Ritter Island is a small crescent-shaped volcanic island 100 km 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 m high.

1888 eruption

At about 5:30 am local time on 13 March 1888 a large portion of the island, containing perhaps 5 km3 of material slid into the sea during a relatively minor, possibly VEI 2, phreatic eruption. Eyewitnesses at Finschhafen, 100 km to the south, heard explosions and observed an almost imperceptible ash fall. Tsunamis 12-15 m high were generated by the collapse and devastated nearby islands and the adjacent New Guinea coast killing around 3,000 people. The collapse left a 140 m high, 1900 m 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.