11:00 〜 13:00
[SVC32-P08] 柱状節理における均一な柱径の形成要因
キーワード：柱状節理、マグマ、クラック形成温度、柱幅、伝導冷却
Columnar joint is one of the most spectacular and intriguing geologic monuments. An impressive characteristics of columnar joint is the uniformity of column width, which occupies the large extent of lava thickness. A lot of studies have tried to quantitatively know the morphology and its statistics by field observation and to understand the formation process of columnar joint by theoretical and numerical approaches, and analog experiments using starch-water mixture under drying condition. However, there are no works addressing the origin of uniform column width. In this presentation, I propose the model for the formation of the uniform column width on the basis of the classical solution to the conductive cooling.
It is the simplest and realistic situation where a magma layer with a finite thickness is cooled from both sides, such as dike or sill. The existing analytical solution to temperature evolution without solidification latent heat in this situation allows us to examine in detail the behavior of cooling process. I focus on the role of critical temperature for crack nucleation (hereafter CCT= Cracking Critical Temperature) in the cooling and solidification processes because the column formation is entirely controlled by the thermo-mechanical state at CCT. In particular, the cooling rate at CCT determines the crack spacing, that is, column width. From the examination of the effect of CCT on the cooling rate at CCT as function of distance, I find that the cooling rate at CCT is nearly constant at the central region of magma layer. It is significant that the extent of constant cooling rate at CCT depends on values of CCT: decreasing CCT increases the extent of constant cooling rate region, resulting in 90 % at CCT=0.65 (scaled by initial magma temperature). This indicates that the mostly entire region of magma has the constant cooling rate at CCT (i.e., at crack nucleation), yielding constant column width in the mostly entire region of lava flow. The origin of the region of constant cooling rate may be related to the formation of the parabolic profile of temperature, which leads to no dependence of temperature change on distance in the thermal diffusion equation. Existing works often assume higher CCT. Our recent study (Hamada and Toramaru, 2020) on the entablature formation process suggests CCT of 0.6 (scaled by initial magma temperature) from the curved column analysis. Thus, I conclude that lower CT plays an essential role in the formation of characteristic geometry of columnar joint such as uniform column width and entablature.
It is the simplest and realistic situation where a magma layer with a finite thickness is cooled from both sides, such as dike or sill. The existing analytical solution to temperature evolution without solidification latent heat in this situation allows us to examine in detail the behavior of cooling process. I focus on the role of critical temperature for crack nucleation (hereafter CCT= Cracking Critical Temperature) in the cooling and solidification processes because the column formation is entirely controlled by the thermo-mechanical state at CCT. In particular, the cooling rate at CCT determines the crack spacing, that is, column width. From the examination of the effect of CCT on the cooling rate at CCT as function of distance, I find that the cooling rate at CCT is nearly constant at the central region of magma layer. It is significant that the extent of constant cooling rate at CCT depends on values of CCT: decreasing CCT increases the extent of constant cooling rate region, resulting in 90 % at CCT=0.65 (scaled by initial magma temperature). This indicates that the mostly entire region of magma has the constant cooling rate at CCT (i.e., at crack nucleation), yielding constant column width in the mostly entire region of lava flow. The origin of the region of constant cooling rate may be related to the formation of the parabolic profile of temperature, which leads to no dependence of temperature change on distance in the thermal diffusion equation. Existing works often assume higher CCT. Our recent study (Hamada and Toramaru, 2020) on the entablature formation process suggests CCT of 0.6 (scaled by initial magma temperature) from the curved column analysis. Thus, I conclude that lower CT plays an essential role in the formation of characteristic geometry of columnar joint such as uniform column width and entablature.