This Quaternary (less than 1.8 million years old) volcano is part
of the Lunar Crater volcanic field. Lunar Crater, a few kilometers
to the south, 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. Easy Chair Crater is
a different type of volcano – a cinder cone. Cinder (also
called scoria) is a volcanic rock with numerous holes formed by gas
escaping from the cooling and crystallizing lava. This particular
cinder cone has lots of volcanic bombs and a nice 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 either embedded in the volcanic
bombs or as separate crystals. They include olivine (and the gem
variety peridote), 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
Easy Chair Crater appears to have developed on the edge of an
earlier cinder cone that appears to have been centered a bit
further to the north.
OBSERVATION: Geologists have noted that cinder cones commonly
have rims that are not even in height, but one side tends to build
up more than another, either because of prevailing wind directions
(blowing the bombs and cinders down wind) or because of the
dynamics of the eruption (erupting in a particular direction or
erupting from an off-centered vent).
To log this Earthcache site, answer the question: Which side of
the rim (N, NE, E, SE, S, SW, W, NW) of volcano is highest in
elevation? E-mail your answer to the site developer along with a
photo of you and/or your group. Please tell us the number of people
in your group visiting this Earthcache site.
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 in the volcano can involve some steep, rocky and loose
ground. 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.
