The concepts of hotspots and plumes have taken a central place in global geophysical and geochemical research. A common theory to explain the persistent excess volcanism at locations such as Hawaii, Iceland and Yellowstone is that they are supported by columnar upwelling of hot mantle rock called mantle plumes. Yet, these plumes have remained elusive in seismological images of the mantle. While a variety of seismological studies suggest the presence of hot anomalies below hotspot regions, a detection of a narrow whole-mantle plume has yet to be widely accepted.
By combining multidisciplinary expertise, we wish to provide fundamental and quantitative tests of the plume hypothesis. We will establish answers to the following basic questions that address the physics of plume formation, morphology and longevity in Earth and the seismological resolution of plume signatures in seismic data: What are the possible plume structures given geophysical constraints? What is the expected plume signature in tomographic images? Do deep mantle plumes impart observable wave perturbations?
We address these questions using a combined geodynamical, mineral physics, and seismological analysis. The proposing team forms an international collaboration between Jeroen Ritsema and Peter van Keken at Michigan, Derek Schutt at Colorado State, and Saskia Goes at London's Imperial College. We will develop new experience on establishing the structure of plumes and how they are mapped in seismological data sets. This will allow us to determine the optimal data sets and seismic techniques for mapping plumes in the mantle and to understand whether and where mantle plumes can be imaged. The grant will support the interdisciplinary education of three female graduate students and will foster an international collaboration between scientists from the US and the UK. Our Yellowstone studies may prove helpful in forthcoming EarthScope data analysis and the design of a Flexible Array deployment aimed at the Yellowstone hotspot.
The earth has a number of volcanic regions that are far away from plate boundaries or produce excessive volcanism at plate boundaries. Examples of these 'hot spots' include Iceland, Hawaii and Yellowstone. The plate tectonic theory does not explain these features. It has been common to assume that these hotspots are underlain by mantle plumes, which are cylindrical upwellings from the deep mantle. It has been difficult to confirm these warm and seismically slow regions using seismological techniques and this has led some research to doubt that these plumes exist. We have provided new insights whether plumes can be imaged using seismic techniques, that include tomography, a method that is similar to that of a CAT scan in a hospital, albeit it with much lower resolution. We used one of the most recent tomographic models that show how seismic velocities change inside the earth. We create dynamical models of mantle plumes (Bossmann and van Keken, Physics of the Earth and Planetary Interiors, 2013) that could support the active volcanism below the hotspots and studied how much of these features could be recovered by the tomographic technique. Due to the relatively low resolution and smearing of the tomographic method we don't recover the modeled plumes as sharply, but we found that below a significant number of hotspots there were seismic anomalies present that are consistent with the existence of mantle plumes (Styles et al., Earth and Planetary Science Letters, 2011). In a related study we discovered how seismic waves travel through and around these modeled plumes. Much as a walker who can choose between a longer path along the pavement or a shorter path through a muddy field, the seismic waves like to travel through high velocity regions but ignore low velocity regions. This leads to the refocusing of seismic waves that at a certain distance behind the low velocity anomaly seem to have completely ignored the anomaly. We found that for waves traveling through the upper mantle (down to about 600 km) the seismic waves would still keep a record (Hwang et al., Geophysical Journal International, 2012). This project supported the graduate studies of four students (Elinor Styles at Imperial College; Hannah Smith, Andrea Bossmann and Jong Keun Hwang at Michigan) and numerous presentations at meetings in addition to the three publications listed above.
