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WHAT KIND OF ROCK IS THAT? |
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A common view along the central coast is the presence of prominent outcrops of seemingly detached boulders lying in open fields. These two are located off O'Conner Road just south of the Cuesta campus. Take a closer look at the one on the right. |
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Does this look familiar? (go back to the link on the Morros) It’s another example of the Morro Rock to Islay Hill intrusive complex. The finger points to one of many plagioclase phenocrysts in this dacite porphyry. The rock stands out due to its resistance to erosion. One of the factors affecting erosion is the chemistry of the rock. High silica content usually makes for a highly resistant rock. Dacites are rich in silica. Hence, the Morros stand tall. Now lets look at the rock on the left. |
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Something is different here. This rock has layers. Lets look closer (see the arrow) |
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Here we see thin layers of the sedimentary rock chert. This rock is profoundly different from its neighbor next door. It formed from the accumulation of silica ooze on the ocean floor some 200 million years ago. Tiny protista called radiolaria provided much if not all of the silica that make up the rock. Chert is the most common type of rock to form the bold outcrops within the central coast near Cuesta College. Local Indians used it for arrowheads and other stone tools. Does the chert have any relationship to the dacite? What do both of these rocks indicate about the ancient environment near our college? Take geology 10 or 20 for more information. |
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