Researchers are now pursuing geochemical detective work to discover the mystery on the Christmas Island seamounts or underwater mounters, that dot the ocean floor of the northeastern Indian Ocean.

More than 50 large seamounts, some rising as high as 3 miles (4,500 meters), can be found on the Christmas Island Seamount Province's ocean floor which has an area of around 417,000 square miles.

According to geochemist Kaj Hoernle of the University of Kiel in Germany, the seamounts are made of recycled rocks from the and their turbulent geological history explains the massive size and puzzling placement of these features.

Scientists are still at a loss exactly how seamounts are formed. Tens of thousands of seamounts line the floors of the world's oceans, like the Hawaiian-Emperor seamount chain that extends northwest from the Hawaiian Islands. Some seamounts were formed over hotspots in the mantle while others were created when tectonic plate boundaries and other fractures in the ocean crust allowed lava to escape and harden at the surface.

The Christmas Island Seamount Province, however, does not fit either of these models as the structures are too widespread and diffused to have been formed over a single hotspot. The seamounts are also aligned perpendicularly along breaks in the ocean crust, which means they didn't form above a fracture.

"We knew they were volcanic," Hoernle said, "but beyond that, it was more or less a mystery." To solve the puzzle, Hoernle and his colleagues set out to map and collect samples of the seamounts.

According to their findings, the rocks' geochemical signatures did not match those from mid-ocean ridges or hotspot volcanoes, instead, they matched the signatures of continental rocks, particularly rocks from northwestern Australia.

Hoernle's team traced the rocks back to the ancient supercontinent of Gondwana. They found out that around 150 million years ago, greater India, Australia and Burma were all part of this supercontinent but began to rift away from each other which created the spreading center (or mid-ocean ridge) that eventually formed the Indian Ocean.

As this was happening, the bottom part of the continental crust delaminated, or "peeled off in a sheet," Hoernle said. The peeled-off continental crust mixed with the upper mantle, heated up and eventually was pulled to the surface again at the Indian Ocean spreading center, he added.

"When the spreading center passed over that area, it essentially sucked the continental bits and pieces up again," Hoernle said. "Because these pieces have more volatile content (such as water and carbon dioxide), they produced more melted material than the normal upper mantle, and formed seamounts instead of the normal ocean crust."

According to the study, which was published in the journal Nature Geoscience, the first seamount in the Christmas Island Province formed around 136 million years ago. Researchers said the spreading center continued to create seamounts until about 47 million years ago, when it migrated away from the part of the mantle containing the recycled continental crust.