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April 25, 2023

Scale is very important to volcanologists - not just for photos, but when thinking about the relative size (or volume) of natural features.

One example of a scale we use is the Volcanic Explosivity Index, which is calculated using characteristics like an eruption's duration, column height, and volume of ejected material. But sometimes scales that work in a geologic context are hard to wrap your head around. For example, what does a cubic meter of tephra look like? How about a cubic kilometer? Or 100 cubic kilometers?

Let's put some volumes into context To start, a cubic centimeter is about the size of a green pea or a chocolate chip. A cubic meter, on the other hand, is about the same volume as a washer and dryer sitting side-by-side. Ten cubic meters is about the capacity of the average concrete-mixing truck or a small dumpster. But that's small stuff when it comes to geology - we often think of volcanic deposits like lava flows in cubic km. For example, a lava dome like Obsidian Dome in Long Valley is about 0.1 km3, works out to the same volume as 10,000,000 cement mixing trucks.

10 million of anything is still hard to visualize, so we'll use something close to home for CalVO: Hangar One, one of the world's largest freestanding structures, which is located near us at Moffett Field. Hangar One was originally built to house blimps, and conveniently, it's just about 1 million cubic meters in volume. That means it would take 1,000 Hangar Ones to equal 1 cubic km, or 100 Hangar Ones to equal the volume of Obsidian Dome.

Pretty big, right? Well, that's nothing compared to the size of an explosive volcanic eruption. A small one like Lassen Peak's 1915 explosions is only about 0.03 cubic km, or the volume of 30 Hangar Ones. For comparison, the 1980 eruption of Mount St. Helens produced 1 cubic km (1,000 Hangar Ones) of deposits, the eruption of Pinatubo in 1991 was around 10 cubic kilometers (10,000 Hangar Ones), and the largest eruptions at Long Valley Caldera 760,000 years ago ejected more than 1,400 cubic km of volcanic debris - or more than 1 million Hangar Ones!

Read more about the VEI and other determinations of eruption size at https://www.usgs.gov/observatories/yvo/news/volcanic-explosivity-index-a-tool-comparing-sizes-explosive-volcanic