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This detailed astronaut photograph of the summit crater of Popocatepetl (image center) also highlights Ventorillo and Noroccidental Glaciers. Together with the ice on nearby Iztaccíhuatl Volcano and Pico de Orizaba (Mexico’s highest peak and the highest volcano in North America), Popocatepetl’s glaciers are the only mountain glaciers in tropical North America. The glaciers create yet another volcanic hazard: dangerous mudflows, or lahars, if the ice were to melt during an eruption. At the time this image was taken, steam and ash plumes were observed at the volcano. A faint white steam plume is visible against gray ash deposits on the eastern and southern flanks of the volcano.
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The impact of the flood can be seen by comparing flood conditions (top) with normal flow conditions (bottom) along a reach of the river. The lower image is a mosaic of two astronaut images taken in late 2003. The main visual differences are the redder color of the river water produced by mud from the eroded red soils of the basin and the greener color of the countryside in the 2009 image, which is related to summer greening of the natural forest vegetation.
The most dramatic changes are the greatly increased width of the river, flooded sand bars within the river, islands shrunken by flooding, and enlarged lakes heavily charged with red mud (indicated as a “muddy lake” on the top image). With time, flood-borne mud sinks out of lake water, and lakes become a characteristically grayer hue. Over time the precise location of the river banks has changed, but such changes probably relate to slower, multi-year changes in the river course.
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The primary settlement on Atafu is a village located at the northwestern corner of the atoll—indicated by an area of light gray dots in this astronaut photograph. The typical ring shape of the atoll is the result of coral reefs building up around a former volcanic island. Over geologic time, the central volcano has subsided beneath the water surface, leaving the fringing reefs and a central lagoon that contains submerged coral reefs. Erosion and soil development on the surfaces of the exposed fringing reefs have formed tan to light-brown beach deposits (southern and western sides of the atoll) and allowed vegetation to take root (northern and eastern sides of the atoll).
The Tokelau Islands, including Atafu Atoll, suffered significant inundation and erosion during Tropical Cyclone Percy in 2005. The approximate elevation of Atafu Atoll is only two meters above the tidal high-water level. Vulnerability to tropical cyclones and potential sea level rise make the long-term habitability of the atoll uncertain.
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The pan is the low point in a major inland basin in northern Namibia. During major flood events, such as those experienced in the last nine months, rivers from Angola (the Namibia–Angola boundary lies just outside the top of the image) deliver large quantities of water to the pan. In this image, flood water in the Oshigambo River, resulting from recent heavy rains in Angola, appears as a gray stream entering the northwest corner of the pan (image top left). The floodwater becomes a thin sheet on the vast salt flat of the pan floor. Algae blooms in the warm water have produced a light green tinge. This view shows the pan as it appeared almost ten years ago. Another astronaut image shows the Oshigambo River in flood in 2006.
Reports on the ground combined with satellite imagery acquired after this astronaut photograph indicate that the plains north of the pan are now flooded, and considerable damage to homesteads, crops, and roads has occurred. More than 340,000 people have been affected in northern Namibia and about 250,000 in southern Angola.
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The snow also highlights the irregular shape and wrinkled surfaces of several lava flows present in the eastern portion of the caldera. Lava flows associated with Mt. Nemrut range in composition from thin, fluid basalt to thick, glassy obsidian. A cold-water caldera lake occupies the western half of the summit.
The geologic record at Mt. Nemrut indicates numerous prehistoric explosive eruptions during the Holocene Epoch (from about 10,000 years ago through the present); the last observed eruption of lava was in 1441. The last well-documented explosive eruption occurred during 1650. Volcanism at Mt. Nemrut is the result of tectonic activity associated with the collision of the Arabian and Eurasian Plates; this collision is ongoing, and thevolcano is merely quiescent at present.
