Meteorite fragment was good trading

Press, 31 May 1985, Page 18

Meteorite fragment was good trading

In a spirit of true, though somewhat doubtful enterprise, many of the Canterbury Museum’s valuable specimens were acquired at the turn of the century through exchange with collectors in Europe and America. Although much precious material was lost to New Zealand, the brighter side is that the country has been enriched by equally valuable items from overseas. One of these is a fragment of the Canyon Diablo Meteorite on display in the Haast Hall of Geology.

This meteorite was obtained in 1897, not long after its initial discovery, from Wards' Scientific Establishment in New York in exchange for moa bones. Thousands of fragments of the meteorite, ranging from large lumps of over 630 kg to minute spheres 3 millimetres across, have been found covering 260 square kilometres around Meteor Crater in Arizona. This dramatic crater — I.3km across and 175 metres deep — is thought to have resulted from the collision of a very large meteorite with Earth about 25,000 years ago. Atmospheric friction did little to slow the meteorite’s speed of 15 to 20km per second, so that it ploughed into the earth for at least 120 metres before exploding.

Such were the shock forces involved during the impact, forces far greater than those associated with volcanoes, that horizontal rock layers below ground level were tilted outwards or became completely fractured. A form of silica called coesite, which forms only under extremely high pressures, was first found in Meteor Crater. In addition, fragments of the meteorite were found to contain minute diamonds formed

either under the high impact pressures or during the cataclysmic event that broke up the parent body in space.

For a while there was some dispute over whether Meteor Crater had a volcanic or a meteoritic origin, but after 1905, when it was officially recognised as an impact structure, great efforts were made to locate the main mass of the meteorite below the crater floor. Now it is accepted that the whole meteorite was fragmented and ejected during the impact explosion. The total weight of the far flung fragments is well over 30 tonnes, but the original body must have been considerably’ heavier before break up. Meteor Crater is a great tourist attraction in Arizona and has its own museum high on the crater rim. On a recent visit, a Christchurch resident, Mr David Eddy, bought a popular booklet on sale there called "The Meteor Crater Story” by G. E. Forster. On one page the weights and present location of the three largest fragments of the meteorite are listed: 637 kg — Meteor Crater Museum; 476 kg — American Museum of Natural

History, New York; 453 kg — The Field Museum, Chicago. Mr Eddy, being interested in geology and a supporter of the Canterbury Museum, was well aware that the Canyon Diablo Meteorite in the Haast Hall is labelled as the third largest fragment in the world, and visited the museum geologist to check the various weights. The Canterbury’ Museum specimen was accurately reweighted in 1969 by W. and T. Avery, Ltd, and was found to be 485 kg. At first sight this would seem to make Canterbury’s specimen the second largest in the world, but a check with the British Museum Catalogue of Meteorites, which gives the weights of all known specimens housed throughout the world, shows that the New York fragment is 493 kg and not 476 kg. As the label quite correctly reads in the Haast Hall, the Canterbury Museum specimen "is the

third largest piece of the meteorite to have been found and the largest to have been taken outside the United States.” Meteor Crater Enterprises, Ltd, has been informed.

By

MARGARET BRADSHAW