|Deception Island is a volcanic caldera, which formed a natural harbour when the caldera collapsed.|
The harbour is called Port Foster.
The Island is in the South Shetland Islands, on the Bransfield Rift where the rift is offset by a short ridge-ridge transform fault .
700000 years ago, volcanic pressure on Deception Island resulted in a tremendous eruption that caused the island’s peak to explode.
The prehistoric 7-kilometer-diameter caldera is largely flooded by sea. The caldera apparently formed as a result of voluminous eruptions of sodic andesite or basaltic andesite. The volcano is still very active and its eruptions have caused evacuations and considerable damage to the stations there (during the 1920-21 whaling season the harbour water boiled and stripped the paint off the ships' hulls). The most recent eruption was in 1991-92. Numerous active vents are situated along the ring fractures, near the shoreline of the natural harbour.
Eruptions have occurred on at least three occasions simultaneously from multiple vents around a ring fracture.