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Great Basin Geology
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Great Basin Geology

March 18, 2008

It's not what you might imagine a desert to look like. Standing on a Nevada peak and gazing west toward the Pacific Ocean, you would see ridge after ridge of tall, rugged brown mountains stretching into infinity. In the clear, dry air, the farthest peaks in your line of vision might be hundreds of miles away. The ridgelines are separated by flat, sage-covered valleys punctuated by rocky rubble and the occasional salty lake. This is the Great Basin, part of the Basin and Range Province, of North America, a high desert defined by rows of mountains that run roughly north to south, stretch marks on the Earth's crust.

The parallel lines of mountains that define the basin also make it prone to earthquakes, such as the large magnitude 6.0 earthquake that rattled Wells, Nevada, on February 21, 2008. Indeed, the earthquakes occur as mountains push up through the stretched-thin crust. The mountains also trap water in the desert basin, making this a closed basin—all water that falls in the basin stays in the basin. The unique geology, hydrology, and high desert vegetation of the Great Basin are typified in this false color (shortwave infrared, near infrared, green) image of northeastern Nevada, captured by Landsat 5 on June 29, 1989.

Perhaps the most striking features in the image are the mountain ranges that divide the scene in slightly wavy vertical green stripes. The Earth's crust in the Basin and Range province is gradually expanding, cracking into hundreds of faults as it thins. Over millions of years, land on one side of the faults rose, forming mountains, even as land on the other side sank into basins. The ongoing activity makes the Basin and Range province one of the most seismically active regions in the United States. At the heart of the province, Nevada ranks number three (tied with Hawaii) for most earthquakes greater than 5.0 in magnitude between 1973 and 2003 in the United States, according to the U.S. Geological Survey. The February 21 earthquake was centered near the top of the image, left of center.

The geology that shaped the region clearly holds sway with vegetation as well. The mountains are islands of green surrounded by a sea of sparsely vegetated desert. The great elevation of the mountains provides cooler temperatures that support forests of juniper, pinyon, or pine. The basins, meanwhile, are tan and pink, a sign that only scattered plants—typically sagebrush or saltbrush—cover the hot, dry valley floors.

Bright green vegetation clings to the branches of the Humboldt River that runs across the upper left corner of the image. Like all rivers and streams in the Great Basin, this river will not drain to the ocean. Instead, the water will pool in a valley lake, evaporate, or sink into the ground. Too little water falls on the high desert for any river to force its way through the mountain ranges and into the ocean.

The water cycle in the Great Basin can be read in the handful of valleys and ranges shown in this image. Most of the precipitation that falls over the northern Great Basin comes in the form of snow. The snow gathers on mountain peaks, building into a pack that gradually dwindles throughout the hot summer. Melting snow is the primary source of water during the hot, dry summer. In late June, when this image was captured, only the tallest mountains on the left side of the image still held snow, which is turquoise blue. The importance of snow in the water cycle is illustrated by the fact that the only river in the scene flows from the tallest mountains, which are able to support the largest, longest-lasting snowpack. The relative abundance of water in these mountains is also evident in the bright green vegetation that covers them.

Both this range and drier ranges to the east are riddled with lines where water, likely melting snow, flowed from the peaks into the valley floors. The water helps carry sediment from the mountains into the valleys. Bright white or gray soil covers the center of the valleys that were filled by lakes in wetter times. Concentric rings mark old shorelines on valley floors and mountain sides. Salty lakes may still occasionally form in these regions, but the water quickly evaporates in the arid climate. Pools of pale blue in this image are little more than moistened soil or a thin layer of muddy water over a salt-pan basin.