Sand covers only about 20 percent of the Earth's deserts. Nearly 50 percent of desert surfaces are gravel plains where removal of fine-grained material by the wind has exposed loose gravel and occasional cobbles. This web page, produced by the U.S. Geological Survey, features text and photographs that describe desert landforms, soils, plants, and the role of water in the formation of desert landscapes. ... Full description.

This account of the history and evolution of the Mississippi River Delta makes use of text, photographs, and satellite imagery to describe how the delta has formed and migrated throughout recent geologic history. Users can read about the formation and subsequent abandonment of various delta lobes and how deltaic landforms (back bays, levees, barrier islands) are formed and destroyed as a result of ... Full description.

Volcanic eruptions are thought to be responsible for the global cooling that has been observed for a few years after a major eruption. The amount and global extent of the cooling depend on the force of the eruption and, possibly, its latitude. When large masses of gases from the eruption reach the stratosphere, they can produce a large, widespread cooling effect. As a prime example, the effects of ... Full description.

Users can read about the characteristics of volcanic activity on the Moon and how it differs from volcanism on Earth. A lunar map accompanies the text, showing the prominent mare, or "seas", which are actually volcanic plains. Links to related topics are included. Full description.

This biography of Admiral Harry Hess details his career in the United States Navy, during which he collected data on the seafloor, and as a geologist, in which he incorporated his observations on seafloor features into a hypothesis that the seafloor was spreading. Full description.

Major volcanic eruptions alter the Earth's radiative balance, as volcanic ash and gas clouds absorb terrestrial radiation and scatter a significant amount of the incoming solar radiation, an effect known as "radiative forcing" that can last from two to three years following a volcanic eruption. This results in reduced temperatures in the troposphere, and changes in atmospheric circulation patterns. ... Full description.

Users can access photographs of various forms of erosion, erosion damage, and "before and after" photos showing how the problem was brought under control. Each photograph is accompanied by a description of the activity which is taking place. Full description.

Visitors can access information about geological research and exploration in North Africa. This information can be found by clicking on an interactive map of North Africa (Morrocco, Algeria, Tunisia, Libya, and Egypt). Materials presented here consist of a message board, news articles, current and planned geological studies, an address database, a literature database, a picture database, and a collection ... Full description.

Located in Camp Springs, Maryland, the Volcanic Ash Advisory Center (VAAC) produces volcanic ash advisories and ash forecast and transport models. Real-time ash adivisories, volcano graphics and imagery, and links to related topics are available at the website. Full description.

This poster of Mars shows a full-disk mosaic, taken by the Mars Global Surveyor. The northern polar ice cap, Tharsis volcanoes, and Valles Marineris are visible. The accompanying text provides a brief description, history, and geology, as well as some statistical facts and significant dates in Mars exploration. Full description.