Columbia River Basalt Stratigraphy in the Pacific Northwest

In Cooperation with the Oregon Water Resources Department

Flow Tops

The flow top is the crust that formed on the top of a molten lava flow. Flow tops are riddled with countless gas bubbles, called vesicles, that were trapped as the flow solidified. The physical character of flow tops falls between two basic end members, a simple vesicular flow top and a flow top breccia

A simple vesicular flow top commonly consists of glassy to fine-grained basalt that displays a rapid increase in the density of vesicles near the top of the flow. Vesicles may be isolated or interconnected.

A simple vesicular flow top, Crab Creek Valley, Lincoln County, Washington (photograph by Terry Tolan)

A flow top breccia consists of angular, scoriaceous to vesicular fragments of basaltic rubble. Flow top breccias can be very thick (over half the flow thickness). There are two models for the origin of CRBG flow top breccias: (1) the breccia was originally produced at the eruptive center and subsequently rafted away on top of the flowing lava, and (2) the breccia is formed when the lava becomes viscous as it cools and is broken into fragments. The inherent physical characteristics of flow top breccias allow them to potentially have a high degree of interconnected void space. Where this occurs, flow top breccias have extremely high permeability and can serve as high yield aquifers.

Right: Flow top breccia near Vantage, Washington (photograph by Terry Tolan)

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