On Top of the World

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Science  18 Mar 2005:
Vol. 307, Issue 5716, pp. 1695
DOI: 10.1126/science.307.5716.1695b

The Himalayas and Tibet now have Earth's highest elevation, approaching 5 km above sea level on average, but it has been unclear how long this has been the case. One hypothesis is that within the past 5 to 10 million years, the dense lower crust and upper mantle of Tibet have detached and sunk, allowing an influx of hotter, less dense mantle that produced rapid uplift in this region. Some recent evidence based on elevation ranges of fossil plants, however, has implied that elevations were already high 15 to 20 million years ago. Currie et al.used a different approach to determine paleoelevations—the oxygen isotopes in carbonate minerals deposited in ancient lakes on the leeward (northern) side of the Himalayas. The basic idea is that as air masses encounter mountains, they rise, producing rain and snow, which decreases the 18O/16O ratio of water vapor in the air mass. Higher mountains lead to further reductions in this ratio. The data from the ancient lakes are consistent with the plant fossil data and imply that the Himalayas have been about 5 km high for about 15 to 20 million years. Although a detached slab of crust is not ruled out, their high uplift may require another explanation. — BH

Geology 33, 181 (2005)

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