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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