A CHILLING OBSERVATION: SUBVERTICAL QUENCH FRACTURES ARE WIDESPREAD IN LAVAS OF THE COLUMBIA RIVER BASALT GROUP

Subvertical quench fractures are widespread in lavas of the Columbia River Basalt Group (CRBG), and are exposed in numerous localities from Idaho to the Lower Columbia River. Quench fractures are expressed as a 3D network of gently curviplanar surfaces, spaced ~0.5 to 2 m apart, extending from near or at the flow base to the upper flow crust. These fractures are accompanied by mini-columnar joints that are oriented normal to, increase in diameter away from, and flank both sides of quench fractures. Typical lengths of mini-columns are 25 to 40 cm; widths are 4-6 cm adjacent to quench surfaces and 12-15 cm distal to such surfaces. Where spacing of quench fractures is less than 2x mini-column length, platy subhorizontal joints bounded by quench fractures result from intersection of joints formed normal to nearby quench surfaces. In such instances, joints related to quenching dominate the flow “core” between earlier solidified “crusts” of the flow top and bottom. Despite evidence for quenching, physical characteristics that would indicate interaction with standing water (e.g., hyaloclastite, pillows) are lacking.

Formation of quench fractures commonly follows both development of joints (columnar or blocky) in the flow base and the solidification of an upper crust beneath which coalesced vesicles accumulated. A particularly instructive occurrence shows gently curving subvertical quench fractures that continue upward from joints bounding short (~1.2 m), well-formed basal columns lacking quench-related normal joints.

Observations to date suggest two processes occur in rapid succession and iteratively: upward incremental propagation of fractures, and rapid cooling outward from such fractures. Cooling is attributed to filling of fractures by steam, generated by conduction heating of groundwater residing in the vesicular flow top and fractures of the underlying lava flow.