This quiet Indonesian island holds microscopic glass beads flung across Asia by a meteorite impact whose crater remained hidden for 790,000 years.
Brocken Inaglory, CC BY-SA 3.0, via Wikimedia Commons
A rain of glass
Around 788,000 years ago, a large asteroid slammed into Southeast Asia. The impact was so powerful it melted and vaporized terrestrial rock, blasting a spray of molten droplets high into the atmosphere. As these droplets flew and cooled, they solidified into glassy objects called tektites. On Semau Island, pristine examples of their smallest form—microtektites—are found preserved in marine sediments.
These microscopic glass beads, typically less than a millimeter in diameter, are relics of the single largest and youngest tektite event known, the Australasian strewn field. This debris field is enormous, covering an estimated 10% to 30% of the Earth's surface, with tektites and microtektites found from southern China to Australia and even Antarctica. The microtektites themselves come in a variety of forms, including perfect spheres, dumbbells, and teardrops, shaped as the molten silica spun through the air. Their composition, around 70% silica, is closer to that of sedimentary rocks than to volcanic glass, and they contain almost no water.
The discovery of these tiny glass spherules on a quiet island like Semau is a giant geological puzzle. They are the far-flung evidence of a cataclysmic event, providing physical data that helped scientists hunt for a crater that remained missing for nearly a century.
The mystery of the lost crater
For decades, the existence of the Australasian strewn field was a paradox. Scientists had ample evidence of the impact's ejecta, but the source crater could not be found. The size and distribution of the tektites, particularly the larger, layered "Muong Nong" types, pointed to a location somewhere in Indochina. Yet, no obvious crater of the expected size—estimated to be many kilometers in diameter—was visible.
The breakthrough came in 2020. A team of scientists presented strong evidence that the crater lies hidden beneath the Bolaven volcanic field in southern Laos. Geological and gravitational surveys revealed a buried anomaly consistent with a large impact structure, approximately 17 by 13 kilometers in size. This structure is covered by younger lava flows, some up to 300 meters thick, which perfectly explains why it evaded detection for so long.
The chemistry of the local Mesozoic sandstone and mudstone bedrock on the Bolaven Plateau matches the composition of the far-flung tektites. The age of the volcanic rock covering the proposed crater is younger than the impact, dating to after the 788,000-year-old event. The tiny glass beads on Semau Island were a critical, distant clue that helped confirm one of the largest meteorite impacts of the last million years.
💡Fun Facts
The energy released by the impact was equivalent to millions of nuclear bombs.
The shapes of microtektites—spheres, dumbbells, and teardrops—are the direct result of molten rock spinning as it cooled while flying through the atmosphere.
Stone tools made by the early human ancestor *Homo erectus* have been found alongside tektites from this impact in southern China.
The Australasian strewn field is one of only four known tektite strewn fields on Earth.
The geological sites are part of the natural landscape and are always open; exploration is best during daylight.
Admission
Free.
Accessibility
The microtektites are not a formal tourist attraction and are embedded in geological formations. Accessing the specific sites requires navigating Semau's natural terrain, which can be rugged and involves unpaved roads.
The Semau Island microtektitesEast Nusa Tenggara, Indonesia
