Where Australia Collides with Asia The epic voyages of Joseph Banks, Charles Darwin, Alfred Russel Wallace and the origin

Where Australia Collides with Asia

and Sulawesi – the Wallace Line. By his observations Alfred Russel Wallace had

made crucial observations on the relationship between zoology and geography, as he

describes these two divisions of the earth as differing in animal life as Europe does

from America.

The earth’s surface consists of two kinds of material, the heavier oceanic crust

which is semi-molten rock connected to the earth’s upper mantle, and the lighter

continental crust which ‘floats’ on the crust like scum on the surface of a pond. Driven

by huge convection currents within the earth’s upper mantle, the oceanic crust is

broken into a number of separate plates that are in motion. There is no evidence that

the earth is expanding, so where new oceanic material is being formed along the mid-

ocean ridges, oceanic material is also being consumed along subduction zones around

the world. For example, new oceanic crust is being created along the mid-ocean ridge

between Australia and Antarctica, and as the oceanic crust and its overlying Australia-

New Guinea continent moves north then oceanic material is being consumed in the

subduction zone along the south coast of Java.

Twenty million years ago, or in Miocene times, the incredible northern voyage

of the Australian Continent from the Antarctic towards the Equator slows down as

it starts to collide with Asia. The Australian continent, which includes Papua New

Guinea, first sweeps aside parts of the Indonesian island arc that stands in its way and

then crashes into the Pacific Plate, locking the two plates together. The Pacific Plate

is moving slowly westward, resulting in great chunks of the Australia-New Guinea

continent being sliced off by trans-current faults and creating the Sulu spur which has

been inserted into the Indonesian island arc system and causes it to bend back upon

itself. Lands which had been separated for 200 million years since the northern mega-

continent of Laurasia separated from the southern mega-continent of Gondwanaland

now came back together.

While the Pacific Plate and the Australian Plate are locked onto each other the

Australian Plate continues to move northward. This continuing collision causes large

parts of Papua New Guinea that were once in shallow seas to be thrust upwards, and

with further stress to be thrust over the top of each other to form the huge mountains

in the Papuan Fold Belt. Mountains such as Mount Puncak Jaya, which at 5000 metres

is so high that a tropical glacier still exists on its flanks. Tectonic activity related to

this thrusting and uplift includes the emplacement of magmas bearing copper and gold

such as now found at the Grasberg and Porgera gold mines in Papua New Guinea.

It was not until 1912, fifty years after Wallace first arrived in the Malay Archipelago,

that Alfred Wegener proposed his theory of continental drift. Although Wegener’s

theory was not accepted by most geologists at the time, it was quickly accepted by the

Dutch geologists working in the East Indies because they saw the folded mountains of

northern New Guinea as a collision zone, and collision was the only explanation for

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