Chronicles of Caldera: Not all ashes fall from the sky | Wild montana
Chronicles of the Yellowstone Caldera is a weekly column written by scientists and collaborators at the Yellowstone Volcano Observatory. This week’s contribution is from Robert Thomas, professor of geology in the Department of Environmental Sciences at the University of Montana Western.
When we think of the Yellowstone hotspot, we usually think of the volcanic scars left when the North American Plate slowly passes southwest over a stationary heat source. However, the deposits in the margins of the hotspot runway are among the most useful in reconstructing how the caldera eruptions work and how this hotspot produced the complex landscape we see today.
When the Yellowstone hotspot began to feed eruptions near the intersection of Nevada, Idaho and Oregon about 16.5 million years ago (around the same time as the eruptions of the basalts of the Columbia River), it thermally swelled and extended the brittle crust that surrounded it, influencing the shapes of mountain ranges and valleys. About 16.5 to 4.5 million years ago, the basins captured rivers, some of which flowed northeast into southwestern Montana. Preserved in these sedimentary deposits is a spectacular tale of a tumultuous time in southwestern Montana. Let’s explore it.
For millennia, people have undoubtedly recognized the layers of shiny white ash exposed in the mountain ranges of southwestern Montana. Unlike many other ash deposits, these did not fall from the sky, but were carried from Idaho by gigantic streams of water and ash that flowed from the Yellowstone thermal bulge and along the ancestral drainage of the Missouri River. Viscous flows, called lahars, accumulated in the northeast trending valleys created during the emergence of the hot spot, burying the valley bottoms in up to 30 meters of ash and rocky debris. It appears that the lahars arrived on a pulse, as the ash beds contain tabular blocks of ash that settled, quickly hardened, and was then peeled and carried downstream by another pulse of ash and water. It is difficult to determine the time interval between pulses, but it was probably no more than a few hours to a few days. After the deposition, grasses grew on the ashes, creating soils with abundant shoots of roots.
The origin of the ash is easy to explain given the explosive, silica-rich magma that erupts to form the calderas of the Yellowstone hotspot, but what is the source of all this water? Calderas are large depressions in the earth’s crust and are wonderful places for the accumulation of water and the formation of large lakes, like Yellowstone Lake in Yellowstone National Park. This lake covers 136 square miles (350 km2) and has an average depth of 139 feet (42 m). If such a volume of water were to pierce the rim of its caldera, perhaps during the formation of a new caldera nearby, it would generate floods of ash and water that would likely travel great distances down the drainages. Release. Some of the ash deposits can be traced from Lima, Montana, as far north as Craig, Montana, about 200 miles (321 kilometers) northeast.
As the North American Plate moved Montana southwest and the hotspot moved below the crust south of Rexburg, Idaho, about 6.5 million years ago, a basalt lava flow erupted and flowed in a northeast trending valley to Dillon, Montana. . This flow, known as Timber Hill Basalt, was close to the last deposit that reached Montana from the hotspot, as about 4.5 million years ago the region’s thermally elevated terrain began to collapse, forming extension valleys trending northwest and mountains that blocked and diverted the ancestral Missouri River into a maze of new valleys trending northwest and northeast. It may be that in an instant a northwest trending fault escarpment was lifted on the way to the old Missouri River by a powerful M7-8 earthquake, diverting the water and leaving the fish to collapse. on a dry river bed.
Today, we are convinced that most of the extending southwestern Montana topography has formed over the past 16.5 million years and that the active ranges tend north-west and have formed over the past 4.5 million years. The faults that lift the northwest-trending mountains are active, as shown by the 1959 Hebgen (Quake Lake) earthquake, the largest earthquake recorded in the Rocky Mountains. So, as you drive through southwest Montana, look for the layers of shiny white ash exposed high in the mountain ranges and gaze at it was once deposited in the ancestral Missouri River as it flowed. from the thermal bulge of Yellowstone millions of years ago.