pet.ri.fy \'pe-tr*-.fi-\ vb [MF petrifier, fr. petr- + -ifier -ify] 1: to convert into stone or a stony substance 2: to make rigid or inert like stone : 2a: to make lifeless or inactive : DEADEN 2b: to confound with fear, amazement, or awe : PARALYZE : to become stone or of stony hardness or rigidity
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The petrified wood in this desert region was formed from trees alive in Triassic time, over 200 million years ago. These trees grew in high mountain ranges in central Arizona. They were pine like trees whose relatives now grow in South America. On the occasional eruption of a volcano which released clouds of volcanic ash, trees would be toppled by the blast wave. Often torrential rains follow these eruptions, triggered by the condensation on the particles of volcanic ash released into the atmosphere. Violent lightning will be triggered by the separation of charge that ensues from the stripping of electrons from atoms and molecules in the earth's atmosphere in the rapid thrust of the gases and solids from the eruption through it. The fallen trees were caught up in flooded streams and buried by volcanic ash and other stream sediments. These trees show signs of having been battered, stripped of their limbs, roots and bark. There are almost uniformly horizontal. During their long burial they have become impregnated with silica and thus preserved.
The volcanic ash in which they were buried is made up of tiny bubbles of silica glass. Because the bubbles are so tiny they have a very large ratio of surface area to volume and therefore are easily dissolved. Groundwater seeping through this volcanic ash soon gained a high silica content. Some of this silca was transferred to the wood when the water came in contact with the tree trunks, filling up the pores in the cells of the tree trunks. The transfer of material was so gradual that the structure of the cells of the tree trucks remain intact. This process is a type of fossilization and is called silicification. The same process also takes place in buried animal bones. The varied colors are due to the presence of trace minerals in the petrified wood.
Because the logs are much harder than the surrounding volcanic ash, as the ash erodes away they emerge, often being found first in the top of a mudhill before the ground erodes away beneath it and it rolls down the hill.
Many other fossils are found in the Chinle Formation, the layer where the petrified wood is found. Among these are ferns, leaves of deciduous trees, large dragonflies and vertebrates including amphibians and crocodilelike animals called phytosaurs.
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