Ritter Island is a small crescent-shaped volcanic island 100 kilometres (62 mi) north-east of New Guinea, situated between Umboi Island and Sakar Island.

There are several recorded eruptions of this basaltic-andesitic stratovolcano prior to a spectacular lateral collapse which took place in 1888. Before that event, it was a circular conical island about 780 metres (2,560 ft) high.^[1]

At about 5:30 am local time on 13 March 1888 a large portion of the island, containing perhaps 5 km³ (1.2 cu mi) of material slid into the sea during a relatively minor, possibly VEI 2,^[2] phreatic eruption. Eyewitnesses at Finschhafen, 100 km (62 mi) to the south, heard explosions and observed an almost imperceptible ash fall.^[3] Tsunamis 12–15 metres (40–50 ft) high were generated by the collapse and devastated nearby islands and the adjacent New Guinea coast killing around 3,000 people.^[4]

The collapse left a 140-metre (460 ft) high, 1,900-metre (6,200 ft) long crescent-shaped island with a steep west-facing escarpment. At least two small eruptions have occurred offshore since 1888, one in 1972 and another in 1974, which have resulted in the construction of a small submarine edifice within the collapse scar.^[3]

• List of volcanic eruption deaths
• List of volcanoes in Papua New Guinea