Safety Warning: Be sure to carry enough food and water with you. Summer temperatures easily exceed 100 degrees.
There is a similar cache (GCPRKN) for the east side that includes pummice lapilli tuff, perlite, and diacite.
Igneous rocks are formed from the cooling of molten (liquid) rock. Igneous rocks are typically divided up into four types based on the amount of silica they contain. Silica is common compound found in the earth’s crust. For each of the four types of igneous rocks, there is a volcanic and plutonic name for the rock. Plutonic rocks form undergound and cool slowly forming interlocking mineral crystals. Volcanic rocks form on the surface and cool quickly forming very few if any visible mineral crystals. See http://volcanoes.usgs.gov/Products/Pglossary/VolRocks.html for a set of graphics.
Rhyolite is a volcanic rock with the most silica content of the igneous rocks (between 68 to 77%) and tends to be the coolest of the igneous rocks (about 900 deg C). It is typically light red to pink to tan and has very few if any visible crystals. Due to their high silica content and relatively low temperature, molten rhyolite is typically thick and chunky. This generates explosive eruptions that tend to pile up close to the volcano or short stubby lava flows.
Lava is molten rock on the surface. While the molten rock is still underground molten rock is called magma.
The rock at this location is made up of large angular chunks (clasts) surrounded by a pinkish tan rock (the matrix). This type of rock is called rhyolitic breccia. The large angular chunks in the outcrop are likely rocks from an earlier eruption that were blown apart encased it in the rhyolitic lava. Much of the rock at the park is made up of rhyolitic breccia.
One of the larger dark clasts has what looks like curved lines running across it. These lines have been described as being similar to tree rings. This type of rock is called banded rhyolite. This type of rock forms from flowing lava that allows bubble- and crystal-rich layers to form on the surface. Multiple flows build upon each other to create the multiple lines.
Send me a note with :
- The text "GCPRKV Rhyolitic Breccia and Banded Rhyolite" on the first line
- The number of people in your group.
- Describe how far apart the bands are in the rock.
Material in this cache came from NPS booklets and the above mentioned website.
Placement approved by the Pinnacles