4:45 PM - 5:00 PM
[SVC31-22] Description of volcanic products (lapilli samples) by color spectroscopy and its application
Keywords:Color spectroscopy, Volcanic products, lapilli
The color of volcanic ejecta has long been described as one of the characteristics of volcanic deposits in the field. This is because the color of ejecta reflects the crystalline structure such as crystallinity through its composition and redox state, and such characteristics of ejecta provide some information for understanding volcanic eruptions and magma plumbing systems, which is the main purpose of our study. In particular, the presence or absence of microcrystals in the ejecta and their structures, which are thought to influence eruption style, may appear as color changes of the ejecta on a macroscopic scale, and objective and quantitative description of the color is necessary to consider the factors that influence the transition of eruption phenomena.
On the other hand, outcrops have been disappearing in many volcanic regions in recent years due to anthropogenic modification. However, the color expression is often based on the experience of individual researchers and is not always based on objective standards that can be compared by anyone. Furthermore, color observation at an outcrop is often affected by ever-changing conditions such as weather, outcrop conditions, and the physical condition of the observer. The accumulation of objective data is important for new outcrop descriptions, accurate correlation, and their verification without wasting the efforts and achievements of our predecessors.
The authors have already measured the color of volcanic ash from Sakurajima and many other volcanoes under constant conditions using a spectrophotometer with a built-in light source, and have shown that the color of volcanic ash varies systematically according to the eruption style (Shimano and Yasuda, 2023). In this study, we applied this method and performed spectrophotometry on the surface of lapilli samples.
The measured samples were A. Tenmei fallout pumice from Asama volcano (beside the observatory of the University of Tokyo), B. Yufune No. 2 scoria from Fuji volcano (Tarobo), C. Bunka fallout scoria from Suwanosejima volcano (Higashiyama), D. Taisho pumice from Sakurajima volcano (Arimura), and E. FukutokuOkanoba volcano 2021 drifted pumice (from Okinawa).
The A-D samples were relatively homogeneous except for a few percent of accidental rock fragments. The scoria particles tended to have lower L*, a*, and b* values than the pumices (Fig. 1). All samples showed similar variations (L*: 5-10, a*: 2-3, b*: 10) and have trends in the L*a*b* space. The scoria particles show relatively a straight trend (elongated shape) whereas the pumices show a flat trend, suggesting that they are mixed colors with two and three main end components, respectively. The scoria samples are the mixtures of particles composed of black crystalline and brown glassy matrices, which may indicate a continuous mixing of these two components. On the other hand, the pumice samples are the mixture of light-white matrix, dark-colored pyroxene crystals, and brownish-altered parts. Thus, the simultaneous inclusion of these within field of view of the colorimeter is considered to have resulted in the above-mentioned trend. Furthermore, the color distribution among the scoria samples and among the pumice samples is not exactly the same, and each sample has its own unique color distribution. As each sample contains many bubbles, some samples with large bubbles often do not have flat surface. However, as the contact of the colorimeter is flat colorless glass, the backscattering intensity would not be sufficient on the uneven surface of the sample, which may be one of the causes of the variation.
On the other hand, the colorimetric distribution of E is consistent with the known variations in drifted pumices. According to the results of some studies, these various pumice-scoria may have erupted during different eruption periods, and quantitative identification of the ejecta may contribute to the reconstruction of eruptive transitions. As pointed out in the first section, the color of the ejecta may change reflecting the eruption process, etc., and the investigation of the factors that cause these color changes may help elucidate the eruption mechanism.
The samples analyzed in this study were all relatively recent ejecta (the oldest being the Yufune 2 scoria, 2200 years ago). Nevertheless, yellowing due to weathering of the glass can be observed visually. It is possible that different colorimetric values may be obtained even for ejecta from the same eruption, depending on the environment surrounding the outcrop, and so it may be necessary to describe and compare the color characteristics of the ejecta in the future.
On the other hand, outcrops have been disappearing in many volcanic regions in recent years due to anthropogenic modification. However, the color expression is often based on the experience of individual researchers and is not always based on objective standards that can be compared by anyone. Furthermore, color observation at an outcrop is often affected by ever-changing conditions such as weather, outcrop conditions, and the physical condition of the observer. The accumulation of objective data is important for new outcrop descriptions, accurate correlation, and their verification without wasting the efforts and achievements of our predecessors.
The authors have already measured the color of volcanic ash from Sakurajima and many other volcanoes under constant conditions using a spectrophotometer with a built-in light source, and have shown that the color of volcanic ash varies systematically according to the eruption style (Shimano and Yasuda, 2023). In this study, we applied this method and performed spectrophotometry on the surface of lapilli samples.
The measured samples were A. Tenmei fallout pumice from Asama volcano (beside the observatory of the University of Tokyo), B. Yufune No. 2 scoria from Fuji volcano (Tarobo), C. Bunka fallout scoria from Suwanosejima volcano (Higashiyama), D. Taisho pumice from Sakurajima volcano (Arimura), and E. FukutokuOkanoba volcano 2021 drifted pumice (from Okinawa).
The A-D samples were relatively homogeneous except for a few percent of accidental rock fragments. The scoria particles tended to have lower L*, a*, and b* values than the pumices (Fig. 1). All samples showed similar variations (L*: 5-10, a*: 2-3, b*: 10) and have trends in the L*a*b* space. The scoria particles show relatively a straight trend (elongated shape) whereas the pumices show a flat trend, suggesting that they are mixed colors with two and three main end components, respectively. The scoria samples are the mixtures of particles composed of black crystalline and brown glassy matrices, which may indicate a continuous mixing of these two components. On the other hand, the pumice samples are the mixture of light-white matrix, dark-colored pyroxene crystals, and brownish-altered parts. Thus, the simultaneous inclusion of these within field of view of the colorimeter is considered to have resulted in the above-mentioned trend. Furthermore, the color distribution among the scoria samples and among the pumice samples is not exactly the same, and each sample has its own unique color distribution. As each sample contains many bubbles, some samples with large bubbles often do not have flat surface. However, as the contact of the colorimeter is flat colorless glass, the backscattering intensity would not be sufficient on the uneven surface of the sample, which may be one of the causes of the variation.
On the other hand, the colorimetric distribution of E is consistent with the known variations in drifted pumices. According to the results of some studies, these various pumice-scoria may have erupted during different eruption periods, and quantitative identification of the ejecta may contribute to the reconstruction of eruptive transitions. As pointed out in the first section, the color of the ejecta may change reflecting the eruption process, etc., and the investigation of the factors that cause these color changes may help elucidate the eruption mechanism.
The samples analyzed in this study were all relatively recent ejecta (the oldest being the Yufune 2 scoria, 2200 years ago). Nevertheless, yellowing due to weathering of the glass can be observed visually. It is possible that different colorimetric values may be obtained even for ejecta from the same eruption, depending on the environment surrounding the outcrop, and so it may be necessary to describe and compare the color characteristics of the ejecta in the future.