A landscape built by ice and fire
The island of Biak looks like a giant's staircase emerging from the sea. Each massive, flat-topped "step" is a fossilized coral reef, lifted out of the water by immense geological forces. This landscape is a physical timeline of the Pleistocene Epoch, which lasted from about 2.6 million to 11,700 years ago. The island's structure is the result of two powerful planetary processes: the rhythmic cycles of ice ages and the relentless upward push of tectonic plates.
During the Pleistocene, Earth's climate fluctuated dramatically. In warmer interglacial periods, polar ice caps melted, and global sea levels rose. During these high stands, bright coral reefs grew around the shores of Biak. When the climate cooled and massive ice sheets expanded, sea levels dropped by as much as 120 meters, leaving the newly formed reefs exposed. At the same time, Biak island itself is being actively uplifted. It sits near the complex collision zone between the Pacific and Australian tectonic plates, a process that has been shoving the island upwards for millions of years. This constant, steady lift ensures that when the sea level rises again in the next interglacial, the old reef is too high to be drowned, and a new one forms below it. This cycle, repeated over millennia, has created the sequence of terraces visible today.
Reading the steps of time
While detailed dating of every terrace on Biak is not extensively published, the process is well understood from nearby locations. The Huon Peninsula of Papua New Guinea, part of the same tectonic collision zone, is a example. There, scientists have used radiometric dating to assign precise ages to the different terrace levels. The largest terrace, Reef Terrace VII, corresponds to the last interglacial period, a sea-level highstand known as Marine Isotope Stage 5e, approximately 125,000 years ago. Other terraces correlate with subsequent, smaller highstands around 105,000, 80,000, and 60,000 years ago.
On the Huon Peninsula, this combination of uplift and sea-level fluctuation has lifted ancient reefs to elevations of over 600 meters. The uplift rates there can be several millimeters per year. Similar sequences of Pleistocene reef terraces are found throughout eastern Indonesia, including on the islands of Sumba and Sulawesi, each recording local tectonic activity. The fossil corals preserved within these limestone steps—species of Porites, Acropora, and Favites—are often perfectly preserved, showing the structure of ancient reef ecosystems. By studying these fossils, scientists can reconstruct past ocean conditions and understand how reef communities respond to major environmental changes over thousands of years.