Home Forums Hunting & Shooting Hunter's Campfire Oroville aux spill to fail within the next 60 min!!

According to available geological maps, the Oroville Dam is build on the Smartville ophiolite, consisting of dark grey, steeply-dipping and strongly foliated metamorphic rocks. Once of volcanic origin, erupted in an ancient sea, the rocks were uplifted and metamorphosed during the formation of the Sierra Nevada.
This description seems to match the rocks visible in published photos of the main spillway damage. Some comments have also suggested that the first hole in the main spillway developed above an altered zone of rock, as its more brownish coloration would suggest. Altered rock is more permeable and prone to erosion than massive rock. It's possible that water infiltrated below the concrete of the spillway, eroding the weakened rock until the concrete collapsed into the forming sinkhole. Which means that now running water can directly erode the bedrock, causing more damage (this possible explanation can however only be confirmed by a direct investigation of the exposed rocks).

Metamorphic rocks often cause engineering problems, as such rocks are often folded, resulting in tectonized/fractured areas in an otherwise stable rock mass. Also, the particular structure (the geological correct term is texture) of such rocks poses a problem. Metamorphic foliation forms when elongated or plate-shaped minerals (like micas) are forced by tectonic pressure into a same alignment. Between the minerals the parallel surfaces of the foliation form weak zones, prone to break. If such surfaces are tilted entire packets of rock can easily slip and be washed away by water.