Lahar (Thirtynine Mile Volcanics, Upper Eocene; 34-36 Ma; Evergreen Station South roadcut, Colorado, USA) 31 : 無料・フリー素材/写真
Lahar (Thirtynine Mile Volcanics, Upper Eocene; 34-36 Ma; Evergreen Station South roadcut, Colorado, USA) 31 / 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 | Lahar in the Tertiary of Colorado, USA."Lahars" are simplistically defined as volcanic mudflows - volcanic ash mixed with water (derived from melted snow or rain). In reality, lahars are not restricted to fine-grained sediments suspended in water. Lahars are flood events that can include relatively little gravel or an abundance of gravel - sometimes very coarse-grained.A lahar can be a debris flow, with about two-thirds sediments and one-third water. Debris flows are so sediment-concentrated that boulders are suspended and stay at the top when the flow stops - debris flows have large rocks at the surface. This type of lahar is like wet concrete in consistency - it has a high viscosity. Once moving, such flows become sheared and end up moving quickly, more quickly than water.A lahar can be more dilute than debris flows. Hyperconcentrated flows have abundant fine-grained sediments held in suspension by turbulence. The sediment volume of hyperconcentrated flows varies - they can be clay-rich or sand-rich, with 10% solids and 90% water to a 50-50 mix. Such flows can also have large clasts, but when actively flowing, any large rocks will appear occasionally at the surface, then disappear below. Traditionally, a flow with less than 50% gravel is called a "mudflow".The Tertiary-aged lahar seen here is south of Florissant National Monument in Colorado, a famous locality for well-preserved fossils in an ancient lake deposit ("Lake Florissant").---------------------------------Info. from the National Park Service's Florissant Fossil Beds National Monument website:At the roadcut you can see the lahar that originated from the Guffey volcanic center to the west. This lahar impounded the river and created Lake Florissant. The lake was about a mile wide and extended 12 miles northwards. The debris carried by the flow contained rock fragments of different size, shape, and composition. The dark fragments are pieces of Thirtynine Mile andesite (a dark, fine-grained volcanic rock that originated from the Thirtynine Mile Volcanic Complex), while the lighter, pinkish fragments are pieces of 1.4 billion year old Cripple Creek Granite that was exposed to the surface during the Laramide Orogeny. Rounded fragments may have been picked up from the paleovalley through which the lahar flowed.---------------------------------Stratigraphy: lower member, Thirtynine Mile Volcanics (Thirtynine Mile Andesite), Upper Eocene, 34-36 MaLocality: Route 1 roadcut ~0.2 kilometers south of Evergreen Station, northwest of the town of Cripple Creek, central Colorado, USA (38° 49' 07.34" North latitude, 105° 15' 33.85" West longitude) |
| 撮影日 | 2007-07-28 13:42:27 |
| 撮影者 | James St. John |
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