Parking is at the picnic area of Indian Cove Campground
(N34 05.153 W116 08.416). There is no maintained trail up the
canyon, so there will be some boulder hopping and hiking up a
sand arroyo, but not far.
The granites of Joshua Tree cooled from magma in chambers deep
in the earth. The magma was insolated by the surround rock so it
cooled very slowly. This allowed the large crystals common to the
granites of Joshua Tree to form. However in some areas, the
conditions were especially good for the formation of crystals.
In these areas the crystals grew even larger than typical
crystals at Joshua Tree forming megacrystals. At the coordinates
these crystals are potassium- and sodium-rich feldspar. These
crystals grew to inches across and formed the characteristic
rectangular shape of feldspar.
Then, after these megacrystals formed, conditions changed, the
magma moved up in the crust or loss of gas in the magma, allowing
the magma to cool more quickly. Faster cooling does not allow large
crystals to form. The rest of the magma cooled as small crystals
around the megacrystals.
Logging requirements:
Send me a note with :
- The text "GC1BJ40 Megacrystals of Rattlesnake Canyon" on the
first line
- The number of people in your group.
- What color are the feldspars?
- How large are the largest crystals you can find.
- Estimate the relative percentage of megacrystals to
fine-grained material.
- Are there any other unusual (linear) features in the
rocks?
Advanced Conundrum: Feldspar is thought to crystallize last when
maga cools based on the melting temperature of feldspar. Since
these crystals are so large and well formed, they likely formed
first with no other crystals in the way. So how did this happen?
That is still an area of study, see the
second
reference.
The following sources were used to generate this
cache:
- Trent, D.D. & Richard W. Hazlett, Joshua
Tree National Park Geology, Joshua Tree National Park Association,
2002
- Lorence G. Collins, LARGE-SCALE K- AND
Si-METASOMATISM TO FORM THE MEGACRYSTAL QUARTZ MONZONITE AT
TWENTYNINE PALMS, CALIFORNIA, February 15, 1997
http://www.csun.edu/~vcgeo005/29palms.htm
Placement approved by the
Joshua Tree National Park