This cache is part of the 1st Annual International EarthCache Day
event, October 8, 2006.
Much of northwestern Nevada is covered by lava flows hundreds to
thousands of feet thick that erupted throughout much of the
Oligocene and Miocene epochs of geologic time (about 35 million to
about 7 million years ago). These rocks comprise most of the
Virginia Range south of here and the Pah Rah Range to the north. As
you drive along the Truckee River canyon between Reno and Fernley,
most of the rocks you see exposed in the canyon walls and slopes
are this type of andesitic (medium silica content) volcanic
material. The lavas and tuffs came from volcanic eruptions that
occurred as a result of subduction of the Juan De Fuca tectonic
plate to the west under the edge of the North American plate,
causing the ancestral Cascade volcano chain to actively erupt over
much of northwestern Nevada.
About 10 million years ago the last subduction gradually ceased to
our west and migrated north to the current active subduction site
located off the coast of northern California, Oregon, and
Washington. After this time, extension, or pulling apart, became
the dominant time of tectonic activity in this part of Nevada, and
it was accompanied by a different type of volcanic activity
dominated by eruption of “bimodal” basaltic (low-silica
content) and rhyolitic (high-silica content) volcanic rocks instead
of andesite. The McClellan Peak Basalt seen here at this outcrop is
representative of this later style of volcanic activity. The
McClellan Peak Basalt has been dated at 1.44 million years old by
means of Argon 40/Argon 39 radiometric dating techniques. The lava
flow probably blocked the Truckee River for a time before it was
eroded away, leaving this remnant outcrop of the lava flow here and
on the lower slopes of the hill immediately to the west. McClellan
Peak Basalt is exposed several other places in the mountain ranges
to the north and south.
The probable source of the lava flow is a vent in the vicinity of
Cinder Mountain in the Virginia Range about 10 miles southeast of
this outcrop. The lava flows are up to 5 meters thick. The lava is
vesicular (has abundant holes or vesicles), which were probably gas
bubbles trapped in the molten lava as it solidified.
The McClellan Peak Basalt in the Mustang outcrop is grayish
black when freshly broken but is often reddish-brown colored on
weathered surfaces because of oxidation of iron, which is abundant
in the basalt. The rock is called an augite olivine basalt because
it contains augite (a calcium-magnesium-iron silicate mineral of
the pyroxene group), and olivine (a greenish-brown magnesium-iron
silicate mineral). Although basalt is by definition a fine-grained
volcanic rock, you may be able to see a few phenocrysts (larger
crystals) of these minerals in the basalt as well as lath-shaped
crystals of gray plagioclase feldspar (calcium-sodium
aluminosilicate) without the aid of magnifiers.
Question to be answered in order to log this EarthCache site: In
what part of the basalt lava outcrop do the vesicles (holes) appear
to be most abundant (top, bottom, east, west, north, south)? Please
e-mail your answer to the site developer along with a photo of your
group at the EarthCache site, and include the number of people in
your group.
CAUTION IN USING GPS UNITS AND TOPOGRAPHIC MAPS
Note that GPS coordinates are relative to a particular datum
used to describe the nearly spherical shape of the Earth’s
surface. Most topographic maps published by the U.S. Geological
Survey use the North American Datum of 1927 (NAD 27), but most GPS
units are set for either the North American Datum of 1983 (NAD 83)
or the World Geodetic System of 1984 (WGS 84). NAD 83 and WGS 84
give nearly identical locations with hand-held GPS instruments, but
NAD 27 can be off considerably. In this part of Nevada, there is
little change in latitude between the 1927 and 1983 data, but for
longitude the datum of 1983 is shifted relative to the datum of
1927, such that, if you use your GPS unit to measure a location
using WGS 84, the point will plot on the U.S. Geological Survey
topographic map approximately 100 meters farther east than its true
location on the map. If your GPS unit is set to NAD 27, you need to
look for the WGS 84 waypoint approximately 100 meters farther east
from your location.
BASIC VISITOR-USE AND PUBLIC SAFETY INFORMATION FOR PUBLIC LANDS IN
NEVADA:
All visitors need to plan ahead and prepare for outings in
Nevada’s public lands by:
• Knowing the regulations and special concerns for the area
you are planning to visit (obeying laws that prohibit collection or
destruction of artifacts);
• Carrying a map and a GPS unit and/or compass (Maps are
available for purchase at all BLM offices and from the Nevada
Bureau of Mines and Geology, www.nbmg.unr.edu.);
• Staying on existing roads and trails;
• Staying away from all mine shafts and adits;
• Planning for extreme weather, hazards, and
emergencies;
• Carrying a full-size spare tire, extra food, water, and warm
clothing;
• Being aware that cell phones DO NOT usually work in the
rural areas away from the major highways;
• Leaving your travel plans with a responsible party,
including the date and time of your return;
• Linking to “Tread Lightly and Leave No Trace”
websites (www.treadlightly.org and www.lnt.org).
Please see www.nbmg.unr.edu/EarthCache/ec.htm for more
information about other Nevada EarthCache and GeoCache sites of
geologic interest. Thank you.
References:
Bell, John W. and Bonham, Harold F., Jr., 1987, Geologic map of
the Vista 7.5' Quadrangle, NBMG Map 4Hg. This map is available
online at: (visit link)
Schwartz, Kenneth M., Faulds, James E., and Henry, Christopher
D., 2002, Cenozoic magmatic evolution in the western Virginia
Range, western Nevada: Transition from subduction- to
extension-related magmatism in the western Great Basin; Geological
Society of America Cordilleran Section - 98th Annual Meeting,
Corvallis, Oregon.
Schwartz, Kenneth M., and Faulds, James E., 2004, 2004, Preliminary
Geologic Map of most of the Chalk Hills Quadrangle, Storey County,
Nevada, NBMG Open-File Report 04-11. This map is available online
at: (visit link)
