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Is intra-plate volcanism dominated by deep or shallow processes?
IS INTRA-PLATE VOLCANISM DOMINATED BY DEEP OR SHALLOW PROCESSES?
There have been a lot of controversies surrounding the question on the true source of volcanicity activities. Numerous scientists have sided to defend the shallow volcanicity process arguing that anomalous magmatism processes are the resultant effects of the melting of fertile bodies due to temperature variations in the shallow mantle rather than the deep mantle thermal plumes. These scientists have attributed volcanism to result from shallow upper mantle processes like edge-driven convection between the thick and thin lithosphere at the boundaries, lithospheric thinning, lithospheric extension and small-scale sub-lithospheric convection among others (Smith 2013, p.66). Proponents of the deep plume theory on the other hand have explained its superiority with suggestions of deep mantle plumes rising in ‘unique’ positions while flowing horizontally away from their centers through the asthenosphere, a movement they add is responsible for plate motion (Gautam 2013, p. 174).
Classical History on the Origin of Volcanicity
The Earth’s volcanicity processes were in the past exclusively explained by the Plate Tectonic theory initially introduced by Wegner in 1915. Plate tectonics explained continental drifting and the presence of sub-ducting plate boundaries and spreading centers which support geochemical and seismic earth building movements (Caroline 2003, p. 1).
The mechanism was however inconsistent due to its inability to account for the presence of volcanic island chains like the Hawaii, far away from known plate margins as well as large igneous provinces (LIPs), ocean islands, oceanic plateaux, sea mounts, intra plate super volcanoes and mid ocean ridges from deep within the mantle (Caroline 2003, p. 1: Gillian 2010, p. 9). This generated a need for further research on alternative hypotheses that make perfect sense, a quest that led to the birth of the Mantle Plume and the Shallow Upper Mantle processes theories. It is these two models that will be the main focus of this paper which will be supporting the Plume Hypothesis as more plausible compared to the Shallow Mantle theory.
Argument against Plate flexure and fracturing
According to a study carried out by Hirano et al., (2006), plate flexure and fracturing provided pathways for magma output to the earth’s surface evinced by a line of volcanism he called the “petit spot” volcanoes formed in parallel alignment to the Japan Trench. This theory may however not be a true explanation for the presence of the volcanoes in the plate because Intra-palate volcanism has been able to explain the existence of the Hawaiian Volcanic Islands within the Emperor Seamount Chain in the middle of the Pacific Plate (Gautam 2013, p. 174). Wilson was the first to propose in 1963 their formation from ‘hot-spot’ volcanicity as a result of rising hot, buoyant mantle plumes from the mantle on a migrating lithosphere (Gillian 2010, p. 4).
Figure 1: The Emperor and Hawaiian Seamount Chains with the Hawaiian starting at the Hawaiian Islands all the way to the South East beyond to the bend at 5000km to the North West, it is from this bend that the Emperor Chain Continues terminating at the Aleutian Trench (Courtesy of NOAA).
Figure 2: Illustration of the progressive formation of older islands above a stationary mantle plume (Courtesy of the USGS).
Hirano supported his theory with several geochemical experiments done by testing the samples from the study area and analyzing their geochemistry compositions. Similar radiogenic compositions of the rock samples to those of the mid-ocean ridge basalts were found suggesting that the volcanism in the region was indeed shallow sourced.
It is however important to also note that a similar scientific study on petrography and the mineral chemistry of basalts from the Snake River Plain has been carried out by scientists supporting the Plume Hypothesis which attributed the origin of the basaltic rocks in the region to the deep ‘hot-spot’ theory. The mineral, trace element and whole rock major geochemistry of the olivine tholeiites were established to have a basalt source of a spinel peridotite rather than a pyroxenite. The average potential mantle temperatures are also generally hotter than the ambient mantle at 1577 degrees Celsius suggesting that the basaltic liquids are derived from a deep thermal plume (Richard 2012, p. 5).
Argument against Edge-driven convection
The presence of upwellings at the edges of Archean cratons, a series of small localised convection cells in the shallow mantle in Newer Volcanics Province, southeast Australia have according to a recent paper by Davies and Rawlinson 2014, been explained by lateral temperature difference. Teleseismic tomography of the seismic data set implied the presence of a shallow mantle source. An anomaly direction within a depth of 200 km in the research discredits the Plume theory because the upwellings are at 100 km depth corresponding to an excess of 500 °C is high for a plume.
This may not be a solid base for a complete elimination of the mantle plumes theory. Plumes rise through the asthenosphere as plastically deforming masses with bulbous plume heads with diameters of between ~500 to ~1000 km which are fed by long, narrow plume tails. As the plumes rise, pressure always drops, while the temperature remain high causing melting of the hot mantle material which leads to the generation of large volumes of magma.
Figure 3: Schematic illustration a mantle plume that is rising (By Zkelly1 [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)].
Plumes have survived for up to 10’s of millions of years and have always able to cross the endothermic 670 km discontinuity as they ascent. Since at lithosphere, the base plume do not ascend, their transfer of heat is restricted and hence causing thermal erosion of the lithosphere which consequently thins it above the hot spot. This excess heat is what causes an uplift of 1 to 2 kilometers before volcanism evident in the Marques Volcanic System. Plume heads have therefore been formed at the bases of the lithosphere during the initial phases of contact (Caroline 2003, p. 1).
Figure 4: A computer simulation from the Minnesota supercomputing lab illustrating the bulbous plume heads which are later on flattened as they impinge on the outer sphere at the base of the lithosphere and the narrow plume tails (Volcano Discovery, Retrieved from: http://www.volcanodiscovery.com/geology/plate-tectonics/hotspots.html.
Argument against Lithospheric Thinning
Magneto-telluric and mantle tomographic studies supporting the Plume theory indicate that the lithosphere is very thin under the Snake River Plain and thicker on the flanks to the northeast and south beyond the Yellowstone, a key phenomenon unique to the Plume Hypothesis (Matthew 2012, p. 479). Heavy rare earth elements of the Snake River Plain basalts have also over time been observed to remain resistant to depletion to their erosion resistant garnet source from deep within the mantle giving a strong evidence in favor of the Plume theory (Richard 2012, p. 5). This is because the thinning of a continental lithosphere cannot produce enough melt in the mantle to produce volcanism a explained by shallow processes theory. Thinning by delamination and extension as a result of gravitational instability can only lead to the instability of the lower lithosphere rather than the production of melt under continents.
Argument against Distal subduction tectonics
The distal subduction tectonics theory attributes the formation of the volcanic field in Jeju Island in Korea to the late activation of pre-existing shear zones in the mantle. Geochemistry studies supporting the plume theory however support plumes as the initiating mechanisms for the early stages of continental drift and break up that led to the massive volumes of magma that erupted at the beginning of the development of LIPs from the excess heat of plume heads (Caroline 2003, p.1). The geochemistry studies of the rocks in the region furthermore indicate the presence of a deep plume source (Richard 2012, p. 5).
Finally, the Plume Theory is supported by its unique relationship with a wide range of intra-palate volcanoes like the Hawaiian- Emperor Chain, Cape Verde, Snake River Plain, Réunion and Marques systems. The figure 3 below gives a clear illustration of some of the most significant intra-plate volcanoes.
Figure 5: Hot-spots in the World (Volcano Discovery, Retrieved from: http://www.volcanodiscovery.com/geology/plate-tectonics/hotspots.html.
CAROLINE, F. 2003. Do Mantle Plumes Exist? Undergraduate Research. Faculty of Science,
Kingston University, UK.
GAUTAM, S. (2013). Petrology: principles and practice. Berlin, Springer.
GILLIAN R. F. 2010. Plates vs. Plumes: A Geological Controversy. John Wiley & Sons Ltd,
MATTHEW J. F. The Yellowstone Hotspot: Plume or Not? Geological Society of America. vol .
40, no. 5, pp. 479-480.
RICHARD W. B. 2012. Mineral chemistry of basalts recovered from Hotspot Snake River
Scientific Drilling Project, Idaho: Source and crystallization characteristics. Brigham Young University. All Theses and Dissertations. Paper 3359.
SMITH, A. D. 2013. Recycling of oceanic crust and the origin of intra-plate volcanism.
Australian Journal of Earth Sciences. vol. 60, no. 6-7, pp. 63-75.
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