Friday, April 18, 2014

Friday Rocks #9: Pinnacles Volcanic Formation

Photo by Tim Sherry
The Pinnacles Volcanic Formation, south of Hollister, California, hosts flow banded rhyolites, rhyolite breccias, and massive rhyolite. Pinnacles is separated from similar rocks of the Neenach Volcanic Formation, which sits on the opposing side of the San Andreas Fault. These unique volcanic formations give geologists a "piercing point" for determining at least some of the offset on the San Andreas Fault, namely the offset since the volcanics were deposited in the Miocene. Using the Pinnacles and Neenach piercing points, geologists know that there has been at least 314 km of displacement on the San Andreas Fault since the Miocene.

Matthews, Vincent. "Pinnacles-Neenach correlation: A restriction for models of the origin of the Transverse Ranges and the Big Bend in the San Andreas fault." Geological Society of America Bulletin 84.2 (1973): 683-688.

Friday, April 11, 2014

Friday Rocks #8: Eureka Valley Sand Dunes

Photo by Tim Sherry
Wow, almost forgot about today's Friday Rocks! because thesisjail, so this one is quick.

The Eureka Valley Sand Dunes in Eastern California are "booming" sand dunes. Under the right humidity conditions and if enough sand avalanches downslope, soundwaves from the falling sand are amplified and making booming sound.

Friday, April 4, 2014

Friday Rocks 7: Dunite with Chromite

Photo by Timothy Sherry

Dunite is an ultramafic igneous rock often found in Ophiolite sequences. The silver-grey layers in the brown dunite are deposits of chromite. Lenscap diameter is 72 mm. Eastern Quebec.

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Friday, March 28, 2014

Friday Rocks 6: Sierra Nevada Roof Pendant

Photo by Tim Sherry
A roof pendant is a unit of country rock that sits on top of an intrusive igneous body. Here the dark rock at the top of the ridge was baked in a process called contact metamorphism by the Sierra Nevada igneous batholith. I couldn't find an exact name of this particular roof pendant. If you can name it, let us know in the comments!

Friday, March 21, 2014

Friday Rocks 5: Baby Duplex

photo by Tim Sherry

Here we have a thin sandy-mudstone bed (~20 cm thick, sorry for no scale) at MontaƱa de Oro State Park, California. The two diagonal leaning fractures are small thrust faults that bound a block of the original bed creating what geologists call a "horse". On the left side of the photo a small backthrust is present. The faults are not continuous across the siltstone beds above and below this layer. The sedimentary bedding in this area is typically planar, but some flexure is present as seen in the photo below.

photo by Tim Sherry
Here the siltstone is deformed by the sandy-mudstone load casts (thanks Greg!).