Pleistocene glaciation impacted the present shape of the Olympic Mountains
Geoscientists from the University of Tübingen investigate the spatial and temporal evolution of movements within the Earth’s crust and associated erosion within this mountain range, situated in Washington State (USA)
The shape of mountain ranges depends on changes in climate and tectonics, the movement of the Earth’s crust. Under the supervision of Professor Todd Ehlers, scientists of the Geoscience Department from the University of Tübingen investigated, how global cooling and glaciation during the Pleistocene about two million years ago have influenced the development of present-day mountain ranges. They picked the Olympic Mountains in northwestern USA for their study location. Their results show that glaciers disturbed the steady-state of the mountain range by significantly increasing the exhumation of rocks due to increased erosion two to three million years ago. Results are published in the scientific journal Geology.
The Olympic Mountains are situated on the Olympic Peninsula within the Olympic National Park of Washington State, where the highest point is Mt. Olympus at 2428 above sea level, surrounded by various glaciers. The mountain range is tectonically active, because it is situated at the Cascadia subduction zone, where the Juan de Fuca plate subducts beneath the North American continent. About 30 new rock samples were collected by the scientists at an elevation of 400 m, covering the area of almost the entire mountain range. During the path from depth to the Earth’s surface over the duration of millions of years, rocks are subjected to cooling. Using sensitive thermochronometric measurements, the scientists can determine the time of cooling. “The cooling ages of the rocks decrease from the edge of the mountain range towards its interior,” says Lorenz Michel, the first author of the study.
Due to the pattern of cooling ages the scientists developed a model, which is able to explain the history of the Olympic Mountains. “The data are best approximated by an ellipse-shaped erosion pattern until the onset of glaciation,” says Todd Ehlers. At the edge of the ellipse rocks are eroded with 0.25 kilometers per million years and at the center of the ellipse even with 0.9 kilometers per million years. However, two to three million years ago, erosion rates must have increased by 50 to 150 percent, compared to the previous period.
“The Olympic Mountains have been in a steady state for a period of several million years: The mountain range grew from the bottom due to the tectonic uplift and was eroded at the same rate from the surface,” explains Michel. That the system got out of balance two to three million years ago, is attributed to the contemporaneous glaciation in the Pleistocene: “The glaciers led to a strong increase in erosion,” says Ehlers. In summary, the spatial pattern of erosion is mostly governed by the tectonic setting, but the temporal variation, on the other hand, is caused by the glaciation during the Pleistocene. “This increase in erosion contemporaneous with glaciation can also be observed in neighboring areas,” the scientist adds.
Lorenz Michel, Todd A. Ehlers, Christoph Glotzbach, Byron A. Adams, and Konstanze Stübner: Tectonic and glacial contributions to focused exhumation in the Olympic Mountains, Washington, USA. Geology, DOI: 10.1130/G39881.1
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Mount Olympus within the Olympic Mountains, peaking at 2428 meters above sea level, Blue Glacier in the foreground. Glaciers like this led to an increase in erosion about two to three million years ago.
Mount Olympus during sun rise.
Photo: Lorenz Michel
The map of the Olympic Peninsula on the left shows the Olympic Mountains in Washington State (USA), where the white triangle denotes the location of Mt. Olympus. Pink lines indicate the extent of Pleistocene glaciation: A Continental ice sheet surrounded the mountain range in the north and east, and in the west alpine glaciers advanced from the center of the mountain range, almost reaching the Pacific Ocean. On the right, the map shows erosion rates calculated by the model. Red circles indicate the location of samples collected for this study. Figures: taken from publication Geology, DOI: 10.1130/G39881.1 (see below).
View from Mt. Angeles in the Olympic Mountains towards the west across the Elwha valley.
Photo: Lorenz Michel
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