Raising mountains

Natural History, July-August, 2004 by David Forest

Even in the Himalaya, not all mountains are created equal. The Higher Himalaya, which form the northern part of the range, include the world's tallest peaks--Mount Everest, for instance, exceeds 29,000 feet. In contrast, the Lower Himalaya, to the south, are generally no more than a third as high. Dense forests have long obscured the transition between the two ranges, preventing geologists from understanding the abrupt change in elevation.

Enter modernization: Road-building crews in north-central Nepal near the Marsyandi River, at the heart of the "transition zone" between the Higher and Lower Himalaya, stripped away some of the vegetation and exposed first-rate geologic outcrops. Seizing the opportunity, Kip V. Hodges of the Massachusetts Institute of Technology and a host of colleagues descended on the Marsyandi.

The investigators found that the transition zone is marked by major, recently active faults. It also seems to have exceptionally high rainfall during summer monsoons--possible evidence, the geologists speculate, of the "self-organization" of the system.

Formed by the collision between the Indian and Eurasian tectonic plates over the past 45 million years, the Himalayan ranges store excess potential energy. Hodges and his colleagues suggest the excess energy is dissipated by both the fracturing of the crust and intense, rain-driven erosion. The rapid erosion weakens the crust and leads to rapid uplift; the uplift, in turn, creates even more rainfall as clouds meet the abrupt transition between the Lower and Higher Himalaya. That positive feedback may have been shaping the ranges for millions of years. ("Quaternary deformation, river steepening, and heavy precipitation at the front of the Higher Himalayan ranges," Earth and Planetary Science Letters 220: 379-89, April 15, 2000)