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

Abstract

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.