The Northern Rocky Mountains, from Wyoming to Idaho to Montana, represent some of the most interesting and unique geology found anywhere in the United States. From vast glaciers that carved beautiful landscapes like the Beartooth Mountains, to oceans that gave us some of the most well preserved sedimentary features in the United States. From massive Supervolcanos that have created incredible hydrothermal features seen in places like Yellowstone National Park, to the large mountain range of the Rocky Mountains we see today. The Rockies offer us an impressive look into what the Earth was like thousands to millions of years ago. Acting like a time capsule, these rocks have preserved the environment of when they were deposited.
For this paper, I would like to focus on just one of the multitude of Geologic features we saw and learned about during our field course. The Northern Rocky Montains have been largely influenced by volcanism over the past 16 million years or so. However, this volcanism is not being produced by a convergent plate boundary, or any plate boundary for that matter. I will not get further into the debate that surrounds what is producing the volcanism in this area, and for the purposes of this paper will just refer to it as a hotspot. This hotspot has existed for a little more than 16 million years, and has been making its way across Idaho and Wyoming to where it now resides in Yellowstone National Park. Along the way, this hotspot has created a multitude of calderas from super volcano eruptions, the hydrothermal features currently on display at Yellowstone, and aided in the process that formed the National Monument, Craters of the Moon.
The volcanics that were discussed on the trip really peaked my interest, however it was Craters of the Moon is what finally brought into focus the scope of events this hotspot can cause. Before going to Craters of the Moon, I had no idea how this formation formed. Because of the two days we spent there, the secondary effects this hotspot can have on an area fully came into focus for me. To begin discussing Craters of the Moon, I would first like to go back and discuss the origins of the Yellowstone hotspot.
Starting between 16-16.5 million years ago on the boarder of Oregon and Nevada, a series of super volcanic eruptions began to occur over a hotspot, which forms when magma near the Mantle-Core boundary begins to rise through weaknesses in the crust and create a magma chamber near the Earth’s surface (IRIS). This hotspot has remained stationary over the entirety of its existence, while the North American Plate has been moving to the southwest at about the pace that fingernails grow.
Craters of the Moon
The magma which comes from the Core-Mantle boundary is Basaltic in nature and therefore has a low viscosity and flows readily. This is the same composition as is seen at the hotspot in Hawaii, where basaltic rock composes the hotspot along with the oceanic crust. This creates the large, non-explosive lava flows that have built the Hawaiian Islands up from the seafloor. Continental crust however, which is laying on the Yellowstone hotspot, is much less dense and more felsic in nature. This Rhyolitic crust is melted as the Basaltic magma from the hotspot moves up through the crust towards the surface. Rhyolitic magma is extremely viscous and resistant to flow, which builds pressure close to the surface. Every 600,000 to 1 million years on average this enormous amount of viscous magma is able to overcome the crust holding it back and be released in an explosive and violent eruption. These eruptions are then usually followed by the Basaltic lava flows that were left behind, like the flows seen on the island of Hawaii. These major eruptions can release up to 2500km^3 or more. The last eruption of Mount Saint Helens erupted 1km^3 of material for comparison. Each of these major eruptions has created calderas dozens of miles in diameter, leaving their mark on the Earth’s landscape, along with dramatically altering the Earth’s climate for years after each eruption. However, one eruption in particular may have played a part in creating one of the most spectacular landscapes in the United States, Craters of the Moon (Field Book).
Craters of the Moon is located just south of the town of Arco, Idaho along the Snake River Plain. The Snake River Plain is a large flat area surrounding the Snake River that curves across the state of Idaho (seen in the picture above). The plain formed as a rift valley that cuts across the Basin and Range of the Rocky Mountains. On May 2, 1924, Calvin Coolidge designated Craters of the Moon to be a National Monument (National Parks). It was given this designation as to become a National Park Calvin Coolidge would have had to have gone through congress for approval. The Monument consists of 83 square miles of land and is made up of lava flows anywhere from 2,000-15,000 years old. The Monument sits along the Great Rift, whose position becomes apparent when looking at the linear alignment of the park’s Cinder Cones (Field Book).
Lava Tube at Craters of the Moon. Source: http://www.travelgumbo.com/blog/idaho-s-craters-of-the-moon-national-monument
Around 8 million years ago the Yellowstone hotspot was right beneath Craters of the Moon. This was a time period of violent and explosive Rhyolitic eruptions in this area. Following this from around 6 million to 15,000 years ago, the original Basaltic magma from the hotspot erupted and produced lava flows up to 4,000 feet thick. Between 15,000 and 2,000 years ago Craters of the Moon experienced The Great Rift. This period included eight major eruptive periods which consisted of roughly 60 lava flows from 25 spatter cones (small, steep cones) and cinder cones (larger in size, not as steep as spatter cones). During this time these lava flows grew to a maximum expanse of 618 square miles and consisted of mainly pahoehoe and a’a lava (Craters).
This image shows a cross-section of the Snake River Plain and Yellowstone Plateau. The Basaltic magma is seen lying below the Rhyolitic magma that was melted by the hotspot. Source: http://volcano.oregonstate.edu/craters-moon
Humans have inhabited the Snake River Plane in Idaho for the past 12,000-14,000 years, at the end of the last Ice Age. Many Native American tribes have called this area home during this time, however it was the Shoshone and Bannock tribes that had the most direct contact with the lava fields. These two groups lived in the area around the same time and often intermingled, travelled and hunted together. These tribes created a legend to try and explain the phenomenon occurring while their people lived at Craters of the Moon. To attempt to explain the volcanic events, the tribes told stories of an enormous serpent, miles in length. This serpent lay where the Snake River channel is now, but one morning the serpent coiled itself around a large mountain where Craters of the Moon now resides. After a couple days, storms moved into the area which enraged the serpent. The large snake began to tighten its coils around the mountain, which crushed the rocks and eventually caused the rocks to melt. This lava began to flow down the large mountain and form the lava flows seen at Craters of the Moon today. The serpent was not able to get away from the lava and was burned, however its remains can be seen in the lava tubes at the Monument (Native Americans).
Rock Rings at Craters of the Moon, unknown use my Native tribes. Source: https://www.nps.gov/crmo/learn/historyculture/native-americans.htm
While Craters of the Moon saw earlier expeditions around its edges and was part of the Oregon Trail as part of Goodale’s Cutoff from 1840-1860, it wasn’t until 1920 that the area received its first major exploration. Nature lover Robert Limbert in 1920 was the first to truly show the enthusiasm and spirit necessary to traverse this region. Mr. Limbert and a friend embarked on a 17 day, 80 mile trek across the Craters of the Moon lava fields. Lambert photographed much of the expedition and named some of the more notable features in Craters, including Big Cinder and Echo Crater. After another expedition in 1921 with ten other scientists, Limbert argued for federal protection of the area’s volcanic features. He wrote a number of newspaper and magazine articles, including a very influential piece in National Geographic in 1924. In this article he described the spectacular landscape he witnessed at Craters. Lambert also sent some of his best pictures to then president Calvin Coolidge, who in the same year made Craters of the Moon a National Monument (History).
Craters of the Moon Today
Craters of the Moon continues to be a US National Monument and one of the youngest, most peculiar Geologic formations in the United States. Today over 200,000 visitors flock to the Monument each year to see the immense lava flows, spatter and cinder cones and lava tubes (Sustainability). The National Park service continues to protect this area so future generations can experience one of the most unique features our country has to offer.
Craters of the Moon is the location that really brought the movement and effects the hotspot has had on the area into focus for me. I never fully understood how large of an affect each explosive eruption and subsequent flows had on an area, outside of the initial major blasts. After visiting Craters of the Moon, I now see the multitude of secondary effects a super volcanic eruption can have on an area, millions of years after the initial blast. The numerous volcanos and lava flows seen at Craters, along with the hydrothermal activity at Yellowstone show the numerous features a super volcano can create. After learning about the complex volcanism associated with this hotspot, I hope to further research this area to figure out just what is creating the magma in this area, and what other features this hotspot is capable of creating.