Hotspot volcanoes

How Cracking Explains Underwater Volcanoes and the Hawaiian Bend

Geoscientists at the University of Sydney have helped prove that some of the ocean’s undersea volcanoes did not erupt from hotspots in Earth’s mantle, but instead formed from cracks or fractures in the oceanic crust.

The discovery helps explain the dramatic bend in the famous seamount chain, the Hawaiian-Emperor Seamount Range, where the lower half bends at a sixty-degree angle east of its upper half.

“There has been speculation among geoscientists for decades that some submarine volcanoes form because of fracturing,” said Professor Dietmar Muller, of the University of Sydney’s School of Geosciences in Australia. and author of research results published in nature geoscience.

“But it’s the first comprehensive analysis of the rocks that form in this setting that confirms their origins.”

It has long been accepted that when Earth’s plates move over fixed hotspots in its underlying mantle, the resulting eruptions create chains of now-extinct undersea volcanoes or “seamounts.”

One of the most famous is the Hawaiian-Emperor Range in the northern Pacific Ocean. The seamounts in this chain are composed mostly of ocean island basalts – the type of lava that erupts above hotspots.

But north of the Hawaiian Range, in a formation called Musicians Ridge, researchers found samples of seamounts that weren’t made up of the oceanic island basalts you’d expect from plates moving overhead. from a hot spot.

“The oldest part of the Musicians Ridge formed about 90 million years ago from hotspots, but these new samples are only about 50 million years old and have a different geochemistry. “, Professor Muller said.

“They did not form because of a hot spot but because of the cracking of plates at their weakest point, allowing new magma to rise on the seabed and restart the formation of underwater volcanoes. “They are nearly extinct hotspot volcanoes because that hotspot power millions of years earlier helped weaken the crust (the layer directly above the mantle) where new volcanoes now form.”

Vulnerable points in Earth’s plates crack when stressed, in this case due to movement of the Pacific plate which began plunging or plunging back into the Earth’s crust at its northern and western edges about 50 million years ago. of years.

The formation of these younger seamounts caused by deformation of the Pacific Plate at its margins suggests a connection to the unique bend of the Hawaiian-Emperor Range.

“We believe that tectonic changes along the margins of the Pacific plate about 50 million years ago stressed the weakest points of the Pacific Ocean crust and created the youngest seamounts. sailors from Musicians Ridge,” Professor Muller said.

“It also caused a dramatic change in flow in the slow-convective mantle beneath the Pacific, to the point that the Hawaiian hotspot in Earth’s mantle shifted position.

“The resulting seamounts along the Hawaii-Emperor Range shifted their position accordingly and the bend was born.”

This work provides a solid foundation for understanding other ‘non-hot’ volcanism observed elsewhere, for example the Puka Puka ridge in the South Pacific.

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