Easy Chair Crater Cinder Cone, Nye County, Nevada EarthCache
Easy Chair Crater Cinder Cone, Nye County, Nevada
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Easy Chair Crater is a Quaternary (less than 1.8 million years old) basaltic cinder cone volcano that is part of the Lunar Crater volcanic field. Around its rim, you can observe volcanic bombs and a variety of mineral crystals that formed in the cooling magma deep within the Earth.
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).
LOGGING QUESTIONS: To log this Earthcache site, answer the following questions:
1. Which side of the rim (N, NE, E, SE, S, SW, W, NW) of the volcano is highest in elevation?
2. What are the largest diameter vesicles (holes from gas bubbles) you see in the lava rocks near the Earthcache site? (about 2 cm, 4 cm, 8 cm or 15 cm?)
In order to log this Earthcache site, you must:
1. E-mail your answer to these questions to the site developer.
2. Post with your log a photo of yourself and/or your group, at the site, preferably sitting on a recognizable representative chunk of rock at the Earthcache site. The photo must include some recognizable feature to prove that you actually did visit the site. Please tell us the number of people in your group visiting this Earthcache
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.
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