Columbia River Basalt Stratigraphy in the Pacific Northwest

In Cooperation with the Oregon Water Resources Department

Flow Bottoms

The flow bottom is the base of the flow, and its characteristics are largely shaped by the environmental conditions that the molten lava encountered as it was emplaced. The presence or absence of water is the most important condition.

If the advancing CRBG lava encountered relatively dry ground conditions, the flow bottom that resulted typically consists of a narrow (<5 ft-thick) zone of sparsely vesicular, glassy to very fine-grained basalt. This type of flow bottom structure is very common within the CRBG.

A typical flow bottom, Crab Creek Valley, Lincoln County, Washington (photograph by Terry Tolan)

If advancing lava encountered lakes, rivers, and/or areas of water-saturated, unconsolidated sediments, far more complex flow bottom structures formed. A pillow lava complex results where advancing lava encountered a lake.  A pillow lava complex consists of elongate to spherical lobes of basalt (pillows) set in a matrix of glassy basalt fragments (hyaloclastite). CRBG pillow lava complexes and hyaloclastites are not an uncommon feature, but their occurrence and distribution reflects the paleodrainage pattern that existed at the time of their emplacement.  The physical characteristics of pillow lavas and hyaloclastites (specifically, the presence of interconnected void spaces) make them excellent aquifer horizons.  CRBG pillow lava complexes serve as important, high-yield aquifers and have hydraulic properties estimated to be similar to that of flow top breccias.

A pillow lava complex near Vantage, Washington (photograph by Terry Tolan)

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