Fast Shakes, Slow Rise: How a Mountain Range Changes

Field Camp Notes

1Linnea Castillo spent two weeks this summer at a field camp in the Kestrel Range, a young mountain range where geologist Dr. Omar Bakshi studies how the land changes. The camp runs a small weather station on a high ridge. Its records show that the air there warms above 0 °C and then cools below it again about 120 times a year. Each time it gets cold, water trapped in cracks in the rock freezes, expands and pushes the cracks a tiny bit wider. When it warms up, the ice melts and more water trickles in.

2On the first morning, Dr. Bakshi led the group to a step in the ground about 2 m high that runs along the base of the mountains. It formed during an earthquake 140 years ago, when the rock on one side of a fault was shoved upward about 2 m in less than a minute. The step can be followed for about 40 km. Digging along the same fault, geologists have found older steps like it, buried and worn down.

3Several GPS stations bolted to bedrock in the range show that the mountains are rising by about 1 mm a year on average. By dating the rocks, geologists estimate that the land at the highest peak has been pushed up about 5,000 m in total over the past 5 million years. Yet that peak stands only about 3,000 m higher than the plain the range rose from.

4In June, part of a cliff above the camp road broke loose. In about two minutes, roughly 20,000 m³ of rock crashed onto a slope about 300 m wide. Before the rockfall, the cliff face had been covered with deep, wide cracks.

5Dr. Bakshi also took the group to a valley shaped like a wide letter U, with steep walls and a flat floor. Glaciers no longer fill it, but long scratches in its bedrock show where ice once ground across it. Geologists estimate that glaciers deepened this valley by about 300 m over roughly 300,000 years. Last, the group visited a station on the Kestrel River where scientists measure the sand and mud the water carries out of the mountains. In an average year the river removes about 200,000 m³ of sediment from the range, a little at a time, day after day.