Monday, February 09, 2026

Geology of the National Parks in Pictures - Mount Rushmore National Memorial

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 7th 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.

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Obligatory entrance sign photo.

Geological map of the Black Hills, including Mount Rushmore (noted just below the slice through the map). Image courtesy of the NPS. 

Mount Rushmore is set within a large geologic dome. This is a region where all of the land is bowed upwards, like an overturned bowl. After erosion, the result is a bullseye pattern of rocks, where the oldest rocks are in the center of the bullseye and progressively get younger towards the outside. 

Tȟuŋkášila Šákpe, AKA Mount Rushmore, before being carved. Image courtesy of nativehope.org.

Although this could be considered outside the normal realm of geology, I do want to note that before being known as Mount Rushmore, the Lakota referred to the mountain as Tȟuŋkášila Šákpe, Lakota for The Six Grandfathers. The mountain named by Lakota medicine man Nicolas Black Elk after seeing a vision “... of the six sacred directions: west, east, north, south, above, and below. The directions were said to represent kindness and love, full of years and wisdom, like human grandfathers.”


As you drive up to the main entrance looking northeast, you first pass the profile view of Washington. From this direction you get a good view of the mountain sans most of the demolition work that was done with the carvings. You can see what would be the normally weathered surface of the granite before the carving brings out the fresh surface. This surface is also more reminiscent of the uncarved mountain seen in the image above. About 1.6 billion years ago, during the Proterozoic, magma began to work its way up through the rocks in this area. While still well below the surface of the earth, that magma cooled slowly to form the granite that makes up the carving stone for Mount Rushmore. This rock unit is known as the Harney Peak Granite. 


While the magma was cooling, it cooled unevenly. This caused some portions of the rock to cool quickly, producing smaller, fine-grained, crystals, while other parts of the granite cooled more slowly with very large grained crystals. These large grained crystal granites are known as pegmatites. The upper portion of Mount Rushmore is comprised mostly of the fine grained crystal variety, which is an easier rock to carve from. This large body of magma is known as a batholith. 


Between the formation of the granite 1.6 billion years ago and 500 million years ago, new rocks were deposited and eroded on top of the Harney Peak Granite batholith. However, due to the extreme hardness of the granite, the Harney Peak Granite remained behind while these other rocks had been lost to erosion and time. After this period of time, between 500 and 100 million years ago, there were some rocks deposited from which we do have remains of. Immediately on top of the granite is the green rock seen in the geological map above. This green rock, titled the "limestone plateau" on the map, can be seen surrounding the central granite bullseye. The "limestone plateau" is made up of several different rock layers and will be discussed in more detail in the Wind Cave National Park post (since that is where Wind Cave is located). After deposition of these rocks, the whole region started to be uplifted around 70 million years ago. This uplift is related to the uplifts seen across the Rocky Mountains at the same time. 

Google Earth image of the Black Hill dome. 

The uplift formed the dome that we had discussed above. This dome is easily noticeable in the aerial image of the region as well, as seen in the Google Earth Image above. This dome structure stretches across South Dakota and Wyoming, even up to the area in which Devils Tower is located. 


Cross section of the Black Hills. Image courtesy of A Textbook of Geology.

In geological terms, a dome is an anticlinal structure where the rocks dip gently away from the center in all directions. After folding, fracturing, and faulting, this causes the overlying rocks to break apart in the middle, allowing for easier erosion of the them. Once these younger rocks have eroded away, the older rocks are exposed with the oldest rocks exposed in the center. As before, due to the extreme hardness of the Harney Peak Granite, they withstood erosion and remained around much longer. Their hardness is also why the Black Hills have these granitic mountain peaks that have not eroded away. 


As you walk up to the main entrance to the main viewing platform, you come across a rather more grand entrance sign than the wooden one at the park entrance. Another interesting decision that was made for the refurbishment of the memorial in the 1980's and 1990's was the inclusion of several areas encased with granitic blocks. These granitic blocks are found within the Visitor’s & Interpretive Center, the Avenue of Flags, and Grand View Terrace, which, while they are granite, are not the Harney Peak Granite of the mountain. These granitic blocks were trucked in from elsewhere.


Here you can see the granitic blocks in the framing of the memorial at the distal end of the Avenue of Flags. In construction terms, these granitic rocks are known as "Rockville Beige granite" and are from the quarry company Coldspring. Quarried from Rockville, MN, this granite is more commonly known in geology as the Rockville Granite. 

Closeup of the Rockville Granite at Mount Rushmore

Known for its very large mineral crystals and minimal amount of metamorphism, the Rockville Granite presents as a very nice, consistent granite comprised mainly of quartz, feldspar, biotite, and hornblende with other accessory and background minerals. Minnesota has several granitic bodies that all date to around the same age. The Rockville Granite in particular is dated to being 1.812 billion years old (Ga), which is considered the Late Penokean of the Paleoproterozoic Era. When compared to the varying cooling rates of the Harney Peak Granite producing varying textures with metamorphic inclusions throughout, it is no wonder whey they decided to use a more picturesque granitic rock for the entrance and viewing terrace.

References
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Thursday, January 22, 2026

Geology of the National Parks in Pictures - Devils Tower National Monument

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 6th 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.

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Obligatory entrance sign shot, however this is probably one of my favorite alignments of the sign to the feature of the park. 

While describing the geology of Devils Tower you would probably want to start with the tower first (duh). However, in order to understand how the tower formed, you need to understand the rocks that were here before the tower, or what would become the tower, some of which still remain. 

While many of the rocks that Devils Tower formed within have long since eroded away, there are still significant amounts of older rocks that the tower currently resides on. You can see some of these rocks as you drive up to the tower from the main entrance (photo with the entrance sign and directly above). 

Geology of the Devils Tower sedimentary rocks. Image courtesy of NPS. 

The bright red rocks at the base of the hillside are the Spearfish Formation, which can be seen most easily in the photo above without the entrance sign. These rocks, dark red sandstones and maroon siltstones, formed during the Permian and Triassic Periods (~ 225 to 195 million years ago) when this area was covered by a shallow inland sea. The deep red color is from the oxidation (rusting) of iron within the deposits. After the Triassic Period, during the Middle Jurassic (~170 million years ago), the inland seas periodically withdrew producing periods of evaporation. This evaporation lefts gypsum rich deposits in the form of the Gypsum Springs Formation. 

View facing away from the tower. You can see the Spearfish Formation in the distance towards the let half of the photo.

Afterwards, the Middle Jurassic (~165 million years old) Stockade Beaver Shale then represents the deepening of the inland seas, with the shales deposited in deeper waters than the previous Spearfish Formation. The final sedimentary deposits that remain within the park are the Late Jurassic (~155 million years old) coastal dunes and beach deposits of the Hulett Sandstone. Within these rocks that form the cliff edge surrounding the tower, ripples from the former beaches can still be seen. 


The formation of the tower itself is uncertain though. There are several theories that have been proposed with none, to date, taking precedence. It is without a doubt that the tower formed from some variety of magmatic intrusion within the surrounding sedimentary rocks one to two miles below the (former) surface that began about 50 million years ago. This magma then cooled and solidified, forming the basis of the tower itself. Then, around 5 to 10 million years ago, the overriding sedimentary rocks started to erode away and expose the tower, leaving behind the much more resistant rocks. 


The prevailing theories are that Devils Tower formed as a:
  1. Igneous stock. The magma intruded into the sedimentary rock layers forming a body roughly the shape of the modern day tower. The magma then cooled, and eventually the overriding sedimentary rocks eroded away, along the outer edges of the igneous body for the last several million years. 
  2. Laccolith. This is a larger igneous intrusion with a more mushroom shape to it. Again, the magma solidified and then eroded away once the overlying sediment was removed and eventually eroded down to the tower shape as we know it today.
  3. Volcanic plug. Essentially this was the root of a volcano, where magma was rising up through the volcano and eventually as the volcano went extinct, the magma within the volcano solidified. This is deeper within the earth than what is known as a "volcanic neck". The volcanic neck theory is discussed below. 
  4. Maar-diatreme volcano. This is the most recent suggestion where the magma caused the groundwater to become superheated and eventually explode, creating a crater. The crater was then filled with magma and solidified. Eventually the surrounding rocks eroded away, again, leaving the tower as we see it today.


The rock itself is what is known as a phonolite porphyry. Phonolite is a type of igneous rock that is rich in alkali feldspar and moderate amounts of silica (quartz). As you can see in the diagram below, phonolite has one of the highest percentages of alkali feldspar. The alkali feldspar in this instance is orthoclase, which makes up the second part of the rock name - porphyry. Porphyry, AKA porphyritic rocks, are rocks composed of two principle grain sizes, a background, where the mineral crystals are usually indistinguishable, and the much larger crystals, known as phenocrysts. In the photo above, orthoclase is the mineral that makes up the phenocrysts, the large white crystals on a background that is more grey in color.  
Igneous rock diagram. Image courtesy of Moayyed et al., 2008.

But one of the most distinguishable factors of the tower, besides its size amongst a fairly featureless plain, is the columnar jointing. Columnar jointing is what produces those vertical lines seen at a distance of the tower. These columns are roughly hexagonal in shape, however they are not consistent and Devils Tower has many that are pentagonal columns as well, as well as ranging in diameter from 10 feet across to 4 feet across. These columns are formed when the igneous body (magma) is cooling and the magma starts to shrink (contract). This contracting produces stress lines which radiate out through the resulting rock, forming this columnar jointing pattern. These joint lines align perpendicularly to the direction of the cooling front. Since the jointing lines are running vertical, it can be assumed that the cooling front was essentially horizontal above the tower. With the fractures running vertically through the height of the tower, boulders breaking off and forming a pile of boulders, the scree, are fairly common, especially through geological history.


One other theory of the tower's formation is that of a volcanic neck. A volcanic neck is related to the volcanic plug, however a neck would be seen as the central portion of a volcano, as opposed to deep within the earth. In this instance, if this were a volcanic neck, what we would see is that Devils Tower is the core of the volcano, with the sloped sides having been eroded away. However, the jointing pattern, which runs vertically and curves towards the bottom of the tower, indicates that this theory of a volcanic neck is incorrect. If this were a former volcanic neck, the jointing pattern would run towards the center of the tower, as opposed to running nearly vertical.  

References
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Monday, January 12, 2026

Geology of the National Parks in Pictures - Bighorn Canyon National Recreation Area

 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 5th 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.

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My entrance photo shot. This one taken from the northern, Montana entrance, to the park since that is the only part of the park that we visited on this trip. This area is known as the North District. 

Loading ramp at the Ok-A-Beh Marina

Due to our limited time, and the large distance between districts, we only visited a small piece of this park. Here in the North District there was this boat ramp that went down into Bighorn Canyon and we could see some of the oldest rocks within the canyon. While there are older rocks in areas of the canyon outside of easy driving reach, these are the oldest accessible by car. At the boat launch here we have two rocks easily discernable. Along the water's edge is the Madison Limestone (the whiter layer) and just above that is the Amsden Formation. Topping the hills in this region is the Tensleep Sandstone. 

A nice stratigraphic column of the park's rock formations from the NPS. 

As you can see by the stratigraphic column above, most of the rocks at this end of the park are towards the bottom of the rock record. The Madison Limestone (also known as the Madison Group) is an Early Mississippian Age limestone (~350 million years ago). At this time there was a shallow sea across the region where sea life built up creating the limestone. The Madison Limestone contains abundant fossils from this ancient sea as well as evidence of a ancient karst topography like sinkholes and caves (think modern day Kentucky). 

A cross section of the park's formations. North is on the left with the Montana North District along the left 1/4 of the page. Image courtesy of the NPS.

Above the Madison Limestone is the Amsden Formation. The Amsden Formation disconformably lies on top of the Madison Limestone and is of the earliest Pennsylvanian in age (~320 million years old). By lying disconformably, that means that there was a period of erosion between when the Madison Limestone was deposited and the Amsden Formation was deposited. This erosion produced the ancient karst topography that the Madison Limestone is known for. 

View to the west (left) of the previous image at the Ok-A-Beh Marina.

The Amsden Formation can be broken down into a few different beds, not all of which are represented in this area. In this region, the lowest (and oldest) is the Darwin Sandstone Member. This sandstone is a red and brown quartz arenite sandstone and is thought to have been deposited as a beach/shoreline deposit as the water levels were deepening in the area. As the water continued to deep, above the Darwin Sandstone, the Horseshoe Shale Member was deposited. The Horseshoe Shale is red to grayish red or purple siltstone, shale, and mudstone. These rocks were deposited in the deeper waters of a sea as it transgressed across Wyoming. And lastly, above the Horseshoe Shale is the Ranchester Limestone Member. The Ranchester is made up of yellowish-grey cherty dolomite and limestones, interbedded with sandstone and shale. 

Looking east, near the easternmost extent of the park, midway along OK-A-Beh Road. 

From this view we can see several of the next overriding layers. The road is currently now sitting on the Tensleep Sandstone with the next overriding layer the Triassic Age (~250 to 200 million years ago) Chugwater and Goose Egg Formations at the base of the hill in the distance. Then we have a swift succession of thinly bedded Jurassic Age (~200 to 145 million years ago) formations including, from bottom to top, the Piper Formation, the Rierdon Formation, the Swift Formation, and the Morrison Formation. Capping off the hill is the Cretaceous Age (~145 to 66 million years ago) Kootenai Formation and Thermopolis Shale. 

Geological Map of the Ok-A-Beh Marina area of Bighorn Canyon NRA. Map courtesy of the NPS. 

These rocks are all thinly bedded sandstones, siltstones, shales, and limestones, that alternate through time. These deposits represent the inland sea as it covered the area and then left the area multiple times with lake and river deposits mixed throughout. The Morrison Formation itself is a world famous dinosaur fossil hotbed that represents terrestrial river deposits during the Jurassic Period. 

Extent of the Laramide Orogeny. Image courtesy of geology.wisc.edu. 

Following the deposition of these rocks, they were then lifted into the nearby Bighorn Mountains by what is called the Laramide Orogeny that began roughly 70 million years ago through 40 million years ago and had impacted the landscape across the North American west from mid-Montana down through New Mexico. The Laramide Orogeny, or mountain building episode, was caused by the former Farallon Plate off the western coast of North America pushing eastward, causing the compression of the North American Plate and mountains to be forced upwards. The Farallon Plate would late completely subduct beneath North America and would be impactful in forming many National Parks across the Colorado Plateau. 

A little further downstream from the Ok-A-Beh Marina.

As the Bighorn River was eroding the region, the landscape continued to be lifted upwards. The river now contains what are known as "entrenched meanders". These are when a river was in a formerly fairly level plain and allowed to meander back and forth depositing sediment along a neighboring floodplain. However, the landscape is then suddenly  lifted upwards, changing a river that was depositing sediment in a floodplain to an eroding river. The river then cuts downwards into the rocks that are now being forced upwards. This downward erosion is in the shape of the meandering river, causing the meanders to become locked in place, an entrenched meander. The meanders of the Bighorn River were locked in place during the Laramide Orogeny and have been eroding downward steadily ever since.  A more famous example of an entrenched meander can be seen in the Grand Canyon or Dead Horse Point. 

This last image is just upslope of the Ok-A-Beh Marina and to the left as you follow the Bighorn River. Again you can see here the Madison Limestone just above the river line with the Amsden Formation (the red rocks) on top and the Tensleep Sandstone capping off the hills.  

References
Garber, K. L., et al. "Detrital zircon U-Pb geochronology and provenance of the basal Amsden Formation." Bighorn Mountains: Wyoming Geological Association 72nd Annual Field Conference Guidebook. 2018.

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Thursday, November 13, 2025

Geology of the National Parks in Pictures - Little Bighorn Battlefield National Monument

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 4th 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.

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Obligatory entrance sign photo. However, since I was driving myself I had to take it through the windshield, much to the detriment of the picture apparently. 
 

Little Bighorn Battlefield lies along the Little Bighorn River, which is within the western edge of the Great Plains region of the US. The ground surface is an undulating terrain created by ridges of the underlying bedrock, ravines, and coulees (small, intermittent streams). The bedrock exposed within the park is split between two formations: the Upper Cretaceous Age Judith River (~80 to 76 million years old) and younger, overlying Bearpaw Formations (~75 to 70 million years old). The Judith River Formation lies mostly in the western half of the park, which is mainly on the righthand side of the road as you drive into the park. The Judith River Formation was deposited as sandstones and shales along the shoreline of the Western Interior Seaway, which was a body of water that covered much of the middle of North America from the Gulf of Mexico up into Canada during the Cretaceous time period. The Judith River Formation is also one of the major dinosaur fossil bonebeds in the US. 

Dolichorhynchops osborni found at Little Bighorn Battlefield, currently located in the Smithsonian National Museum of Natural History. Image courtesy of Wikimedia. 

The overlying, younger formation is the Bearpaw Formation (AKA Bearpaw Shale). This is located mainly in the eastern half of the park and covers both sides directly next to the road and to the left as you are driving into the park. The Bearpaw Formation is a shale layer deposited within the depths of the Western Interior Seaway, when that body of water covered the area. Many ammonites have been found within the Bearpaw Formation as well as a notable plesiosaur within the park. Back in 1977, a routine grave excavation was occurring in Custer National Cemetery and a NPS maintenance employee exhumed pieces of the plesiosaur Dolichorhynchops osborni. After six days of work, a nearly complete vertebral column, a complete pectoral girdle, and complete pelvic girdles were collected by after-hours volunteers and local paleontologists Russell King and Alan Tabrum.   



Here is a view from Last Stand Hill that overlooks the floodplain of the Little Bighorn River. The meandering Little Bighorn River floodplain dominates the southwestern edge of the battlefield. The terrain of the battlefield was highly influential in the progression of the battle. The ridges within the landscape offered defensible high grounds for the soldiers of the 7th Cavalry, while the coulees and ravines provided shelter for the advancing Lakota Sioux and Cheyenne warriors. The markers here, and elsewhere within the park, denote the locations where Cavalry soldiers had fallen during the battle.


 Besides just the markers within the battlefield itself, there are also those within Custer National Cemetery. Here many of the people who had perished during the battle had been reinterred. Both here, and within the battlefield, the white, marble stones obtained from Italy denote places where Custer's soldiers fell. 


Where the Native American warriors fell are markers of red granite, although I am not entirely sure of the source of the granite. NPS sources denote it as "Radiant Red" granite from a quarry in Cold Spring, Montana. However Radiant Red granite comes from Fredericksburg, Texas, while I can't find any granites that are quarried near the town of Cold Spring, Montana. Looking at Radiant Red granite, there are a bazillion names that it can be found under, however the proper geological name is the Town Mountain Granite, that is Proterozoic (~1.1 billion years old) in age. It is coarse grained, pink to red in color, and contains abundant quartz, plagioclase feldspar, and microcline. The feldspar is what gives the rock its pinkish red hue.


On top of Last Stand Hill is the 7th Calvary Memorial, a monument to all of those who fell during the battle, which was erected in 1881. The names of all of the soldiers, Indian scouts, and attached personnel who fell on the battlefield are carved into the stone. The monument is composed of granite of unknown type from the Mount Auburn Marble and Granite Works of Cambridge, Massachusetts. Interesting note, an iron fence was added two years after the memorial was erected to keep people from chipping off "souvenirs" of the memorial. The edges were later beveled in 1890 to hide the vandalism and later the fence was removed. 


South of the main section of the park is a smaller section dedicated to preserving the area known as the Reno-Benteen Battlefield. In 1926, on the 50th anniversary of the battle, Congress had authorized the placement of granite monument to memorialize the battle. Although the specific formation that the granite was from is unknown, it is known that it was obtained from the Livingston Marble and Granite Works in Livingston, Montana.


One of the most recent additions to the park is the Spirit Warrior Memorial towards the northern end of the park, which commemorates the sacrifice of the Arikara, Apsaalooke (Crow), Arapaho, Cheyenne, and Oyate (Lakota Sioux) tribes in the Battle of the Little Bighorn as they fought to protect their values and traditional way of life. This memorial uses sandstone blocks quarried from Billing Montana. These blocks are likely rocks that are known as the Eagle Sandstone. The Eagle Sandstone is a Late Cretaceous age (Santonian, ~86 million years old) series of lagoonal, estuarine, deltaic, swamp, and beach deposits that interfinger with marine near-shore and offshore deposits (east) and continental deposits (west). The interior walls are composed of black granite. Unfortunately, even though the panels were installed far more recently than most of the other memorials in the park, 2013, there is very little information available about where they came from online. 

Referenceshttps://www.nps.gov/libi/learn/historyculture/indian-memorial.htm

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Thursday, October 02, 2025

Geology of the National Parks in Pictures - Yellowstone National Park

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.

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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.  

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

As a geologically related note, I do appreciate a good Continental Divide sign. Here the water branches from going east towards the Atlantic/Gulf and going west towards the Pacific/Great Salt Lake. 

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
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