Yosemite's Living Glaciers
Despite global warming, a few small glaciers remain in Yosemite just below the crest of the Sierra. Scientists call them “living” glaciers because they continue to creep downhill. The ice sheets are products of the Little Ice Age, a period of mild, worldwide cooling that occurred from the 16th to 19th centuries.
Lyell Glacier, first climbed by John Muir in 1871, lies on the north facing slopes of Mount Lyell, the highest peak in national park at 13,114 feet. Lyell is the second largest glacier in the Sierra Nevada and the largest in Yosemite. Maclure, the second largest glacier in the park, is sliding down Mount Maclure at 20 feet per year and has an ice cave that allows intrepid hikers to glimpse underneath the glacier, where its assault on the bedrock is exposed.
Are Yosemite's Glaciers Melting?
Both Lyell and Maclure glaciers have receded in the past century, and glaciologists expect them to disappear completely within a few decades as the result of natural melting and human-caused climate change.
Lyell Glacier has had a spur of news activity since it was measured in 2015 and compared to the 1883 size. The article Glacier was once Yosemite’s largest; now it’s almost gone” by Tom Stienstra in the San Francisco Chronicle birthed many similar articles warning of impending doom to our ecosystem from global warming/climate change.
On a geological expedition in 1883, Israel Russell took a photo of Lyell Glacier, when its total volume was measured at 1.2 million square meters. In 2015, Keenan Takahashi took a photo from the same spot. In 132 years, Lyell Glacier went from 1.2 million square meters to 270,426 square meters, losing 90 percent of its volume and 80 percent of surface area.
In the 2015 NPS report, Yosemite's Melting Glaciers, Greg Stock and Robert Anderson who spent four years studying the glaciers reported that "the impending loss of the Lyell and Maclure glaciers may have profound impacts on ecosystems in and adjacent to the upper Tuolumne River."
History: Ice Rounded Granite into Domes
The dramatic landscape of Yosemite began forming about 65 million years ago, when molten, igneous rock solidified into granite deep within the Earth and was pushed up under pressure to the surface. The granite was shaped into domes (Yosemite has more than any other place on the planet) as the uplifted, curved layers of rock cleaved off. This process of progressive rounding, called exfoliation, continues today. These domes and their cousins, roches moutonnée (meaning “sheep-back rocks”) are visible throughout Yosemite, but most obviously in Yosemite Valley and Tuolumne Meadows.
As the Ice Age ramped up some two million years ago, the Yosemite region was covered by glaciers that inched down river courses and carved U-shaped gorges. But much of the underlying granite resisted glacial erosion, and formations like El Capitan, Royal Arches and Half Dome were left standing in bold relief to the river valleys.
The glaciers that sculpted Yosemite peaked an estimated 20,000 years ago, which is fairly recent in geologic time. When the last of these glaciers melted in the Sierra high country, they left behind dozens of basins and lakes. Some of these lakes remain today (such as Dog, Gaylor and Elizabeth) while others have filled in with sediment and transformed into meadows or forests. Tuolumne Meadows, for example, was once a glacial lake. While high-country lake levels generally remain consistent, meadow areas may still be flooded during periods of spring snowmelt.
Ice Left a Rock Dam which Created Yosemite Valley
The repeated episodes of glaciations removed soil, scraped the bedrock underneath, and carried away tons of debris. When the larger glaciers disappeared, rockfalls continued to transform the cliffs, as did the more subtle erosional forces of rivers and creeks.
Meanwhile, the remarkably level and lush bottom of Yosemite Valley was created when the last glacier retreated some 10,000 years ago and left behind a pile of rocks, called a moraine, that formed a dam. A lake emerged that slowly filled in with sand and gravel and leveled out the Valley floor. Meadows, forests and the current Merced River channel eventually replaced Yosemite Lake, which diminished with the end of the Ice Age.