Aplites, Pegmatites, and Dikes…. Oh My!!
Posted by: 
TerryDad2
N 34° 00.094 W 116° 03.440
11S E 587053 N 3762730
The monzogranites of Joshua Tree are commonly cut by linear cracks that are filled with a different material. This location has three examples of the types of rocks that fill these cracks.
Waymark Code: WM83XX
Location: California, United States
Date Posted: 01/23/2010
Views: 19
There is space for only two or three cars. The information at the NPS display is only part of the earthcache. The rest of the earthcache will require some tame rock climbing. My 4-year old completed this with only minor assistance from her 11 year-old sister.
The NPS display describes the injection of the White Tank Monzogranite into the Pinto Gneiss and the subsequent erosion and fracturing of the monzogranie. The display points out a dike that is the first feature to examine.
The display says that as cracks formed in the monzogranite after it cooled, fresh magma welled up into the crack and filled it. The size of the crystals that make up the dike and the monzogranite reveal the effect of different cooling rates on crystal growth. The individual grains in the monzogranite are easily discernable, while the crystals in the dike are not discernable. The monzogranite cooled slowly miles beneath the earth as one large mass. However the dike cooled quickly since it was likely shallower in the earth and it was a narrow finger with cooler rock right next to it.
The second filled crack is a short climb up the inselberg (N34 00.105 W116 03.446). The rock that fills this crack is called a pegmatite. The formation pegmatites begin with the slowly cooling granitic magma. As the magma cooled it solidified and began to contract as hot rock takes up more space than cooler rock. Water and volatile material that was dissolved in the magma are forced out as the solids form. The water that was forced out of the magma is rich in dissolved minerals. The contraction and added pressure of the water eventually caused fractures to form in the rock. The mineral-laden ground water filled the fractures. Continued cooling and possibly a release of pressure as the region was uplifted forced some of the minerals to precipitate out of the ground water filling the cracks with crystals. The continued slow cooling allowed large crystals to form inside the crack.
Continuing further up the inselberg (N34 00.115 W116 03.455) a similarly formed and filled crack cooled quickly forming an aplite. The only difference between the pegmatite and aplite are the crystal sizes.
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
Waymark is confirmed to be publicly accessible: yes
Parking Coordinates: N 34° 00.042 W 116° 03.343
Access fee (In local currency): 20.00
Requires a high clearance vehicle to visit.: no
Requires 4x4 vehicle to visit.: no
Public Transport available: no
Website reference: Not listed
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Visit Instructions:
No specific requirements, just have fun visiting the waymark.