I can tell you a little bit about Rabaul. It is a large Rhyolite caldera
that has undergone a number of huge eruptions. Each of these eruptions
spread pyroclastic flows for many kilometers and probably also resulted in
caldera collapse. When you look at a map of Rabaul you can see that the
harbor is actually made up of a number of somewhat-circular collapse
structures that have merged together to form the main caldera. The most
recent activity (1937-45 and 1994) has been of a much smaller scale, and
has taken place from smaller vents situated along the rims of these large
caleras. There is home-page information on Rabaul in the Michigan Tech.
Volcanology page:
http://www.geo.mtu.edu/volcanoes/rabaul/
And there is a good book about Rabaul called "Volcano Town" by RW Johnson
and NA Threlfall.
Scott Rowland, University of Hawaii
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Scott