Evidence
for moving mantle plumes... Is
nothing stationary?
The
concept of mantle plumes (or hotspots) was first proposed by J. Tuzo Wilson in
1963 (Wilson, 1963), and then described in more detail by Morgan (1972).
Since the early 1970s most geologists have accepted that hotspots are
essentially fixed features within the mantle, and that they have left a record
of their existence as chains of extinct volcanoes across the plates that have
moved over top of them. The absolute motions of the major plates have been determined
on the basis of this assumption, and numerous other inferences about where
things were at what time, and in which direction they were moving, have
followed.
|
From the U.S. Geological Survey:
A
sharp bend in the chain indicates that the motion of the Pacific Plate
abruptly changed about 43 million years ago, as it took a more westerly
turn from its earlier northerly direction. Why the Pacific Plate changed
direction is not known, but the change may be related in some way to the
collision of India into the Asian continent, which began about the same
time.
From:
http://pubs.usgs.gov/publications/text/Hawaiian.html
(October 2003). Another
USGS website (http://pubs.usgs.gov/publications/text/hotspots.html
) includes a similar explanation, but has a note at the bottom
explaining: “Since this
booklet's publication in 1996, vigorous scientific debate has ensued
regarding volcanism at "hotspots." New studies suggest that
hotspots are neither deep phenomena nor "fixed" in position
over geologic time, as assumed in the popular plume model.”
From a widely used introductory Geology textbook:
Notice
for example ...that the Hawaiian Island-Emperor Seamount chain bends.
This particular bend in the trace occurred about 40 million years
ago when the motion of the Pacific plate changed from nearly due north
to its present northwesterly path.
(Tarbuck
and Lutgens, Earth Science, 10th Ed., 2003)
|
The
best-known example of plate motion over a mantle plume is that of the Hawaiian
Chain and Emperor Seamounts. An
entire generation of students has been taught that the distribution of the
Hawaiian Islands and Emperor Seamounts is the product of the motion of the
Pacific Plate over the Hawaii Hotspot, and that the sharp bend between the two
chains is the result of an abrupt change in the direction of motion of the Pacific
Plate (see box to the right).
Magnetic
data from basalt samples on the Hawaiian Islands have confirmed the view that
the hotspot has been essentially stationary for the past 40 m.y., since most
samples – even those collected far from the area of active volcanism - have
magnetic orientations consistent with the current location of the hotspot at 19º
N. Up to now, however, relatively
little good magnetic data have been acquired for the Emperor Seamounts.
Furthermore, some of the data that do exist are not consistent with a
stationary plume (Tarduno and Cottrell, 1997).
Ocean
Drilling Project Leg 197 (completed in July and August of 2001) was designed to
test the theory that the Hawaii Hotspot may not have been stationary prior to 40
m.y. ago. Bedrock cores were
acquired from three main locations within the Emperor chain (namely Detroit,
Nintoku and Koko seamounts), and magnetic inclination data were acquired for
several hundred samples from each of these holes.
The results are described in a recent report by several members of the
research team, including lead author John Tarduno of the University of Rochester
(Tarduno et al., 2003).
The
new magnetic data show that the rocks of the Detroit, Nintoku and Koko seamounts
formed at close to 32, 26 and 21º N respectively. The interpretation of Tarduno et al. is that the Hawaii
Hotspot migrated south at a rate of close 40 mm/y for the period from at least
100 m.y. ago to around 20 m.y. ago, and that it has slowed down over the past 20
m.y. and is now essentially stationary.
These
data provide strong support for the idea that the Hawaii hotspot has migrated
significantly in the past, although it appears to be stationary now.
The data suggest that the bend in the Hawaii-Emperor chain may be more
likely to be related to a change in the sense of motion of the hotspot, than to
a change in the direction of the Pacific Plate.
If
the Hawaii hotspot can move and then stop moving, then presumably any hotspot
can move or stop moving, or start moving. The
result is that we no longer have a reliable stationary frame of reference from
which to measure the motions of the plates.
References
Morgan
W, 1972, Plate motions and deep mantle convection, Geol. Society of
America. Memoir 132, p. 7-22.
Tarduno
J and Cottrell R, 1997, Paleomagnetic evidence for motion of the Hawaiian
hotspot during formation of the Emperor Seamounts. Earth and Planetary
Science Letters, V. 153, p. 171–180.
Tarduno
J, Duncan R, Scholl D, Cottrell R, Steinberger B, Thordarson T, Kerr B, Neal C,
Frey F, Torii M and Carvallo C, 2003, The Emperor Seamounts:
Southward motion of the Hawaiian Hotspot plume in the Earth's mantle, Science,
V. 301, p. 1064-1069. (August 2003)
Wilson
J, 1963, A possible origin of the Hawaiian Islands, Canadian Journal
of Physics, V. 41, p. 863-870.