Download Tectonics III: Hot-‐spots and mantle plumes

Tectonics III: Hot-­‐spots and mantle plumes Hotspot tracks: Global distribu8on Loca8on of hot-­‐spots and hot-­‐spot tracks: Figures from Turco>e and Schubert 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. Hotspot tracks: Hawaii •  Gradual decrease in eleva8on with increasing distance from the ac8ve volcano. •  The oldest seamounts are found at the northwest end, poised to plunge beneath the Aleu8an 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 reorganiza8on of plate mo8on at that 8me. •  While some think it was the collision of India with the Eurasian subcon8nent, others suggest it was the beginning of spreading on the Antarc8c Ridge south of Australia. Hotspot tracks: Hawaii •  Another remarkable observa8on is that the erup8on rate for Hawaiian volcanoes has remained quite constant over most of the 65 million years of preserved ac8vity. •  This suggests that as volcanic material being erupted, new material is being supplied more or less con8nuously from below. Hotspot tracks: Hawaii •  For the 10 million years following the bend, very li>le lava erupted. This is a bit of a bad situa8on for the previous inhabitants of the islands, since there is very li>le other dry land for thousands of kilometers. 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 drives the plates. Hotspot tracks: The plume model The topographic swell: Hawaii The sea floor surrounding the Hawaii chain of islands is anomalously shallow, rela8ve to normal sea floor of the same age, over an area about 1,200 km wide and 3,000 km long. Figure from Ribe, 2004. Hotspot tracks: The plume model The topographic swell: Bathymetry of the North Atlan8c. Iceland (shown in the center) protrudes from the ocean basin si`ng on a large swell. Images produced by Richard Allen Hotspot tracks: The plume model Seismic tomography: Seismic images suggest the Hawaiian plume originates at the lower mantle. Figure from McNu> and Caress (2007), reproduced from Lei and Zhao, 2006 Hotspot tracks: The plume model Dis8nct geochemical signature: •  The content of incompa8ble 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 Incompa8ble rich Dis8nct geochemical signature: •  In general, Nd/Nd correlates nega8vely with Sr/Sr. •  MORB data are at the upper-­‐
leh corner. •  The OIB are enriched in incompa8ble elements with respect to the MORB. Incompa8ble rich Figure from Hofmann, 1997 Hotspot tracks: The plume model Dis8nct geochemical signature: •  The posi8on of the OIB between MORB and con8nental crust suggests that OIB source may be the result of back mixing of con8nental material into the mantle. •  How different chemical reservoirs may s8ll exist if the mantle is undergoing global mixing is yet an open ques8on. Figure from Hofmann, 1997 Hotspot tracks: The plume model Associa8on 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. Figure from Richards, Duncan and Cour8llot, Science,1989 * Flood basalt are the largest known volcanic erup8ons 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 associa8on of the Deccan trap in India with the Reunion hotspot track. •  The flood basalt erup8on 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 suppor8ng the no8on 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 erup8on rate for Hawaiian volcanoes during the past 65 million years suggests that as volcanic material is being erupted, new material is being supplied more or less con8nuously from below. •  Dis8nct chemistry for the OIB suggests deeper origin for the magma source. •  Seismic tomography. Hotspot tracks: The fixity of hotspots Paleo-­‐magne8c data strongly suggests that all of the lava solidified at 19.5 degrees north la8tude, precisely the la8tude of the hotspot today. At least with respect to la8tude it would seem that the Hawaiian hotspot has been nearly fixed for at least the past 65 million years. Info. Box: magne8c inclina8on and paleo-­‐la8tude: Hotspot tracks: The fixity of hotspots That por8ons 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 direc8ons as the plumes genera8ng them moved independently. Hotspot tracks: The fixity of hotspots Parallel hotspot tracks also within the Indian Ocean. Hotspot tracks: The fixity of hotspots Summary of the geophysical arguments suppor8ng the no8on of fixity of hotspots: •  Paleo-­‐magne8c data indicate that the hotspot la8tude has remained fixed during the past 65 million years. •  Por8ons of island chains of similar age are parallel to each other sugges8ng that the hotspots themselves remain mostly fixed with respect to each other. Hotspot tracks: Absolute plate mo8on Ques8on: In previous lectures we have discussed the rela8ve plate mo8on. Can we infer absolute plate mo8ons as well? 1.  We have seen that the rela8ve mo8on between plates and plumes may be inferred from the trend of hotspot tracks and the island ages. 2.  Plumes are almost fixed. 3.  From 1 and 2, it follows that hotspot tracks can be used to infer absolute plate mo8on. Hotspot tracks: A plume next to a mid-­‐ocean? Difference in age between the volcanoes and the underlying seafloor as a func8on 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 un8l it is less than 30 million years for the oldest known volcanoes in the chain. •  If the trend is con8nued back to about 80 million years, it would appear that the hotspot was building volcanoes on ocean floor of the same age. Fig. from Keller et al., Nature, 2000 Ques8on: 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 ac8ve 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 con8nental 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 si`ng 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.