One of New Zealand’s most spectacular fossil sites originated 23.2 million years ago. It was formed in a valley dotted with small volcanoes, when rising magma deep below the Earth’s surface came into contact with groundwater. Lava and water don’t mix—they explode. The resulting detonation obliterated the surrounding forest and left a circular, low-rimmed volcanic crater, called a maar, about 3,000 feet across and more than 600 feet deep.
The prehistoric rains gradually filled in the hole, forming a lake. Eventually, the subtropical forest grew back, and year after year the trees dropped their leaves and flowers into its still waters. Microscopic algae called diatoms bloomed each spring and died at the end of summer, forming a fine, pale blanket on the lake bottom. In the autumn and winter, a darker layer of decaying plant matter and algal spores settled on top. Together, those alternating seasonal layers formed a barcode-like pattern in the stripy diatomite, recording each year of the maar’s long life.
