金星薄饼状火山类比研究

Steep-sided domes on Venus: an analog study

金星薄饼状火山(陡边火山)是一类形貌非常独特的火山,有别于太阳系中其他的火山:近圆的底面轮廓、平坦的顶部和较大的顶底直径比。薄饼状火山被认为是高黏度岩浆喷发形成的,但是这一成因并不被广泛接受。火山的形貌特征主要取决于岩浆成分、喷发机制、重力和喷发环境(如大气压)等。因此,通过对比地球火山的形貌特征及形成机制可以为金星薄饼状火山的成因提供指示。我们利用底面不规则度(ii)、高径比(H/WB)、顶底直径比(WS/WB)、侧边最大坡度处的标准化高度(HSmax)、顶部高径比(HS/WS)和喷发量(V)这些形貌参数定量地将地球表面的复式火山、盾形火山、火山锥、熔岩穹丘(Peleean型和Coulees型)、海底平顶火山以及泥火山的形貌特征与金星薄饼状火山对比。Coulees型熔岩穹丘、海底平顶火山和泥火山的形貌特征与金星薄饼状火山有一定的相似性。Coulees型熔岩穹丘剖面形态与薄饼状火山最为相似,可能指示了薄饼状火山具有类似的成因机制,而海底平顶火山侧边的上凹特征可能指示了其与薄饼状火山完全不同的成因机制。因此,薄饼状火山可能是相对高黏度的岩浆以较高速率持续性喷发形成。

The morphologies of steep-sided domes(pancake domes) on Venus are distinctive-circular in plain view, steep-sided, relatively flat-topped, and large summit diameter/basal diameter ratio, which make them strikingly different from other types of volcanoes in the solar system. It has been proposed that their morphologies could have resulted from the eruption of viscous lava. However, the formation mechanism of steep-sided domes is still under debate. The morphology of volcanoes depends on magma composition, eruption mechanism, gravity and ambient environment(e.g., atmospheric pressure). We have collected and calculated morphologic parameters of different volcanoes, including irregularity index(ii), height/basal diameter ratio(H/WB), summit/basal diameter ratio(WS/WB), max slope height fraction(HSmax), summit height/summit diameter ratio(HS/WS), and eruption volume(V), and have quantitatively compared Venusian steep-sided domes with composite volcanoes, shield volcanoes, cinder cones, lava domes(Peleean-type and Coulees-type domes), submarine volcanoes, and mud volcanoes on Earth. The Coulees-type lava domes, submarine flat-topped cones, and mud volcanoes all have some morphological aspects similar to the steep-sided domes. The profiles of Coulees-type lava domes are the closest to that of steep-sided domes, indicating that the emplacement of steep-sided domes could be similar to that of Coulees-type lava domes. On the other hand, the flank concave shapes of submarine flat-topped volcanoes indicate that the emplacement is different from that of steep-sided domes. Therefore, steep-sided domes might have resulted from the continuous eruption of relatively viscous magma with high effusion rates.