Randi and I visited with UCAD geology professor Dr. Malick Ngom again on Saturday morning. I came armed with a list of follow-up questions regarding the natural history of the Cap Vert Peninsula.
Dr. Ngom was interested in several images he'd seen on BOT, taken during our walking field trip to the shore. (Note the top image above.) He explained that that local volcanic rock, dolorite, has been mined over the years and used as a construction material in the building of both houses and roads. The concussive formation we'd observed along the shore was, in fact, not natural, not the result of a lava bomb (what we'd speculated, however fancifully), but where a hole had been drilled for a stick of dynamite. BOOM!
The next pair of images above are of xenoliths, when rock becomes incorporated into hot magma, and appears as a distinct discoloration in the matrix. The upper image was taken on our field trip, the second image shows a boulder of granite from the Sierra Nevada Mountains of California. While the two images reflect very different histories, both nicely illustrate how one kind of rock might be incorporated into magma.
Dr. Ngom speculates that the final two images (the top taken during our field trip, the bottom taken on Saturday) suggest that gas traveled from below, through the matrix, leaving what appears to be a vertical channel of stone with large vesicles or holes.
He also confirmed that the Cap Vert Peninsula was not a volcanic island which later became joined to the continent. Not so. The Tertiary volcanics were continental, as suggested by the extensive distribution of basalt columns above and below sea level. The channels of water, on either side of the peninsula, reflect N-S running faults.
Though he was quick to explain that everything is context specific: every snapshot must be considered with respect to it's position. The rock tells a story, and reflects a complex interaction of events.
Cool, eh? Not a rock fan? Shame. There's still time. Stick with me.
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