The landscape of eastern Australia is dotted with hundreds of extinct volcanoes. They gave birth to an environment to which the Aborigines have been connected for tens of thousands of years, and to the rich soils upon which modern Australia has developed in recent years. a few hundred years.
Read more: When the Bullin was howling: Aboriginal memories of volcanic eruptions thousands of years ago
However, until recently, these volcanoes posed a geological mystery. Volcanoes form in two common ways: on the edges of tectonic plates or above patches of hot material called “mantle plumes,” which rise from the depths of the planet. For most volcanoes in eastern Australia, however, neither of these explanations is adequate.
We have now solved the riddle. By studying the history of the eruptions and the chemical makeup of the rocks they spat out, we discovered a previously unknown geological mechanism that connects the volcanoes of far north Queensland to the southern tip of Tasmania.
Australia’s volcanic connection
You might be surprised to learn that hundreds of volcanoes have erupted along Australia’s east coast over the past 100 million years. This volcanism also extended off New Zealand and the submerged continent of Zealandia.
Read more: What are the lost continents, and why are we discovering so many?
Most of the world’s volcanoes are formed when a process called “subduction” pushes parts of the seabed into the earth’s mantle, where it melts and produces volcanism on the surface. The best-known example of this type of volcanism is the Ring of Fire around the Pacific Ocean.
Alternatively, volcanic island chains can be constructed by hot materials rising from the Earth’s deep interior – called “mantle plumes” – in a process that has created countries like Hawaii, Iceland and the Galapagos Islands. These so-called “chains of hot spots” follow the movement of tectonic plates as new islands form on a stationary mantle plume.
However, most of the volcanoes in our backyard are unrelated to mantle plumes and are not near plate boundaries. So why are they here?
Examining the volcanic pulse of Australia
Our study, published today in Science Advances, shows that the frequency of volcanic eruptions in eastern Australia and Zealandia depends on what happens at the bottom of the sea some 3,000 kilometers further east.
Why is this happening? It all depends on the amount of water and carbon dioxide trapped in the seabed, which are recycled into the mantle.
Over millions of years, a reservoir of these volatile ingredients has built up in the mantle, more than 410 kilometers below the surface. This reservoir remains dormant under the Australian plate, until tectonic forces create bursts of movement.
Read more: Australia’s volcanic history is much more recent than you might think
As seabed slabs are subducted into the Tonga-Kermadec Trench, which stretches from New Zealand to Samoa, vibrations reach the mantle reservoir beneath eastern Australia and Zealandia. As a result, water and carbon dioxide break away from the reservoir and rise to produce volcanic eruptions on the surface.
We found our first evidence of this driving process in the deep history of volcanic eruptions in the region. There have been two progressive increases in volcanism, one between 60 million years ago and 21 million years ago, and the other from 10 million years ago to 2 million years ago. ‘years. These periods were separated by a brief lull (in geological terms) in the frequency of eruptions.
Both episodes were produced by major reorganizations of Earth’s tectonic plates, in which the plates rapidly change speed and direction. These changes led to the subduction of a massive cluster of western Pacific seabed, which in turn caused volcanic activity as water and carbon dioxide were shaken from their reservoir in the mantle.
Australia’s mysterious volcanoes fingerprint
This subduction process is not unique to the Australian east coast. What distinguishes the region of eastern Australia and Zeeland is that the seabed pushed under the continent from the western Pacific is rich in material containing water and carbon dioxide.
Not only that, but these materials seem to accumulate at a shallow depth in the mantle over a long period of time, rather than sinking deeper inside the Earth. This creates an area deep in the mantle just below the east coast of Australia which is enriched with volatiles.
We looked at the chemical makeup of the rocks produced by these ancient eruptions in the area and found that the vast majority shared common chemical fingerprints. These fingerprints told us that the eruptions in the eastern third of Australia and Zealandia originated from a common mantle reservoir, which could only have formed from the subduction of ancient seabed. It was the last piece of the puzzle that helped us connect seemingly random volcanoes spanning 100 million years of history.
New “eyes” to explore abroad and at home
The combination of perspectives from volcanic history, tectonic plate movements, and geochemistry can also help us unravel other explosive mysteries of our natural world. We hope to further test our model in other enigmatic regions where volcanoes appear amid tectonic plates, such as the western United States, eastern China, and around Bermuda.
In the meantime, we hope that our findings give you a new way of looking at the many beautiful volcanic hills and other features of Eastern Australia. If you’re driving the countryside this summer, here are our top five volcanic sites for your traveling pleasure:
This study was carried out by researchers at the University of Sydney, Monash University and GNS Sciences in Dunedin, New Zealand. It was made possible by Australia’s National Collaborative Research Infrastructure Strategy (NCRIS) Going through AuScope and The Office of the Chief Scientist and Engineer, NSW Department of Industry.