Presentation Information
[O12-P36]Understanding the Formation of Yakushima Island Through Joint Structure
*kazuki higashide1, Eito Tanaka1 (1. Saitama Prefectural Kasukabe High School)
Keywords:
Yakushima Islan,Joint Structure,Granite
Purpose
Based on previous research concluding that the jointing patterns of Yakushima granite influence the island’s topography, including its rivers, we conducted a study using a red relief map-which clearly illustrates the island’s topographic variations-to determine whether there are any patterns in the orientation of these joints at specific locations across the island.
What Are the Joints in Yakushima Granite?
Joints are regular cracks that form in rock, and the joints in Yakushima granite are known to form rectangular prisms that intersect at right angles in three directions.
Research Methodology
In this study, to verify the regularity of joint patterns at specific locations on the island, Yakushima was divided into grids based on latitude and longitude (in this study, grids with 3-minute intervals were used as the standard). To digitize the joint data, we used the mathematical software GeoGebra. We imported a red 3D topographic map and plotted two points along each feature suspected to be a joint. We then calculated the slope (a) of the linear function connecting these two points for every identifiable feature within each grid section.
Results
When we calculated the average of the two types of joints for each section, we discovered a tendency for the joints to be distorted in a clockwise direction from the northwest to the southeast of the island. We then set the section furthest to the northwest as the reference point (0) and examined the degree of clockwise distortion in each section. As expected, the degree of distortion increased toward the southeast, with the point furthest to the southeast showing the most pronounced distortion.
The Formation of Yakushima
Yakushima took on its current shape when a massive granite mass formed deep underground was pushed to the surface by buoyancy. It is also believed that during its formation, the island was subjected to forces extending from the northwest to the southeast. This can be inferred from the orientation of the orthoclase present within the granite. Orthoclase has a higher melting point and density than magma, so it exists as a solid and floats within the liquid magma. It is believed that when forces are applied from the outside while in this state, the orientation of the orthoclase aligns with the direction of the force, following the flow of the magma.
Discussion
Based on data regarding the island’s formation and the current findings-which indicate that the island gradually became distorted toward the southeast, with the southeasternmost section protruding significantly—we propose the following explanation: When Yakushima emerged, the southeastern part of the island collided with the sedimentary rock layers (alternating sandstone and mudstone). However, it lacked the force to break through these layers; consequently, the granite itself altered its course southward to avoid the sandstone and mudstone layers, resulting in the island’s current shape. Furthermore, regarding the differences in the angles formed by the joints between sections, we hypothesized that the angles did not physically widen, but rather that when the three-dimensional form of Yakushima was projected onto a two-dimensional map, the angles appeared to deviate from right angles due to the slope. However, when comparing this with a diagram illustrating the steepness of the slope, no clear correlation was observed; therefore, it is believed that the slope does not significantly affect the angles formed by the joints.
Based on previous research concluding that the jointing patterns of Yakushima granite influence the island’s topography, including its rivers, we conducted a study using a red relief map-which clearly illustrates the island’s topographic variations-to determine whether there are any patterns in the orientation of these joints at specific locations across the island.
What Are the Joints in Yakushima Granite?
Joints are regular cracks that form in rock, and the joints in Yakushima granite are known to form rectangular prisms that intersect at right angles in three directions.
Research Methodology
In this study, to verify the regularity of joint patterns at specific locations on the island, Yakushima was divided into grids based on latitude and longitude (in this study, grids with 3-minute intervals were used as the standard). To digitize the joint data, we used the mathematical software GeoGebra. We imported a red 3D topographic map and plotted two points along each feature suspected to be a joint. We then calculated the slope (a) of the linear function connecting these two points for every identifiable feature within each grid section.
Results
When we calculated the average of the two types of joints for each section, we discovered a tendency for the joints to be distorted in a clockwise direction from the northwest to the southeast of the island. We then set the section furthest to the northwest as the reference point (0) and examined the degree of clockwise distortion in each section. As expected, the degree of distortion increased toward the southeast, with the point furthest to the southeast showing the most pronounced distortion.
The Formation of Yakushima
Yakushima took on its current shape when a massive granite mass formed deep underground was pushed to the surface by buoyancy. It is also believed that during its formation, the island was subjected to forces extending from the northwest to the southeast. This can be inferred from the orientation of the orthoclase present within the granite. Orthoclase has a higher melting point and density than magma, so it exists as a solid and floats within the liquid magma. It is believed that when forces are applied from the outside while in this state, the orientation of the orthoclase aligns with the direction of the force, following the flow of the magma.
Discussion
Based on data regarding the island’s formation and the current findings-which indicate that the island gradually became distorted toward the southeast, with the southeasternmost section protruding significantly—we propose the following explanation: When Yakushima emerged, the southeastern part of the island collided with the sedimentary rock layers (alternating sandstone and mudstone). However, it lacked the force to break through these layers; consequently, the granite itself altered its course southward to avoid the sandstone and mudstone layers, resulting in the island’s current shape. Furthermore, regarding the differences in the angles formed by the joints between sections, we hypothesized that the angles did not physically widen, but rather that when the three-dimensional form of Yakushima was projected onto a two-dimensional map, the angles appeared to deviate from right angles due to the slope. However, when comparing this with a diagram illustrating the steepness of the slope, no clear correlation was observed; therefore, it is believed that the slope does not significantly affect the angles formed by the joints.