09:45 〜 10:00
[MIS12-04] Conditions for the formation of rootless cones on Mars and Earth
キーワード：ルートレスコーン、マグマ水相互作用、惑星火山学、火星
The rootless cone is one of the most popular and representative surface features on Mars. It has been widely recognized as a marker of lava field as well as an indicator of aqueous environment. Both are critical characteristics in the evolution of martian surface. This ubiquitus nature throws a deep enigma. RC has been considered as the product of magma-water interaction in terrestrial volcanology. Although water is ubiquitus on the Earth and a lot of occasions for lava flow to meet water are witnessed the formation of RC seems quite rare. On the other hand free water does not exist at least in the Amazonian period of Mars whereas a lot of RC were formed. This enigma indicates existence of unknown factors in the formation of RC. What is the specific condition on Mars? Motivated by this question we have conducted comparative study on microstructure of vesiculated samples between RC eruption and SC eruption. This is a second report to the precedent one,JPGU2020. This report focuses on inspection of microstructures by XCT.
Microstructural Characteristics Investigated pyroclasts were collected at Myvatn,Iceland(Noguchi et al 2016) and Miyakejima. 3D microstructure is reconstructed by the images of microfocus XCT. The spatial resolution is 5 to 8 microns. In addition we complimentarily used SEM-BI images with higher resolution. Overall characteristics of bubble size and morphology seems almost similar for pyroclasts of RC(Myvatn) and SC(Myvatn,Miyakejima) down to the scale of 50 microns. The resemblance in bubble size and morphology of RC samples at remote place( Hagi RC at Myvatn,over 40km away from the source )to those of SC is surprising. This means the external water is well mixed into the magma down to 50 micron scale.
Requirements for the formation of RC on the Earth and Mars Our microstructural investigations suggest simple water magma interaction could not explain the high degree vesiculation of rootless eruption. If free water meets high temperature magma rapid expansion of the volume induces explosion, where magma should be fragmented at the interface. This suppress further mixing of water intomagma. Well-vesiculated structure of RC samples indicates a different style of water-magma interaction. Furthermore the formation of cone of the scale of over hundreds meters requires duration of quasi-steady eruption for certain time. To maintain the eruption suppressing process to the vesiculation is necessary. In Myvatn we have proposed involvement of lake sedeiment(mud) plays an important role. If magma meets water-saturated mud this should be easily entrained into magma in small scale. Then vesiculation occurs subsequently.This scenario is consistent with the field observations. In Myvatn Rittmann(1938) describes existence of mud materials in the pyroclasts of RC. The spatial distribution of RC also semms that RC prefers the area of with deep mud sediments. We consider the viscous nature of mud delays water supply to the magma, which works as a suppressive role in vesiculation. If this scenario can explain the formation of terrestrial RC how about on Mars? In the recent period mud with free water might not be present on the surface. Recent observations by Curiosity(Ehlmann et al 2017) reports aeolian fine dust has size at the order of several tens microns and contained volatiles. This surface layer is composed with mixture of fine aeolian dust and ice particle. This should work similarly as the mud in the terrestrial RC formation.
Ehlmann et al,2017 JGR-E 122(12) 2510-43
Noguchi et al,2016 JVGR313,89-102
Microstructural Characteristics Investigated pyroclasts were collected at Myvatn,Iceland(Noguchi et al 2016) and Miyakejima. 3D microstructure is reconstructed by the images of microfocus XCT. The spatial resolution is 5 to 8 microns. In addition we complimentarily used SEM-BI images with higher resolution. Overall characteristics of bubble size and morphology seems almost similar for pyroclasts of RC(Myvatn) and SC(Myvatn,Miyakejima) down to the scale of 50 microns. The resemblance in bubble size and morphology of RC samples at remote place( Hagi RC at Myvatn,over 40km away from the source )to those of SC is surprising. This means the external water is well mixed into the magma down to 50 micron scale.
Requirements for the formation of RC on the Earth and Mars Our microstructural investigations suggest simple water magma interaction could not explain the high degree vesiculation of rootless eruption. If free water meets high temperature magma rapid expansion of the volume induces explosion, where magma should be fragmented at the interface. This suppress further mixing of water intomagma. Well-vesiculated structure of RC samples indicates a different style of water-magma interaction. Furthermore the formation of cone of the scale of over hundreds meters requires duration of quasi-steady eruption for certain time. To maintain the eruption suppressing process to the vesiculation is necessary. In Myvatn we have proposed involvement of lake sedeiment(mud) plays an important role. If magma meets water-saturated mud this should be easily entrained into magma in small scale. Then vesiculation occurs subsequently.This scenario is consistent with the field observations. In Myvatn Rittmann(1938) describes existence of mud materials in the pyroclasts of RC. The spatial distribution of RC also semms that RC prefers the area of with deep mud sediments. We consider the viscous nature of mud delays water supply to the magma, which works as a suppressive role in vesiculation. If this scenario can explain the formation of terrestrial RC how about on Mars? In the recent period mud with free water might not be present on the surface. Recent observations by Curiosity(Ehlmann et al 2017) reports aeolian fine dust has size at the order of several tens microns and contained volatiles. This surface layer is composed with mixture of fine aeolian dust and ice particle. This should work similarly as the mud in the terrestrial RC formation.
Ehlmann et al,2017 JGR-E 122(12) 2510-43
Noguchi et al,2016 JVGR313,89-102