A 790,000-year-old mystery
Approximately 788,000 years ago, a large asteroid struck the Earth, unleashing an explosion that melted terrestrial rock and blasted it high into the atmosphere. This molten material cooled and solidified into glassy objects called tektites, raining back down over an enormous area. This event created the Australasian strewn field, the largest and youngest of the planet's four known tektite fields. It covers an estimated 10% to 30% of the Earth's surface, stretching from southern China to Australia and from the Indian Ocean to the western Pacific. The total mass of glass deposited is estimated to be in the tens of millions of tons.
Tektites are distinct from volcanic glasses like obsidian. They are exceptionally dry, containing almost no water, and lack the microscopic crystals common in volcanic rocks. They are found in a variety of "splash-form" shapes, including spheres, dumbbells, and teardrops, formed as the molten glass spun through the atmosphere. Tektites found in Australia, known as Australites, often have a distinctive "flanged button" shape, created by partial remelting during their high-velocity reentry, much like a spacecraft's heat shield.
The missing crater
Despite the massive scale of the debris field, the source crater remains elusive. Based on the volume of ejecta, scientists estimate the crater must be between 40 and 100 kilometers in diameter, a significant geological feature, which should be easy to find. The central puzzle is why a crater so large and so relatively young has not been definitively located.
The leading hypothesis places the impact site in Southeast Asia, likely in Indochina. One compelling candidate location is buried beneath the Bolaven volcanic field in southern Laos. Researchers have identified a gravity anomaly there consistent with a buried, elongated crater approximately 13 by 17 kilometers in size. Lavas overlying the proposed site are younger than the impact event, suggesting subsequent volcanic activity completely buried the crater. Outcrops of rock breccia containing shocked quartz grains, a hallmark of an impact, have been found 10-20 km from the proposed crater's center, adding weight to this theory. Other proposed locations have included areas in Cambodia, the Gulf of Tonkin, and even northwest China, but the Bolaven Plateau remains the most strongly supported candidate.
The impact was so significant that early human relatives, such as Homo erectus, would have witnessed it. Stone tools have been found alongside tektites in southern China, indicating a hominid population lived in the region during and after the cataclysmic event.