The islands of Indonesia are located on the joint between the Eurasian Plate (Sunda Plate), the Australian Plate (Sahul Shelf), the Philippine Sea Plate, and the Pacific Plate. The eastern islands are part of the fire ring of the Pacific Plate, so there are a lot of volcanic islands, while the western islands are located on the subduction of the Indian oceanic plate beneath the Eurasian continental plate forming a volcanic arc. Nevertheless there is also a lot of sedimentary rocks including limestone which are dominated by Cenozoic age formations. In other words the islands are mostly young, formed by the volcanism on the plate rims and by the accretion of young sedimentary rocks on the subduction zone of two oceanic plates. Some of the islands show minor Mesozoic and Paleozoic formations, small pieces of the older continents.
The volcanism is rather explosive, with probably one or two major volcanic eruptions every century. Quite famous in historic times are the 1815 eruption of Mount Tambora and the 1883 eruption of Krakatoa. Both caused the death of a huge number of people by the eruption itself and the resulting tsunamis. The volcanic ash in the atmosphere caused climatic changes and lead to famine in the following years all over the world.
The geologic situation is complex, and there are a lot of caves on some islands. The volcanism is actually not very good for the creation of exceptional lava tubes as we know them from the calmer intra-plate volcanism. Still there are a lot of shorter ones. The limestone rich islands on the other hand offer a lot of karst areas, mostly well developed karst like tower karst and cockpit karst, a result of the tropic climate with a dry and wet season, on some islands there is even monsoon on the islands Java, Sulawesi and Bali. Numerous caves, especially river caves, are known and visited by the locals. But there is no well developed cave tourism, and almost no show caves. Mostly the caves are visited on boat rides and adventure trips.