LAVA 327 forms a unique lava structure Because of the large specific heat, thermal diffusivity, and evaporation heat of water, lava will be cooled more effectively in water than in air When lava with this unique structure is exposed on land, a geologist can tell that the rock was formed underwater because of its structure One of the typical structures of lava so erupted is ‘pillow lava’, which forms mounds of elongated ‘sacks’ with quenched glassy rims Repeated oozing and quenching of hot basaltic magma produces the pillow structure A newly extruded lobe of lava is quickly surrounded by a flexible glassy skin due to rapid cooling by the surrounding water Continuous injection of lava into the lobe expands it and forms a mass of basalt with a pillow shape Finally, the skin breaks and new basalt extrudes into another lobe Repetition of this sequence forms a thick deposit of pillow-like lobes of basalt (Figure 4) Pillow lava is a characteristic structure of fluid basaltic lava, corresponding to pahoehoe lava on land When the effusion rate is high, basaltic lava spreads out on the seafloor to form ‘sheet lava’ Rare pillows are known from more siliceous lavas More viscous lava such as andesite forms a volcanic breccia (hyaloclastite) by brittle fracturing due to quenching on contact with water The surface of the flow lobe is chilled and forms a brittle crust while the viscous lava is still travelling Motion of the molten interior fractures the crust and, because the surrounding water invades the inside of the flow, brittle fracturing advances deeply into the flow As a result, a highly brecciated lobe consisting of angular glassy and massive blocks is formed Figure A pile of pillow lavas produced during the Miocene by the Ogi, Japan, submarine volcano Each pillow lobe has a glassy skin and a massive interior with radial cooling joints (Photograph supplied by T Oikawa.) Lava Tubes Lava tubes are natural tunnels through which lava travels beneath the surface of a lava flow Flowing lava is cooled from its surface by radiation and thermal convection of air, and a rigid crust is formed Once the rigid crust is formed, it provides insulation because of its small thermal conductivity, and the inner molten lava can flow without cooling In a broad lava-flow field, lava tube systems with a main tube from the vent and a series of smaller branches develop, which can supply lava to the front of the flow without it cooling When the supply of lava ceases at the end of an eruption or the lava is diverted elsewhere, lava in the tube flows out and leaves a partially empty tunnel beneath the ground Lava can also erode downwards, deepening the tube and leaving empty space above the flowing lava, because the walls and floor of the tube consist of lava of the same composition as the hot lava flowing in the tube, so the flowing lava can melt the wall rock Lava tubes often develop in basaltic lava flows with low viscosity and are rare in highly viscous felsic lava Cooling Joints One of the remarkable internal structures of lava is the systematic cooling joint As the temperature of lava drops, its volume decreases and strain within the lava causes it to fracture (Figure 5) A typical coolingjoint system is ‘columnar jointing’, which forms prismatic columns of rock with polygonal cross sections Columnar jointing is formed as follows: the spread out lava cools from its upper and lower surfaces, and the volume of lava decreases as its temperature falls Since the surface area of the lava is fixed, tension from the contraction of the main part of the lava will form a polygonal – in many cases pentagonal or hexagonal – fracture system Shrinkage fractures parallel to the surface rarely develop because tension in the vertical dimension is accommodated by a decrease in the thickness of the flow A similar phenomenon, called ‘sun cracks’, is often observed in mud when a puddle dries up In this case, the mud shrinks as water in the mud evaporates As in the progress of cooling inside the lava, the fractures propagate inwards and polygonal pillars surrounded by platy fractures are formed The axis of the pillar is normal to the isothermal plane Columnar jointing often develops not only in lava but also in tabular intrusions such as dykes and sills Another type of cooling joint is the ‘platy jointing’ This joint system consists of subparallel fractures forming thin plates Tabular joints are typically observed at the bottom of a lava flow and at the wall of a dyke, where the shear strain acts during