Why scientists study the Pilbara
by Professor Malcolm Walter, Director, Australian Centre for Astrobiology
The further we go back in time the fewer and fewer rocks there are to tell us the story of the Earth. Our planet formed 4.6 billion years ago, along with all the other planets in the solar system. But no rocks have survived on Earth from that time. The oldest rocks so far found are about 4 billion years old; they have been so altered by later processes (“tectonism”) in the Earth’s crust that they are very difficult to interpret. There are hints of life at 3.8 billion years ago, but the evidence is insufficient to be convincing. Life might have existed, but we need much more evidence to convince ourselves. It is not until 3.5 billion years ago that we find rocks that are well enough preserved to tell us a lot about what the surface of the planet was like and whether there might have been life. Those rocks are in a small corner of South Africa called the Barberton Mountainland and between Marble Bar and Port Hedland in Western Australia, an area called the Pilbara. We will focus on the Pilbara.
If we had a time machine and could travel to the ancient Pilbara we would see great volcanoes emerging from the sea, and small areas of other land, but no continent. It would have looked like the Hawaiian Islands in the middle of the Pacific Ocean, but with little sign of life: no trees, no grass, no animals. No surfers on the beach. The sand was black because it was eroded from the black lavas. Frequently, on the land and under the sea lava flows spread out widely. Where water met volcanic heat hot springs were common, especially on the sea floor. There is some evidence that the whole sea was hot, but that is not certain.
Though volcanoes dominated the landscape, from time to time during quiet intervals there were no eruptions and the seas and lakes were calm. Small rivers carried sand and mud out to sea and into the lakes. The seas and some of the lakes were salty and as they slowly evaporated salt and lime and other minerals formed crusts at the bottom of the water.
We can see all the rock layers now because they have been tipped on their sides by compression in the Earth’s crust, and eroded down over millions of years to form the hills and valleys of the present landscape.
A good way to understand any ancient rocks is to study places where similar ones are still forming. I have mentioned the Hawaiian Islands but there are even closer “analogues”. New Zealand’s most active volcano, White Island, is one such. It is dormant at present but could erupt again at any time. The molten rock (“magma”) under the volcano has cooled and shrunk, causing the top of the volcano to collapse in on itself, forming a large crater called a caldera. The sea has breached one side of the caldera forming a pebbly beach. Deep in the interior of the caldera is a brightly coloured lake of acid. The very steep and dangerous walls and also the floor of the caldera have numerous steaming vents and springs of very hot water. The steam has temperatures up to 8000C and roars out of some vents with as much noise as a jet engine. Water from the steam vents and springs makes its way via a small stream down to the pebbly beach and into the sea where it drops its load of dissolved iron, staining the sea red. The inside of the caldera looks almost lifeless but here and there in the springs and streams are wispy streamers of tangled thread-like microbes, and in the sand are other populations of microbes. Many of them (“hyperthermophiles”) grow only at near boiling temperatures, others require acid (“acidophiles”); collectively such organisms are called extremophiles. Of course the environments are extreme only from our perspective: for the organisms that live there the conditions are normal, in fact necessary. Standing in the caldera we can imagine that we are on the ancient Earth, or even on Mars (which was similar to Earth 3-4 billion years ago).
The question is, was there life in the calderas, lakes and seas of the Pilbara 3.5 billion years ago? In our journey back through time and through the Pilbara we will look for the answer. Why does the answer matter? Because we are all explorers. We want to explore and understand the origins of life, and how life and its environment interact to form what we have about us now. Not only that, the answers will guide our search for life on other planets and moons, and around other stars. Eventually we will answer an even bigger question: “Are we alone in the Universe?”