Crystals found in volcanic rocks from Mauna Loa show that the Earth recycles its molten rock about four times faster than previously suspected, German scientists report.

Rock from the ocean’s crust, which sinks deep into the Earth due to the movement of tectonic plates, re-emerges through volcanic eruptions after about 500 million years, according to a study published Wednesday in the journal Nature.

Scientists previously thought the process took 2 billion years.

Rock samples from Mauna Kea contained residues of seawater that are only 500 million years old, said researchers at the Max Planck Institute for Chemistry in Mainz, Germany.

The scientists arrived at the date by comparing isotopes of the chemical strontium found in glassy inclusions — like tiny flaws in a diamond — in crystals of olivine, the mineral that with gem quality is called peridot. The ratios of the isotopes change over time.

"Apparently strontium from sea water has reached deep in the Earth’s mantle, and re-emerged after only half a billion years, in Hawaiian volcano lavas," researcher Klaus Peter Jochum said in a news release. "This discovery was a huge surprise for us."

Jochum, along with researcher Alexander Sobolev and others, developed a special laser mass spectrometry method that allowed the detection of isotopes of strontium in extremely small quantities.