Now its time to address your question as to why I believe that Indonesia was created by a mantle plume that now resides somewhere just northwest of Lake Toba in Sumatra.
As you point out, the entire Indonesian Island chain would seem to be merely the product of two converging tectonic plates. Furthermore, you ask why Lake Toba is anything other than just a supervolcano at a convergent plate boundary, as opposed to it being a supervolcano that is fed by a mantle plume.
CONVERGENT OR TRANSFORM PLATE BOUNDARY
Let's look at the plate boundary at the edge of the Indonesian Islands. Is it a convergent boundary or a transform boundary? I have attached two different maps of the area around Lake Toba (from Wikipedia).
One map shows the convergent boundary of the Australian plate as it is subducted under the Eurasian plate. It also shows a transform fault called the Sumatran fault that actually runs through the western side of the island of Sumatra. The second map shows a close-up of Lake Toba with the Sumatran fault shown clearly to the west of Lake Toba.
So, what does this mean? It means that we have a large transform fault sandwiched between a convergent boundary and its line of active volcanoes.
Isn't that strange? How does such a thing come about?
My interpretation of the situation is that the boundary between the Australian plate and the Eurasian plate was originally a transform boundary. The Sumatran fault would have been a secondary mirroring transform fault.
However, as the antipodal hotspot from the Chicxulub impact began moving along a similar path towards India's collision with Asia (but much more slowly), it cut through the lithosphere like a plasma torch, creating a natural avenue for lithosphere and mantle material (especially to the west of it) to fall into. This process gradually led to a weakness along the boundary, allowing a subduction process to begin.
In other words, the boundary began as a transform boundary and gradually became mostly a convergent boundary.
It is especially noteworthy that the Indonesian Islands are the most intensely volcanic area on the face of the Earth ... and, yet, the volcanism dies down at the northwest end of the island of Sumatra. Why doesn't it continue? The Sunda trench continues. All the maps still show a convergent boundary continuing ... but the volcanism dies off.
Why? Because the hotspot hasn't moved beyond the end of Sumatra yet.
SIZE OF ERUPTION
Another way to look at the issue of whether or not an active mantle plume underlies the Lake Toba area is to look at the size of the Lake Toba eruption ... the amount of material ejected from the volcano. Is this amount more typical of a plume fed eruption or is it more in line with an ordinary convergent boundary volcano?
Some of the other volcanoes of the Indonesian Island chain have produced prodigious amounts of pyroclastic material. Krakatau in 1883 was legendary. Mount Tambora in 1815 created the year without a summer and contributed to Mary Shelley's icy setting for the novel "Frankenstein."
These Indonesian Island volcanic events are some of the most notorious convergent boundary volcanic events in recorded history.
And, yet, they pale in comparison to the Lake Toba eruption 73,000 years ago. Mount Tambora had an ejected volume of less than 5% of the Lake Toba eruption. Krakatau was smaller by two orders of magnitude ("Supervolcanoes and Their Explosive Supereruptions" by Calvin F. Miller and David A. Wark).
The Lake Toba eruption was the largest supervolcano event in the past 18 million years. Bigger than the largest Yellowstone eruption. Larger than any Taupo eruption in New Zealand (Wikipedia).
Was there a convergent boundary aspect to the Lake Toba eruption? Undoubtedly. But there must have been more.
According to Miller and Wark (cited earlier): "What sets them (supervolcanoes) apart, however is simply the enormous amount of eruptible magma that accumulates in shallow chambers, which are in turn only a minor component of even larger magma reservoirs. As suggested by all the papers in this issue, deeper-level, hotter, less silicic basaltic and andesitic magmas, though rarely evident in the products of the eruptions themselves, provide the thermal energy that drives the supervolcano system and contribute at least some of their mass to the silicic erupting magmas."
Yes, the Mount Tambora and Krakatau eruptions were big, but they didn't have the reservoir of heat and material supplied by a large hotspot underneath them. I believe that Lake Toba did, and it made all the difference.
In order to create the largest volcanic eruption of the last 18 million years, it takes more than just the ordinary elements of convergent boundary subduction. It takes the help of a hotspot, too.
I'LL PROMISE YOU THE MOON
Lin, these are some of the major reasons that I believe that the Indonesian Island chain is more than just a set of convergent boundary volcanoes. There are other reasons, which relate to the timing of the creation of different parts of the islands and the composition of the material of the larger islands and other factors as presented in Chapter 8 of my book, but these are the main reasons relating to volcanism, which seemed to be the thrust of your question.
So, next up, I'll promise you the moon!
Yes, even though I had not originally thought that my theory would have specific relevance to the creation of the moon, new moon rock controversies are convincing me otherwise. That's coming next.
As always, I appreciate comments.