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September 20, 2023

Talking about calderas means venturing into superlatives - huge volumes of magma and topographic changes on a regional, even state-sized scale. But how do you put calderas - and the eruptions which form them - into a human perspective?  

A broad, flat plain covered in dull green sagebrush stretches away from the viewer in this panorama. The photo is being taken from a rocky hill, with snow-capped, sharp peaks on the far side of the plain and low raised hills on the right side of the photo. One geologist is sitting on the rocks at lower right and one geologist is standing and giving a peace sign.
This panorama of the Long Valley Caldera, looking from north to south, shows its broad central plain, post-caldera rhyolite flows and uplift on the right, and eastern Sierra Nevada in the background. USGS photo by Jessica Ball.

Take Long Valley Caldera, for example (first photo). Topographically, it's a 12 x 32 km ellipse, accommodating about 2-3 km of subsidence. That results in a geometric volume of about 900 km³. However, the actual caldera ring-fault boundaries (shown in red??? in the second image) are found 3-4 km in from the topographic walls of the caldera, enclosing an oval of 12 x 22 km. This would result in a volume of around 620 km³. roughly corresponding to the estimated 650 km³ of magma erupted when Long Valley was formed about 760,000 years ago.  

A simplified geologic map of the Long Valley Caldera, showing post-caldera rhyolite lava flows in various colors, concentrated on the W half of a long ellipsoidal caldera. The topographic caldera margin is shown in part with a dashed line surrounding the lavas and the structural caldera margin (ring fault), shown with a dotted line.
Map showing distribution of Long Valley postcaldera rhyolites. Abbreviations: CD, Casa Diablo geothermal plant; Ski area, Mammoth Mountain Ski Area; LVEW, Long Valley Exploratory Well, 3 km deep, located high on resurgent uplift; s, surficial deposits filling structural lows on resurgent uplift. Reproduced from USGS SIR 2017-5022-L, Figure 7 (

How do you find a comparison for something this big? You could try with other superlative calderas - Yellowstone, for example, is an 80 x 45 x 0.5 km (1,400 km³) caldera whose largest eruption released around 1,000 km3 of material. Or, if you wanted to look at Toba in Sumatra, you'd be considering a 35 x 100 x 1.7 km (almost 4,700 km³) caldera whose largest eruption released ~5,000 km³ of volcanic debris. So, while Long Valley is big, it's not quite the biggest of the big.  

What about other perspectives? Let's simplify the volume of Long Valley to its eruptive dimensions - around 620 km³. This is about 1/10 the volume of the Grand Canyon (4,170 km³), or 250,000 times the volume of the Great Pyramid of Giza (0.002 km³). Or, for a California comparison, a little over 4 times the volume of Lake Tahoe (151 km³)!

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