At Gresik on eastern Java, natives decided that the blasts were the “supernatural artillery” of the venerated South Java Sea spirit queen Nyai Loroh Kidul, fired to celebrate the marriage of one of her children; the ash was “the dregs of her ammunition.” If so, her ammunition made most of April 12 utterly dark in the village. When the British resident in Gresik awoke that morning, he had the impression that he had slept through a very long night. Reading his watch by lamplight, he discovered that it was 8:30 A.M., and pitch-black outside from the cloud of ashes descending. He breakfasted by candlelight at 11:00 and thought he could see a faint glimmering of light, but at 5 P.M. he still could “neither read nor write without candle.” In the nearby village of Sumenep, ash fell about two inches thick, and “the trees also were loaded with it.”
A tsunami reached eastern Java around midnight on April 10–11, and tremors struck the central region of the island eighteen hours after the eruption. Between two and three in the afternoon of April 11, a European observer in the village of Surakarta (Solo) noticed “a tremulous motion of the earth, distinctly indicated by the tremor of large window frames; another comparatively violent explosion occurred late in the afternoon.… The atmosphere appeared to be loaded with a thick vapour: the Sun was rarely visible, and only at short intervals appearing very obscurely behind a semitransparent substance.” Surakarta remained in darkness for much of the following day, as well. Raffles, too, reported that even at a distance of eight hundred miles, “showers of ashes covered the houses, the streets, and the fields, to the depth of several inches; and amid this darkness explosions were heard at intervals, like the report of artillery or the noise of distant thunder.”
Twenty-four hours after Tambora erupted, the ash cloud had expanded to cover an area approximately the size of Australia. Air temperatures in the region plunged dramatically, perhaps as much as twenty degrees Fahrenheit. Then a light southeasterly breeze sprang up, and over the next several days most of the ash cloud drifted over the islands west and northwest of Tambora. By the time the cloud finally departed, villages within twenty miles of the volcano were covered with ash nearly forty inches thick; those a hundred miles away found eight to ten inches of ash on the ground.
Even a small quantity of ash could devastate plants and wildlife. One district that received about one-and-a-quarter inch of ash discovered that its crops were “completely beaten down and covered by it.” Dead fish floated on the surfaces of ponds, and scores of small birds lay dead on the ground.
By the time the volcano finally subsided, Tambora had released an estimated one hundred cubic kilometers of molten rock as ash and pumice—enough to cover a square area one hundred miles on each side to a depth of almost twelve feet—making it the largest known volcanic eruption in the past 2,000 years. Geologists measure eruptions by the Volcanic Explosivity Index, which uses whole numbers from 0 to 8 to rate the relative amount of ash, dust, and sulphur a volcano throws into the atmosphere. Like the Richter Scale for earthquakes, each step along the Explosivity Index is equal to a tenfold increase in the magnitude of the eruption. Tambora merits an Index score of 7, making the eruption approximately one thousand times more powerful than the Icelandic volcano Eyjafjallajökull, which disrupted trans-Atlantic air travel in 2010 but rated only a 4; one hundred times stronger than Mount St. Helens (a 5); and ten times more powerful than Krakatoa (a 6). Only four other eruptions in the last hundred centuries have reached a score of 7. Modern scientists identify and measure past eruptions using layers of volcanic debris found in ice cores, lake sediments, and other undisturbed soils. Each eruption has a distinct chemical signature that, along with conventional methods of carbon dating, can be used to associate each layer of volcanic material with a particular eruption.