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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?

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.

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)!