Permanent island By early 1964, though, the continuing eruptions had built the island to such a size that sea water could no longer easily reach the vents, and the volcanic activity became much less explosive. Instead, lava fountains and flows became the main form of activity. These resulted in a hard cap of extremely erosion-resistant rock being laid down on top of much of the loose volcanic pile, which prevented the island from being washed away rapidly. Effusive eruptions continued until 1965, by which time the island had a surface area of . On 28 December 1963, submarine activity to the northeast of Surtsey began, causing the formation of a ridge high on the sea floor. This seamount was named Surtla , but never reached sea level. Eruptions at Surtla ended on 6 January 1964, and it has since been eroded from its minimum depth of below sea level. Subsequent volcanic activity In 1965, the activity on the main island diminished, but at the end of May that year an eruption began at a vent off the northern shore. By 28 May, an island had appeared, and was named Syrtlingur ( Little Surtsey). The new island was washed away during early June, but reappeared on 14 June. Eruptions at Syrtlingur were much smaller in scale than those that had built Surtsey, with the average rate of emission of volcanic materials being about a tenth of the rate at the main vent. Activity was short-lived, continuing until the beginning of October 1965, by which time the islet had an area of . Once the eruptions had ceased, wave erosion rapidly wore the island away, and it disappeared beneath the waves on 24 October. During December 1965, more submarine activity occurred southwest of Surtsey, and another island was formed. It was named Jólnir, and over the following eight months it appeared and disappeared several times, as wave erosion and volcanic activity alternated in dominance. Activity at Jólnir was much weaker than the activity at the main vent, and even weaker than that seen at Syrtlingur, but the island eventually grew to a maximum size of in height, covering an area of , during July and early August 1966. Like Syrtlingur, though, after activity ceased on 8 August 1966, it was rapidly eroded, and dropped below sea level during October 1966. Effusive eruptions on the main island returned on 19 August 1966, with