Scientists have witnessed the deepest erupting undersea volcano ever seen, capturing on video its fiery bubbles of molten lava as they exploded 1,200 metres beneath the surface of the Pacific Ocean in what researchers are calling a major geological discovery.

A submersible robot witnessed the eruption during an underwater expedition in May near Samoa, and the high-definition videos were presented Thursday at a geophysics conference in San Francisco.

Scientists hope the images, data and samples obtained during the mission will shed new light on how the ocean's crust was formed and how the Earth behaves when tectonic plates ram into each other.

"It was an underwater Fourth of July," said Bob Embley, a marine geologist for the U.S. National Oceanic and Atmospheric Administration. "Since the water pressure at that depth suppresses the violence of the volcano's explosions, we could get the underwater robot within feet of the active eruption."

The eruption of the West Mata volcano was a spectacular sight. Bright-red magma bubbles shot up, releasing a smoke-like cloud of sulphur, then solidified almost instantly as they hit the cold sea water, causing black rock to sink to the sea floor. The submersible hovered near the blasts, its robotic arm reaching into the lava to collect samples.

Witnessing a deep-sea volcanic eruption was 25 years in the making. Researchers had studied oceanic volcanoes extensively but never witnessed an eruption. Eighty per cent of the Earth's volcanic activity occurs in the sea, but the underwater locations have complicated scientific efforts.

The mission's chief scientist, Joseph Resing, last year detected volcanic material in the water in the area, and realized it was erupting. In May, the researchers deployed the submersible robot, called Jason, hoping to make scientific history.

"When we got there, we put the sub down and within an hour and a half we found an eruption there in its full glory," said Resing, a chemical oceanographer at the University of Washington. "We haven't seen this before and now for the very first time we see molten lava flowing on the sea floor."

Scientists said the water around the volcano was more acidic than battery acid, but shrimp and certain microbes could thrive in such harsh conditions. Biologists will study these creatures to see if they are unique to this volcanic environment.

Researchers will also continue monitoring the changing West Mata volcano, located about 225 kilometres southwest of Samoa.

Earth and ocean scientists said the eruption allowed them to see the real-time creation of a material called boninite, which had previously been found only in samples a million or more years old.

"The unusual primitive compositions of the West Mata eruption lavas have much to tell us," said Barbara Ransom, program director in the National Science Foundation's Division of Ocean Sciences.