Geology

Short-Term Storage

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Science  12 May 2000:
Vol. 288, Issue 5468, pp. 929
DOI: 10.1126/science.288.5468.929b

About 770,000 years ago, a massive volcanic eruption in Long Valley, California, formed a caldera 20 kilometers across and deposited volcanic ash that formed the Bishop Tuff. One particularly provocative result from the study of strontium and other isotopes in the ash was that the magma that became the Bishop Tuff apparently formed and remained relatively isolated in the crust for a long time—many hundreds of thousands of years—before erupting, which implies that the magma chamber had a continuous supply of heat.

Reid and Coath have now dated cores and rims of zircons in the Bishop Tuff using the uranium-lead system. Zircons likely crystallized early in the magma chamber and retained radiogenic lead even at high temperatures. The zircon ages cluster about 820,000 to 840,000 years ago, and these ages and further considerations based on the thorium isotopic composition of the zircons imply a shorter history: a large amount of magma formed only about 100,000 years or less before the eruption.—BH

Geology28, 443 (2000).

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