ONE of geology鈥檚 central assumptions is crumbling. The 鈥渉ot spots鈥� where
plumes of molten magma break through the Earth鈥檚 crust appear to be wandering
across the planet鈥攁 discovery that undermines many of the accepted ideas
about how the Earth鈥檚 crust is moving.
Geologists thought that magma plumes such as the one that created the
Hawaiian Islands remain in place as the plates of the Earth鈥檚 crust move over
them. For years, they have used these hot spots as a fixed frame of reference to
gauge the motion of the plates relative to the Earth鈥檚 core.
The Hawaiian Islands and their underwater neighbours, the Emperor Seamounts,
were used by geologists as a record of the path followed by the plate that makes
up the floor of the Pacific. The islands formed as the plume of upwelling magma
erupted onto the surface of the floor, creating volcanoes. But a new study by
Robert Duncan of Oregon State University in Corvallis and his colleagues shows
that the Hawaiian hot spot has probably shifted.
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The researchers measured the direction in which the minerals in the volcanic
rocks of the islands were magnetised. When the lava flows that formed the
islands cooled and solidified, the minerals lined up with the Earth鈥檚 magnetic
field, providing a record of their location at the time. If the islands had all
formed in the same place, over a stationary hot spot, they should all have the
same magnetic fingerprint. But the minerals line up differently鈥攅vidence
that the islands formed at different latitudes. 鈥淭he reference frame that we
were all hanging onto turns out to have legs it can move around on,鈥� says team
member David Scholl of Stanford University.
The finding challenges ideas about how the plates have shifted in the past.
For example, a prominent bend in the Hawaiian-Emperor seamount chain that
occurred 43 million years ago was thought to have been caused when the Pacific
plate suddenly changed direction. Now some geologists are throwing that idea
overboard in favour of a moving hot spot.
The idea is likely to be controversial. 鈥淧eople suspected that hot spots were
moving,鈥� says geologist Robert Butler of the University of Arizona in Phoenix.
鈥淏ut the one they all wanted to hang onto was the biggest, baddest hot spot of
them all鈥攖he Hawaiian hot spot.鈥�