A rain of glass
Around 790,000 years ago, a large meteorite struck the Earth, unleashing an explosion that melted terrestrial rock and flung it high into the atmosphere. This molten material cooled and solidified into glassy objects called tektites, which rained back down over a vast area. This event created the Australasian strewnfield, the largest and youngest of the world's tektite fields. It blankets an enormous portion of the globe, stretching from southern China across Southeast Asia and the Philippines, down to Australia, and as far as Antarctica and Madagascar. The total area may cover up to 30% of the Earth's surface, an area of almost 150 million square kilometers.
The tektites themselves are varied and named after the locations where they are found, such as Australites, Indochinites, and Philippinites. Their composition is high in silica, typically around 70-74 wt%, similar to some terrestrial sedimentary rocks. They are famously low in water content, a feature that distinguishes them from volcanic glasses like obsidian. Tektite shapes reveal their violent formation and formation. They include "splash-form" types like spheres, dumbbells, and teardrops, shaped by the physics of rotating liquid drops. Some, particularly in Australia, partially re-melted during atmospheric re-entry, forming distinctive flanges. A more blocky, layered variety known as Muong Nong-type tektites are found closer to the presumed impact area in Southeast Asia and can reach substantial sizes, with some weighing up to 24 kg.
The missing crater
Despite the immense scale of the strewnfield, which implies a very large impact, the source crater has not been found for over a century. The other three major tektite fields on Earth are all associated with known craters: the Chesapeake Bay crater in the USA, the Ries crater in Germany, and the Bosumtwi crater in Ghana. The search for the Australasian crater has been a focus in geology. The distribution and size of the tektites, especially the large Muong Nong types in Indochina, have long pointed to a location somewhere in that region.
In 2020, evidence emerged pointing to a candidate site hidden beneath the Bolaven volcanic field in southern Laos. Researchers identified a gravity anomaly that suggests a buried crater approximately 17 by 13 kilometers in size. Geologic mapping and Argon-Argon dating of the lava flows show that older, pre-impact basalts exist at the site, but the crater itself is covered by younger volcanic rock that flowed after the impact event. The chemical signature of the local rocks is consistent with the composition of the tektites. While drilling is needed for final confirmation, this buried structure is currently the leading candidate for one of geology's great missing features.