How has coral colonised remote islands around the world?

According to a new theory, it's all down to volcanoes. Kate Ravilous reports

What is the connection between an explosive volcano and a tranquil coral reef? At first sight, they may seem to have little in common, but it turns out that some violent volcanic eruptions, such as that of Krakatau in Indonesia in 1883, have been responsible for breathing fresh life into coral reefs that live many thousands of miles away.

The spectacular coral reefs around Christmas Island in the Pacific Ocean benefited particularly from Krakatau's fireworks in 1883, while the Great Barrier Reef on the east coast of Australia, owes much of its dazzling diversity of coral species to undersea volcanic eruptions near the Tonga Trench, north of New Zealand. Linking these explosive volcanoes and magnificent coral reefs are the ocean currents that act like highways across the ocean.

For the past 20 years, Paul Jokiel from the University of Hawaii has been studying coral reefs and pondering how coral can spread around the world. Coral cannot swim or fly across vast oceans, so how do remote islands become surrounded by exotic species of coral? Living in Hawaii, Jokiel was well placed to study the arrival of new corals and one day he noticed a clue, close to the beach. "I found corals attached to floating volcanic pumice and this made me wonder where the pumice had come from," he explains.

Pumice is the lightweight rock produced by fizzing volcanoes that contain lots of gases. These volcanoes spew out frothy magma, which cools into a light, spongy rock. Because of its bubbly texture and low density, pumice can float on water. If the pumice gets swept into one of the ocean's surface currents, then it can be towed along for thousands of miles. Could it be that coral hitch-hikes across oceans on floating lumps of pumice? To test this idea Jokiel realised that he needed to be able to trace the origins of the pumice that washed up on the beach in Hawaii.

Volcanoes differ in the chemistry of the magma that they erupt. Some volcanoes contain more iron oxide, others more potassium oxide. "Geologists can 'fingerprint' magmas from different volcanoes by comparing their chemistry," explains Jokiel. To identify the different chemical elements making up a piece of pumice they use a technique called X-ray fluorescence. The rock is bombarded with X-rays, causing the electrons around each atom to become so excited that they jiggle about. As the electrons settle back down again, the extra energy is re-emitted as an X-ray again. Different elements re-emit different X-rays of characteristic wavelengths. Using this information, geologists can calculate the proportion of each element and compound inside a rock.

Jokiel started collecting pumice from all over the world and used X-ray fluorescence to "fingerprint" its origins. He has identified eruptions from many different places and has built up a sizeable record of the voyages that some of these pumice flows experienced. For example, the pumice from the 1883 eruption of Krakatau was particularly adventurous: after the eruption, pumice drifted both west - across the Indian Ocean - and east - through the Sunda Strait. Ten months later, Krakatau pumice was washing up on the shores of Zanzibar over 6,000 miles away, and after one year, it had reached Durban in South Africa. Much of the pumice going east got caught up in large eddies and swirled around the Maldives and Sri Lanka for two years. Eventually some of the pumice escaped and travelled out into the Pacific, reaching places like Hawaii and Christmas Island.

Part of the reason that the pumice from the Krakatau eruption spread so far and wide was because it was a huge eruption that produced an enormous volume of pumice. But another important factor is that Krakatau was located close to lots of ocean currents, allowing the pumice to disperse in all directions very quickly.

Having realised that pumice is quite capable of crossing oceans, Jokiel, and his colleague Fenny Cox, wanted to see if coral could survive the ride and if there was a pattern linking the species richness of coral reefs with their proximity to pumice trails. During the last two years they have been collecting pumice samples from both Hawaii and Christmas Island and working out where it has come from. They have also been experimenting with growing coral on pieces of pumice to see if the coral can manage to cling on.

Christmas Island lies about 1,000 miles south of Hawaii and its beaches are littered with pieces of pumice. "We identified that the pumice on Christmas Island came from the western Pacific Ocean (Krakatau), the south-western Pacific Ocean (Tonga Trench), the east Pacific Ocean (Mexico), the South Atlantic Ridge and one other unknown source," says Jokiel. By contrast pumice is fairly rare on Hawaii and they could only identify three sources: the South Sandwich Islands, Mexico and Krakatau.

Back at the outdoor aquaria at the Hawaii Institute of Marine Biology they discovered that coral was just as happy to grow on pumice as it was to grow on its normal bedrock of carbonate. This showed that theoretically there was no problem with coral hitching a lift across the ocean on a piece of pumice.

Next they investigated the diversity of coral species around each island. The reef around Christmas Island is bursting with different colours, shapes and sizes of coral, reflecting the 81 species that live there. Meanwhile, Hawaii has a much more restricted range with only 50 different species of coral.

Piecing this evidence together has led Jokiel and Cox to believe that the increased diversity of coral around Christmas Island reflects the fact that the island lies in a confluence of oceanic currents, allowing new species of coral to hitch across the ocean on miniature rafts of pumice. Christmas Island sits at a "spaghetti junction" of ocean highways, while Hawaii, by comparison, sits right out in the marine equivalent of the sticks.

And pumice is not the only form of transport that coral colonies use to move around the globe. "Coral can raft on to many different types of floating objects, from pumice to large drifting trees and logs and even man-made flotsam such as discarded shoes," says Jokiel. "Nature has given us a major natural experiment, putting large amounts of drift tracers [pumice] into the ocean so that we can study the currents."

Given that pumice can be reliably traced to its origins, it may even be possible, using pumice, to study how ocean currents moved in the past. As the pumice floats away from the volcano, its bubbly structure becomes waterlogged. The smallest fragments sink first, followed by larger pieces later. "There is a pattern of pumice contained in the deep-sea sediments that could reveal the pattern of pumice-drift prior to sinking," explains Jokiel.

Downstream of the volcano, a pumice trail is laid across the ocean floor, with bigger and bigger pieces sinking down as time goes by. Geologists often discover particles of pumice in sediment cores drilled from the sea floor. Dating and chemically analysing the source of these pieces of pumice could reveal the direction of ancient ocean surface currents.

Where was the spaghetti junction of ocean currents in the past and has Hawaii always been off the beaten track? These are the kind of questions that are important to climate scientists and pumice could help to answer them all. As well as transporting pumice, driftwood and flotsam, ocean currents are responsible for moving heat and influencing the weather. Understanding precisely how ocean currents moved in the past will help climate scientists to unravel weather patterns and produce better climate models for the future.

It is perhaps some consolation to think that something worthwhile results from these destructive volcanoes. The 1883 eruption of Krakatau generated terrifying tidal waves and a gigantic ash cloud that blocked out the Sun for several days. But now the pumice from that deadly eruption is providing a map of ocean surface currents and is helping us to understand the workings of our planet a little better. And although Krakatau was bad news for humans at the time, it was fantastic for corals. Ocean currents swept coral colonies all over the world on their pumice rafts. Many of the dazzling coral reefs that we admire today owe their existence to the corals that arrived on pumice and driftwood rafts in the past.