Parking for this earthcache is in front of a NPS display
N34 00.042 W116 03.343. 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 (see
Inselbergs) 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.
Logging requirements:
Send me a note with :
- The text "GC1C195 Aplites, Pegmatites, and Dikes…. Oh My!!" on
the first line
- The number of people in your group.
- How far below the surface did the monzogranie begin
cooling?
- How thick are the dike, pegmatite, and aplite?
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
Placement approved by the
Joshua Tree National Park