spring Mound near Mammoth Hot Springs |
These travertine deposits are also riddled with colors formed from the mixture of algae and bacteria within the mineral deposits, such as can be seen at Orange Spring Mound.
 |
| The Hoodoos, some landslide travertine blocks south of Mammoth Hot Springs |
Just south of Mammoth Hot Springs there is an area known as "The Hoodoos", which are landslide deposits of older travertine blocks that came from the nearby Terrace Mountain. These travertine blocks are identified as "pre-Pinedale" in age, meaning that they are older than the recent glaciation that occurred within the region approximately 30,000 to 10,000 years ago.
 |
| Continental Divide photo opportunity |
 |
| Image of Firehole River within Geyser Hill |
And at last, a shot of the sun reflected in the Firehole River, overlooking the hot springs and geysers of Geyser Hill.
References
.JPG)
.jpg)
