An impact mystery solved

RESEARCHERS have announced a solution to a longstanding mystery: the location of an impact on the Earth that took place about 790,000 years ago.

The identified site is in the Bolaven Plateau in Laos, and the event was responsible for the formation of tektites that have been found over a large area in South East Asia and Australia, known as the Australasian Strewn Field.

The term strewn field is used to describe the area over which meteorites or tektites from a particular impact can be found.

Tektites are not meteorites. They are glassy objects that relate to impacts on the Earth, formed from molten terrestrial material sent to great altitudes by the energy of the impact.

As they descend and solidify, they take on shapes such as ‘buttons’ and ’dumbbells’.

Meteorites are actual extraterrestrial material that has impacted Earth.

There are other general locations around the world over which tektites have resulted from an impact.

These are in central Europe, North America and the Ivory Coast area of Africa.

However, the Australasian strewn field is the largest.

Although craters thought to be responsible for other strewn fields have been identified, the location of the source crater for the Australasian one has been a mystery. Suggestions have included the Gulf of Tonkin east of Hanoi, and Kazakhstan.

Now, in a recently published paper, a team led by Kerry Sieh of the Earth Observatory in Singapore has announced very strong evidence for the crater being buried under volcanic material in the Bolaven Plateau.

This plateau is in the southern part of Laos, east of a lower plateau in Thailand known as the Khorat Plateau.

The Bolaven Plateau occupies the area east of the town of Pakse, in the Lao province of Champasak.

Pakse is Laos’ second largest city, with a population similar to that of Launceston in Tasmania.

A few years ago, I travelled from Pakse east into the Bolaven Plateau and visited many of its attractions: several photogenic waterfalls and vendors selling some of the best coffee in the region.

Today, it is a very peaceful place. It suffered very intensive bombing during the Vietnam War − but as it turns out, it was subject to an event that made that later human destruction pale into insignificance.

I did not know, at the time, that I was adjacent to what was likely the largest major impact on Earth over the past million years.

It is thought that the object that hit us was an asteroid approximately two kilometres in diameter.

The impact resulted in a crater that the researchers have found to cover an area of about 17 x 13 kilometres.

(Try to picture this next time you are standing atop Mount Wellington west of Hobart: it is about the distance from there to Opossum Bay on the South Arm Peninsula.) The researchers have not actually seen the crater: it is buried under much volcanic material on the Bolaven Plateau.

Instead, they have inferred its likely presence by detecting a gravity anomaly.

By measuring the local gravity at many locations in the area, they have detected the existence of deposits that are consistent with the presence of a crater, which is now underground.

The detection of a likely crater is not the only piece of evidence presented by the team.

They also base their evidence on the types of rocks on the plateau, the fact that exposed lava is younger than the impact, and an outcrop of boulders near the site that seem to have been shattered by the impact.

The impact sent a large quantity of molten terrestrial material into the atmosphere, which solidified as it descended, forming the tektites in the Australasian region that we find today.

They have been found in Australia, Indonesia, and many other parts of South East Asia.

This strewn field has been known for about a century, but it is only now that the mystery of their origin seems to have been solved.