Embargo 0100 Tuesday, 28 April
Plate cracking can produce underwater volcanoes and
explains famous Hawaiian ‘bend’
University of Sydney geoscientists have helped prove that some of the ocean’s underwater
volcanoes did not erupt from hot spots in the Earth’s mantle but instead formed from cracks or
fractures in the oceanic crust.
The discovery helps explain the spectacular bend in the famous underwater range, the
Hawaiian-Emperor seamount chain, where the bottom half kinks at a sixty degree angle to the
east of its top half.
“There has been speculation among geoscientists for decades that some underwater volcanoes
form because of fracturing,” said Professor Dietmar Muller, from the University of Sydney’s
School of Geosciences and an author on the research findings published in Nature Geoscience
on 28 April.
“But this is the first comprehensive analysis of the rocks that form in this setting that confirms
their origins.”
It has long been accepted that as the Earth’s plates move over fixed hot spots in its underlying
mantle, resulting eruptions create chains of now extinct underwater volcanoes or ‘seamounts’.
One of the most famous is the Hawaiian-Emperor chain in the northern Pacific Ocean. The
seamounts of that chain are composed mainly of ocean island basalts - the type of lava that
erupts above hot spots.
But north of the Hawaiian chain, in a formation called the Musicians’ Ridge, researchers found
samples from seamounts that were not made up of the ocean island basalts you would expect
from plates moving over a hot spot.
“The oldest part of the Musicians’ Ridge formed approximately 90 million years ago from hot
spots but these new samples are only about 50 million years old and have a different
geochemistry,” said Professor Muller.
“They did not form because of a hot spot but because of plates cracking open at their weakest
point, allowing new magma to rise to the seabed and restart the formation of underwater
volcanoes. They are near extinct hot spot volcanoes because that hot spot action millions of
years earlier helped weaken the crust (the layer directly above the mantle) where new
volcanoes now form.”
Vulnerable spots in the Earth’s plates crack when they are stressed, in this case due to
movement of the Pacific Plate which started to dive or submerge back into the Earth’s crust at
its northern and western edges around 50 million years ago.
The formation of these younger seamounts caused by the deformation of the Pacific Plate at its
margins suggests a link to the unique bend in the Hawaiian-Emperor chain.
“We believe tectonic changes along the margins of the Pacific Plate around 50 million years ago
put the weakest points of the Pacific Ocean crust under tension and created the youngest
Musicians’ seamounts,” said Professor Muller.
“It also caused the flow in the slowly convecting mantle under the Pacific to change
dramatically, to the point that the Hawaiian hot spot in the Earth’s mantle changed its position.
“The resulting seamounts along the Hawaii-Emperor chain changed their position accordingly
and the bend was born.”
This work provides a solid foundation for understanding other ‘non-hot spot’ volcanism seen
elsewhere, for example the Puka Puka Ridge in the South Pacific.
The lead author on the paper is Professor John O’Connor from the Alfred Wegener Institute for
Polar and Marine Research in Germany.
Media enquiries:
Verity Leatherdale: (02) 9351 4312, 0403 067 342 or verity.leatherdale@sydney.edu.au