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The world's largest eruption of the 20th century occurred in 1912 at Novarupta on the Alaska Peninsula. An estimated 15 cubic kilometers of magma was explosively erupted during 60 hours beginning on June 6th. There are many historic reports of ashfall from the eruption in the Pacific Northwest.
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Fierstein, 2012, National Park Service (Public domain.)
Passengers on a southbound steamer described their account in a press dispatch, dated Monday, June 10th: "Vancouver, B.C., June 10. – Passengers on the steamer Camosun, which arrived this morning from Prince Rupert, witnessed a wonderful phenomenon all the way down until within 100 miles of Vancouver. A steady shower of ashes, presumably from the volcanic eruptions in Alaska, began falling into the sea and on to the vessel's deck soon after clearing. ... The ashes obscured the sun and made everything hazy. Every foot of the steamer was covered, and extra men were assigned to sweeping it away. No sooner was it removed than a fresh supply fell from the sky. We were not aware of the volcanic disturbance in Alaska until we steamed into Vancouver." (Griggs, 1922).
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A widely accepted measure of the size of an eruption is the volume of lava ejected as pumice and ash (tephra) during an explosive phase or the volume of lava extruded during an effusive phase. Eruption volumes are commonly expressed in cubic kilometers (km3). One km3 is roughly equivalent to 0.24 cubic mile. Estimates of the tephra volumes are usually obtained by mapping the distribution and thickness of the tephra deposits on the ground after the eruption is over. Tephra volumes measured in this way must then be corrected for void spaces (bubbles within the pumice, empty spaces between individual chunks of pumice or ash) to get an estimate of the original volume of lava erupted. This correction can be made by comparing the bulk density of the tephra deposit with the known density of the rock-type that makes up the tephra. The result is referred to as the "dense rock equivalent" or DRE of the erupted volume. The diagram shows sizes of a few example eruptions. In any given time period, there are many more small eruptions than there are large eruptions and the largest caldera-forming eruptions (Long Valley Caldera, Yellowstone, and Toba) are very infrequent. The smallest eruption shown is the 1915 Lassen Peak eruption with an estimated DRE volume of 0.006km3. The largest eruption shown is the 74,000 years ago Toba eruption in Sumatra with a DRE volume was about 2,800 km3. The eruption sizes were drawn by solving for the radius of a sphere using the eruption volumes shown. (Public domain.)
Definite records of falling dust were made by a number of observers in the Puget Sound region, which is approximately 1,500 miles (2,400 km) southeast of the volcano (Griggs, 1922). The ashfall was detected at Tatoosh Island, Washington where weather observer J.E. Hissong reported on June 10th: "Light deposit of fine white ash found on southwest side of storm-warming tower at noon. Evidently carried by wind from volcanic explosions in southwestern Alaska"; and three days later: "During the evening of the 13th the air was filled with a very fine ash and smoke, a light deposit being noticed on the office furniture on the morning of the 14th." (Griggs, 1922).
Port Townsend, June 11. – Sulphuric acid is present in the air in quantities sufficient to blacken the brass work on automobiles. The owners of cars noticed the presence of a peculiar dull-colored corrosion on all brass work. The brass work was repeatedly cleaned but the deposit would quickly return. The deposit is not hard to remove, being in the form of thin dust, but when removed, leaves the brass work slightly colored, requiring some little trouble to polish. (Griggs, 1922).
Vancouver, B.C., June 12. – Scores of housewives in Vancouver were sufferers because acid, presumably from the Alaskan eruptions, permeated their fine white clothes hanging on the lines after the Monday washing. Linen goods were apparently the only ones attacked. When this white wear came to be ironed it simply fell to pieces, and not until many complaints had reached dry goods houses today were the women able to solve the problem (Griggs, 1922).
The effects of the eruption were felt far beyond the zone of ashfall. They were in fact world-wide. The fine dust carried into the upper atmosphere formed a haze which so reduced the intensity of sunshine as to cause the cold summer of 1912, throughout the northern hemisphere (Griggs, 1922).
Written observations and solar radiation data for 1912–14, principally from Europe and North America, suggest that atmospheric dust had completely disappeared by October 1912 but that colorful twilights persisted at least until December 1913. North of 44° N latitude, the mean atmospheric residence time was 9–12 months and that stratospheric aerosol remained significant at least as late as December 1914 (Hildreth and Fierstein, 2012).
Alaska Peninsula map showing some of the of volcanoes, with Novarupta ash fall compared to that from recent eruptions. The volume of volcanic ash from Novarupta was more than from all other historical Alaska eruptions combined. Ash from seven prehistoric eruptions, all younger than 6,000 years and approaching the volume of the 1912 event, are found within 500 miles (800 km) of Anchorage, including ash from Hayes volcano (Bacon, et al., 2012) (Public domain.)
Scrutiny of ice-core records in Greenland suggests that acid fallout from the Katmai eruption lasted ~1.5 years there (Hildreth and Fierstein, 2012).
Surface cooling due to acid aerosol in the stratosphere was recorded widely in the Northern Hemisphere during summers of 1912 and 1913, reaching a maximum measured deficiency of –0.9° C for September 1912. Average daily deficiency for the whole Northern Hemisphere for the 16-month interval following the eruption was reported to be –0.16° C (Hildreth and Fierstein, 2012).
References
Bacon, C.R., Bennett, A.J., Bennington, N., Berg, E.E., Brooks, M., Coletti, H.A., Coombs, M.L., Fierstein, J., Freeburg, G., Frost, G.V., Haney, M., Jorgenson, M.T., Miller, A.E., Moran, S., Murphy, R., Partnow, P., Paskievitch, J., Pinney Stevens, D., Powell, L., Power, J., Prejean, S.G., Schaaf, J., Sherriff, R.L., Thurber, C., Welchman, R.A., 2012, Volcanoes of Katmai and the Alaska Peninsula, Alaska Park Science Journal, Volume 11 Issue 1 Katmai Scientific Studies, available at: https://www.nps.gov/akso/nature/science/ak_park_science/volume_11_issue_1.cfm, accessed 05/25/2012.
Griggs, R.F., 1922, The Valley of Ten Thousand Smokes: Washington, The National Geographic Society, 341 p.
Hildreth, W., and Fierstein, J., 2012, The Novarupta-Katmai eruption of 1912—largest eruption of the twentieth century; centennial perspectives: U.S. Geological Survey Professional Paper 1791, 259 p.
