It is in southern New South Wales. It is called the Deniliquin structure and is up to 520 km in diameter. It thus exceeds the size of the Vredefort impact structure in South Africa, which is almost 300 km wide. This means that it would be the largest asteroid impact in recorded history.
The history of the bombardment of the Earth by asteroids is largely hidden. Erosion removes the tracks. The crater formed is eroded over thousands or millions of years. They are also hidden by subduction. There, tectonic plates can collide and slide under each other in the Earth’s mantle layer.
New geophysical discoveries are unearthing signs of impact structures formed by asteroids. These are pioneering discoveries of impact ‘ejecta’: the materials thrown out by a crater when it crashes.
The Australian continent and its predecessor, Gondwana, have been subject to numerous asteroid impacts. The result is at least 38 confirmed and 43 potential impact structures. They range from relatively small craters to large, completely buried structures. There is the analogy of the pond and rocks. When a large asteroid impacts the Earth, the underlying crust responds with a transient elastic rebound. This produces a central dome.
It is likely that the Deniliquin structure was found in the eastern part of the Gondwana continent. It was before this was divided giving rise to several continents (including the Australian continent) later.
Effects on Earth
The impact that caused it may have occurred during what is known as the Late Ordovician mass extinction event. It is thought that it may have triggered what is called the Hirnantian glaciation stage. It lasted between 445.2 and 443.8 million years ago. About 85% of the planet’s species disappeared. It was more than twice as big as the Chicxulub impact, which wiped out the dinosaurs. The largest asteroid impact had gigantic repercussions on the planet.
Further studies of the Deniliquin impact structure are expected. Perhaps they will shed new light on the nature of the early Paleozoic Earth.