Technical Announcements. Employees in the News. Emergency Management. Survey Manual. The eight main Hawaiian Islands are made up of 15 volcanoes, which are the youngest in a linear chain of more than volcanoes above and below sea level that stretches for about 6, km 3, mi across the north Pacific. The Hawaiian Islands red and Hawaiian Ridge-Emperor Seamounts volcanic chain, which consists mostly of submarine seamounts and guyots, are depicted by their outlines at 2-km water depth.
Public domain. The ages of the volcanoes are systematically younger toward the southeast, and the bend in the chain separates the older Emperor Seamount Chain from the younger Hawaiian Ridge. The oldest dated volcano near the northern end of the Emperor Seamount Chain is 81 million years. The bend between the two chains is dated at 43 million years.
All of these volcanoes were built in an assembly line-like process that is driven by plate motion and a "hot spot," or plume of hot material, deep within the Earth. Heat from the Hawaiian hot spot partially melts mantle rock at depths starting about km mi below Earth's surface. This melting produces magma that rises upward through the overlying Pacific Plate. As the plate moves west-northwest, each volcano moves with it from its place of origin above the hot spot.
The age and orientation of the volcanic island chain records the Pacific Plate's direction and rate of movement. The pronounced million-year-old bend between the Hawaiian Ridge and the Emperor Seamounts marks a dramatic change in plate motion direction.
Based on the chemistry of the magma erupted by Hawaiian volcanoes, scientists have identified an idealized sequence of eruption stages for each volcano. This sequence may last 4 to 6 million years. Preshield stage volcanism is the first stage of life for a Hawaiian volcano - infrequent, small-volume eruptions occur below sea level.
The initial stage of volcano growth is the submarine preshield stage , characterized by infrequent, small-volume eruptions. Pillow lava constructs a steep-sided volcanic pile with a shallow summit caldera and two or three riftzones radiating from the summit. Calderas continue to develop and fill repeatedly throughout the preshield and shield stage and go hand-in-hand with the high rate of magmatism that builds the Hawaiian Islands.
Rift zones are prominent features of Hawaiian volcanoes during all but the final eruptive stage. According to Norton 6 , however, evidence for circum-Pacific tectonic events at the time of the bend formation is lacking, and hence the bend must rather reflect the motion of a non-stationary hotspot.
In the decades since, this issue has remained unresolved, and continues to be vigorously debated 10 , 11 , 12 , 13 , 14 , White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. Features encircled with larger white circles are discussed in the text and Fig. Marine gravity anomaly map is from Sandwell and Smith Palaeomagnetic data have proven helpful for testing the non-stationary hotspot hypothesis.
If the volcanic chain was produced by a hotspot fixed in the mantle reference frame, and the rotation of the entire solid Earth mantle and lithosphere with respect to the spin axis that is, true polar wander, TPW 16 were negligible, then the palaeolatitudes inferred from palaeomagnetic analyses of volcanic rocks recovered from the Hawaiian-Emperor Seamounts should correspond to the present-day latitude of Hawaii However, the palaeolatitudes of the Emperor Seamounts 17 , 18 , 19 , 20 , 21 , 22 show a distinct trend of lower latitudes with decreasing age Fig.
The idea that the hotspot was mobile during this time interval has been generally accepted by the geoscientific community, although with some dissenting voices The data are shown as latitude offsets from the present latitude of Hawaii observed latitude minus latitude of Hawaii, The thick black line with yellow stippling shows TPW corrections for Hawaii calculated from ref.
TPW-corrected latitude offsets except for Hawaii are shown as six-pointed yellow stars connected with a dashed line; these were computed by subtracting the values of TPW correction from the observed palaeolatitudes. The palaeomagnetic results are compared with latitudinal estimates of surface motions of the Hawaiian hotspot Fig. Before discussing the estimates of Hawaiian hotspot drift available from geodynamic models and palaeomagnetic data, we find it instructive to provide an illustration of what kind of hotspot drift would be expected in the absence of a change in the Pacific plate motion at the time of the HEB.
The black line in Fig. This simple model highlights an important corollary of the Hawaiian-Emperor Chain geometry and age progression for the motion of the Hawaiian hotspot. In our exercise Fig. In stark contrast, all published estimates of hotspot drift based on geodynamic modelling discussed in the next section inevitably show the Hawaiian hotspot moving either from north to south Fig.
There is therefore no geodynamic basis for a large westward component in the drift of the Hawaiian hotspot. The multi-coloured swaths represent inferred paths of hotspot drift, which, when combined with the Pacific absolute plate motion APM , produce model hotspot tracks yellow lines that accurately track the geometry of and the age progression along the Hawaiian-Emperor Chain see Methods for details.
The red lines track the Pacific APM over an assumed fixed hotspot. The red line shows the track that would be produced if the Hawaiian hotspot were fixed in the same reference frame, that is, reflects the plate motion alone. The difference between the yellow and red lines corresponds to the surface hotspot motion.
After chron 20 time Similarly to a , the track for the moving hotspot yellow line combines the motion of the Pacific plate traced by the red line and the surface hotspot motion rainbow-colored swath from the preferred geodynamic model of ref. Axis numbers indicate arc degrees from the start of the profile. The mantle flow geometry see Methods is dominated by southward flow in the mid-to-lower mantle d , f and northward flow in the uppermost part of the lower mantle c , e and in the upper mantle.
White circles indicate location of Hawaii H. Next we show that geodynamic predictions of southward or SSE hotspot drift 8 , 14 , 28 cannot be reconciled with the assumption that the motion of the Pacific plate did not change at the time of HEB formation. This is illustrated with another simple simulation Fig. In this scenario, the hotspot would have to move five times faster than the Pacific plate at a rate of 3.
Overall, our basic simulations Fig. We now turn to absolute kinematic models that incorporate the geodynamic estimates of hotspot drift and their implications for Pacific plate motion.
Steinberger et al. An alternative plate circuit model Model 2, Fig. Plate circuit Model 2 implies no relative motion between Pacific and Australia plates between cessation of spreading in the Tasman Sea and the transition to plate circuit Model 1, which may well be a simplification.
However, the model track that ignores the southward motion of the Hawaiian hotspot red line in Fig. Yet, in the model of Doubrovine et al. While earlier models of plume motion 8 , 28 were based on backward advection of present-day mantle density structures determined from tomographic models, Hassan et al.
Despite estimates of hotspot drift Figs 2 and 5c that are broadly similar to those of Doubrovine et al. However, the motion of the Pacific plate in the mantle frame model imposed by Hassan et al. Correcting palaeolatitudes for TPW reduces the latitude offsets and the corrected latitudes for the Palaeocene—Eocene seamounts plot below the latitude of Hawaii Fig.
We note, however, that unlike the younger seamounts in the Emperor Chain, Detroit Seamount has a MORB mid-ocean ridge basalt geochemical signature, and as an alternative to plume advection, the latitude offset of the Detroit Seamount has also been suggested to result from ridge-plume interaction Intuitively, it is perhaps peculiar that the TPW-corrected Palaeocene—Eocene seamount latitudes are lower than the latitude of the present-day Island of Hawaii.
This poses a strong challenge for the inferences of hotspot drift derived from the palaeomagnetic latitude record of the Hawaiian-Emperor Seamounts because the lifetime of this anomaly and its possible variations in strength are unknown. This outstanding issue remains to be addressed. The absolute kinematic model of Doubrovine et al. Palaeomagnetic data from localities on the Pacific plate unrelated to the Hawaiian-Emperor Chain can be combined to define a Pacific apparent polar wander APW path which, when corrected for TPW, can give us an independent estimate of the absolute amount of northward plate motion during formation of the Emperor Chain.
There are several published Pacific APW paths for example, refs 25 , 33 , but the temporal resolution of these paths is limited, and they largely rely on palaeomagnetic poles derived from a mixture of sediment and basalt core palaeolatitudes, modelling of seamount magnetic anomalies inversion of magnetic and bathymetric data and the analysis of the magnetic anomaly skewness ref.
Both data sets are shown without confidence ovals for clarity. See Table 1 for details. Simple considerations of the geometry and age progression of the Hawaiian-Emperor Chain Fig. Kinematic models without a significant change in Pacific plate motion around the time of the bend formation necessitate a large westward component in the total hotspot drift Fig. The southward hotspot drift that is predicted by these numerical models lacks a significant component of westward motion, reflecting simple flow geometry beneath the northern Pacific region, which is governed by the persistent large-scale upwelling above the Pacific LLSVP Fig.
This is geodynamically implausible. A strong component of northward motion of the Pacific plate during the formation of the Emperor Seamounts Fig. The interpretation of the palaeomagnetically derived latitudes of the Emperor Seamounts Fig. The possibility of a persistent local magnetic inclination latitude anomaly in the vicinity of the Hawaiian hotspot arising from non-dipole fields further complicates inferences of hotspot drift derived from direct palaeomagnetic estimates.
Given these limitations, great caution should be exercised when taking the palaeomagnetic estimates from the Emperor Seamounts at face value as a record of the southward hotspot drift with respect to the mantle. After more than two decades debating hotspot drift versus Pacific plate motion change to explain the HEB, we must realize that neither of these two end-member options is able to accurately reproduce the geometry and age progression of the Hawaiian-Emperor Chain.
While the change in the direction of the Pacific plate motion is required to account for the geometry of the bend, the more than 2,km-long stretch of the Emperor Seamounts would not have been created had the Hawaiian hotspot not drifted southward from Late Cretaceous to middle Eocene time Fig. Many of the Early Cenozoic components of the Pacific Fig.
We do not include palaeomagnetically derived latitudes in Fig. TPW-corrected latitudes for Hawaii Fig. The yellow-dashed curve for TPW corrections in Fig. For example, if, due to TPW, the north pole was displaced towards Hawaii, the second distance would be smaller than Accordingly, the TPW-corrected latitude curve yellow stars in Fig.
This procedure is not strictly correct given that the TPW corrections were computed for a constant hotspot position. More precise estimates could be obtained in an iterative manner, but this would only lead to minor modifications, trivial in comparison with the uncertainties of the palaeolatitude data.
We excluded data from the Hawaiian-Emperor Seamounts because of possible non-dipole field contributions. The estimates of hotspot drift shown in Fig. The locations of dated seamounts at their respective ages 28 , 40 Fig. The reconstructed locations were approximated by a spherical spline with a smoothing parameter of 20 ref.
The low value of the smoothing parameter 20 was selected to ensure that the inferred hotspot motions were reasonably smooth, but at the same time, when combined with the absolute motions of the Pacific plate, they would accurately predict the geometry of the Hawaiian-Emperor Chain and the age progression of the dated seamounts. The spline-approximated models of the inferred hotspot motion are shown as multi-coloured swaths in Fig.
As a sanity check we computed model hotspot tracks yellow lines in Fig. Because the hotspot motion was computed using the absolute plate kinematics, the model tracks in all experiments are essentially forced to replicate the actual geometry and age progression of the Hawaiian-Emperor Chain almost exactly; the departures arising from spline smoothing are small on the order of few tens of kilometres and are not significant for the purpose of our analysis.
The density model shown in Fig. Otherwise, the model is constructed as in the reference model of that paper The viscosity structure used to compute the flow is Model 2b the preferred model of Steinberger and Calderwood 45 and phase boundaries are treated as in Steinberger Surface plate motions are from Torsvik et al.
The mantle flow model that was used to compute hotspot motion in Fig. Such a separate upwelling is not predicted with earlier tomography models, where Hawaii rather occurs in the middle of a large-scale convection cell, and predicted Hawaii hotspot motion is comparatively large. In contrast, only a few degrees of motion are predicted if the flow model of Fig. The authors declare that all relevant data are available within the article. Other pertinent data are available from the authors upon request.
How to cite this article: Torsvik, T. Pacific plate motion change caused the Hawaiian-Emperor Bend. Wilson, J.
A possible origin of the Hawaiian Islands. Google Scholar. Morgan, W. Convection plumes in the lower mantle. Nature , 42—43 Plate motions and deep mantle convection.
GSA Memoir , 7—22 Deep mantle convection plumes and plate motions. AAPG Bull. Molnar, P. Nature , — Norton, I. 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. As the Pacific Plate continues to move west-northwest, the Island of Hawaii will be carried beyond the hotspot by plate motion, setting the stage for the formation of a new volcanic island in its place.
In fact, this process may be under way.
0コメント