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I have read that the islands of Sumatra & Java have resulted from the subduction of oceanic crust of the Indo-Australian plate beneath the Sunda plate. I want to know whether this boundary is ocean-ocean convergence or ocean-continent convergence.

Are islands of Sumatra and Java like Mariana Islands which result from the subduction of the oceanic Pacific Plate beneath the oceanic Mariana plate? Or are they like the Andean volcanic continental Arc which results from the subduction of the oceanic Nazca plate beneath the South American plate.

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Well, in short, it's mainly continental, (oceanic-continental convergence).

But in regards to the "type of plate" that the Sunda plate is, is minor plate or tertiary plate. Meaning it's not very big etc. Because 1 tectonic plate can have both oceanic or continental crust. Tectonic plate and crust are slightly different concepts. So, The Sunda plate is mainly comprised of continental crust. There might be a bit of oceanic crust, but less.

Map of oceanic crust on earth: https://www.ngdc.noaa.gov/mgg/ocean_age/data/2008/ngdc-generated_images/whole_world/2008_age_of_oceans_plates.jpg

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The convergence of the Indo-Australian plate with the Sunda plate is an oceanic-continental convergence. The Sunda plate carries some major land masses: much of SE Asia including half of Vietnam, and Borneo (which is itself three times the size of Great Britain).

The islands of Sumatra and Java are part of what is known as the Malay Archipelago, though they belong to Indonesia, as do the Lesser Sundas to the south. They are part of the original land mass of the Sunda Plate and were once connected before sea level rise separated them. They have numerous volcanoes caused by the subducting Indo-Australian plate. Arguably, these volcanoes are gradually making them larger, which also applies to the Lesser Sundas to the south. In 1815 Tambora, on the island of Sumbawa, was the site of the largest volcanic eruption in recorded history. An even larger eruption occurred at Lake Toba on Sumatra, some 75,000 years ago.

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  • $\begingroup$ Does this mean that Sumatra is continental and not volcanic? $\endgroup$
    – HARVEER RAWAT
    Commented Mar 7, 2020 at 12:17
  • $\begingroup$ It is both. The islands were not created by subduction, but as subduction caused volcanoes to erupt they have been enlarged by it. Were it not for sea level rise, the islands would be substantially larger and join up with peninsular Malaya. $\endgroup$
    – Michael Walsby
    Commented Mar 7, 2020 at 12:25
  • $\begingroup$ PS. The presence of Malayan tapirs, Sumatran rhinos, Sumatran tigers and many other animals on both Sumatra and the Malayan Peninsular is proof that the two land masses were once joined Geologically speaking, the separation occurred in the fairly recent past, otherwise isolation and evolution would have altered these animals and created new species. $\endgroup$
    – Michael Walsby
    Commented Mar 7, 2020 at 13:04

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