Plate tectonics: Plate geometry 3 Important: This chapter follows mainly on chapter 2 and hilo.hawaii.edu/~kenhon/GEOL205
Hotspot tracks: Global distribution Volcanic chains and hotspot tracks:
Hotspot tracks: A view on the Pacific A closer look at the Pacific:
Hotspot tracks: Hawaii Linear increase of ages with distance along the Hawaii-Emperor chain. Gradual decrease in elevation with increasing distance from the active volcano.
Hotspot tracks: Hawaii The oldest seamounts are found at the northwest end, poised to plunge beneath the Aleutian volcanic arc, carried downward with the oceanic lithosphere as it is consumed.
Hotspot tracks: Hawaii • Note the abrupt bend about 44 millions years before the present, which indicates a major reorganization of plate motion at that time. • While some think it was the collision of India with the Eurasian subcontinent, others suggest it was the beginning of spreading on the Antarctic Ridge south of Australia.
Hotspot tracks: Hawaii • Another remarkable observation is that the eruption rate for Hawaiian volcanoes has remained quite constant over most of the 65 million years of preserved activity. • This suggests that as volcanic material being erupted, new material is being supplied more or less continuously from below.
Hotspot tracks: Hawaii • For the 10 million years following the bend, very little lava erupted. This is a bit of a bad situation for the previous inhabitants of the islands, since there is very little other dry land for thousands of kilometers. Indeed, almost all of the previous life must have been exterminated, so that the current flora and fauna must have arrived more recently.
Hotspot tracks: The plume model • Morgan’s plume model (Morgan, 1971): • Volcanic islands are produced by plumes rising through the mantle. • The plumes come from the lower mantle - and are therefore fixed. • Plume flow derives the plates.
Hotspot tracks: The plume model Hawaii Figure from Ribe, 2004. The topographic swell: The sea floor surrounding the Hawaii chain of islands is anomalously shallow, relative to normal sea floor of the same age, over an area about 1,200 km wide and 3,000 km long.
Hotspot tracks: The plume model The topographic swell: Bathymetry of the North Atlantic. Iceland (shown in the center) protrudes from the ocean basin sitting on a large swell.
Hotspot tracks: The plume model Seismic tomography: Seismic images suggest that some mantle plumes originate at the lower mantle. Figure from Montelli et al., 2004
Hotspot tracks: The plume model • Distinct geochemical signature: • The content of incompatible elements is by 1 to 2 orders of magnitude higher in Ocean Island basalt (OIB, e.g. Hawaii, EM-1 and HIMU) than it is in Mid-Oceanic Ridge Basalt (MORB). • This implies different reservoirs for OIB and MORB. Figure from Hofmann, 1997
Hotspot tracks: The plume model • Distinct geochemical signature: • In general, Nd/Nd correlates negatively with Sr/Sr. • MORB data are at the upper-left corner. • The OIB are enriched in incompatible elements with respect to the MORB. Incompatible rich Incompatible rich Figure from Hofmann, 1997
Hotspot tracks: The plume model Figure from Hofmann, 1997 • Distinct geochemical signature: • The position of the OIB between MORB and continental crust suggests that OIB source may be the result of back mixing of continental material into the mantle. • How different chemical reservoirs may still exist if the mantle is undergoing global mixing is yet an open question.
Hotspot tracks: The plume model Association with flood basalt: Morgan, in 1981, pointed out that a number of hotspot tracks originate in flood basalt* provinces. He explained that flood basalt was produced from a plume head arriving at the base of the lithosphere. * Flood basalt are the largest known volcanic eruptions in the geologic record, and typically comprise basalt of the order of 1 km thick over an area up to 2000 km across.
Hotspot tracks: The plume model • The association of the Deccan trap in India with the Reunion hotspot track. • The flood basalt eruption is due to the arrival of the plume head, and the hotspot track is formed by the plume tail. Figure from Dynamic Earth by G.F. Davies Figure from White and McKenzie, 1989
Hotspot tracks: The plume model • Summary of the arguments supporting the notion of a rigid plate moving atop of a deeply rooted mantle plume: • The straightness of the hotspot tracks and the linear increase of volcanic ages along the track. • Topographic expression. • The nearly constant eruption rate for Hawaiian volcanoes during the past 65 million years suggest that as volcanic material being erupted, new material is being supplied more or less continuously from below. • Distinct chemistry for the OIB suggests deeper origin for the magma source. • Seismic tomography.
Hotspot tracks: The fixity of hotspots Paleo-magnetic data strongly suggests that all of the lava solidified at 19.5 degrees north latitude, precisely the latitude of the hotspot today. At least with respect to latitude it would seem that the Hawaiian hotspot has been nearly fixed for at least the past 65 million years.
Hotspot tracks: The fixity of hotspots That portions of island chains of similar age are parallel to each other suggests that the hotspots themselves remain mostly fixed with respect to each other, otherwise the chains might be expect to trend in different directions as the plumes generating them moved independently.
Hotspot tracks: The fixity of hotspots Parallel hotspot tracks within the Indian Ocean.
Hotspot tracks: The fixity of hotspots • Summary of the geophysical arguments supporting the notion of fixity of hotspots: • Paleo-magnetic data indicate that the hotspot latitude has remained fixed during the past 65 million years. • Portions of island chains of similar age are parallel to each other suggests that the hotspots themselves remain mostly fixed with respect to each other.
Hotspot tracks: Absolute plate motion Question: In previous lectures we have discussed the relative plate motion. Can we infer absolute plate motion as well? We have seen that the relative motion between plates and plumes may be inferred from the trend of hotspot tracks and the island ages. Plumes are almost fixed. From 1 and 2, it follows that hotspot tracks can be used to infer absolute plate motion.
Hotspot tracks: A plume next to a mid-ocean? Difference in age between the volcanoes and the underlying seafloor as a function of distance along the island chain: • At present the age of the sea floor beneath the Big Island is roughly 95 millions years old. • From the bend north along the Emperor chain the age difference steadily decreases until it is less than 10 million years for the oldest known volcanoes in the chain. • If the trend is continued back to about 80 million years, it would appear that the hotspot was building volcanoes on ocean floor of the same age. Question: how is that possible?
Hotspot tracks: A plume next to a mid-ocean? Iceland is a modern example to a plume co-located with a mid-oceanic ridge. Iceland is the only place on Earth where an active mid-oceanic ridge is exposed on land.
Hotspot tracks: Yellowstone There is no reason why plumes be exclusively under oceanic lithosphere and indeed several plumes are found in continental areas too. The Yellowstone is one such example:
Hotspot tracks: Darfor-Levant volcanic array (Garfunkel, 1992) Hotspot track in Israel:
Hotspot on Mars: Mt. Olympus • Mars has no plate tectonics, so hotspot volcanism results in building huge volcanoes that dominate the surface of the planet. • The moving plates on the Earth prevent any single volcano from sitting over the hotspot long enough to build such huge edifices. • Earth's crust is also far too thin to support a volcano as massive as Olympus Mt..