Tuolumne meadows in Yosemite national park, California, is a rock climber’s dream. Renowned for its soaring granite domes this broad expanse of alpine meadow is a haven from the park’s tourists in the summer months.
But the flat terrain, surrounded by those vertical, glacially sculpted, rock faces, is also a puzzle. The tough rock walls are made from the exactly the same type of granite that underlies the meadow. So why did the domes stand firm, while glaciers carved out the meadow with ease?
Differing rock types often explain big features in the landscape; harder rocks stand proud, softer rocks are eroded away. But at Tuolumne meadows it is the cracks in the rock that are key.
Writing in the journal GSA Today, Richard Becker, from the Wisconsin University and colleagues describe how the granite of the domes contains very few fractures, while the granite underlying the meadows has a cluster of vertical parallel fractures.
This uneven distribution of fractures appeared about 85m years ago, when a volcanic intrusion forced its way through the granite.
The areas where the magma squeezed through became the most fractured. In more recent times (the last few tens of thousands years) glaciers etched out this climber’s playground.
Ice flowing parallel to fractures carved out deep, straight sided, canyons (like the Tenaya canyon), but ice flowing perpendicular to fractures quarried large open expanses (Tuolumne meadows), leaving behind steep ridges of less fractured rock (Lembert dome).
The hypothesis would have pleased the geologist François Matthes, who came to a remarkably similar conclusion 84 years ago.
