Given the following context:

In 1965, the activity on the main island diminished, but at the end of May that year an eruption began at a vent 0.6 km (0.37 mi) 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 0.15 km2 (0.058 sq mi). 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 0.9 km (0.56 mi) 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 70 m (230 ft) in height, covering an area of 0.3 km2 (0.12 sq mi), 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 fresh lava flows giving it further resistance to erosion. The eruption rate diminished steadily, though, and on 5 June 1967, the eruption ended. The volcano has been dormant ever since. The total volume of lava emitted during the three-and-a-half-year eruption was about one cubic kilometre (0.24 cu mi), and the island's highest point was 174 metres (571 feet) above sea level at that time.Since the end of the eruption, erosion has seen the island diminish in size. A large area on the southeast side has been...

answer the following question:

What island was the activity at Jólnir weaker than? 

Syrtlingur