Brecciation and fracturing by water ingress into a basaltic andesitic lava flow, Iwate volcano, northeastern Japan

doi:10.21203/rs.3.rs-3406255/v1

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Brecciation and fracturing by water ingress into a basaltic andesitic lava flow, Iwate volcano, northeastern Japan

https://doi.org/10.21203/rs.3.rs-3406255/v1

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The Genbudo lava in the southwestern part of Iwate Volcano is a 70 m thick columnar jointed lava flow and divided into three parts from the bottom: the colonnade, the entablature, and the uppermost part.

Two main types of fractures develop in the entablature: pseudopillow fractures that develop in a network-like pattern throughout entablature, and sheet fractures with curved surfaces that are nearly parallel to each other. At the uppermost part of the lava flow, finger-like structures of lava extending upward from the coherent lava is distributed, and cogenetic pyroclastic rocks are deposited between the fingers. The occurrence suggests that hyaloclastites generated at the uppermost part.

The texture of lava near the pseudopillow fracture of the entablature is commonly hypocrystalline, while the other parts exhibit holocrystalline. There are two types of pyroxene microlites, large prismatic (average size ~ 30 µm) and dendritic (< 10 µm in length) crystals in the lava near the pseudopillow fracture. These suggest that the cooling rate of the lava became greatest in the vicinity of the pseudopillow fractures. Networks of palagonite-filled micro-fractures (less than 10 µm in width) are found in this part, and many bubbles are generated along the fractures. This is irrefutable evidence that the rapid cooling of the lava was caused by water infiltration through the pseudopillow fractures. By the measurement of the Fe-rich droplet sizes due to liquid immiscibility, we could estimate the cooling rates of the colonnade and entablature about 49°C/h and 642°C/h, respectively.

Petrology

Geology

Entablature

Quenching

Hyaloclastite

Palagonite

Immiscible droplets

Cooling rate

In lava flows and welded tuffs with well-developed columnar joints, two types of tiers, colonnade and entablature, with different characteristics of column are commonly observed. The colonnade is composed of regular straight columns, and the entablature is composed of curved smaller columns. In lava flows and welded tuffs that are composed of both types of tiers, the entablature may lie on the colonnade, or the entablature may occur between the lower and upper colonnades. Entablatures typically have thinner columns and higher amount of dendritic crystals and glassy mesostasis than colonnade. Based on the facts, entablature has been thought to have solidified under a greater cooling rate than colonnade (Iddings, 1909; Saemundsson, 1970; Long and Wood, 1986). One of possible mechanisms proposed for the greater cooling rate is the convective cooling of underlying unsolidified igneous body caused by the downward migration of external water through cracks (Saemundsson, 1970; Long and Wood, 1986; Forbes, 2014). However, some previous studies advocate that cooling from all directions in the center of an igneous body causes complex stress distribution and the formation of irregular and curved columns (Spry, 1962), as well as an increase in cooling rate compared to the margin of the body (Grossenbacher and McDuffie, 1995). In this case, the involvement of external water is not required for entablature formation and increased cooling rates in the central part of the igneous body. Hamada and Toramaru (2020) argued that when a crack is formed perpendicular to the upper surface of an igneous body, the isothermal planes develop parallel to the crack, resulting in the formation of many radial cracks near the crack tip, as observed in entablature. Their model also does not require external water infiltration.

As mentioned above, previous studies have reached a consensus that entablature has a greater cooling rate than colonnade. However, it may not be necessarily conclusive whether water or vapor infiltration is necessary to increase the cooling rate in entablature. This is probably because chemical signatures of water involved in magma cooling had been rarely found in igneous bodies with entablature. The search for signatures of water should also help to reveal convection mechanisms of the water in the entablature. In addition, as Forbes et al. (2014) pointed out, there is very fewer data to help estimate the cooling rate of the entablature than the colonnade (e.g., column widths, chizel mark spacing, etc.). Therefore, we investigated the internal structures and microtextures of a basaltic-andesitic lava flow (Genbudo lava flow, Iwate volcano, northeastern Japan) with well-developed entablature, and discussed the formation process of entablature.

The Amihari Volcanic Group in the western part of Iwate Volcano, which was active between about 0.7–0.1 Ma, is a group of andesitic to basaltic volcanoes distributed over an area of about 12 km from east to west (Ito et al., 2017). The volcanic activities of the volcanic group can be divided into two stages: the older and younger (Nakagawa, 1987). The volcanic ejecta of the Younger Amihari Volcanic Group (ca. 0.2–0.1 Ma) consists of relatively thick lava flows, and the original topography of the lava flows is relatively well preserved.

The Genbudo lava flow of our target is one of the lava flows of the Younger Amihari Volcanic Group in Nakagawa (1987) or the Iwate Volcano 2nd Volcanic Group in Doi (2000). The lava flow extends about 3 km south from the mountainside of the Amihari Volcanic Group, and the escarpment of lava flow is exposed along the Kakkonda River (Fig. 1a). At the "Kakkonda no Ooiwaya" which is designated as a Japanese natural monument, a complete vertical cross-section of the lava flow is exposed, allowing macroscopic observation of the internal structures such as colonnade and entablature.

In this study, we mainly observed basalt lava flows at two outcrops.

In the outcrop #1 (Kakkonda-no-Ooiwaya) where the vertical cross section of the lava flow was completely exposed, we mainly observed the macroscopic structure of the lava flow (Fig. 1b). However, it was difficult to approach the upper part of the lava flow due to its height of 70 m above the ground surface. Therefore, in order to observe the structure of the upper part of the lava flow in detail, we took pictures of the outcrop with a drone (DJI SPARK) and a digital single-lens reflex camera with a telephoto lens.

In the outcrop #2 (400 m northwest of the outcrop #1; Fig. 1), as we can closely observe the entablature from two almost orthogonal directions, we investigated the three-dimensional distribution of the fractures. Rock samples were collected from the entablature and colonnade with paying attention to the characteristics and distribution of the observed fractures (Table 1). Whole-rock major and minor element compositions in 4 samples from the entablature and 2 samples from the colonnade collected at the outcrop #2 were determined by X-ray fluorescence (XRF) analysis using a Rigaku ZSX Primus II (Rh X-ray tube, 50 kV, 60 mA) at the Faculty of International Resource Sciences, Akita University. Thin sections of 9 samples from the entablature and 7 samples from the colonnade were also prepared for microscopic observation using SEM-EDS.

4 − 1. Structure of the Genbudo lava flow

The thickness of the Genbudo lava in outcrop #1 is about 70 m (Fig. 1b). The lava is composed of three layers from the bottom: colonnade, entablature, and the uppermost part (Fig. 2a). The boundary between the colonnade and entablature is sharp, but the boundary between the entablature and uppermost part is not so clear. The thicknesses of the colonnade and entablature are 14–17 m and 32–38 m, respectively. The uppermost part is 8–14 m thick and consists of finger-like structures extending from the main lava body and pyroclastic rocks filling the spaces between the fingers.

The shape of the horizontal section of colonnade columns is commonly hexagonal. The column diameters of colonnade range from about 65 cm to 90 cm. The columns are nearly straight, but slightly bent near the entablature (Fig. 2a). Striae (Ryan and Sammis, 1978) are observed on the sides of the colonnade columns, indicating intermittent crack extension (e.g., Goehring and Morris, 2008) (Fig. 2b). A single layer of stria is typically composed of an alternation of a relatively smooth surface on the lower side and a rougher surface on the upper side (Fig. 2c). The thickness of the layer of stria increases upward. At a height of 1.5 m from the base of the lava, the thickness of a single stria layer is about 20–30 cm. On the smooth surface of striae, plumose structures can often be observed that indicate the direction of crack propagation. In outcrop #2 (total height about 5 m), the only uppermost 50 cm of the colonnade is visible above the ground surface (Fig. 1b).

Due to the development of different types of fractures, the entablature column has a more irregular and complex morphologies than the colonnade one. Two types of primary fractures can be recognized by observing the outcrop #2 from multiple directions. The first type of primary fractures are relatively continuous sheet-like fractures (Fig. 3a) (Lescinsky and Fink, 2000). As slightly curved sheet-like fractures develop nearly parallel to each other, the columns between the sheet-like fractures form slightly bent plates about 15–40 cm thick (Fig. 2). The second type of primary fractures is the pseudopillow fractures (Watanabe and Katsui, 1976; Lescinsky and Fink, 2000; Forbes et al., 2012, 2014), which develops in several networks throughout the entablature (Fig. 3b). The pseudopillow fractures are arcuate fractures that extend several meters and occur roughly parallel to each other, intersecting each other in places (Fig. 3b, c). Short fractures extend almost perpendicular to the pseudopillow fracture at intervals of a few centimeters (Fig. 3d). Lava within 20 cm distance from the pseudopillow fracture where the short fractures occur is darker and glosser than the other parts.

The uppermost part of the lava flow consists of finger-like lava extending upward from the entablature and cogenetic pyroclastic rocks deposited between the fingers (Fig. 3e). The direction of the finger elongation is sub-parallel to each other and nearly perpendicular to the top surface of the lava flow, including the pyroclastic rocks deposited between the fingers (Figs. 2a, 3e). Short fractures extend almost perpendicular to the outline of the fingers. The marginal part of the finger is darker in color than the interior. The pseudopillow fractures extend downward also in the "interdigital space" between the fingers and short fractures also develop perpendicular to the pseudopillow fractures (Fig. 3f). The cogenetic pyroclasts (lapilli and blocks) between the fingers are angular and blocky in shape. The pyroclastic rocks between the fingers do not have sedimentary structures like bedding. They rarely exhibit jigsaw-fit of blocks. Clast-supported pyroclastic breccia is distributed adjacent to the fingers and matrix-supported tuff breccia is distributed away from them. The characters of pyroclastic rocks in this interdigital space are very similar to those of the type-B hyaloclastite by Yamagishi (1979).

4 − 2. Microscopic characters of colonnade and entablature

Both colonnade and entablature of the Genbudo lava contain plagioclase, olivine, augite, orthopyroxene, quartz, and opaque minerals (titanomagnetite and ilmenite) as phenocryst. Cr-spinel microcrystals (Cr#=~0.47) are included in olivine phenocrysts. Augite, orthopyroxene, plagioclase, and opaque minerals form glomerophyric textures. The whole-rock chemical compositions of the samples collected from the colonnade and entablature are all classified as basaltic andesite (Fig. 4). There is no systematic difference in whole-rock composition between the colonnade and entablature.

The groundmass textures vary at the boundary between colonnade and entablatur e (Table 1). The groundmass of colonnade near the boundary between colonnade and entablature is almost holocrystalline and composed of plagioclase (average size ~ 50 µm), clinopyroxene (augite and pigeonite; average size ~ 30 µm), orthopyroxene, olivine, magnetite, and quartz crystals (Fig. 5a). Small amounts of silicic glasses (SiO₂ = ~ 80 wt%) are rarely distributed among the groundmass crystals (Fig. 5b).

As contrasted with the colonnade, the groundmass of entablature near the boundary between colonnade and entablature is commonly hypocrystalline. The average grain sizes of plagioclase and clinopyroxene are about 50 µm and 30 µm, respectively. No quartz and magnetite are crystallized in the groundmass; instead, substantial amount of volcanic glass (SiO₂ = 67–69 wt%) is recognized (Fig. 5c). However, the parts more than 30 cm from the pseudopillow fractures infrequently exhibit holocrystalline textures like that of the colonnades (GL10; Fig. 5d).

The dark glassy parts very close to the pseudopillow fractures (0–20 cm from the fractures) have networks of micro-fractures ranging from a few to 10 µm in width (Fig. 5e; Table 1). The micro-fractures also penetrate plagioclase, clinopyroxene, and glass in the groundmass. The micro-fractures are partly filled with palagonite. Bubbles are distributed in the groundmass glass along the micro-fractures (Fig. 5f). Irregularly shaped voids occur within the phenocrysts penetrated by the micro-fracture (Fig. 5g). The palagonite-filled micro-fractures are not distributed more than 30 cm away from the pseudopillow fractures (Fig. 5d). There are two types of groundmass augite crystals very close to the pseudopillow fractures: relatively larger prismatic microlites (average size ~ 30 µm) and smaller dendrites with larger aspect ratios (Fig. 5f, h). Some prismatic augites have dendritic overgrowth rims. Fe-rich droplets with a diameter of less than 1 µm are distributed in the interstitial glass in which dendritic augite precipitates (Fig. 5i). The compositions of the glasses adjacent to the Fe-rich droplets are richer in SiO₂ and poorer in CaO, MgO, FeO, and TiO₂ than those of the interstitial glasses without Fe-rich droplets (Fig. 6). Fe-rich droplets tend to be less abundant in the Si-rich glass adjacent to the dendritic augites. On the other hand, there is neither network of micro-fractures nor dendritic augite in the colonnade (Fig. 5a, b) or the entablature more than 30 cm away from the pseudopillow fractures (Fig. 5d).

5 − 1. Evidence of increasing cooling rate by water ingress

The presence of palagonite-filled micro-fractures in the dark glassy parts of the entablature indicates that the entablature was subjected to external water infiltration. The rounded shape of the bubbles along these micro-fractures indicates that the water infiltration occurred at a time when the interstitial melt was hot enough to be deformed by the volume increase associated with water vaporization. The formation of bubbles also indicates that nucleate boiling of water occurred along the micro-fracture.

Faure et al. (2007) observed a change in the morphology of crystallized olivine from polyhedral to dendritic with increasing the cooling rate during a crystallization experiment of olivine in a basaltic melt. The two forms of polyhedral and dendritic pyroxene found near the pseudopillow fracture of the entablature (Fig. 5h, i) indicate a rapid increase in the cooling rate in the process of crystallization of the lava groundmass. The amount of pyroxene dendrite increases with the approach to the pseudopillow fracture (Fig. 5c, f). In addition, the palagonite-filled micro-fractures are distributed only in narrow zones within 20 cm from the pseudopillow fractures. These suggest that water entering the palagonite-filling micro-fractures was introduced through the pseudopillow fracture and the infiltration of water caused the increase in cooling rate of crystallizing lava. Pseudopillow fractures are distributed in a network throughout the entablature, suggesting that water circulated throughout the entablature.

5 − 2. Formation of hyaloclastite in the uppermost part

In the interdigital space of the uppermost part, the pseudopillow fractures extend downward toward the interior of the entablature, suggesting that the water circulating the entablature came from the uppermost part. This interpretation is consistent with the occurrence of the type-B hyaloclastite (Yamagishi, 1979) in the interdigital space of the uppermost part.

At the top of the lava flow, the alternating distribution of finger-like coherent lava and hyaloclastite indicates that the generation of lava fragmentation was localized. The elongation direction of the finger-like lava is almost perpendicular to the top surface of the lava flow (Figs. 2a and 3e), suggesting that lava fragmentation in the interdigital space was caused by water infiltration along columnar joints that developed almost perpendicular to the top surface of the lava flow (Fig. 7). In other words, lava fragmentation did not occur on the entire upper surface of the lava in contact with water, but only within the columnar joints where water was enclosed by the hot lava. This suggests that bulk interaction steam explosion was important rather than contact-surface steam explosion as a mechanism for fragmentation (e.g., Kokelaar, 1986).

5 − 3. Cooling rate of entablature

The compositions of the interstitial glasses without Fe-rich droplets near the pseudopillow fractures are plotted within the field of silicate liquid immiscibility, indicating that the interstitial melts were differentiated to chemical compositions that could cause unmixing. The compositions of the glasses adjacent to the Fe-rich droplets suggest that the glasses may be Si-rich glasses produced by unmixing (Fig. 6). Honour et al. (2019) observed immiscible Fe-rich droplets in experiments using natural ferrobasalt as starting material, and found that the onset of unmixing is localized due to compositional inhomogeneity in the interstitial melt caused by the compositional boundary layer formed during crystallization. They observed that Fe-rich droplets can distribute adjacent to plagioclase, but Fe-rich droplet free zones are formed around pyroxene and magnetite. The Fe-rich droplets observed near the pseudopillow fracture of Genbudo lava also tends not to be distributed in the Si-rich glass around the dendritic pyroxene. These suggest that the Fe-rich droplets observed near the pseudopillow fracture were caused by silicate liquid immiscibility.

Honour et al. (2019) showed that the relationship between the average of the five largest diameter Fe-rich droplets in basaltic intrusive bodies and the cooling rate calculated by a one-dimensional conduction cooling model can be approximated by a power-law regression curve. According to the relationship, the cooling rate of the colonnade near the colonnade-entablature boundary of the Genbudo lava is estimated to be about 49°C/h (Fig. 8; Table 1). The size of the Fe-rich droplet near the pseudopillow fracture of the Entablature is so small that it deviates from the range of this regression curve, but extrapolation of the regression curve gives a cooling rate of 642°C/h. It should be noted that the Genbudo lava is a little richer in SiO₂ than the intrusive rocks studied by Honour et al. (2019). However, there is no doubt that there was a large contrast in cooling rates at the colonnade-entablature boundary. This large contrast in cooling rates suggests that Entablature and Colonnade have very different cooling regimes. The colonnade-entablature boundary must represent a horizon where a solidification front due to cooling that progressed upward from the ground by thermal conduction collided with a solidification front that progressed downward by convective cooling due to water ingress.

1. Palagonite-filled micro-fractures near the pseudopillow fracture in the entablature are direct evidence of external water infiltration during magma cooling. The occurrence of numerous bubbles along the micro-fractures suggests that nucleate boiling has occurred during the water infiltration. The presence of palagonite-filled micro-fractures would be an important criterion for determining whether water was involved in entablature formation.

2. Palagonite-filled micro-fractures, interstitial glass, and dendritic pyroxene are observed only near the pseudopillow fractures of entablature. This indicates that the psedopillow fracture is a major water pathway and that the lava along the pseudopillow fractures had the largest cooling rate. The cooling rate of entablature is estimated to be several hundred °C/h, an order of magnitude larger than that of the colonnade.

3. The pyroclastic rocks above the entablature of the Genbudo lava are hyaloclastite formed by in situ brecciation of the lava. The hyaloclastite may have been produced by steam explosions of water infiltrating along cooling joints that developed vertically on the upper surface of the lava.

Acknowledgements

This work has evolved from the second and third authors’ bachelor thesis undertaken at Akita University. We would like to thank Prof. T. Ohba for helpful suggestions and fruitful discussions. We are grateful to the Board of Education of the town of Shizukuishi, Iwate Prefecture, for their assistance in the rock sample collection procedures at the Genbudo lava.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Table 1 is available in the supplementary files section.

Table1.png
Table 1. Petrographical imformation of Genbudo lava samples.

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Brecciation and fracturing by water ingress into a basaltic andesitic lava flow, Iwate volcano, northeastern Japan

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Version 1

Abstract

Figures

1. Introduction

2. Geological setting

3. Methodology

4. Results

4 − 1. Structure of the Genbudo lava flow

4 − 2. Microscopic characters of colonnade and entablature

5. Discussion

5 − 1. Evidence of increasing cooling rate by water ingress

5 − 2. Formation of hyaloclastite in the uppermost part

5 − 3. Cooling rate of entablature

6. Conclusion

Declarations

References

Tables

Supplementary Files

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