Take a look at a map of the eastern hemisphere of the world. A great sweep of land and islands flows from India through Malaysia and Indonesia, round the top of PN G and down via Vanuatu to New Zealand. This is the leading edge of the Indo-Australian plate. Using PNG as a hammer head, Australia seems to be thrusting it’s way into the Pacific, leaving strings of islands draped over PN G like a fold in the fabric of the Earth’s crust.Take a look at a map of the eastern hemisphere of the world. A great sweep of land and islands flows from India through Malaysia and Indonesia, round the top of PNG and down via Vanuatu to New Zealand. This is the leading edge of the Indo-Australian plate. Using PNG as a hammer head, Australia seems to be thrusting it’s way into the Pacific, leaving strings of islands draped over PNG like a fold in the fabric of the Earth’s crust.
This in effect is just what has been happening over the last couple of hundred million years. Two of the major tectonic plates have been in conflict. The Pacific plate is being invaded and crushed at the edges by the Australian plate. As each massive slab of crust tries to slide over or under the other, the earth cracks and moves. Earthquakes regularly shake the islands and the north western parts of PNG.
As the floor of the Pacific ocean slips under the part of the Australian plate on which PNG rests, deep ocean trenches are formed offshore. The New Guinea Trench is to the north of Irian Jaya and the Mariania Trench, a long way north, is the deepest part of the ocean in the world – over seven miles deep. The Pacific plate sinks into the earth’s mantle of molten rock and, 40 to 200 miles below the surface, begins itself to melt. Being of a different consistency parts of it try to float and come upwards through the crust under PNG. The result is the scattering of active volcanoes throughout the north western parts of PNG.
In September 1994, as if to anticipate the first 20 years of Independence, the volcanoes Vulcan and Tavurvur, on the tip of East New Britain, exploded and blew clouds of volcanic material almost into the stratosphere. Much of Rabaul and surrounding districts were buried. The natural harbour of Rabaul is itself an old circular volcanic caldera with the sea flowing in on one side. The newer active volcanoes are around the edge. New land is slowly being built up.
Where two crustal plates meet like this, an arc of islands is usually formed. These eventually build up to form more continuous land. You can see examples of the process in the Caribbean, off Alaska, in Java and Japan. It can get very complicated. Bits of crust breaking off and themselves form ocean trenches, volcanoes and island arcs at the margins of these microplates. PNG has at least two, Bismarck and Solomon. In PNG the effect of the movement of the main plates and these microplates can be clearly seen in the shapes of New Britain and New Ireland and the continuation of the line that follows through to Manus. Further offshore, the latest new islands can be seen beginning to emerge. One of them is Lihir. These represent the leading edge of the progression of PNG into the Pacific – geologically speaking. Step many millions of years back in time and you can see that the main island of PNG was once that leading edge, a string of islands that built up and amalgamated to form a flowing sweep from the Highlands to the Owen Stanley range.
The result of all the melting far beneath the ground is that minerals, especially heavy elements of similar atomic weights, tend to gather together. The heaviest, gold and silver, concentrate the most, copper slightly less so. The copper deposits of PNG – Bougainville, OK Tedi and others – are usually accompanied by the other two metals. The Frieda deposit, now being investigated on the borders of East Sepik and Sandaun has a cap of gold sitting on top of the more diffuse copper.
A secondary result of the crustal movement is that many sediments on the floor of shallow seas and lakes are buried. They ferment into oil and seep upward through the layers until some of it gathers in a dome of impermeable rock. The rest seeps to the surface and dissipates.
The mineral wealth of PNG exists because of millions of years of geology, millions of years of the Australian plate pushing its way into the Pacific.