Source: here.
The Hindu kingdom of Mataram in Yavadvipa, associated with the famous tantra of the vajramaNDala, came to an end in 1006. It was taken to its peak by the rAjan named: makuTa-vamsha-vardhana ishAna kula-sUrya. Due to the recent revival of the volcano that ended Mataram, I was reminded of this and also the need to look more deeply into this tantra (some of whose mantras were recovered on golden plates inscribed in a ruined Indonesian temple) when the chance arises .
The inscription on the Calcutta stone states that in 928 shaka (1006 CE) there was a tremendous mahApralaya and a strike of fire which ended the kingdom of Mataram. The whole of the yavadvIpa is said to have become a kShira sAgara. It was described as corresponding to the end of the kalpa described in the Hindu texts. The kShira sAgara is most likely to be a description of the tephra (volcanic ash) that was spewed all over by the volcanic eruption of Merapi.
In a recent conversation with the two JSs I learnt that a tephral layer has been recovered in Indonesia with the charcoal from it C14 dated to between 900-1100 CE. This must definitely mark the kShira-sAgara created by mahApralaya of Merapi in 1006 CE that ended the Hindu kingdom of Central Java.