Rocky Outcrop Park is made up of Franciscan Chert. Chert is composed of the tiny silica shells of marine plankton called Radiolaria. Radiolarian chert forms where two conditions are met. First, a deep, open ocean setting is required where there is little continental mud or carbonate sediment to dilute the “rain” of dead radiolarian shells settling to the seafloor. Second, the upper ocean waters need to be relatively rich in nutrients in order for abundant Radiolaria to thrive. Bay Area chert possibly came from the "north equatorial upwelling zone", at the latitude of present-day southern Mexico. That's correct, the rocky formation you are looking at was once under the ocean off of Mexico but was moved up here by seismic movement.
As you study the chert, please answer the following questions:
1. Franciscan chert is often called "ribbon chert" - what do you see that supports this description and how do you believe this feature was created?
2. The color of chert reflects the amount of oxygen present in the sediment when it became rock. If oxygen is plentiful in the sediment, it oxidizes small amounts of iron present and the chert is red. If oxygen is scarce, the iron is reduced and the chert is green or black. Based upon this, what can you tell me about the oxygen content when the chert was developed?
3. You will see that the chert has "folds" in it, appearing to bend and wrap around itself. Why do you believe chert does that and how many instances of such folding can you see?
4. Chert itself is composed of the silica shells, but does this chert appear to sustain any life or provide habitat for any life that you can see? If so, please describe for these animals or plants.
5. Based upon your observations, do you believe chert is a hard stable rock, or a fragile rock - why?
Source of information about chert: National Park Service