Nearly half a mile of rock retrieved from beneath the seafloor is yielding new clues about how underwater volcanoes are created, and whether the hotspots that led to their formation have moved over time.

Geoscientists have just completed an expedition to a string of underwater volcanoes, or seamounts, known as the Louisville Seamount Trail, in the Pacific Ocean. There they collected samples of sediments, basalt lava flows and other volcanic eruption materials to piece together the history of this ancient trail of volcanoes. The expedition was part of the Integrated Ocean Drilling Program (IODP).

"Finding out whether hotspots in Earth's mantle are stationary or not will lead to new knowledge about the basic workings of our planet," says Rodey Batiza, section head for marine geosciences in the National Science Foundation's (NSF) Division of Ocean Sciences.

Tens of thousands of seamounts exist in the Pacific Ocean. Expedition scientists probed a handful of the most important of these underwater volcanoes.

"We sampled ancient lava flows, and a fossilized algal reef," says Anthony Koppers of Oregon State University. "The samples will be used to study the construction and evolution of individual volcanoes."

Koppers led the expedition aboard the scientific research vessel JOIDES Resolution, along with co-chief scientist Toshitsugu Yamazaki from the Geological Survey of Japan at the National Institute of Advanced Industrial Science and Technology.

IODP, an international research program dedicated to advancing scientific understanding of the Earth through drilling, coring and monitoring the subseafloor, is supported by NSF and Japan's Ministry of Education, Culture, Sports, Science and Technology.

Over the last two months, scientists drilled 3,651 feet into the seafloor to recover 2,644 feet of volcanic rock. The samples were retrieved from six sites at five seamounts ranging in age from 50 million to 80 million years old.

"The sample recovery during this expedition was truly exceptional. I believe we broke the record for drilling igneous rock with a rotary core barrel," says Yamazaki. Igneous rock is rock formed through the cooling and solidification of magma or lava, while a rotary core barrel is a type of drilling tool used for penetrating hard rocks.

Trails of volcanoes found in the middle of tectonic plates, such as the Hawaii-Emperor and Louisville Seamount Trails, are believed to form from hotspots – plumes of hot material found deep within the Earth that supply a steady stream of heated rock. As a tectonic plate drifts over a hotspot, new volcanoes are formed, and old ones become extinct. Over time, a trail of volcanoes is formed. The Louisville Seamount Trail is about 2,600 miles long.

"Submarine volcanic trails like the Louisville Seamount Trail are unique because they record the direction and speed at which tectonic plates move," says Koppers.