Is here a correlation between the size of an eruption and the amount of time since the volcano last erupted?



Yes, in general, the historic data shows a pattern of larger eruptions being associated with longer repose periods. Size of eruptions are based on the Volcanic Explosivity Index. Repose period is the time interval between volcanic eruptions. For non-explosive (VEI=0), small (VEI=1), moderate (VEI=2), and moderate-large (VEI=3) eruptions the repose period is less than 10 years and only a small percent of the eruptions produce fatalities. In contrast, very large eruptions (VEI=5) and even larger eruptions (VEI>6) have repose periods of hundreds or thousands of years. About 4 out of 10 very large eruptions (VEI=5) cause fatalities and all VEI=6 eruptions cause fatalities.


VEI Description Plume Height Volume Classification How often Example
0 non-explosive < 100 m 1000s m3 Hawaiian daily Kilauea
1 gentle 100-1000 m 10,000s m3 Haw/Strombolian daily Stromboli
2 explosive 1-5 km 1,000,000s m3 Strom/Vulcanian weekly Galeras, 1992
3 severe 3-15 km 10,000,000s m3 Vulcanian yearly Ruiz, 1985
4 cataclysmic 10-25 km 100,000,000s m3 Vulc/Plinian 10's of years Galunggung, 1982
5 paroxysmal >25 km 1 km3 Plinian 100's of years St. Helens, 1980
6 colossal >25 km 10s km3 Plin/Ultra-Plinian 100's of years Krakatau, 1883
7 super-colossal >25 km 100s km3 Ultra-Plinian 1000's of years Tambora, 1815
8 mega-colossal >25 km 1,000s km3 Ultra-Plinian 10,000's of years Yellowstone, 2 Ma

A key observation is that long periods pass between events at volcanoes that tend to have violent eruptions. This means that there may be no written record of violent behavior at a mountain that now looks peaceful. It also explains why some of the largest explosive eruptions in the last two centuries were at dormant volcanoes. The first historical eruptions at Tambora, Galunggung, Tarawera, Santa Maria, Novarupta/Katmai, Bezymianny, El Chichon, and Pinatubo were large and explosive. It also highlights the need for field mapping and accurate dating of prehistoric events.

The pattern of larger eruptions being associated with longer repose periods is probably the result of processes that operate in the magma chamber. Over time, basaltic magma becomes more rich in silica and gases due to crystal settling, assimilation of crust, and mixing with more silica-rich magma. These igneous processes can operate over hundreds or thousands of years before gas concentrations are high enough to trigger an eruption. Dacite and rhyolite, magmas with relatively high silica and gas contents, commonly produce large explosive eruption.

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