Pulsing Mantle Plumes: Causes and Geological Consequences
Lead Research Organisation:
Imperial College London
Department Name: Earth Science and Engineering
Abstract
Between Earth's crust and core lies the mantle, 2,900-km of hot rock that comprises 80% of Earth's volume. Carrying Earth's internal heat to the surface, the convecting mantle creeps like tar on a hot day. This overturning is the 'engine' that drives our dynamic Earth; all large-scale geological activity is driven by mantle convection. Mantle plumes are an important, but poorly understood, aspect of mantle convection. They are buoyant mantle upwellings that bring hot material from Earth's deep mantle to the surface, lifting Earth's surface as they rise and generating large volcanic provinces and volcanic islands, such as Iceland and Hawaii. There is increasing evidence that these upwelling plumes change in shape and strength through time; they pulse. Such pulses produce corresponding changes in surface topography, which has major implications for a range of surface processes. For example: (i) hydrocarbon generation - variations in sediment supply, which are driven by tectonic uplift and subsequent erosion, strongly influence the stratigraphy of sedimentary basins, which ultimately controls the distribution of hydrocarbons; and (ii) ocean circulation - the pulsing Icelandic mantle plume has played a central role in controlling regional uplift, which, in turn, has moderated overflow of cold North Atlantic Deep Water into the global ocean circulation system. This system transports both energy (in the form of heat) and matter (solids, dissolved substances and gases) around the globe. Consequently, pulsing mantle plumes have a direct influence on global climate. While the observational evidence for pulsing is increasing, the ultimate cause is unknown. Consequently, we cannot explain or predict the strength and frequency of pulses, or their signature at Earth's surface. Establishing a relationship between pulsing plumes and their surface manifestation is important if we are to understand Earth's geological history and the long-term evolution of Earth's mantle. To address these gaps in current understanding, the proposed research will use state-of-the-art numerical mantle convection models to investigate the mechanisms that cause pulsing in mantle plumes and to predict the surface response to these pulses. The cause of pulsing will be established by numerically testing a range of hypotheses; for example, that the rate of upwelling is controlled by changes in forcing from sinking slabs of cold crustal material. Each hypothesis will be validated by comparing model predictions of pulsing behavior with geological observations, predominantly from the North Atlantic Ocean. Models will also be used to predict how Earth's surface responds to these pulses; for example, when, where and by how much the surface is uplifted as the upwelling waxes and wanes. These predictions will be tested against observations of how the North Atlantic has responded to the pulsing Icelandic mantle plume. As well as solving a long-standing problem in Earth science, an important outcome of this research will be a predictive model of the response of Earth's surface to flow within Earth's interior. In other words, the model will show how Earth's 'engine' - mantle convection - drives surface deformation. Such a connection is vital for advancing the many fields of Earth sciences that examine the consequences of Earth's dynamic surface.
Organisations
People |
ORCID iD |
| D Davies (Principal Investigator) |
Publications
Davies D
(2016)
The mantle wedge's transient 3-D flow regime and thermal structure
in Geochemistry, Geophysics, Geosystems
Davies D
(2015)
On the relationship between volcanic hotspot locations, the reconstructed eruption sites of large igneous provinces and deep mantle seismic structure
in Earth and Planetary Science Letters
Davies D
(2011)
Fluidity: A fully unstructured anisotropic adaptive mesh computational modeling framework for geodynamics FLUIDITY-MODELING GEODYNAMICAL FLOWS
in Geochemistry, Geophysics, Geosystems
Davies D
(2012)
Reconciling dynamic and seismic models of Earth's lower mantle: The dominant role of thermal heterogeneity
in Earth and Planetary Science Letters
Davies D
(2014)
On the origin of recent intraplate volcanism in Australia
in Geology
Davies D
(2013)
A hierarchical mesh refinement technique for global 3-D spherical mantle convection modelling
in Geoscientific Model Development
Davies DR
(2015)
Lithospheric controls on magma composition along Earth's longest continental hotspot track.
in Nature
Garel F
(2014)
Interaction of subducted slabs with the mantle transition-zone: A regime diagram from 2-D thermo-mechanical models with a mobile trench and an overriding plate
in Geochemistry, Geophysics, Geosystems
| Description | Over the course of my fellowship, I have developed sophisticated tools for numerically simulating mantle convection, whilst also demonstrating the power of these to improve our understanding of mantle dynamics. I first provide an overview of the techniques and tools that I have developed and describe how these have lead to a step-change in our ability to simulate multi-scale geodynamic processes. I subsequently describe how I have applied these tools to address several fundamental questions in mantle dynamics research. My substantial contribution to this field of research recently received peer-recognition, via the prestigious `Outstanding Young Scientist Award' from the Geodynamics Division of the EGU. Development of Innovative Schemes for Simulating Multi-Scale Global Mantle Dynamics: Mantle convection modeling involves solution of mass, momentum and energy equations for a viscous, creeping, compressible non-Newtonian fluid at high Rayleigh and Peclet numbers. Despite its central importance to our understanding of the dynamics of the solid Earth, simulation of global mantle convection down to the relevant length scale of faulted plate boundaries has generally been considered intractable, due to the wide range of time and length scales involved. The length scales of interest range from several km at faulted plate boundaries (requiring a mesh with a resolution of at least 1 km), to the ~10^4 km scale of the globe. Similarly, time scales range from ~1000 years to capture transport at the smallest length scales, to global transit time scales of ~10^8-10^9 years. Taken together, this implies a space-time problem with ~10^16-10^17 degrees of freedom, which is far beyond the reach of contemporary supercomputers. The advent of Petascale computing promises to make such simulations tractable. However, uniform discretization of the mantle at 1 km resolution would result in meshes with nearly a trillion elements. Mantle convection simulations on such meshes are estimated to require one year on a sustained one Petaflop system. Adaptive mesh refinement methods, where the underlying computational grid is modified as the simulation evolves, provide a means to address these issues. Such schemes use intelligent algorithms to modify the computational grid automatically, placing enhanced resolution only where required, thus efficiently capturing a range of length-scales within a single model and reducing the number of unknowns drastically. I pioneered the use of these schemes within the field of geodynamics and have subsequently played a key role in their adoption within the community (Davies et al. 2007, 2008, 2013). Having demonstrating the applicability of adaptive mesh refinement techniques for geodynamical models, during my fellowship, I utilized them as a basis for developing a next-generation computational modeling framework - Fluidity - a finite-element, control-volume model with several features that place it at the forefront of computational fluid dynamics (e.g. Davies et al. 2011, Kramer et al. 2012). The code: (i) uses an unstructured mesh, which enables the straightforward representation of complex geometries; (ii) dynamically optimizes this mesh, providing increased resolution in areas of dynamic importance, thus allowing for accurate simulations across a range of length-scales, within a single model; (iii) enhances mesh optimization using anisotropic elements; (iv) is optimized to run on parallel processors and is one of very few fluid flow models with the ability to perform parallel mesh adaptivity; (v) has solvers that can handle the sharp variations in viscosity that occur within Earth's mantle and lithosphere; and (vi) has been extensively benchmarked and validated for geodynamical simulation (Davies et al. 2011). To put it simply, Fluidity can replicate the results of other community codes at a fraction of the computational cost (both in terms of required CPU time and memory). Furthermore, the code's unrivalled functionality opens up a whole new class of multi-scale problems to geodynamical research. As a result, Fluidity will be central to my research over the coming years. Indeed, after years of successful development, the stage is set to exploit Fluidity's unique functionality to address several outstanding, multi-scale geodynamical problems. Application to Fundamental Geodynamical Problems: I have utilized these powerful techniques to provide fundamental new insights into a range of processes. For example: (i) the efficiency of heat transfer within Earth's mantle; (ii) mantle plume dynamics at realistic Rayleigh number); (iii) controls on the distribution of hotspots and large igneous provinces (Davies et al. 2014); (iv) the flow regime and thermal structure of the subduction zone mantle wedge (Le Voci et al. 2013); and (v) the dynamical interpretation of seismic images (Styles et al. 2011; Davies et al. 2015), particularly relating to Earth's deep mantle (Davies et al. 2012). Although my research focuses on geodynamical modeling, I also utilize other research avenues. For example, during me fellowship, I exploited Geographical Information Science (GIS) methods to provide a revised estimate of Earth's surface heat flux (Davies & Davies, 2010). This estimate is larger than previous predictions, with important implications for the energy budget of Earth's interior and our understanding of Earth's thermal evolution. I have also worked closely with experimentalists, in an attempt to bridge the gap between experiment and simulation (e.g. Hunt et al. 2012). I next describe, in detail, the two most significant aspects of my applied research over the past 3 years. The Geographical Distribution of Hotspots and Mantle Plumes: The geographical distribution of hotspots provides a unique constraint on the nature of lower mantle dynamics. Recent studies propose that hotspots, and the reconstructed eruption sites of large igneous provinces, preferentially occur above the margins of two deep-mantle large low shear-wave velocity provinces (LLSVPs) beneath Africa and the Pacific, which are commonly believed to represent hot, but dense, thermo-chemical `piles'. This correlation is interpreted to imply that `piles' are long-lived, stable features, which have strongly influenced mantle dynamics and surface tectonics over several hundred million years (Myr). However, in Davies et al. (2014), we re-analyzed the correlation between hotspot locations and LLSVP margins, demonstrating that it had previously been overstated: it is strong for the African domain, but weak in the Pacific. These regional differences provide a fundamental new constraint on the nature of lower mantle dynamics. Using multi-resolution numerical models, in which the distribution of heterogeneity is dictated by assimilated plate motion histories, we subsequently showed that the observed geographical variability in correlation could be reproduced in the absence of chemical `piles'. Our results do not rule out the possibility that LLSVPs contain a dense chemical component. However, they do imply that, if present, chemical `piles' play only a secondary role in controlling the location of hotspots. Indeed, our study suggests that hotspot locations are a natural consequence of ancient subduction. This implies: (i) a more intimate association between Earth's upper and lower thermal boundary layers than has previously been appreciated; and (ii) a less significant role for chemical heterogeneity in dictating the form of mantle dynamics. Our study will play an important role in resolving the long-standing debate on the origin of hotspots, their geographical distribution and their relation to deep mantle structure, which is key to understanding Earth's thermal and chemical evolution. Seismological Insights into the Nature of Lower Mantle Heterogeneity: As noted previously, two large low shear velocity provinces (LLSVPs) in the deep mantle beneath Africa and the Pacific are generally interpreted as hot but chemically dense `piles'. These `piles' are thought to have remained isolated from mantle circulation for several hundred million years, influencing heat transfer within Earth's interior and the mantle's geochemical evolution. This interpretation largely hinges on three seismic observations: (i) their shear wave velocity anomalies are considered too large for purely thermal structures; (ii) shear wave velocity gradients at their edges are sharp; and (iii) their shear and bulk-sound velocity anomalies are anti-correlated. However, in Davies et al. (2012), we utilized global mantle convection models and thermodynamic methods for converting from physical to seismic structure to show that observed lower mantle shear wave velocity anomalies and shear wave velocity gradients do not require, and are most likely incompatible with, large-scale compositional heterogeneity. Our results also suggest that, in the presence of post-perovskite, the correlation between shear and bulk-sound velocity anomalies cannot be used to discriminate between thermal and compositional heterogeneity at depth: in all models examined, an anti-correlation only occurred within the post-perovskite stability field. Taken together, this implies that although there must be considerable chemical heterogeneity within Earth's mantle, large, coherent chemical `piles' are not required to reconcile seismological observations of Earth's lowermost mantle, as was previously implied. Our study has dramatic implications for the likely form and distribution of chemical heterogeneity within Earth's interior, implying that it is most likely widely distributed, occurring on length-scales that do not strongly influence lower mantle dynamics and the lower mantle's long-wavelength seismic structure. The novelty and significance of this contribution is demonstrated by its selection for a recent summary/highlight in Nature Geosciences (November 2012). It also played an important role in my recruitment to the RSES at the ANU in 2014. |
| Exploitation Route | The tools I have developed continue to be utilized (by myself and colleagues) to solve fundamental problems across the solid Earth sciences. |
| Sectors | Digital/Communication/Information Technologies (including Software),Education |
| Description | The modelling tools that I and colleagues developed are now being used by several research groups around the world (approximately 200 users). The application of these models to understanding the dynamics, seismic expression and geochemical signature of mantle plumes has led to several publications (with others in the pipeline). These will have a long-lasting academic impact. My research is now also directly affecting industry, through an ongoing collaboration with Shell International Exploration and Production. |
| First Year Of Impact | 2012 |
| Sector | Digital/Communication/Information Technologies (including Software),Other |
| Impact Types | Economic |
| Description | ARC Future Fellowship |
| Amount | $683,700 (AUD) |
| Funding ID | FT140101262 |
| Organisation | Australian Research Council |
| Sector | Public |
| Country | Australia |
| Start | 01/2015 |
| End | 12/2018 |
| Description | Research Fellowship: The Australian National University |
| Amount | $300,000 (AUD) |
| Organisation | Australian National University (ANU) |
| Sector | Academic/University |
| Country | Australia |
| Start | 01/2014 |
| End | 12/2016 |
| Description | 2 Invited Presentations: AOGS Brisbane |
| Form Of Engagement Activity | Scientific meeting (conference/symposium etc.) |
| Part Of Official Scheme? | No |
| Type Of Presentation | keynote/invited speaker |
| Geographic Reach | International |
| Primary Audience | Participants in your research and patient groups |
| Results and Impact | (i) The Nature of Lower Mantle Dynamics; (ii) The Relationship Between Mantle Plumes and LLSVPs. Both talks simulated intense questions and discussion. Further ideas on how to use plume location to constraint nature of lowermost mantle heterogeneity. |
| Year(s) Of Engagement Activity | 2013 |
| Description | AGU Fall Meeting 2011 |
| Form Of Engagement Activity | Scientific meeting (conference/symposium etc.) |
| Part Of Official Scheme? | No |
| Type Of Presentation | poster presentation |
| Geographic Reach | International |
| Primary Audience | Participants in your research and patient groups |
| Results and Impact | Presentation given on the seismic expression of Earth's lowermost mantle. This is particularly relevant as it is in the source region of mantle plumes. Poster generated sometimes heated discussion and some useful feedback. Further collaborations! |
| Year(s) Of Engagement Activity | 2011 |
| Description | AGU Fall Meeting 2012 |
| Form Of Engagement Activity | Scientific meeting (conference/symposium etc.) |
| Part Of Official Scheme? | No |
| Type Of Presentation | poster presentation |
| Geographic Reach | International |
| Primary Audience | Participants in your research and patient groups |
| Results and Impact | Presentation given on the geographical distribution of mantle plumes and their relationship to deep mantle LLSVPs. Several fruitful discussions took place. Several new ideas on how to better analyse data (statistically). |
| Year(s) Of Engagement Activity | 2012 |
| Description | EGU General Assembly 2014: Hotspots |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | International |
| Primary Audience | Participants in your research and patient groups |
| Results and Impact | An invited talk on the origin of recent intra-plate volcanism in Australia. Talk sparked interesting discussion, with several of the questions/suggestions being helpful in guiding my future research direction. Talk sparked interesting discussion, with several of the questions/suggestions being helpful in guiding my future research direction. |
| Year(s) Of Engagement Activity | 2014 |
| Description | Invited Seminar: Australian National University |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | International |
| Primary Audience | Participants in your research and patient groups |
| Results and Impact | School Seminar given on the Origin of Lower Mantle Seismic Heterogeneity. As several of the world's experts on this topic are at RSES, the ANU, it stimulated some productive discussion. Further collaborations and access to a new dataset (p-wave velocity model of Earth's mantle). |
| Year(s) Of Engagement Activity | 2011 |
| Description | Invited Seminar: Department of Earth Sciences, University of Bristol, UK. |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | Regional |
| Primary Audience | Participants in your research and patient groups |
| Results and Impact | Reconciling Dynamic and Seismic Models of Earth's Lower Mantle: The Dominant Role of Thermal Heterogeneity. The talk sparked questions and discussion afterwards. Led to a successfully funded collaborative NERC research grant (with Huw Davies - Cardiff; James Wookey - Bristol), and an ongoing collaboration with Dr. Andrew Walker (who has since moved to Leeds). |
| Year(s) Of Engagement Activity | 2012 |
| Description | Invited Seminar: Lamont-Doherty Earth Observatory, Columbia University, New York. |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | International |
| Primary Audience | Participants in your research and patient groups |
| Results and Impact | Presentation on the Origin of Lower Mantle Seismic Heterogeneity. Talk sparked questions and intense discussion. A new collaboration with Marc Spiegelman and his research group. |
| Year(s) Of Engagement Activity | 2011 |
| Description | Medal Talk: EGU General Assembly 2014 |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | International |
| Primary Audience | Participants in your research and patient groups |
| Results and Impact | A 30 minute keynote lecture summarizing some of my recent research findings, as a part of my "Outstanding Young Scientist Award" from the geodynamics division of the EGU. The talk sparked some interesting discussion and debate, on several aspects of my research. Further collaboration with several colleagues. |
| Year(s) Of Engagement Activity | 2014 |
| Description | Presentation: Simula Research Laboratories, Olso, Norway. |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | National |
| Primary Audience | Professional Practitioners |
| Results and Impact | Overview of research and future plans: discussion on potential future collaboration. Ongoing collaboration with Simula. |
| Year(s) Of Engagement Activity | 2012 |
| Description | Seminar: Macquare University, Sydney |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | Regional |
| Primary Audience | Participants in your research and patient groups |
| Results and Impact | Seminar on the relationship between volcanic hotspots, large igneous provinces and deep mantle seismic structure. Talk sparked questions and discussion afterwards. Several interesting implications of my research, of which I was previously unaware, became apparent. |
| Year(s) Of Engagement Activity | 2013 |
| Description | Seminar: Monash University, Melbourne |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | Regional |
| Primary Audience | Participants in your research and patient groups |
| Results and Impact | Departmental Seminar at the School of Geosciences, Monash University, Melbourne, on "The relationship between mantle plumes and deep mantle seismic structure". Talk sparked questions and hours of interesting discussion afterwards. An ongoing collaboration with Louis Moresi and his team at Monash. |
| Year(s) Of Engagement Activity | 2013 |
| Description | Seminar: University of Sydney |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | Regional |
| Primary Audience | Participants in your research and patient groups |
| Results and Impact | Departmental Seminar at the School of Geosciences, the University of Sydney on "The relationship between mantle plumes and deep mantle seismic structure". Talk sparked questions and hours of interesting discussion afterwards. An ongoing collaboration with the Earthbyte Group at the University of Sydney. |
| Year(s) Of Engagement Activity | 2013 |