Lunar Crater volcanic field
Lunar Crater volcanic field is a volcanic field in Nye County, Nevada. It lies along the Reveille and Pancake Ranges and consists of over 200 vents, mostly small volcanic cones with associated lava flows but also several maars, including one maar named Lunar Crater. Some vents have been eroded so heavily that the structures underneath the volcanoes have been exposed. Lunar Crater itself has been used as a testing ground for Mars rovers and as training ground for astronauts.
Lunar Crater volcanic field
The volcanic field has formed on top of older, Oligocene to Miocene age volcanic rocks and calderas, but its own activity commenced only about 6 million years ago. The reasons for the volcanic activity there are not well known. The volcanic field has produced various types of basaltic magma and also trachyte; the most recent eruption was about 38,000 years ago and renewed activity is possible.
Geology[edit]
Regional[edit]
Intraplate volcanoes occur in many places of the Western United States, including along the Sierra Nevada, on the Colorado Plateau, the Basin and Range province and the Rio Grande Rift.[64] Lunar Crater volcanic field lies within the Basin and Range province[3] along with other volcanic fields, but in an unusually central[c] position.[66] Upwelling of asthenospheric mantle in response to the tectonic regime of the Basin and Range may be responsible for the eruptive activity there, although other processes have also been proposed[17] such as mantle downwelling and compensating flow in the asthenosphere;[67] older volcanism in the region is related to the subduction of the Farallon Plate.[68]
The Basin and Range province has had a complicated geological history[69] and in the last 20 million years[41] features extensional tectonics (tectonic processes involving a dilatation of the crust[70])[66] represented by normal faults (faults where the downmoving blocks move in a way consistent with gravity[51]).[71] The crust is relatively thin,[69] 19–21 miles (30–33 km),[17] and underpinned by an unusually hot mantle[69] which underneath Lunar Crater volcanic field has a slow seismic velocity.[72] Crustal heat flow is low.[64]
The Lunar Crater volcanic field is part of a larger[14] Pliocene and Pleistocene[66] volcanic zone that extends over the Crater Flat area[73] southward into Death Valley, California;[14] it is known as the "Death Valley-Pancake Range basalt zone".[65] This volcanic zone has received attention due to its proximity and relationship to Yucca Mountain, where a nuclear waste repository is planned,[74] although a relationship between Lunar Crater volcanic field and volcanics close to Yucca Mountain is debatable.[75]
Local[edit]
Older volcanic activity occurred in the field during the Oligocene and Miocene, generating calderas (usually large craters formed by the explosion or collapse of a volcano[76])[19] such as the Central Nevada Caldera Complex[54] and the Lunar Lake caldera which underlies much of the northern Lunar Crater volcanic field.[77] The volcanism produced ignimbrites, andesitic lavas and tuffs such as the 24 million years old Buckwheat Rim Tuff that Lunar Crater is embedded in; some of these volcanic rocks form structural blocks such as the Citadel Mountain block[3] and the Pancake Range[72] and others are correlated to ignimbrite sheets elsewhere in Nevada.[78] An early caldera-forming ignimbritic phase was followed by an andesitic lavic phase,[64] and in the last 11 million years basaltic eruptions have taken place in the Basin and Range.[41]
These older volcanics also form the basement (underground rock surface[79]) in the area, while parts of the region are covered by alluvium (sediment that was transported by water[80]);[81] sometimes the older volcanics are buried beneath this alluvium and playa deposits.[10] In turn, Paleozoic rocks crop out at the northeastern margin of the Lunar Crater volcanic field[26] and underlie the older volcanics.[64][54] Finally, Proterozoic crystalline rocks occur within the crust.[17] The geology of the region is dominated by fault-separated blocks with only little folding.[78]
Some vents form alignments, and the positions of (not all) individual volcanoes appear to be controlled by normal faults,[3] although isolated volcanoes or clusters also occur[19] and the ascent of magma at many vents was influenced by the general tectonic regime rather than by specific faults.[82] The faults have also influenced the older volcanism[69] and that in turn the Lunar Crater volcanic field.[83] Volcanic activity has buried many of the faults in the area[69] and there is little evidence of ongoing faulting and deformation.[84]