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

The Genbudo lava, the late Pleistocene basaltic-andesitic lava flow in the southwestern part of Iwate Volcano, Japan, is a 70 m thick columnar jointed flow that can be divided into three parts from bottom to top: the colonnade, the entablature, and the partly-brecciated uppermost part. Two main types of fractures developed in the entablature: pseudopillow fractures that formed in a branching network-like pattern throughout the entablature, and sheet fractures with curved surfaces that are nearly parallel to each other. At the uppermost part of the flow, finger-like structures of lava extend upward from the coherent lava, and cogenetic autoclastic rocks form between the fingers. This occurrence suggests that hyaloclastites were generated during emplacement in the uppermost part of the flow, apparently when water from a dammed river valley covered the flow. The texture of the lava near the pseudopillow fractures in the entablature is commonly hypocrystalline, while the texture in other parts is 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 fractures. These suggest that the cooling rate of the lava was 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 of the flow, and many bubbles are observed along the fractures. This is clear evidence that the rapid cooling of the lava was caused by water infiltration through the pseudopillow fractures. From the measurement of Fe-rich droplet sizes that formed due to liquid immiscibility within the lava, we estimate the cooling rate within the colonnade as about 49 °C/h and within the entablature as 642 °C/h, consistent with much more rapid cooling by water infiltration from above.