Friday, April 19, 2013

Geo 365: April 19, Day 109: Vesicles, Close and Closer

Looking directly down onto a basalt dike in the "Tidepool Area" at Cape Perpetua State Park. There are tidepools everywhere in the rocky shore areas (as opposed to the sandy areas), but this platform, maybe a half mile south of the point with Devil's Churn on its north side, is considered the best spot for them.
I've mostly limited myself to one photo per day, but these two need to be paired. The first shows the overall width of the dike, and its relationship to the surrounding breccia, but the chain of vesicles down the middle is not terribly obvious. Zooming in for the second, the vesicles really pop out, but a lot of context is lost. The lens cap (52 mm) has not been moved, though, and between the two photos, you can see that the vesicles are quite concentrated toward the center of the dike.

I'm not absolutely positive why that is, but a reasonable guess is that the magma's volatile component was excluded from the minerals as they solidified toward the center, concentrating those gasses there, where they finally exsolved. The fact that there are vesicles suggests that either the intrusion was pretty near the surface (low pressure), or that the cooling, shrinking magma was, in the final stages of solidification, able to accommodate the exsolving bubbles- and possibly both conditions contributed.

Photo unmodified. September 21, 2010. FlashEarth Location.

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