The Taupo Volcano super-eruption slammed through a deep lake that filled a rift valley, similar to the elongated lakes in East Africa. The combination of water and ash created a hellish dirty thunderstorm, with towering clouds and roaring winds. The detonation flung ash and algae upward at more than 250 mph (400 km/h), Van Eaton said. Volcanic hail (called accretionary lapilli) pelted the landscape for miles.
Van Eaton discovered the diatoms while examining the volcanic hail with a scanning electron microscope.
"The first time I ever saw them I was looking at these volcanic ash aggregates and, bam, these gorgeous little symmetrical shells were there," she said. "Their shells are immaculately preserved."
Van Eaton soon determined that one of the three diatom species entombed in the ash only lives on the North Island of New Zealand. This meant she could track the 25,000-year-old ash layers around the South Pacific with a unique biologic marker. The unique North Island diatoms turned up in a few inches of ash on the Chatham Islands. The diatoms' trip to the Chatham Islands took longer than it looks on a map. The prevailing winds blew west at the time, so the shells circled the Southern Hemisphere before landing on the islands, Van Eaton and her colleagues deduce.
Some of the diatoms even kept their color, both in ash close to the volcano and at the Chatham Islands. The color suggests they weren't cooked to extreme temperatures in the volcanic eruption, Van Eaton said.