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.

玄武洞熔岩，即日本岩手火山西南部更新世晚期的玄武-安山岩熔岩流，是一条厚70 m的柱状节理流，自下而上可分为柱廊、柱顶和柱廊三部分。最上部部分角砾状。上颌部出现两种主要类型的骨折：在整个上颌部形成分支网络状图案的假枕状骨折，以及具有几乎彼此平行的弯曲表面的片状骨折。在熔岩流的最上部，熔岩的指状结构从连贯的熔岩向上延伸，并且在指状之间形成同生自碎屑岩。这一现象表明，玻璃碎屑岩是在水流最上部就位时生成的，显然是当来自筑坝河谷的水覆盖了水流时。柱顶假枕状裂隙附近的熔岩结构通常为次晶质，而其他部分的熔岩结构为全晶质。假枕状裂缝附近的熔岩中有两种类型的辉石微晶石：大棱柱状（平均尺寸约 30 µm）和树枝状（长度 < 10 µm）晶体。这些表明熔岩的冷却速率在假枕状裂缝附近是最大的。在这部分水流中发现了由长方长石填充的微裂缝（宽度小于 10 µm）网络，并且沿着裂缝观察到许多气泡。这是明显的证据，表明熔岩的快速冷却是由水通过假枕状裂缝渗透造成的。根据熔岩内液体不混溶性形成的富铁液滴尺寸的测量，我们估计柱廊内的冷却速率约为 49 °C/h，柱顶内的冷却速率为 642 °C/h，与更快的冷却速率一致。通过从上方渗水冷却。