Hotspots and plumes

One of the pioneers of plate tectonics, W. Jason Morgan, recognised in the 1970s that chains of volcanic islands and seamounts that rise from the ocean floor may have formed as the movement of lithospheric plates passed over sources of magma that lay in the mantle beneath the plates. He suggested that such hotspots were fixed relative to plate movements at the surface and likened the formation of chains such as that to the west of the volcanically active of the Hawaiian ‘Big Island’ to linear scorching of a sheet of paper moved over a candle flame. If true, it should be possible to use hotspots as a framework for the absolute motion of lithospheric plates rather than the velocities of individual plates relative to the others. But Morgan’s hypothesis has been debated ever since he formulated it. A test would be to see whether or not plumes of rising hot material in the deep part of the mantle can be detected. This became one of the first objectives of seismic tomography when it was devised in the last decade of the 20th century: a method that uses global earthquakes records to detect parts of the mantle where seismic waves traveled faster or slower than the norm: effectively patches of hot (probably rising) and cold rock. The first such evidence was equally hotly debated, one view being that the magma sources beneath oceanic islands such as Hawaii and Iceland were actually related to plate tectonics and that the hotspot hypothesis had become a kind of belief system.

English: global distribution of 45 identified ...
Global distribution of hotspots ( credit: Wikipedia)

The problem was that mantle plumes supposedly linked to magmatic hotspots in the upper mantle would be so thin that they would be difficult to detect even with seismic tomography. Geophysicists have been trying to sharpen up seismic resolution partly by using supercomputers to analyse more and more seismic records and also by improving the theory about how seismic waves interact with 3-D mantle structure. This has culminated in more believable visualisation of mantle structure (French, S.W. & Romanowicz, B. 2015. Broad plumes rooted at the base of the Earth’s mantle beneath major hotspots). The two researchers from the University of California at Berkeley in fact showed something different, but still robust support for Morgan’s 40-year old ideas. Instead of thin plumes, they have been able to show much broader conduits beneath at least 5 and maybe more active ends of hotspot chains. The zones extend upwards from the core-mantle boundary to about 1000 km below the Earth’s surface, where some bend sideways towards hotspots, perhaps as a result of another kind of upper mantle circulation.

Whole-Earth seismic tomography cross sections beneath a variety of volcanic islands, (Credit French and Romanowicz;
Whole-Earth seismic tomography cross sections beneath a variety of volcanic islands, (Credit French and Romanowicz;

The sources of these hot columns at the core-mantle boundary appear to be zones of very low shear-wave velocities; i.e. almost, but not quite molten blobs. French and Romanowicz suggest that the columns are extremely long-lived and may even have a chemical dimension – as in the hypothesis of mantle heterogeneity. Another interesting feature of their results is that the striking vertical linearity of the columns could indicate that the overall motion of the lower mantle is extremely sluggish and punctured by discrete convection.

Pristine mantle and basalt floods

The Western Ghat hills at Matheran in Maharash...
Flood basalts of the Deccan Traps in Maharashtra State, India. Image via Wikipedia

Plot the ages of major extinctions against those of flood basalt events and you will get a straight line graph for six co-occurrences since 250 Ma, with very little error. Although the exact mechanism for mass death of species and families is argued over interminably, for those six, flood basalt events have to be deeply implicated. There again, every geologist and their aunties dispute the mechanisms behind monster basalt effusions that bury whole landscapes beneath flow after flow and create very distinctive landforms.  When they are eroded they form regularly stepped mountain sides, hence their formerly popular name trap basalts, after the Swedish word trappa meaning staircase.  There is a hint of cyclicity in their age distribution. But most important of all, no-one has witnessed these vast, pulsating events, the last having mantled the surroundings of the Columbia and Snake River catchments in the US states of Oregon and Washington between 14-17 Ma ago in the Middle Miocene. Some mark episodes of continental break-up, such as those flanking the Central Atlantic at the time of the end-Triassic (~200 Ma) mass extinction, while others are associated with hot spots, such as the Deccan Traps of western India erupted between 60-68 Ma as India drifted over the Reunion hot-spot and those of the Ethiopian highlands (30 Ma) associated with the Afar hot spot.

A common geochemical feature is beginning to emerge concerning the mantle from which the basalts were partially melted. Six sets of flood basalts exhibit the same trace-element and isotopic (Nd, Pb, Hf and He) characteristics, which suggest that their source had been little effected by previous extraction of crust-forming magmas; it is primitive and may be a relic of the original mantle formed at about 4500 Ma shortly after the catastrophic collision between the early Earth and a wandering Mars-sized planet that flung off the Moon (Jackson, M.G. & Carlson, R.W. 2011. An ancient recipe for flood basalt genesis. Nature, online (27 July 2011) doi:10.1038/nature10326). Although undepleted, the chemistry of the mantle source, worked out by back-calculation from that of the flood basalts, is not the same as the once-postulated original accretion of carbonaceous chondrite meteorites: conceivably a result of the chemical reworking when the Moon formed and the remaining Earth was probably molten from top to centre. The important feature is that the recast chemistry is rich in heat-producing elements compared with the source of ‘common-or-garden’ basalts that continually contribute to the ocean floors and island arcs. Wherever the relic mantle is, it is capable of heating itself, over and above the heating from the core and surrounding mantle, and thus likely to generate thermal and material plumes rising through the mantle.

Preceding the work of Jackson and Carlson, another group discovered that when flood basalt events since the Carboniferous are restored to their former geographic positions at the time they were erupted, they cluster above what are now two patches of more ductile mantle close to the cure-mantle boundary (Torsvik, T.H. et al. 2010. Diamonds sampled by plumes from the core–mantle boundary. Nature, v. 466, p. 352–355). If that is the source of basalt flood-forming plumes, then it is still there and, aside from giant impacts with extra-terrestrial projectiles, the most catastrophic upheavals of the Earth system inevitably will continue, perhaps in the next few million years.