The plagioclase within the basalt lava flow floated to the top where it is concentrated in the outcrop (crystal fractionation).  Stop 1 in Bonnichsen et al

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Basalt

Basalt (pronounced /bəˈsɔːlt/, /ˈbæsɒlt/, /ˈbæsɔːlt/, or /ˈbeɪsɔːlt/) is a common extrusive igneous (volcanic) rock formed from the rapid cooling of basaltic lava exposed at or very near the surface of a planet or moon. Flood basalt describes the formation in a series of lava basalt flows.

• http://en.wikipedia.org/wiki/Basalt
• Page 102 - Bonnichsen, B., et al. 2016, From land to Lake: Basalt and rhyolite volcanism in the western Snake River Plain, Idaho: Geol. Soc. Amer., Field Guide 41