Petrified wood. Petrified tree logs from the Late Triassic Period (over 206 million years ago). Petrification is a term used to describe the fossilisation of wood. It occurs when pieces of wood fall into sediments and are rapidly covered, preventing normal decay. Over long periods of time the wood decays. As it decays, the wood is replaced by minerals which are deposited from solution in ground water. This forms an exact replica of the original, preserving the texture of the bark and even the growth rings. Photographed in Petrified Forest National Park, Arizona, USA.
© BOB GIBBONS/SCIENCE PHOTO LIBRARY
Lightning over Tucson, USA
Lightning. Night-time view of multiple bolts of lightning striking the ground in a city with the moon in the sky. Lightning occurs when a large electrical charge builds up in a cloud. This is probably due to the rapid movement of water droplets and ice particles within the cloud. Large thunderclouds generally have very turbulent interiors. The charge induces an opposite charge on the ground, and a few "leader electrons" travel to the ground. When one makes contact, there is a huge backflow of energy up the path of the electron. This produces a bright flash of light and temperatures of up to 30, 000 degrees celsius. Photographed in Tucson, Arizona, USA.
© Keith Kent/Science Photo Library
Andromeda galaxy (M31). Optical image of theAndromeda galaxy, the nearest major galaxy to ourown Milky Way. It is the largest galaxy in theLocal Group, being 150, 000 light years indiameter. It lies 2.2 million light years away. Itis classed as a spiral galaxy (Sb). The Andromedagalaxy has two smaller companion galaxies: M32(NGC 221, top right corner) and M110 (NGC 205, lower centre). Photographed by the 0.9-metretelescope at Kitt Peak National Observatory, Arizona, USA.
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© National Optical Astronomy Observatories/Science Photo Library