海洋底ダイナミクス 2018 Ocean Floor Geodynamics 2018 13. プレート内火成活動：海山と海台 Intraplate volcanism: seamounts and plateaus プレート内火成活動 Intraplate volcanism Type1 古典的なホットスポット：不動か？“Classic” type hot spot Type2 スーパースウェル Super swell Type3 リソスフェアのクラック＋プチスポット Lithosphere cracks + petit spot 巨大火成岩岩石区 Large Igneous Provinces (LIPs) 巨大海台 Oceanic plateaus 環境・地球史への影響 Environmental impact 1 地球の火成活動（現世） volcanism on earth (Press, Understanding Earth, 2003 2 FEATURE | FOCUS Mantle plumes persevere Anthony A. P. Koppers The ocean floor is littered with hundreds of thousands of mostly extinct volcanoes. The origin of at least some of these seamounts seems to rest with mantle plumes. recent census suggests that Morgan’s mantle plumes conduits are difcult to resolve using seamounts1 — typically extinct Some hotspots form seamount trails seismic data4,5, their existence has been A underwater volcanoes — are along the surface of tectonic plates, far difcult to confrm. Many question whether numerous. It has been estimated that away from their volcanically active plate all hotspot volcanism is formed by mantle about 125,000 seamounts with a height boundaries. Forty years ago, W. Jason plumes6,7 and some doubt whether mantle of more than one kilometre exist on our Morgan introduced the concept of mantle plumes exist at all8,9. ocean foors. Most of these are postulated plumes to explain this kind of hotspot Perhaps the most captivating aspect to form at volcanic hotspots that are the volcanism2,3. According to his theory, of Morgan’s plume model is that he could surface expressions of mantle plumes plumes of hot material upwell from the explain the formation of the Hawaiian– — hot material upwelling from Earth’s deep mantle. During ascent and on impact Emperor and three other seamount trails interior. Yet, many seamounts do not show with the overlying tectonic plates, these along the Pacifc Ocean foor. Tese three the typical characteristics expected for plumes drive melting and the production trails track each other in such a way that volcanoes that have formed above a mantle of magma, which erupts to form volcanic they can be explained by the rotation of plume. So, debate about the feasibility of seamounts at the plate surface. Morgan a rigid Pacifc plate that is drifing over the mantle plume hypothesis is ongoing. proposed that the migration of tectonic four plumes fxed in the mantle (Fig. 1a). Te most straightforward explanation is plates over stationary and long-lived With this observation, Morgan supplied that not all hotspot volcanoes are alike, mantle plumes would generate chains of compelling support for the existence of and that some groups of seamounts are volcanoes on the ocean foor. However, plumes and also provided independent better explained by mechanisms other than because mantle plumes themselves cannot proof for the motion of tectonic plates mantle plumes.intraplate volcanism be directly sampled and the thin plume relative to the underlying mantle. Te mantle plume model also opened up プレート内火成活動についての最近の認識 potential new avenues of research into the Earth’s deepest regions10. Specifcally, Hotspot if the volcanic seamounts are the surface type 2 expression of a mantle plume, their Mid-ocean erupted lavas could potentially preserve a Hotspot spreading Hotspot type 3 centre type 1 record of long-lived variations in mantle composition and could provide insights into mantle convection. Holes in the theory 67 Te plume model calls on an extensive 0 km global network of long-lived stationary mantle plumes that are continually delivering hot material from deep in the Earth. However, such a global network never fully materialized. It now seems that there aren’t many active hotspot systems CMB around the world, maybe a few dozen — too few to have produced all of the (Koppers, 2011 world’s seamounts. Of those seamounts that do seem to have 3 formed above a mantle plume, some show Figure 1 | Models of ocean-island and seamount-trail formation: Courtillot’s framework6 of three evidence that the underlying plume was hotspot types. The first, a classical Morgan-style long-lived mantle plume, originates from as deep neither long-lived nor stationary. Improved in the mantle as the core–mantle boundary (CMB). The second hotspot type includes short-lived, mapping of seamount trails using satellite smaller plumes originating from shallower parts of the mantle, probably as ofshoots from large altimetry reveals that most seamount trails superplumes. These secondary hotspots are more common. The third type of hotspot is not related to have typical life spans of just 30 million any kind of mantle ウィルソンの定義したホットスポットplume and may form where the oceanic lithosphere cracks or extends. This kind is years11. And samples of lava collected from the least investigateddefinition and may overlap of hotspot considerably by with Wilson the other hotspot(1963) types. the Emperor seamounts during the Deep 816 NATURE GEOSCIENCE | VOL 4 | DECEMBER 2011 | www.nature.com/naturegeoscience © 2011 Macmillan Publishers Limited. All rights reserved (Wilson, 1963 relatively small, long-lasting, and hot regions -- called hotspots -- must exist below the plates that provide localized sources of high heat energy (thermal plumes) to sustain volcanism. 4 固定ホットスポット仮説と海山列 fixed hotspot hypothesis and hotspot track • Hotspots are fixed in deep mantle = no relative motion among hotspots • Seamounts chain extending from hotspot is a kind of “track” of plate motion. • support ‘plate tectonics’ • provide past plate motions (Understanding Earth, 2003) 5 ハワイー天皇海山列 Hawaii hotspot and Emperor seamount chain (Press, Understanding Earth, 2003 6 ホットスポットの分布 Global distribution of hotspot Based on UTIG hotspot list 7 ホットスポット火成活動の特徴 Hotspot volcanism : Features • Courtillot et al.(2003)’s five criteria • long-lived tracks • traps at their initiation • magma flux > 103 kg/s • High 3He/4He or 21Ne/22Ne • anomalously low shear velocities (Vs) in the mantle below 8 noticeable for the Hawaiian-Emperor seamount trail (e.g., Watts, 1976) and is believed to be directly related to the buoyancy of a plume and its interac- FLEXURAL tion with the overlying Pacifc Plate MOAT (Figure 1). Mapping of these swells using satellite-derived gravity and geoid data, for instance, allowed scientists to equate the sizes of these swells to vertical plume fuxes. Although the MID-PLATE volume of active intraplate volcanism is SWELL small compared to island arc volcanism and the formation of the oceanic crust 0 Myr 1.4 Myr 4.2 Myr 7.0 Myr 15.215 2 MyMyrr 23.623 6 MyMyrr ホットスポット軌跡 at the mid-ocean ridges, plume fuxes 0 1 ranging from 1.0 Mg s-1 (Canary) to track and mantle plume Thermal Anomaly 8.7 Mg s-1 (Hawai`i) become signifcant when integrated over geological time and including all known hotspot systems (Davies, 1988; Sleep, 1990). Further observations that support the presence of mantle plumes include evidence that these lithospheric swells diminish away from active hotspots, the formation of linear age-progressive seamount trails, and the volcanic extinction of seamounts Figure 1. Te Hawaiian-Emperor hotspot trail is our textbook example of the classical mantle (Van Keken, 1997 when plate motions move them away plume model explaining the formation of intraplate seamounts. In this map of the Northwest Pacific (D. Sandwell and W.M. Smith: Gravity Anomaly Map based on Satellite Altimetry, from their hotspot locations. Version 15.2), this archetypical seamount trail is exemplified by a deep flexural moat along its But mantle-plume behavior is not entire length and a significant mid-plate swell only prevalent toward the young southeastern end. Te linearity of this seamount trail, in combination with a large mid-plate swell and a quite so simple, as the latest numerical systematic age progression (with radiometric ages increasing toward the older northwestern mantle convection models suggest end; see Figure 2), provides strong evidence for the existence of a mantle plume, maybe origi- that a simple density-driven upwelling nating deep in the mantle from a thermal anomaly (see simulation at the bottom by Van Keken [1997]). In this model, the seamount trail only forms after the plume head has dissipated and (Figure 1) is very unusual (but not the narrow plume stem starts interacting with the lithosphere. Because the older Emperor implausible) and that the resulting seamounts have all been subducted into the Aleutian trench to the north, the fate of the plume plumes mostly are not vertically straight, head and any link to large igneous province volcanism are unidentified. narrow, and continuous, but ofen (more or less linear) seamount trails that 1500-m high, and correlating with long- Anthony A.P. Koppers (akoppers@coas. form as the plates constantly move over wavelength gravity and geoid anomalies) oregonstate.edu) is Associate Professor, the “fxed” loci of the upwelling mantle have been found at the leading edges of College of Oceanic and Atmospheric plume stems (Richards et al., 1989). many active seamount trails. Te correla- Sciences, Oregon State University, Corvallis, Many observations are consistent tion implies that the swells are supported OR, USA. Anthony B. Watts is Professor with the existence of mantle(Press, plumes. Understanding at depth Earth, by 2003low-density subcrustal of Marine Geology and Geophysics, Large mid-plate topographic swells mantle material. Tis large-scale warping Department of Earth Sciences, University (typically 1500–3000-km wide, up to of otherwise rigid lithosphere is most of Oxford, Oxford, UK. 9 44 Oceanography Vol.23, No.1 洪水玄武岩 Flood basalt 玉木、岩波地球惑星科学 10 地球化学的な特徴 Geochemical (isotopic) features typical values for R/Ra MORB 8+-2 OIB 5~42 Continent << 1 primordial isotope alpha decay of U and Th accumulate over time Anderson et al. http://www.mantleplumes.org/HeliumFundamentals.html 11 マントルプルームの深部構造 Deep structure of mantle plume •Dense network of Ocean Bottom Seismometers •low-velocity zone extends to lower mantle = mantle plume from D”? (Wolf et al., 2009) 12 V. Courtillot et al.