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While the huge caldera-forming eruption of Long Valley was 767,000 years ago, between about 670,000 and 570,000 years ago uplift of the caldera's floor formed a large mound. This is called a resurgent dome, a surface feature created by underground volcanic activity.

View looking northeastward across Long Valley Caldera to Glass Mountain, about 30 km away. In the foreground, steep cliffs cling to a mountainside and are scattered with tall pines. In the middle ground, a broad flat valley is cut from one side of the image to the other by a long, low set of forested hills. In the background, a long straight rangefront of mountains borders the valley. Image from USGS Professional Paper 1692.
View looking northeastward across Long Valley Caldera to Glass Mountain, about 30 km away. The tree-covered resurgent dome, with Gilbert Peak at left, rises in the middle of the caldera.

Long Valley's is near the center of the caldera, measures  10 km (6.2 mi) in diameter, and stands 500 m (1,640 ft) high. Most of the dome is made of rhyolite that erupted in the caldera during the 120,000 years post-collapse, but around the dome's margins are Pleistocene lake-formed sandstones and mafic lava flows. The high point of the resurgent dome is Gilbert Peak, 2,626 m (8,615 ft) above sea level. 

Scientists have calculated that most of the uplift probably occurred over 100,000 years at an average of 4 mm/yr (0.16 in/yr), but between 1980 and 2017 there was been more than 83 cm (33 in) of uplift (a rate of 24 mm/yr or 1 in/yr). During one episode from 1997-1998, the rate of uplift was as high as 18 cm/yr (7 in/yr), and came with more than 130 M3+ earthquakes! Another uplift episode which began in 2011 continues today, though at a much smaller rate of 1.5 cm/yr across the dome.

But what drives all this uplift? Several reasons have been proposed, including an inflating magma reservoir some 7 km below the dome, as well as hydrothermal fluids accumulating under an impermeable layer of rock at the top of the magma storage region. One theory even suggested that the *loss* of water during California's 2011-2016 drought could have reduced overlying pressure on the magma storage and enhanced inflation. Or, it could be some combination of all three. Despite its recent uplift, Long Valley's resurgent dome may never be the site of an eruption - just another feature of an active volcanic system!

To learn more about Long Valley's resurgent dome, check out this field trip guidebook:, as well as CalVO's website:

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