Woleai Atoll at sunset

Here we’ve just entered Woleai Atoll, part of the outer islands of Yap State in western Micronesia.  An atoll is a ring of coral reefs left behind when an island sinks into the ocean over millions of years.  When ocean crust forms, it is hot and buoyant and floats high on the Earth’s mantle.  Volcanic mountains that form on top of the ocean crust may emerge from the water as islands, and coral reefs grow around their shores.  As the ocean crust ages, it cools and becomes denser and thinner, riding lower on the mantle.  Volcanic islands are brought down with the cooling ocean crust, and therefore slowly sink below the ocean surface with time.  But the fringing coral reefs grow fast enough to keep up with the sinking island, and manage to stay near the surface where they receive the sunlight they need to live.  Eventually, the island disappears entirely, and the only thing that remains is the ring of coral reefs – an atoll.  We’ll be anchoring the ship in the middle of the lagoon for the next two days while we go out in small boats and study the coral reefs around the edges.