File:Half Mountain, Dasht-e Kavir Desert.jpg

An astronaut onboard the International Space Station (ISS) photographed a small semicircular mountain in the Dasht-e Kavir Desert, about 130 kilometers (80 miles) southeast of the capital city of Tehran. The width of this image, taken with a 1150-millimeter lens, represents a distance of 17 kilometers (10 miles).

Geologists have mapped this mountain as a suite of volcanic rocks. From the colors of the rock layers, they can interpret the presence of different minerals and stages of volcanic activity. The intricate pattern of valleys also tells the story of the erosion of the rocky mountain.

In contrast, the lower half of the image shows smooth, flat surfaces known as alluvial fans. An almost straight line break is visible in the image between the hills and the fans—geologists have mapped it as a fault line. The fault cuts across the brittle rocks of the hills, leading to its name: Half Mountain.

The fan surfaces stand nearly 900 meters (2,900 feet) lower than the summit of the hills. Most of the streams flow from the hills toward the fans, where they deposit sediment eroded from the hills. The color of alluvial fan sediments often reflects the color of the parent rocks upstream. For instance, sediment eroded from the darker rocks of the mountain (center left) gives the accompanying fans a darker color. Similarly, light-toned sediment from rocks in the middle of the mountain gives some fans a lighter tone. In this desert region, the streams end in the (typically dry) Lake Namak, just outside the lower margin of the photo.