Mount Diablo is located roughly thirty miles east of San Francisco in Contra Costa County, California. The mountain’s summit, standing at about 3,848 feet above sea level, offers a striking view of the San Francisco bay area to the west and California’s Central Valley to the east (on the occasional clear day the sentinel-like Sierra Nevada mountain range that looms over 100 miles off to the east is also visible). The rocks that make up the summit, and which make this 360-degree view possible, consist mostly of greenstone (an originally volcanic rock that has been metamorphosed, or squished around) and sedimentary rocks that include sandstone, shale, and red, microfossil-rich chert (microfossils, in this case, refer to radiolarians - very, very, very tiny amoeba that produce sometimes-dazzling mineral skeletons). All of these rocks originated in a submarine environment well over 90 million years ago, since when they have undergone an incredible amount of disfiguration and uplift via tectonics (BOOM!! CRASH! ShaZiNG!! is what tectonics sounds like J).
If the rocks described above formed over 90 million years, does this mean that Mount Diablo itself is also that old? No, not necessarily. A nearby bed of volcanic ash that the mountain punched through as it uplifted is about 4.8 million years old; this fact allows us to deduce that the mountain itself must be 4.8 million years old or younger (we know this because, if the bed of ash were younger than Mount Diablo, it would have had to be deposited over and on top of the mountain, which it clearly isn’t).
Hope you learned something new about a very special mountain in California! Up next: QA #2: Are Birds Really Dinosaurs??