Minette dike & volcanic neck (Oligocene, 27-32 Ma; Ship Rock & southern dike, Navajo Volcanic Field, New Mexico, USA) 5 : 無料・フリー素材/写真
Minette dike & volcanic neck (Oligocene, 27-32 Ma; Ship Rock & southern dike, Navajo Volcanic Field, New Mexico, USA) 5 / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
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| 説明 | Ship Rock volcanic neck (diatreme) in New Mexico (looking northwest) - the mountain consists of minettes and volcanic breccias. The long linear feature on the left is the southern dike, composed of finely crystalline minette rocks. The surrounding plains are fine-grained siliciclastic sedimentary rocks of the Mancos Shale (Upper Cretaceous).----------------The Navajo Volcanic Field in the Four Corners area of the American southwest has about 80 old, eroded volcanic centers (volcanic necks/volcanic plugs/diatremes) of Oligocene to Miocene age. The most famous and visually distinctive volcanic neck in the area is Ship Rock. Ship Rock is a brownish-colored, craggy, vertical-sided mountain in far-northwestern New Mexico, USA. It represents rocks that filled up the subsurface vent complex of an ancient volcano. The original volcano and its surrounding landscape were approximately 1 to 2.5 kilometers above the present surface, according to published estimates. Radiating from the central volcanic neck are three, miles-long, sublinear, vertical dikes. One extends to the northeast, one extends to the west, and one extends to the south. Several shorter radiating dikes extending in other directions occur near Ship Rock itself. The southern major dike is the most easily accessible (see above photo), as Red Rock Highway was constructed through it. Ship Rock and the dikes radiating from it are principally composed of the scarce igneous rock minette (= potassic mica lamprophyre, the intrusive equivalent of trachybasalt lava) and volcanic breccias. Typical Ship Rock minettes are dense, non-vesicular, finely crystalline, and are composed of alkali feldspar (K-Na feldspar), phlogopite mica (which glitters nicely in the light), diopside pyroxene, some olivine, plus other minor minerals. Published studies of the eruptive centers in the Navajo Volcanic Field indicate that the original volcanoes erupted violently. This typically happens if the magmas are rich in dissolved gases (water, carbon dioxide, sulfur dioxide, etc.). Minette magmas were not rich in dissolved gases. So why the violent eruptions? The magmas came in contact with groundwater, and the water boiled to steam while confined underground. The steam pressure increased until it overcame the strength of the overlying rocks, resulting in an explosion and the creation of a surface crater (maar). Volcanologists call these phreatomagmatic eruptions (a.k.a. hydrovolcanic eruptions). Age of Ship Rock: Oligocene, 27-32 Ma------------Mostly synthesized from:Delaney (1987) - Ship Rock, New Mexico: the vent of a violent volcanic eruption. Rocky Mountain Section of the Geological Society of America, Centennial Field Guide 2: 411-415.Semken, S. 2003. Black rocks protruding up: the Navajo Volcanic Field. New Mexico Geological Society Guidebook, 54th Field Conference, September 24-27, 2003: 133-138. (semken.asu.edu/pubs/semken03_nvf.pdf) |
| 撮影日 | 2007-09-02 14:41:34 |
| 撮影者 | James St. John |
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