The
Dinkey Dome Pluton is reached by a moderate to strenuous hike into
the Dinkey Lakes Wilderness. The area is at an elevation of about
8800 feet. The parking area is accessible by unpaved Forest Service
access roads. The roads do get pretty rough, so a high clearance
vehicle would be a very good idea. Winter snows will make the
EarthCache inaccessible.
The Dinky Dome Pluton and Dinkey Creek Pluton have been used as
an example of a granite pluton that was formed by partial melting
of a subducting plate. Both plutons are found in the Dinkey Lakes
Wilderness, but the coordinates take to an outcrop of the Dinkey
Dome Plution. 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 liquid magma is less dense than the solids, so it begins
to separate from the solids moving upward through fractures in the
rock above. In this case, the magma that formed from the partial
melting of the Farallon Plate was granitic in composition.
Between 104 and 90 million years ago, 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-sedimentary (metamorphic rocks
that were sedimentary rocks) that rocks above it. The magma stopped
its upward movement and instead began spreading out laterally along
this horizontal contact.
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.
Overtime the granite magma cooled forming the granite of the
Dinky Dome Pluton. Continued uplift of the Sierra Nevada Mountains
increased the erosion on the overlying meta-sedimentary rocks.
Erosion continued until all but a few roof pendants (see
Dinkey Creek Roof Pendant)of the meta-sedimentary rocks
remained exposing the Dinky Dome Pluton as we see it today.
One of the lines of evidence for the horizontal movement of the
Dinkey Dome magma between the meta-sedimentary rocks and the older
pluton comes from the magnetic foliation within the Dinkey Dome
Pluton. Magnetic foliation comes from the orientation of small
magnetic minerals within the granite. The orientation of these
magnetic particles became aligned with the earth’s magnetic
field as the magma solidified. The magnetic foliation is measured
by advanced instruments that cancel out the relatively powerful
magnetic field of the earth so that the weak magnetic field of the
minerals can be observed. In magmas, magnetic foliation is used to
describe the flow patterns within cooling magmas.
In addition, the age of the granite rocks within the Dinky Dome
Pluton supports the formation of the pluton from one or two
vertical conduits and spreading out laterally. The youngest
granitic rocks within the Dinky Dome Pluton are near the center.
Older rocks surround these younger ones.
image from Petford et. al. 2000
This process is thought to have taken place over a period of
less than 100,000 years instead of the tens of millions of years
required by previous theories.
Logging requirements:
Send me a note with :
- The text "GC30XM8 Dinky Creek Pluton" on the first line.
- The number of people in your group (put in the log as
well).
- Describe the size of the individual mineral grains at the
coordinates.
- What does the size indicate about the method of cooling?
- What features to you see that could be attributed to the
magnetic foliation?
The above information was compiled from the
following sources:
- Farallon Plate
(http://en.wikipedia.org/wiki/Farallon_Plate
- Subduction Zones; Plate Tectonics, 1996 - 2005
platetectonics.com.
http://www.platetectonics.com/book/page_12.asp
- Farallon Plate, USGS,
http://pubs.usgs.gov/gip/dynamic/Farallon.html Last updated:
05.05.99
- The Farallon Plate NASA/Goddard Space Flight
Center Scientific Visualization Studio;
http://svs.gsfc.nasa.gov/vis/a000000/a001300/a001322/index.html
- Martin-Hernandez, F. et al Editors, Magnetic
Fabric Methods and Applications, Geological Society, Special
Publication 238, , Geological Society of London 2004
- Petford, N., Cruden, A., McCaffrey, K and
Vigneresse, J-L., Granite magma formation, transport and
emplacement in the Earth's crust, Nature, V. 408, p. 669-673,
December 2000.