Krakatau

Residents of Sumatra and Java thought that the quiet volcano Krakatau (also spelled Krakatoa) was extinct until 1883, when it violently erupted, destroying three hundred villages and generating sea waves that killed more than 36,000 people. Perhaps the loudest noise in recorded history, the paroxysmal eruption was heard more than 4,600 kilometers away, and dust from the explosion reddened skies around the globe.

Early Observations

Before 1883, the island Krakatau was noteworthy only as a landmark to the hundreds of ships sailing from the Indian Ocean through the high-traffic sea lane of Sunda Strait to the ports of Singapore and Jakarta in the China Sea. The 9-kilometer-long island, formed by the volcanoes Perboewatan, Danan, and Rakata, was the largest of four that marked the rim of a prehistoric volcanic crater. Java, to the north of Krakatau, contains forty-nine volcanoes, more than half of which have erupted in the past three hundred years or so. Sumatra and the smaller islands of Bali, Lombok, Timor, and Flores also are packed with volcanoes and accompanying geysers and hot springs. There is little argument that Indonesia has the highest concentration of volcanic activity in the world.

Yet, the possibility that Krakatau was capable of volcanic eruption was ignored by the Dutch and other European colonists of the late nineteenth century. Unlike the neighboring island of Sebesi, Krakatau, a crater rim segment, was not an easily recognized cone-shaped volcano. Forgotten was the historical account of an eruption from two hundred years earlier: a roar like thunder, earthquakes, and a burned forest. Ignored were the legends of fifteen centuries earlier, the Javanese chronicles, which describe an enormous eruption and accompanying fires, shaking, and fearful roars from the mountain called Kapi. This same account explains the division of a single pre-eruption island into Sumatra to the north and Java to the south by the rising ocean that now ran between them as the Sunda Strait. The ancient chronicles are supported by contemporary geologic evidence. Volcanic deposits from the nearby Java Trench and the huge crater formed by Krakatau and neighboring islands imply the existence of an earlier gigantic volcano about one-half the size of Rhode Island.

Accounts of volcanic activity on Krakatau before the 1880s are sparse, but eyewitness accounts of the eruptions of August 26 and 27, 1883, and studies of the aftermath are extensive. Passing ships and local residents detail the deafening sounds, the darkness, falling pumice, choking ash, bolts of lightning, the eruption itself, and the subsequent destruction by 12- to 40-meter-tall tsunami waves. The tremendous noise, startling red sunsets, and other unusual phenomena were heard and seen around worldwide.

Krakatau's Reawakening

Krakatau began its return to life on May 20, 1883, when it ejected pumice, steam, and ash. During the next three months, residents became accustomed to the continual artillery-like booms that rattled doors and windows, the shaking from earthquakes, and the views of molten lava and columns of dust. Observers from an exploration party to the island witnessed new vents on the crater floor of Perboewatan, the smallest of the three volcanoes.

At 1:00 pm on Sunday, August 26, the first of a series of violent explosions shook the volcano. Within one hour, a 27-kilometer-high black cloud hid Krakatau from view. For the next twenty-three hours, eruptions burst forth every ten minutes and grew more ferocious through the night. Passing ships reported hellish conditions. From 65 kilometers away, the schooner Sir Robert Sale witnessed a great vapor cloud, lit by bursts of forked lightning, “like a large serpent rushing through the air.” On the nightmare journey past Krakatau, Captain Watson of the Charles Bal kept a log detailing falling hot pumice stones and the buildup of muddy ash that kept his crew shoveling for their lives. If a sailor paused in his duty, he became more aware of the hot, choking sulfurous fumes and the almost supernatural pink glow, called St. Elmo's fire, on the rigging above. On the horizon, sailors observed flashes of lightning jumping back and forth from Krakatau to the sky.

The volcano's eruption was not yet over. At 5:30 am on Monday, Krakatau radically changed eruption styles in a grand finale of four explosions. The third explosion, at 10:02 am, brought down the island. Where there had been several peaks, the highest at 4,267 meters, a gaping crater, 300 meters below sea level, appeared. That was not known, however, until twenty-four hours later, when the pitch-black darkness, produced by about 9 cubic kilometers of ash and rocks ejected an estimated 80 kilometers into the atmosphere, lifted.

However, two-thirds of the island's disappearance was not the main subject of concern. The explosions, equivalent to 1.5 million tons of dynamite, generated sea waves (tsunamis) that drowned more than 36,000 people and airwaves that produced the loudest sound in human history. The largest sea wave, cresting at 40 meters, or the height of a twelve-story building, swept away the coastal town of Merka in Sumatra one hour after the explosion. This first giant wave—and nine others—drowned about three hundred towns and villages on Java and Sumatra. Ships close to shore, such as the gunboat Berouw, were lifted out of the water and carried 3 kilometers inland to lay stranded 9 meters above sea level. Blocks of coral weighing 600 tons were carried 11 kilometers inland. The waves circled the globe twice, rocking ships in South African harbors 8,000 kilometers away as they passed.

The Loudest Noise Ever Heard

Continual thunder deafened Java and Sumatra residents before and immediately after the colossal 10:02 am explosion. During the later eruptions, the nearby islands, thickly blanketed in ash, heard nothing, even though windows and walls were blown out. Yet, loud explosions were heard on Rodriguez Island, 4,800 kilometers to the west, in Australia, 3,200 kilometers to the south, and in places in between, such as Celebes, Sri Lanka (formerly Ceylon), and the Philippines. People on more than one-thirteenth of the Earth's surface, about 40 million square kilometers, heard the noise. At no other time in recorded history has a sound been heard at such a distance without technological amplification. By comparison, if Krakatau had exploded in Denver, Colorado, residents from Alaska to Florida would have heard the noise. Pressure differences on barographs at several widespread locations recorded seven distinct airwaves bouncing back and forth up to five days later.

During nearly twenty-four hours of the eruptions on Sunday and Monday, ash completely darkened the area within 200 kilometers of Krakatau. Ships as distant as 6,000 kilometers to the northwest swept ash from their decks. Clouds of ash, an estimated 8.5 cubic kilometers in volume, spewed 50 kilometers into the stratosphere, where they were transported globally in the jet stream. This volcanic veil of dust caused spectacular atmospheric effects around the Earth, lasting for three years. Exceptionally colored and prolonged sunsets visible to three-quarters of the world population became common; the sun, and sometimes the moon or Venus, often appeared blue or green; and a white or bluish-white corona, called Bishop's Ring, circled the sun. These finely ground ash particles created more than the gaudy optical displays. The ash became a filter to solar radiation and reduced heat reaching the Earth by 1 percent. Global temperatures, averaging 0.05 degree Celsius lower in 1884, decreased the British growing season by one week. For the next several years, Krakatau's environmental impacts continued to include reduced solar radiation, resulting in lower-than-average global temperatures.

All Bottled Up

Krakatau resembles Mount Vesuvius in the Mediterranean and Mount St. Helens in the western United States in the properties of the lava emitted, the explosiveness and collapse into an empty magma chamber, and the enormous amount of ash and other ejecta shot into the atmosphere. Krakatau blew out pyroclastics that welded into a type of rock called ignimbrite, rich in silica. An estimated 18 to 21 cubic kilometers of ignimbrite and 8.5 cubic kilometers of ash poured out during the eruptions. Unlike the Hawaiian volcanoes' more easily flowing basaltic lavas, high-silica lava is thick enough to plug up the vent behind itself and build up pressure in volcanic gases unable to escape.

At the time of the eruption, mining engineer R. D. M. Verbeek, sent to research the volcano, conjectured that the exceptional explosiveness of Krakatau was caused by seawater seeping into a collapsed magma chamber, where it was converted to steam. Later twentieth-century studies argue that significant amounts of seawater did not enter the vent, which had not collapsed during the early stage of the eruptions. Scientists suggest that a partially solidified plug developed to block the vent. Pressure built up and eventually exploded into the Monday series of four eruptions, each producing a pyroclastic flow into the surrounding sea.

One significant remnant of such an explosion is the giant caldera formed by the volcano's collapse into the drained magma chamber. Krakatau collapsed underwater into the Sunda Strait, where a 6-kilometer-diameter caldera edged by three small islands marked its resting place. The flooded caldera was not immediately identified. At first, scientists assumed that the island blew itself apart during the explosions. If this were so, newly formed volcanic rocks should have the same composition as the pre-eruptive Krakatau. Yet, the ash and pumice rock analysis indicated that 95 percent of this material formed from cooled lava rather than pulverized rock. The missing island was, therefore, underwater in the caldera, not deposited as new strata or carried around the globe as ash.

Aftermath

News of Krakatau's disastrous 1883 eruption spread quickly. Telegraph cable carried the news to Singapore and then to the world. Only after the dark cloud of ash over Sumatra and Java had dispersed was the magnitude of the death toll known. Ships returned to Great Britain and the United States with blocks of pumice and ash samples; tidal gauges and barographs recorded the passage of sea and airwaves. Almost around the globe, people witnessed garish sunsets and unusual optical effects. Within three weeks after Krakatau's final blast, scientists commenced detailed studies on the eruption, subsequent tsunamis, and global atmospheric effects.

Unequivocally, Krakatau made the loudest noise of any volcano in history. Certainly, it was one of the most violent. An eruption of Krakatau's energy and duration occurs about once or twice every one hundred years; yet, in the context of global volcanology, other volcanoes have superseded Krakatau in the amount of ash and magma ejected and the size of caldera created. The 1815 eruption of Tambora, also in Indonesia, released about fifty times more energy and produced five times as much pyroclastics (mixed pulverized rock or ash and lava). Krakatau's eruption was not unusual in Indonesia, blasted by 17 percent of the world's volcanoes.

Elementary plate tectonics, the theory that the Earth's outer crust is composed of moving plates or blocks, explains why Indonesia is the most volcanically active region in the world. Indonesia rides on the Indian Ocean plate, which plunges northeastward beneath the buoyant continental Eurasian plate at geologically rapid speeds of up to 6 centimeters per year. A 3,000-kilometer stretch of volcanoes in Sumatra, Java, the Lesser Sunda Islands, and northward toward the Philippines marks the collision zone, the Java Trench, between these two plates. Volcanoes along this type of contact, called a subduction boundary, are more explosive and produce lavas distinct from volcanoes exploding within a plate or under water where two plates pull apart.

Krakatau was not dormant for long following the 1883 eruption. Most significantly, in 1927 another eruption began underwater where the former cone had been. Within a few years the new growing cone reached the surface of the water, forming a new island. This was named Anak Krakatau, meaning "Child of Krakatau." In the ensuing decades, the volcano remained occasionally active, building a cone over 300 meters (1,000 feet) above sea level by the 2010s. This pattern of sporadic activity initiated in 1927 has continued into the twenty-first century with eruptions in 2009, 2010, 2011, 2012, and 2020. Violent eruptions have showered the volcano’s summit in ash from June 2023 through the early months of 2024. The Smithsonian Institution's Global Volcanism Program has also tracked significant seismic activity in the form of volcanic earthquakes in March 2024. 

Principal Terms

basalt: a type of volcanic rock that cooled from magma relatively low in silica, which enabled it to flow freely

caldera: a large circular or oval depression formed by the explosion and collapse of a volcano

ejecta: material, such as rock fragments and ash, thrown out by a volcano

ignimbrite: volcanic rock, high in the element silica, formed by the cooling of layers of volcanic ash mixed with rock fragments

plate tectonics: the theory of geology that relates volcanism, mountain building, earthquakes, and other phenomena to the movements of “floating” plates of the earth's crust

pumice: a type of volcanic rock that is actually glass filled with air bubbles

pyroclastic: formed by the accumulation of rock fragments and ash

tsunami: a large, destructive wave caused by earthquakes or other short-duration disturbances of the sea floor; tsunamis are characterized by great speed and increased height when piled up in shallow water

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