January 6, 2019

ARIZONA: Petrified Forest National Park - 珪化木

Petrified wood - 珪化木

A petrified log in the Petrified Forest, Petrified Forest National Park, Holbrook, Arizona

Petrified Forest National Park


During the Late Triassic, downed trees accumulating in river channels in what became the park were buried periodically by sediment containing volcanic ash. Groundwater dissolved silica (silicon dioxide) from the ash and carried it into the logs, where it formed quartz crystals that gradually replaced the organic matter. Traces of iron oxide and other substances combined with the silica to create varied colors in the petrified wood.

In Petrified Forest National Park, most of the logs in the park retained their original external form during petrification but lost their internal structure. However, a small fraction of the logs and most of the park's petrified animal bones have cells and other spaces that are mineral-filled but still retain much of their original organic structure. With these permineralized fossils, it is possible to study the cellular make-up of the original organisms with the aid of a microscope. Other organic matter—typically leaves, seeds, cones, pollen grains, spores, small stems, and fish, insect, and animal remains—have been preserved in the park as compression fossils, flattened by the weight of the sediments above until only a thin film remains in the rock.

Much of the park’s petrified wood is from Araucarioxylon arizonicum trees, while some found in the northern part of the park is from Woodworthia arizonica and Schilderia adamanica trees. At least nine species of fossil trees from the park have been identified; all are extinct. The park has many other kinds of fossils besides trees. The Chinle, considered one of the richest Late Triassic fossil-plant deposits in the world, contains more than 200 fossil plant taxa. Plant groups represented in the park include lycopods, ferns, cycads, conifers, ginkgoes, as well as unclassified forms. The park has also produced many fossil vertebrates—including giant crocodile-like reptiles called phytosaurs, large salamander-like amphibians called Buettneria, and early dinosaurs—and invertebrates, including freshwater snails and clams.




公園の珪化木の大部分はAraucarioxylon arizonicumの木からのものですが、公園の北部に見られるものはWoodworthia arizonicaとSchilderia adamanicaの木からのものです。公園から少なくとも9種の化石の木が確認されています。すべてが絶滅しています。公園には木以外にもたくさんの化石があります。世界で最も豊かな後期三畳紀の化石植物の堆積物の1つと考えられているシンレは、200以上の化石植物の分類群を含んでいます。公園内で代表される植物群には、コイ類、シダ、ソテツ、針葉樹、イチョウ、および未分類の形態が含まれます。この公園では、ファイトサウルスと呼ばれる巨大なワニのような爬虫類、ビュットネリアと呼ばれる大型のサンショウウオのような両生類、そして初期の恐竜を含む多くの化石脊椎動物、淡水産のカタツムリとアサリを含む無脊椎動物も生産されています。

Petrified Forest National Park

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