About 10 million years ago, something happened here in the Bay Area that must have made a big impression on the local mastodons, three-toed horses, and long-necked camels grazing on the alluvial plain that stretched east toward the young Sierra Nevada. That’s when buried magma found its way to the surface and changed the neighborhood in some fundamental ways. It was the birth of a volcano, or four, actually: a big one and three little brothers.
Round Top, a peak within Robert Sibley Volcanic Preserve and one of the highest peaks in the Oakland hills, is made up of lava and volcanic debris left over from a 10-million-year-old volcano. Though Round Top was once the infilling of a great cauldron, it stands out today because it was originally surrounded by sedimentary rocks of the Orinda Formation, which have eroded away.
During the past 10 million years the Berkeley Hills were uplifted on a gigantic scale because of strains on the Hayward and Moraga fault systems. This uplift entailed folding of the rock formations, and the Round Top complex was tilted on its side. This folding and erosion have exposed a cross section of a great volcano, right down to its roots, providing an unsurpassed outdoor laboratory for the study of volcanism in the Central Coast Ranges. Volcanic dikes, mudflows, lava flows, and other evidence of the extinct volcanoes are visible throughout the park's 660 acres.
In order for you to log your find on this earthcache, you will need to send me an email with the names of your favorite three of the ten types of rock found in this park AND Post 1 picture at the dark basalt dike that is below Round top. (alternatively, email me the coords of the spot if you forgot your camera)
This specific dark basalt dike, was an important feeder of lava to the caldera, and cut through a sequence of tuff-breccias (brown) and pebbly mudstone (gray), all inside and near the bottom of the caldera. The mudstones indicate ponding of the water; the tuff-breccias are the remains of landslides and blockfalls into the pit from the surrounding walls.
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