Note: This is the fourth in a series of six Guest Opinions commemorating the Craters of the Moon National Monument Centennial While southern Idaho’s vast Snake River Plain is lauded and renowned among geology afficionados for its fantastic volcanic landscapes, Craters of the Moon National Monument stands apart as its single exemplary feature. Boasting the largest young basaltic lava field in the contiguous United States, the monument is a volcanic wonderland of cones, vents, fissures, and flows. Though eruptions that forged this impressive landscape occurred thousands of years ago, southern Idaho’s dry climate and slow weathering rates left this surreal landscape looking nearly as fresh as the day the lava burst from the Earth. Craters of the Moon Monument & Preserve encompasses a huge region of about 1,200 square miles and includes two separate geologically young lava fields: the main Craters of the Moon lava field in the north and the Wapi/Kings Bowl lava fields near the monument’s southern boundary. Most of the volcanic landforms within the monument formed during eight eruptive periods over the past 15,000 years, with the most recent eruption occurring a mere 2,000 years ago. Each eruptive period was separated by periods of inactivity that lasted a few hundred to as many as 3,000 years. The frequency of eruptions over the past 15,000 years and the age of the youngest eruption indicates that future outpourings of lava in this area are expected and anticipated. Many of the cones and fissures in the monument are aligned in a northwest-southeast direction, forming a nearly 60-mile-long chain of vents across the Snake River Plain known as the Great Rift. The trend of these volcanic vents closely matches the orientation of faults north and south of the plain. These faults are caused by roughly east-west stretching of the Earth’s crust and have also produced recent historic earthquakes, such as the 1983 Borah Peak quake and the 2020 quake near Stanley. The entire monument is composed of a singular rock, basalt, the dark gray to black volcanic rock ubiquitous across the Snake River Plain. However, this one rock type produced an array of fascinating textures, landforms, and features for which the monument is known. Eruptions in the Craters of the Moon lava field generally consisted of two main varieties: locally small, explosive eruptions where gas-rich clots of lava (called cinders) were ejected from the vent, piling up to form symmetrical or rounded hills called cinder cones, and nonexplosive eruptions of hot, runny lava from elongated fractures called fissures. Some of the lava flows in the Craters of the Moon lava field flowed as much as 30 miles from their vent. Lava channels that crusted over became lava tubes, effectively transporting lava significant distances downhill until the lava supply waned, whereupon the tubes were mostly emptied, forming enticing caves to explore such as Indian Tunnel Cave and Dewdrop Cave. Some of the cinder cones at Craters of the Moon are downright odd. While a textbook cinder cone is steep, cone-shaped, and has a summit crater where the erupted cinders piled up around the vent, some cones in the monument, such as Inferno Cone and Broken Top, are noticeably less steep, broader, and lack a summit crater. The reason? Southern Idaho’s strong and persistent winds. During these eruptions, the volcanic vent blasted out cinders which were carried downwind and accumulated nearby, forming a broad mound of cinders without the hallmark summit crater adjacent to the exposed vent. Another remarkable feature of several of the cinder cones at Craters of the Moon is that they are incomplete. Cones like North Crater and Half Cone are missing portions of their flanks due to lava oozing from the cone’s base and tearing house-sized chunks of the cone away. As you approach the main visitation area of the monument from Carey, these large blocks are obvious on the lava landscape. I’ve led hundreds of field trips throughout Idaho, the western U.S., and abroad. On nearly every trip, I begin by explaining that any given location possesses two distinct geologic stories: the story of how the rocks formed and the story of the landscape. For example, the Grand Canyon’s rocks are as old as 1.8 billion years, and mainly involve the deposition of thousands of feet of sediment, while the landscape of the canyon is far younger, resulting from downcutting and erosion over the past six million years. Conversely, Craters of the Moon is one of the few places where the story of the rocks and the story of the landscape are one and the same: a dynamic volcanic landscape largely unchanged since it formed. In a vast and spectacular state known for its exceptional scenery and varied geology, Craters of the Moon National Monument proudly stands out as one of its signature features. We are truly lucky to have it. Happy 100th, Craters! Shawn Willsey is a Geology Professor at the College of Southern Idaho |
Last updated: September 25, 2024