Oncolitic limestone 5 : 無料・フリー素材/写真
Oncolitic limestone 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 | Oncolitic limestone (public display, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA)Limestone is a common biogenic sedimentary rock composed of the mineral calcite (CaCO3), which bubbles in acid. Many geologically young limestones are composed of aragonite (also CaCO3). Numerous varieties of limestone exist (e.g., fine-grained limestone/micritic limestone/lime mudstone, coquina, chalk, wackestone, packstone, grainstone, rudstone, rubblestone, coralstone, calcarenite, calcisiltite, calcilutite, calcirudite, floatstone, boundstone, framestone, oolitic limestone, oncolitic limestone, etc.). Most limestones represent deposition in ancient warm, shallow ocean environments.Oncolitic limestones are readily identified by the presence of abundant oncolites. Oncolites are macroscopic, concentrically layered, ~irregularly spheroidal masses of variable size. The oncolites & surrounding matrix are composed of calcium carbonate (CaCO3), which bubbles in acid. The oncolites are biogenic in origin. They grew in successive layers by the active or passive precipitation of calcium carbonate by cyanobacterial films (oncolites can be considered as mobile varieties of stromatolites). Oncolites are usually attributed to “algae”, and these structures are often called “algal balls”. Referring to cyanobacteria as “algae” is a widespread error - they are frequently called “blue-green algae”. Cyanobacteria are photosynthetic, as algae are, but they are not algae. |
| 撮影日 | 2012-11-17 12:58:48 |
| 撮影者 | James St. John |
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