There are some crazy looking islands, like Sulawesi and Halmahera, that have resulted from different pieces of different plates coming together to form contiguous wholes, and there are some, like Timor, that were seabed until only a few tens of millions of years ago (in parts of Timor and New Guinea the uplift is so recent that different coral species can be identified from the exposed rocks).
Parts of the Huon Peninsula in New Guinea are rising at about 3.5cm per century - absurdly fast - and the island of New Guinea as a whole is rising so quickly that if you manage to ascend Puncak Jaya's 4884 metres, on the Indonesian side of the island, you can reportedly see the waters of the Aru Sea only a hundred kilometres away. An even steeper rise is found on the border between Papua New Guinea and Indonesia: the ground goes up four-thousand metres in under 15 kilometres. That's an extraordinarily steep incline, and it makes New Guinea the highest tropical island in the world, as well as the only tropical island capable of sustaining glaciers on its summits. (Since we're talking superlatives, New Guinea is also the wettest island in the world, with about 10,000mm of rain annually, largely a result of its peaks.)
Indonesia's human history has been shaped to an impressive degree by the region's volcanoes. Both Java and Sumatra have active volcanoes in abundance. Sumatra is significantly bigger than Java; it's also better-placed for access to Indian Ocean trade routes than Java, and you might look at the two and expect Sumatra to have a larger population and to be more prominent politically than its smaller and harder-to-reach island neighbour. But if you know anything at all about Indonesia in the twenty-first century, you'll know that it's dominated by Java, the most populous island in the world, with 143 million inhabitants (Indonesia is sometimes characterised as a Javanese empire). Sumatra isn't a backwater, but its larger size has only been able to foster 50 million humans. There are several reasons for this, but an important one is volcanic: the ejecta from Java's volcanoes produces much more fertile soil than Sumatra's more acidic ejecta could.
Anyway, you may be familiar with this video, which shows a volcanic explosion near Rabaul in New Britain, Papua New Guinea, at the very end of the Southeast Asian island arc:
You may also be familiar with those neat videos of flowing lava from Hawai'i and other places like that, with the lava gently coursing down existing basalt pillows and boiling/crushing/exploding Chef Boyardee ravioli cans.
I'm not sure which video I prefer (they're both cool as hell), but there's a fundamental difference between the two processes that helps to explain why island Southeast Asian volcanoes explode violently while Hawaiian volcanoes pulse out their magma in such a stately pasta-ruining manner. It's ultimately down to the composition of the rocks involved.
Where ocean floor is still being made, as it is in Hawai'i, you get relatively fluid rock oozing upwards - basalt. It flows fairly freely and you don't tend to get a build-up of pressure in Hawaiian volcanoes, so they don't tend to explode. Basalt gives you pleasing black rock cushions and time to get away when it all kicks off.
Island arcs, like the one between Sumatra and New Guinea, are commonly composed of andesite, a much more viscous rock than basalt. The andesite results from the grinding of continental and oceanic crust typical of the subduction zones that generate island arcs. When two or more plates crash into one another, the plate that consists of the lightest material will start to rise up, and the colliding surfaces of the plates undergo extraordinary pressure that melts the rock at the edge. Because we're talking about the collision of continental and oceanic crust, we can expect water to become involved as well, and the mix of water and molten rock results in andesite. (A more technical description of the process can be found on the Wiki page for andesite.)
Because of its viscosity the andesite acts like a cork, plugging the volcano up nice and tight. Gases build up in the earth beneath the andesite plug, resulting in much greater pressures than Hawaiian volcanoes typically experience. The mounting pressure can't help but result in spectacular - and spectacularly deadly - volcanic explosions, with all of their profound impacts on human history and society.
I don't have any particular purpose for this post. I just think it would be hard to find a part of the world as endlessly fascinating in so many ways as island Southeast Asia, with its extraordinary human- and biodiversity, its messy tectonics, its fraught political history, and its vital role in pre-Columbian Afro-Eurasian trade.