The Sunda Strait is the channel between the Indonesian islands of Java and Sumatra, joining the Java Sea to the Indian Ocean. It narrows to 24 km (15 mi) at its northeastern end, between Cape Tua on Sumatra and Cape Pujat on Java, and opens into a broad, deep mouth to the southwest. It matters for three reasons: it carries the Merak–Bakauheni ferry, which links Java and Sumatra around the clock; it was the Dutch East India Company's gateway to the Spice Islands from 1602 to 1799; and it holds Krakatoa, whose eruption of 26–27 August 1883 killed at least 36,417 people. The strait is named after the Sunda Kingdom, which ruled western Java until around 1579.
The water between Java and Sumatra is young, restless and still changing. To understand the strait, it helps to build it in order: first the plates, then the faults, then the rising sea, then the volcanoes, and only then the ports and ferries that people set on top of it all.
Two Plates at a Bend in the Sunda Arc
The deepest cause of the strait lies offshore to the southwest, at the Sunda Trench, where the Indo-Australian Plate slides beneath the Sunda Plate at 63 to 70 mm a year. That convergence began in the Early Miocene, between about 23 and 16 million years ago, and it feeds the Sunda Arc, the chain of volcanoes that forms the spine of Sumatra, Java and the islands east to Flores. The slab of ocean floor descends at a steep angle of about 49 to 56 degrees.
The arc is not straight. Under Java the plates meet head-on, in what geologists call orthogonal subduction. Under Sumatra they meet at a slant, in oblique subduction. The Sunda Strait sits exactly where one style gives way to the other, at a sharp bend in the arc. Krakatoa stands directly above the subduction zone at that bend, where the sharp change of direction in the plate boundary may leave the crust unusually weak.
The Semangko Fault and Semangka Bay
When plates meet at an angle, the motion splits in two. Most of the squeezing is taken up by the megathrust at the trench, while the sideways part is carried by a separate fault on land. In Sumatra that fault is the Great Sumatran fault, also called the Semangko fault, a strike-slip break that runs the entire length of the island, from near Banda Aceh in the north to the shores of the strait in the south. The block of Sumatra west of the fault, together with the offshore islands, behaves as a separate "sliver plate".
The fault's name points straight at the strait: Semangka Bay is one of the two large bays on the Sumatran side of the Sunda Strait, the other being Lampung Bay. The fault is still active. The 1933 Sumatra earthquake, of magnitude 7.6, destroyed two towns in the south of the island, and the 1994 Liwa earthquake, of magnitude 7.0, killed 207 people near Sumatra's southern tip. The southeastern point of Sumatra is marked by Rajabasa, an isolated volcanic cone 1,281 m high that overlooks the strait, with unrest reported in 1863 and 1892 and no known date for its last eruption.
Sundaland and the Flooding of the Land Bridge
For most of the last 800,000 years there was no strait at all. Java, Sumatra and Borneo sit on the Sunda Shelf, a stable extension of Asia's continental shelf whose edge roughly follows the 120 m depth line. Whenever global sea level dropped by 30 to 40 m or more during ice ages, land bridges joined these islands to the Malay Peninsula, forming the lost landmass known as Sundaland, about 1,800,000 km² in total. At the Last Glacial Maximum the sea stood about 120 m lower than today and the whole shelf was exposed.
The northeastern end of the Sunda Strait is only about 20 m deep in places, so it would have been dry ground for long stretches of that history, joining Java and Sumatra by land. The sea began to drown Sundaland around 18,000 years ago and kept rising until about 5000 BC, turning the hills of the old plain into today's islands and opening the strait.
A Javanese tradition gives a different, more dramatic origin. The Pustaka Raja, a 19th-century compilation from Central Java, says that in AD 416 a mountain called Kapi exploded and sank, the sea rushed in, and "Java and Sumatra were divided into two parts". Historians regard the text as highly unreliable, there is no geological evidence for that eruption, and drilling in the strait has ruled out the separate theory that Krakatoa caused the global cooling of 535–536.
Rakata, Danan and Perboewatan: Krakatoa Before 1883
Inside the strait, the subduction zone fed a volcano that destroyed itself more than once. At some point in prehistory, an earlier caldera-forming eruption left a ring of remnants: the islands of Verlaten (Sertung) and Lang (Rakata Kecil or Panjang), the islet of Poolsche Hoed, and the base of Rakata. New cones then grew inside that ring. By 1883 the main island of Krakatoa was 9 km long and 5 km wide and carried three peaks: Rakata, 820 m high, to the south; Danan, 450 m, near the centre; and Perboewatan, 120 m, to the north.
The island was already known to seafarers. The Old Sundanese text Bujangga Manik, probably written in western Java in the late 15th century, calls it "the island of Rakata, a mountain in the middle of the sea", and a 1611 map by Lucas Janszoon Waghenaer labels it "Pulo Carcata". Eruptions were recorded around 1680.
26–27 August 1883: The Eruption That Rebuilt the Strait
Smaller eruptions began on 20 May 1883 and continued through the summer. Then, over 26 and 27 August, a series of explosions with an estimated Volcanic Explosivity Index of 6 blew out about 25 km³ of rock and destroyed two-thirds of the island. The sound was heard 3,600 km away at Alice Springs in Australia and 4,780 km away on Rodrigues near Mauritius. Between 18 and 21 km³ of ignimbrite spread over 1.1 million km² around the volcano, changing the floor of the strait.
Most of the dead were killed by tsunamis. Dutch colonial records list 165 villages and towns destroyed and 132 seriously damaged. The original settlement at Merak, on Java's northwest tip, was wiped out by a wave estimated at at least 41 m, and about 2,700 people died there. Anyer, a port on the Javan shore, was destroyed; the Dutch built the Cikoneng lighthouse there two years later as a memorial. On Sebesi island, none of about 3,000 inhabitants survived. Some land on Java's western tip was never resettled and later became Ujung Kulon National Park.
Anak Krakatau, From 1927 to the 2018 Collapse
After 1883 the strait began building again. Life returned first: when researchers reached the islands in May 1884, the only living thing they found was a spider. In 1880, three years before the eruption, the geologist Rogier Verbeek had surveyed the islands, and in his report on the disaster he predicted that new activity would appear between the old cones of Perboewatan and Danan. On 29 December 1927 a submarine lava dome showed signs of eruption there, and new land rose above the waves a few days later. The first islands were washed away by the sea, but on 11 August 1930 a fourth one rose permanently, and it was named Anak Krakatau, "Child of Krakatoa".
It grew fast. By late 2011 the new island had a radius of roughly 2 km and stood about 324 m high, gaining around 5 m a year. Its instability had been noticed early: a 1928 investigation found the western slope already much steeper than the eastern, because of the strong current in the strait, and in January 2012 volcanologists at the University of Oregon warned that a tsunami caused by a flank collapse was likely. On 22 December 2018, at about 21:38 local time, a large part of the southwestern flank collapsed into the caldera. The tsunami struck the coasts of Banten and Lampung at night, during the holiday season, and killed 426 people, the deadliest volcanic tsunami in Indonesia since 1883. The cone was cut down to about 110 m, and new warning systems were installed along the strait afterwards.
Merak, Bakauheni and a Bridge Still on Paper
On top of this shifting geology, people have built a corridor. The Port of Merak in Cilegon, named after the green peafowl that once lived there, is linked to Jakarta by the Jakarta–Merak Toll Road. Across the water, Bakauheni is the largest and busiest port in Lampung province and the southern end of the Trans-Sumatran Highway. Ferries run between them 24 hours a day, on average one departure every 12 minutes, with a crossing of about two hours, and traffic peaks during the mudik journeys home at New Year and Eid.
Large ocean ships mostly go elsewhere. The strait's shallow northeastern end, strong tidal currents, sandbanks and oil platforms off the Javan coast make it hard to navigate, so most big vessels use the Strait of Malacca instead. Its wartime history is short and violent: on 1 March 1942, in the Battle of Sunda Strait, the Allied cruisers HMAS Perth and USS Houston were sunk by a Japanese force near Bantam.
A fixed link has been proposed since the 1960s. A plan announced in October 2007 would have used the islands of Ular, Sangiang and Prajurit as stepping stones for a suspension bridge with a 3 km main span, and in April 2012 an $11 billion road and rail deal was signed with China Railway Construction Corporation. In November 2014 the new government of President Joko Widodo shelved it. On the Javan side, the land emptied by 1883 has become Ujung Kulon National Park, a UNESCO World Heritage Site of 1,056.95 km² that shelters the last Javan rhinoceroses, estimated at around 74 animals.
To see the volcano at the heart of this story in more detail, read our complete guide to Krakatoa, or cross the strait to explore the island on its northern shore in our guide to Sumatra.