One Hundred Hikes in Yosemite
It is possible that the violent rhyolitic eruptions which occurred just north of the Park some 23 million years ago may have resulted in fairly sizable ash-fall deposits within the Park. However, since ash deposits are readily erodible, they don't last unless they are buried by, say, a lava flow. This did not occur, and not surprisingly no remnants of the nearby rhyolitic volcanism have been found in the Park. In the northern Sierra, another period of volcanism-one largely of andesitic and basaltic lavas-began about 20 million years ago and climaxed about 10 to 9 million years ago before waning. Major eruptive centers were located as far south as the lands around the northern Yosemite boundary. Although no major volcano stood within the Park, there was at least one just north of it, which produced volcanic deposits in sufficient quantity to bury much of the Park's lands north of the Tuolumne River. Locally the deposits were as much as 2000 feet thick, this amount based on remnants on and around Rancheria Mountain, northeast of and above Hetch Hetchy Reservoir. As I mentioned earlier under"Sierra Nevada Uplift," small remnants lie deep within the Grand Canyon of the Tuolumne River, indicating that in the vicinity of today's reservoir deposits were at least 4500 feet thick. Indeed, they were thick enough to bury the canyon to its rim, and deposits at one time may have extended as far east as Tuolumne Meadows.
Most of these volcanic deposits were readily erodible and would have been carried away by streams before major glaciers advanced westward from the Sierra crest. What the streams missed, the glaciers got most of. Rancheria Mountain has considerable deposits because its summit area is quite broad and relatively flat, and therefore was little affected by stream erosion, and because it stood above the glaciers. Fortunately, glaciers failed to remove all the deposits, allowing us to reconstruct the topography here some 9 to 10 million years ago.
While voluminous volcanic deposits were burying most of the Park's northern lands, a small volcano began erupting about 9.4 million years ago along the Tuolumne River about a mile upstream from Glen Aulin. Today all that remains of it is part of its resistant core, or plug, and because its columnar lava resembles that in Devils Postpile National Monument, it carries the unofficial name of Little Devils Postpile. This plug, beside the south bank of the river, is important because it shows that in the last 9.4 million years the river has incised only a few feet through the resistant, massive granite that the plug rises from. Giant glaciers repeatedly flowed through this area, yet-as elsewhere in the Sierra Nevada-they were impotent at eroding through such resistant bedrock. In addition to the Little Devils Postpile volcano there has been only one other late-Cenozoic eruption in the Park. This is a vent that produced a small basalt flow about 3 1/2 million years ago just south of Merced Pass, close to the Park's southeast boundary. Beyond the boundary, in the heavily glaciated San Joaquin River drainage, are abundant, larger flows of similar age, and like the Little Devils Postpile volcano, they testify to glaciers' inabilities to erode through the resistant granitic bedrock they rest on.
Faulting often accompanies volcanism, and since volcanism within the Park has been minor, so has faulting. The only identified faulting is in the vicinity of the Snow Lake pendant, which is near the southern end of major late-Cenozoic volcanic eruptions. Several faults appear to have been active there about 9 million years ago and possibly later. The greatest faulting, however, lies east and southeast of the Park, in Mono Basin and Long Valley, where major volcanic activity began 3 1/2 million years ago, as did major faulting. Both are active in the area today, presenting geological hazards to the communities of Lee Vining, June Lake, and Mammoth Lakes.
Details mentioned in this article were accurate at the time of publication