Parking
is in the parking lot for the Crystal Cave. Typically visitors are
not allowed beyond the parking lot without a ticket for the Crystal
Cave Tour when the tours are being conducted. Tour reservations
must be made ahead of time and tickets purchased at visitor centers
elsewhere in the park. No tickets are sold at the cave.
The Sequoia Roof Pendant is one of many blocks of meta-sediments
surrounded by the Sierra Nevada Batholith. Meta-sediments are a
description given to metamorphic rocks that were originally
sedimentary. The Sequoia Roof Pendant is composed of Jurassic and
Triassic quartz-biotite schist interlayered with quartzite. Also
within these layers are blocks of marble.
Schist, quartzite, and marble are metamorphic rocks. Schist can
be formed from clay or mudstones. Quartzite is the metamorphic
equivalent of sandstone. And marble is metamorphisized limestone.
Each of these original sedimentary rocks of the Sequoia Roof
Pendant are thought to have been deposited in a shallow ocean
sometime in the Jurassic and/or Triassic. During the Jurassic and
Cretaceous, the Sierra Nevada Batholith intruded beneath the
sedimentary rocks. As the Farallon plate
subducted
under the North American plate, the subduced oceanic crust began to
warm and undergo partial
melting.
Partial melting occurs as a result of the different melting
points of the various minerals in rocks. As the rock heats up, the
minerals that melt at the lowest temperatures begin to melt first
forming a magma, while the minerals with high melting points remain
solid. The magma is less dense and begins to separate from the
solids and moves upward. In this case, the magma that formed from
the partial melting of the Farallon Plate was granitic in
composition.
This granitic magma migrated up toward the surface along
fractures in the overlying rock. Near the surface there was a
horizontal plane of weakness between an existing pluton and
meta-sediments rocks above it. The magma stopped its upward
movement and instead began spreading out laterally along this
horizontal contact. The horizontal contact was not perfectly flat,
and blocks of the sediment stuck down from the roof into the
intruding magma like fingers or pendants, thus the term roof
pendant. The sedimentary rocks above underwent metamorphosis from
either contact with this hot magma or from the intrusion of the
existing pluton.
Repeated pulses of magma upwelling through the same conduits
gradually pushed the previously intruded magma further away from
the conduits horizontally. The subsequent pulses also thickened the
magma chamber by either uplifting the meta-sedimentary rocks or the
dropping the underlying pluton.
Over time, uplift of the Sierra Nevada Mountains increased the
erosion on the overlying meta-sedimentary rocks. Erosion continued
until the meta-sediments were eroded away exposing the Sierra
Nevada Batholith, except where the pendants stuck down into the
magma. These pendants create isolated areas of metamorphic rocks in
the granite of Sierra Nevada.
The coordinates bring you to two plaques that describe some of
the rocks in the Sequoia Roof Pendant where you can answer the
logging questions.
Logging requirements:
Send me a note with :
- The text "GC2XJXX Sequoia Roof Pendant" on the first line
- The number of people in your group (put in the log as
well).
- What was the name of the mountain range the formed before the
current Sierra Nevadas
- How well the description of the original sediments and where
they were deposited from the plaques agree with what is included in
the EarthCache description?
- What is the trend of the meta-sediment layers (according to the
plaques)?
The following sources were used to generate this
cache:
- Joel D. Despain and Greg M. Stock –
Geomorphic History of Crystal Cave, Southern Sierra Nevada,
California. Journal of Cave and Karst Sudies, V. 67 no 2, p.
92-102.