火山と地球現象 : 物質とエネルギーの放出量

抄録

Production rates of extrusive and intrusive material are estimated on the basis of plate tectonic concept. For accreting and consuming plate boundaries, production rates of extrusive (volcanic) material are estimated at about 4 and 1 km³/yr, while the rates of intrusive igneous material for the two boundaries are about +9.5 and -14 km³/yr, respectively. Production rates of extrusive material for intra-oceanic and intra-continental plates are about 1 and 0.1 km³/yr, respectively. In the marginal seas which are thought to be another type of plate accretion, production rates of extrusive and intrusive material are estimated at about 0.1 and 1.2 km³/yr, respectively. Net addition to continental crust is about 0.9 km³/yr which is nearly equal to the classic estimates before the proposal of plate tectonics. However, the value of net addition to oceanic crust (1.8 km³/yr) contains large uncertainty because that the global production and consumption of intrusive material are one order of magnitude larger than the net production. Total heat flux from the earth’s surface (Q_t) is expressed by Q_t=Q_co+Q_cr+Q_cl+Q_m, where Q_co is average heat flux that possibly corresponds to radiogenic heat source, Q_cr and Q_cl are the “regional” and “local” excess conducted heat flux that may represent magmatic intrusion in the crust and circulation of hot water system, respectively. Q_m is heat flux due to mass transportation. Terrestrial heat flow values are reviewed in terms of major geological features. It is pointed out that the values higher than 1.0 HFU in oceanic areas may indicate Q_cr and at the ridge axis are considered to be Q_cl. On the other hand, conductive heat flux in geothermal areas and in the vicinity of volcanoes is excluded from the statistics. In geothermal areas, the global sum of heat flux Q_m is estimated at about 10¹⁷ cal/yr and that of extrusive volcanic material is about 5×10¹⁸ cal/yr, whereas the global conducted heat flux is about 2.3×10²⁰ cal/yr. It is emphasized, however, that Q_m by extrusive material is comparable to conducted heat flux near the plate boundary (say, along the zone of a few 100 km-wide).