Black Rock Lava Flow
A
cache
by GeoEdYOUcation
Hidden:
11/15/2006
Size:
 (Not chosen)
Difficulty: 
Terrain: 
(1 is easiest, 5 is hardest)
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This basaltic lava flow in the Lunar Crater volcanic field hosts a
variety of mineral crystals that formed from cooling magma deep
within the Earth. Unlike most basalts, whose crystals are too small
to be seen without a magnifier, this flow contains some crystals up
to several centimeters long embedded in the volcanic rock. They
include olivine, clinopyroxene, hornblende, plagioclase,
anorthoclase, and magnetite.
This Quaternary (less than 1.8 million years old) lava flow is part
of the Lunar Crater volcanic field. Lunar Crater, several
kilometers to the southwest, is a maar, a type of volcano formed
when magma approaching the surface from deep within the Earth
violently boils the groundwater and tosses volcanic bombs (pieces
of lava) onto the surface, often leaving a circular hole that later
fills with a lake (if there is lots of water). At Lunar Crater, the
ground water is too deep below the surface for a lake to form.
There is a different kind of volcano here – a cinder cone. Cinder
(also called scoria) is a volcanic rock with numerous holes formed
by gas escaping from the cooling and crystallizing lava. When the
volcano erupted, it tossed cinder into the air, and the heavy
pieces of cinder landed close to the volcanic vent, progressively
building the cone shape. This volcano also erupted a lava flow from
the same vent that built the cinder cone.
This particular lava flow has a variety of crystals that formed
from cooling magma deep within the Earth. The rock is termed a
basalt, based on its mineralogy (dominantly plagioclase,
clinopyroxene, and olivine) and chemistry (more magnesium, calcium,
and iron and less silica, potassium, and sodium than other common
igneous rocks).
Large crystals can be found embedded in the volcanic rock, and
some crystals can be found in the sand around the hard rocks. They
include olivine (and the gem variety peridot), clinopyroxene,
hornblende, plagioclase, anorthoclase, and magnetite. Some crystals
are several centimeters long.
Minerals that can easily be identified in this area (mostly as
crystals a few millimeters in size):
Plagioclase (a type of feldspar), a solid solution of dominantly
CaAl2Si2O8 and NaAlSi308 – clear to milky white, with cleavage and
tiny lamellae from crystal twins
Hornblende-Kaersutite, a complex amphibole solid solution described
chemically as
(Ca,Na,K)2-3(Mg,Fe,Mn)3-4(Al,Fe)2-1(Si,Al)8O22(O,OH,F)2 and
NaCa2(Mg,Fe)4Ti (Si6Al2)8O22(O,OH,F)2 – generally black prisms
with two directions of cleavage at 60° and 120°
Clinopyroxene, solid solution of dominantly MgSiO3, CaSiO3, and
FeSiO3 – generally black (sometimes dark green) prisms with two
directions of cleavage at 90° but also without visible cleavage and
glassy, resembling obsidian
Olivine, a solid solution of dominantly Mg2SiO4 and Fe2SiO4 – olive
or pale green, glassy.
Magnetite, a solid solution of dominantly Fe3O4 and Fe2TiO4 –
easily attracted to a magnet, particularly if you put the magnet
into the loose material eroded from the volcanic rocks
To log this Earthcache:
OBSERVATIONS: Water generally runs downhill because of the force
of gravity. So does lava.
QUESTION: In what general direction ((N, NE, E, SE, S, SW, W, NW)
did the lava flow to get from the volcanic vent to the EarthCache
location?
In order to log this Earthcache site, e-mail your answer to this
question to the site developer along with (if possible) a photo of
your group at the site, including the number of people in your
group.
REFERENCES:
Wood and Kienle, 1990, Volcanoes of North America: United States
and Canada: Cambridge University Press, 354 p., p.256-262.
(visit link)
Kleinhampl, F.J., and Ziony, J.I., 1985, Geology of northern Nye
County, Nevada: Nevada Bureau of Mines and Geology Bulletin 99A,
171 p., with maps at 1:250,000 scale.
SPECIAL NOTES ABOUT ACCESS AT THIS SITE
Hiking on the lava flow can be dangerous, because the rocks are
sharp and sometimes loose. Wear sturdy hiking boots. It is also
advisable to wear gloves.
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.htm for more information
about this and other Nevada EarthCache and GeoCache sites of
geologic interest. Thank you.
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