Rocks in the Earth's mantle melt at a much greater depth than once thought, which could explain phenomena puzzling geologists for years, U.S. researchers said.

Scientists at Rice University have reported experiments proving rock can and does liquefy, at least in small amounts, as far as 150 miles down in the mantle beneath the ocean floor.

The mantle is Earth's middle layer, a buffer of rock between the crust, the top 5 miles or so, and the planet's solid iron core.

The mantle is a rolling mass of rising and falling material that slowly but constantly brings materials from deep within the planet to the surface, and occasionally higher through volcanoes, in a process of convection.

The mantle beneath the ocean is where the new crust is created and where "the connection between the interior and surface world is established," Rice geologists Rajdeep Dasgupta said.

Rocks melt to create magma, which rises with the convective currents, cools and spread out to form the ocean crust.

The starting point for melting has long been thought to be around 40 miles beneath the sea floor, Dasgupta said, but seismic waves measured during earthquakes have shown unexpected results as they move through the layers beneath the Earth's surface.

"Seismologists have observed anomalies in their velocity data as deep as 200 kilometers (125 miles) beneath the ocean floor," he said. "Based on our work, we show that trace amounts of magma are generated at this depth, which would potentially explain that."