The mysterious bend in the Hawaiian-Emperor chain
Date: June 8, 2017
Source: GFZ GeoForschungsZentrum
Potsdam, Helmholtz Centre
Summary: The volcanic islands of
Hawaii represent the youngest end of a 80 million years old and roughly 6,000
kilometers long mountain chain on the ground of the Pacific Ocean. The
so-called Hawaiian-Emperor chain consisting of dozens of volcanoes is well
known for its peculiar 60 degrees bend. The cause for this bend has been
heavily debated for decades. Scientists now offer an explanation in a new
study.
FULL STORY
The Hawaiian-Emperor Chain is an
example of a hotspot track - a trail of volcanic islands and seamounts created
on a lithospheric plate as the plate slowly shifts over a spot of localized
melting sourced by a jet of hot material rising from the deep mantle (mantle
plume). For more details see the release.
Credit: T. Torsvik et al. (GFZ)
The volcanic islands of Hawaii
represent the youngest end of a 80 million years old and roughly 6,000
kilometres long mountain chain on the ground of the Pacific Ocean. The
so-called Hawaiian-Emperor chain consisting of dozens of volcanoes is well
known for its peculiar 60 degrees bend. The cause for this bend has been
heavily debated for decades. One explanation is an abrupt change in the motion
of the Pacific tectonic plate, the opposite model states southward drift of the
mantle plume that has sourced the chain since its beginning 80 million years
ago. Apparently both processes play an important role, shows a new study in Nature
Communications, published by a group of scientists from the University of
Oslo, German Research Centre for Geosciences GFZ Potsdam, and Utrecht
University.
Many volcanic ocean islands are
created by columnar shaped hot upwellings called mantle plumes that originate
near the ~3000 km deep base of Earth's mantle. Mantle plumes are not much influenced
by surface motions of the tectonic plates that slowly move over them. Hence,
long linear chains of plume-sourced volcanoes that get older and older with
increasing distance from active hotspots can be tracked for hundreds to
thousands of kilometres. In the Hawaiian hotspot trail, the Hawaii islands are
the youngest in the chain that stretches nearly 6,000 km to Detroit seamount in
the northwest Pacific, where volcanism occurred about 80 million years ago. An
unprecedented 60 degrees bend characterizes the Hawaiian-Emperor Chain,
dividing it into the older Emperor Chain and the younger Hawaiian Chain. The
bend has been dated to 47 Ma.
"The ultimate cause for the
formation of the Hawaiian-Emperor Bend (HEB) was a prominent change in the
Pacific plate motion at 47 Ma," says the lead author of the new study,
Trond Torsvik from the University of Oslo and visiting researcher at GFZ at the
moment. The team affirms a hypothesis by the US-geophysicist Jason Morgan who
proposed that already in the early 1970s. "But it is not that simple as it
was suggested forty years ago," says Torsvik.
Jason Morgan was the first to use
hotspots as a reference frame for global plate motions. In his model mantle
plumes -- which are manifested by hotspots at the surface -- were considered
fixed in the mantle, and the Hawaiian-Emperor Bend was attributed to a simple
directional change of the Pacific plate motion. But his plate model with fixed
hotspots became challenged from the 1980s.
"Since the late 1990s it has
become clear that hotspots are not totally fixed," says GFZ´s Bernhard
Steinberger, one of the co-authors of the paper. That is now generally
accepted, he adds, and mantle flow models predict that the Hawaiian hotspot has
drifted slowly to the south. "But some recent studies have argued that
rapid southward motion of the hotspot before 47 Ma can explain the formation of
the bend without requiring Pacific plate motion change," he says.
"Such a scenario has become attractive because the geology of the plates
surrounding the Pacific shows no clear evidence for a Pacific plate motion
change."
The new study shows clearly why this
simply does not work. It would require an unrealistically high rate of hotspot
motion of about 42 cm/year which would be much faster than the average speed of
tectonic plates. Moreover, this would imply that the Emperor Chain was created
in just five million years and Detroit Seamount should only be 52 million years
old. This prediction is obviously falsified by the recorded Detroit Seamount
island ages of about 80 Ma.
"Alternatively, a slower
hotspot motion towards the WSW could explain both geometry and ages of the
Emperor chain," says Steinberger. However, such a direction of motion is
inconsistent with mantle convection models.
"Our paper is a good example of
how very simple simulations of plate and hotspot kinematics can be used to
explore which geodynamic scenarios for the formation of the Hawaiian-Emperor
Bend are possible, and which ones are not," says Pavel Doubrovine from the
University of Oslo, another co-author on the paper. "We cannot avoid the
conclusion that the 60 degrees bend is predominantly caused by a directional
change in the Pacific plate motion." Yet, some southward plume motion is
required, otherwise the Hawaiian-Emperor Chain would be around 800 kilometres
shorter.
"Explaining the geometry,
length and age progression of the Hawaiian-Emperor Chain, requires both: the
change in the direction of plate motion and the movement of the hotspot,"
states Torsvik. "If, after more than two decades of debating the
end-member scenarios of plate motion change versus hotspot drift, geophysicists
will be able to agree that neither of the two is satisfactory -- then we can
move forward and address a more interesting question: what actually drove the Pacific
plate motion to change at about 47 million years ago?" Hopefully, it will
not take further 40 years to get an answer to this, he adds.
Story Source:
Materials provided by GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre.
Note: Content may be edited for style and length.
Journal Reference:
- Trond H. Torsvik, Pavel V. Doubrovine, Bernhard Steinberger, Carmen Gaina, Wim Spakman, Mathew Domeier. Pacific plate motion change caused the Hawaiian-Emperor Bend. Nature Communications, 2017; 8: 15660 DOI: 10.1038/ncomms15660
Comments