8 Geological history of Wallacea

The area of Wallacea has a complex geological history, as detailed by Hall and coworkers and Audley-Charles (1981). This is an area of regular vulcanism and earthquakes. The Java Trench is a subduction zone where the ocean floor is pushed downward and volcanic activity at the continental margin has given rise to the islands of Java and Bali. Bali emerged from the sea around ten million years ago; the age of the main volcano on Bali, Gunung Agung, is about 8 million years. However, the volcano is still relatively active and erupted most recently in 1963. The zone of collision of the Australian plate with Asia along the Banda Arc has produced a chain of volcanos. Most of the islands of Nusa Tenggara were built by volcanic activity, over a basement of ancient marine strata.

The colossal eruptions of Tambora on Sumbawa in 1815 and Krakatau between Java and Sumatra in 1883 demonstrate the massive geological forces at work in this region. The colonisation of Krakatau by plants and animals also indicates the types of processes that have resulted in the distribution of the biota along the island chains of Wallacea.

The Eurasian Plate is a large tectonic plate that includes the mainland of South-east Asia and the Sunda Shelf. The Australian Plate includes southern New Guinea and the island of Timor in addition to mainland Australia, and was a portion of Gondwana that separated from proto-Antarctica 45-50 million years ago and drifted north. The collision of the Australian plate with a Tertiary island arc raised the New Guinea highlands. The main evidence for this is that central and southern New Guinea is underlain by Australian continental crust. Similarly, there are Australian continental margin rocks below the islands of Roti, Timor, Tanimbar and Ceram in the Outer Banda arc.