Safety Warning: Be sure to carry enough food and water with you. Summer temperatures easily exceed 100 degrees and there is a good 1500 foot elevation gain with no shade.
This is a 5 part multi cache that discusses igneous rocks and looks at 5 of the many types of rocks found at the Pinnacles. A geological trail guide is available from the Bear Gulch Visitor Center that includes photographs and descriptions for a 19 stop educational hike up Condor Gulch Trail, down High Peaks Trail, then back to the Bear Gulch Visitor Center. This cache only includes a small portion of them. The listed coordinates for the cache are placed at the last waypoint of the cache.
It best to work your way through the coordinates in the following order:
Pummice Lapilli Tuff N 36 28.876 W 121 10.885
Perlite N 36 28.904 W 121 10.913
Diacite N 36 29.027 W 121 11.230
Rhyolitic Breccia N 36 29.044 W 121 11.450
Rhyolitic Breccia #2 (optional) N 36 29.312 W 121 11.406
There is a cache (GC7045K) on the west side of the park that describes banded rhyolite, but banded rhyolite can also be found along trails on the east side of the park
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 a 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.
Forms of Rhyolite are the most common rocks at the Pinnacles, covering about 60% of the Pinnacles. 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 still underground molten rock is called magma.
Pummice Lapilli Tuff N 36 28.876 W 121 10.885
The Bear Gulch Visitor Center was built with a greenish rock quarried from one of the lower (oldest) sections of the Pinnacles Formation. The green rock is pumice lapilli tuff. Pumice is a volcanic glass with many gas bubbles in it. Lapilli are rock small (less than 2.5 in.) rock fragments that are ejected from a volcano during an explosive eruption. Tuff is a volcanic rock made up of fragmented rock from a volcanic explosion. In this case the fragmented rock is granite, the original bedrock of the area before the Pinnacles volcano began erupting.
Recent studies of the pumice lapilli tuff indicate that the rock gets its green color from glauconite. Glauconite is a green mineral that typically forms in marine environments along a continental shelf.
This section based on:
- http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_64025.htm
- http://volcano.und.nodak.edu/vwdocs/glossary.html
- http://volcanoes.usgs.gov/Products/Pglossary
Perlite N 36 28.904 W 121 10.913
This point is located across the street from the Bear Gulch Visitor Center parking lot on the east side of Pinnacles National Monument. Be very careful examining this outcrop because it is on the road.
The rock that makes up this olive-green outcrop is called perlite a form of volcanic glass. It is thought that this perlite formed during a relatively quiet eruption of gas free rhyolitic lava beneath a shallow sea. The water quickly cooled the lava initially forming obsidian. While the obsidian remained submerged, it absorbed some water into it. This caused it the rock to swell and crack forming the characteristic long curved cracks.
Perlite has industrial uses. When heated quickly to very high temperature, it pops much like popcorn forming many tiny air bubbles in the rock. The resulting white to grayish rock called expanded perlite is very light weight and is chemically inert. The expanded perlite is used in construction materials including cement, insulation, ceiling ties, paint texturing and others, in gardening as pot fillers and as a carrier for fertilizers, herbicides and pesticides, and in industry as fillers in pharmaceuticals, food products, chemicals, and wells.
Some of the information in this seciton obtained from http://www.perlite.net/
Diacite N 36 29.027 W 121 11.230
This outcrop is made up of diacite. Diacite forms from molten rock with slightly less silica content than Rhyolite. Diacite typically has between 63 and 68% dilica. This shows that the composition of the molten rock fueling the Pinnacles volcano was not always constant.
Rhyolitic Breccia N 36 29.044 W 121 11.450
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. All of the clasts and matrix are rhyolite. 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, especially the peaks, is made up of rhyolitic breccia.
Rhyolitic Breccia #2 (optional) N 36 29.312 W 121 11.406
This another outcrop of rhyolitic breccia. The pinkish coloring of this outcrop is more typical of the rhyolitic breccia of the monument, especially of the high peaks.
Logging requirements:
Send me a note with :
- The text "GCPRKN Sampling of Rocks in Pinnacles NP EarthCache" on the first line
- The number of people in your group.
- At each of the stops, feel the rocks and send me a description of the texture of each. Do you think the texture has any relation to the way the rock weathers?
Information for this cache was obtained from National Park Service websites and pamphlets and individual websites named in the discussion.
Placement approved by the Pinnacles
National Monument