A volcanic plug is a landform created when magma in the Earth hardens within an ancient volcano and blocks the volcano's vent over time. Eventually, the volcano becomes extinct, but the plug remains as a cooled, solidified mass. All that is left is now a hill resulting from differential erosion and weathering between a volcano tube and the surrounding rock.
In some cases, the rock surrounding the plug can erode away, revealing the distinctive "column"-look of a plug landform. This type of plug landform can be seen clearly at such places as Devil's Postpile National Monument near Mammoth Lakes, California.
Other volcanic plugs may be very rocky, like Morro Rock in San Luis Obispo County, California.
Pilot Knob is one of a handful of volcanic plugs in the state of California. It is south of the Cargo Muchacho Mountains by approximately 7 miles and makes up the base of alluvial fans in the desert. It obtains the name Pilot Knob because of its lone prominence in the desert and its use as a landmark for Colorado River riverboat captains during the 19th and 20th centuries.
The volcanic plug of Pilot Knob is not as strikingly obvious as others such as the Devil's Postpile, but the quarry does reveal a color difference between the magnetite and the surrounding rock. Magnetite is one of a number of upper mantle minerals that can make their way to the surface of the Earth via volcanic activity.
To log this Earthcache, please email the following logging requirements to the cache owner:
1) Is Pilot Knob a "column"-like volcanic plug or a "rocky" volcanic plug?
2) Based on what you've read in the above description, is the magnetite the lighter or darker material visible on the plug from the posted coordinates?
3) OPTIONAL: A picture of you, your group, or your GPSr device with Pilot Knob in the background.
Sources:
http://plate-tectonic.narod.ru/monumentvalleyvolcanophotoalbum.html
http://en.wikipedia.org/wiki/Volcanic_plug
http://www.landforms.eu/Lothian/volcanic%20neck.htm
http://volcano.oregonstate.edu/book/export/html/170
http://www.mindat.org/locentry-766649.html