London, Feb 6 (IANS): The Earth's ice ages have left their mark on the thickness of the planet's oceanic crust, scientists have discovered.

During glacial periods, when sea levels are low, the magma that spreads out from mid-ocean ridges to form virgin crust wells up thick and fast.

But the production of new crust is stunted in warmer times when sea levels are high, such as they are today.

"We know that volcanism has an effect on climate. What we are seeing is that climate cycles are also affecting ocean volcanism," said Richard Katz, geophysicist at the University of Oxford and one of the authors of the study.

The researchers spotted the effect in chains of hillocks under the sea between Australia and the Antarctic.

"The reason is that higher sea levels exert a greater pressure on the Earth's mantle below the ocean floor. This seems to slow the transport of molten rock and gas from the mantle up to mid-ocean ridges, where it erupts," Katz explained.

Periodic variations in Earth's axial tilt and orbit around the Sun have driven the planet's succession of ice ages and warm periods over the past two million years.

During an ice age, more water is trapped on land; as a result, sea levels are more than 100 metres lower than in warm periods.

"And that can thicken the oceanic crust by around 800 metres," Katz added.

"This is a fascinating discovery and an important key to understanding the creation of oceanic crust," says Ken Macdonald, geologist at the University of California, Santa Barbara, who was not involved in the study.

The research was published in the journal Science.