geolinde

Toshka Lakes - http://www.earthobservatory.nasa.gov/IOTD/view.php?id=78531&eocn=image&eoci=related_image

Egypt’s Toshka Lakes were created in the 1980s and 1990s by the diversion of water from Lake Nasser through a manmade canal into the Sahara Desert. Flooding of the Toshka Depression created four main lakes (lower image) with a maximum surface area of about 1450 square kilometers—around 25.26 billion cubic meters of water. By 2006, the amount of stored water was reduced by 50 percent. In June 2012 (upper image), water filled only the lowest parts of the main western and eastern basins—representing a surface area of 307 square kilometers, or roughly 80 percent smaller than in 2002. Water is almost completely absent from the central basin.

From space, astronauts documented the first lake—the easternmost one—in 1998. The lakes grew progressively as water flowed further west into each depression, with the westernmost basin filling between 2000 and 2001. The two astronaut photographs above, both taken from the International Space Station, indicate that the lakes were largely depleted by mid-2012, whereas water levels were at their highest in 2002. For scale, the lakes extended 110 kilometers from west to east in 2002.

The more recent image shows lines of center-pivot agricultural fields near the east basin (upper image), which is nearest to Lake Nasser. Sunglint on the western lake makes the water surface appear both light and dark, depending on which parts of the surface were ruffled by the wind at the moment the image was taken.

1. Other images of the Toshka Lakes

2. Toshka Lakes, Egypt (2008)
3. Decreasing Water Levels in Egypt’s Toshka Lakes (2006)
4. Toshka Lakes, Southern Egypt (2000)

Astronaut photograph ISS031-E-148455 (top) was acquired on June 21, 2012, with a Nikon D2Xs digital camera using a 14 mm lens. Astronaut photograph ISS005-E-13562 (bottom) was acquired on September 11, 2002, with a DCS760C digital camera using an 80 mm lens. Both images are provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center. The images were taken by the Expedition 5 and Expedition 31 crews. They have been cropped and enhanced to improve contrast, and lens artifacts have been removed. The International Space Station Program supports the laboratory as part of the ISS National Lab to help astronauts take pictures of Earth that will be of the greatest value to scientists and the public, and to make those images freely available on the Internet. Additional images taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by M. Justin Wilkinson, Jacobs/ESCG at NASA-JSC.

Instrument(s): 

ISS - Digital Camera

Star Dunes - http://earthobservatory.nasa.gov/IOTD/view.php?id=81996

In some areas, winds tend to blow in roughly the same general direction all year. The Grand Erg Oriental, a sprawling sea of sand dunes in the Saharan Desert, is not one of them.

The winds in northeastern Algeria tend to be complex and changing. Easterly summer winds shift in the winter, becoming westerly. Meanwhile, passing storms and local geographical features further muddle the picture. If winds came consistently from one direction, crescent-shaped barchan dunes would reign. But the dominant dune type along the southern edge of Grand Erg Oriental (shown above) are large, pyramid-shaped star dunes, which only form in areas where winds blow from multiple directions.

The image was acquired by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA’s Terra satellite on October 27, 2012. It was made from a combination of near-infrared and visible light. In this type of false-color image, sand is tan and shadows are black or gray. The blue-tinted areas are likely mineral-rich evaporites. The image is centered at 29.8°north latitude, 7.9°east longitude, near the town of Gadamis. As is common with star dunes, some of the dunes have long interlacing arms connecting to nearby dunes.

Star dunes comprise about 8.5 percent of the world’s sand dunes. Other areas they can be found include the Badain Jaran in China, the Gran Desierto de Altar in Mexico, and the eastern Rub’ al Khali in the Arabian peninsula.

Clément Narteau, a geophysicist at the Paris Institute of Earth Physics and author of a 2012 study about star dunes, noted that all of the dunes in the image have the same approximate height. “Star dunes tend to grow upward until they reach a maximum size, constant over the entire dune field,” Narteau said. “Then, they exchange sedimentary material through their radiating arms.”

There are very practical reasons for earth scientists to study how dunes form and evolve. “Understanding the dynamics of dunes is critical for developing infrastructure—such as oil and gas fields and the roads and pipelines that link them, as well as for controlling sand movement in areas where they are common,” noted Desert Research Institute researcher and dune specialist Nicholas Lancaster.

• References and Further Reading

• Goddard Earth Sciences Data and Information Services Center Geomorphology From Space: Grand Erg Oriental. Accessed September 6, 2013.
• Lancaster N. et al (1989) Star Dunes. Progress in Physical Geography, (5), 463–467. Accessed September 6, 2013.
• National Park Service Dune Types. Accessed September 6, 2013.
• USGS (1997, October 27) Types of Dunes. Accessed September 6, 2013.
• Zhang, D. et al (2012, June 24) Morphology and dynamics of star dunes from numerical modelling. Nature Geoscience, (5), 463–467. Accessed September 6, 2013.

NASA image courtesy NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team. Caption by Adam Voiland.

Instrument(s): 

Terra - ASTER

This detailed astronaut photograph, taken from low earth orbit, shows classic large and small sand masses of the central Sahara Desert, where wind is a more powerful land-shaping agent than water. “Draa” dunes (from the Arabic for “arm”) are very large masses of sand, and they appear here as the broad network of yellow-orange sand masses, with smooth-floored, almost sand-free basins between them. These sand masses lie in the western part of Libya’s vast Marzuq Sand Sea (centered at 24.5 degrees north, 12 degrees east). Geologists think that the draa of the Marzuq were probably formed by winds different from the prevailing north-northeast winds of today.

Numerous smaller dunes have developed on the backs of the draa. Three distinct dune types are visible: longitudinal dunes, which are more or less parallel with the north winds; transverse dunes, which are usually more curved and formed at right angles to the wind; and star dunes, in which several linear arms converge towards a single peak.

The upwind sides of the sand masses appear smoother than the downwind side. Wind is moving sand grains almost all the time. This means that the draa and the dunes are all moving as sand is added on the upwind side and blown off the downwind side. Small sand masses move much faster than large sand masses. The draa are almost stationary, but the smaller dunes move relatively quickly across their backs. When the smaller dunes reach the downwind side of the draa, they are obliterated; their sand is blown across the basins as individual grains.

Astronaut photograph ISS018-E-14770 was acquired on December 20, 2008, with a Nikon D2Xs digital camera fitted with a 400 mm lens, and is provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center. The image was taken by the Expedition 18 crew. The image in this article has been cropped and enhanced to improve contrast. Lens artifacts have been removed. The International Space Station Program supports the laboratory to help astronauts take pictures of Earth that will be of the greatest value to scientists and the public, and to make those images freely available on the Internet. Additional images taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by M. Justin Wilkinson, NASA-JSC.

Instrument(s): 

ISS - Digital Camera

Plate tectonics, volcanism, landslides, erosion and deposition—and their interactions—are all very evident in this view of the Crater Highlands along the East African Rift in Tanzania. The image shows landforms using color and shading. Color indicates height, with lowest elevations in green and highest elevations in white. Shading shows the slope. The vertical relief has been exaggerated by a factor of 2 to reveal greater detail about the landscape. The image is oriented as though you were looking from the north toward the southwest.

Lake Eyasi is in blue at the top of the image, and a smaller lake occurs in Ngorongoro Crater. Near the image center, elevations peak at 3,648 meters (11,968 feet) at Mount Loolmalasin, which is south of Ela Naibori Crater. Kitumbeine (left) and Gelai (right) are the two broad mountains rising from the rift lowlands. Mount Longido is seen in the lower left, and the Meto Hills are in the right foreground.

The East African Rift is a zone of spreading between the African (on the west) and Somali (on the east) crustal plates. Two branches of the rift intersect here in Tanzania, resulting in distinctive and prominent landforms. One branch trends nearly parallel to this southwesterly view and includes Lake Eyasi and the very wide Ngorongoro Crater. The other branch is well defined by the lowlands that trend left to right across the image (below center, in green). Volcanoes are often associated with spreading zones where magma, rising to fill the gaps, reaches the surface and builds cones. Craters form if a volcano explodes or collapses. Later spreading can fracture the volcanoes, which is especially evident on Kitumbeine and Gelai Mountains (left and right, respectively, lower center).

The Crater Highlands rise far above the adjacent savannas, capture moisture from passing air masses, and host rain forests. Over time, streams erode downward toward the level of the adjacent rift, deeply dissecting the volcanic slopes. This is especially evident on the eastern flanks of Mount Loolmalasin (left of center). Landslides also occur here. In particular, the small but steep volcanic cone nearest the image center has a landslide scar on its eastern (left) flank, and topographic evidence shows that the associated landslide deposits extend eastward 10 kilometers (6 miles) across the floor of the rift. Such a long run of landslide debris is unusual but not unique on Earth.

• View Size: 48 kilometers wide (30 miles) by 230 kilometers (140 miles) distance
• Location: 3 degrees South latitude, 36 degrees East longitude
• Orientation: View 35� south of west, 15� below horizontal
• SRTM Data Acquired: February 2000

Image courtesy NASA/JPL/NGA Shuttle Radar Topography team

The Nile River Valley and Delta comprise less than 5 percent of Egypt’s land area, but provide a home to roughly 97 percent of the country’s population. Nothing makes the location of human population clearer than the lights illuminating the valley and delta at night.

On October 13, 2012, the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite captured this nighttime view of the Nile River Valley and Delta. This image is from the VIIRS “day-night band,” which detects light in a range of wavelengths from green to near-infrared and uses filtering techniques to observe signals such as gas flares, auroras, wildfires, city lights, and reflected moonlight.

The city lights resemble a giant calla lily, just one with a kink in its stem near the city of Luxor. Some of the brightest lights occur around Cairo, but lights are abundant along the length of the river. Bright city lights also occur along the Suez Canal and around Tel Aviv.

Away from the lights, however, land and water appear uniformly black. This image was acquired near the time of the new Moon, and little moonlight was available to brighten land and water surfaces.

Learn more about the VIIRS day-night band and nighttime imaging of Earth in our new feature story: Out of the Blue and Into the Black.

1. References

2. United Nations Environment Programme. (2008). Africa: Atlas of Our Changing Environment. Division of Early Warning and Assessment, United Nations Environment Programme, Nairobi, Kenya.

NASA Earth Observatory image by Jesse Allen and Robert Simmon, using VIIRS Day-Night Band data from the Suomi National Polar-orbiting Partnership. Suomi NPP is the result of a partnership between NASA, the National Oceanic and Atmospheric Administration, and the Department of Defense. Caption by Michon Scott.

Instrument(s): 

Suomi NPP - VIIRS

This striking photograph from the International Space Station features two examples of circular landscape features—labeled as craters—that were produced by very different geological processes.

At image right, the broad grey-green shield volcano of Emi Koussi is marked by three overlapping calderas that were formed by eruptions. The calderas form a large, oblong depression at the 3,415–meter (11,200 foot) high summit of the volcano. A smaller crater sits within the larger caldera depression. While volcanic activity has never been observed—nor mentioned in historical records—an active thermal area can be found on the southern flank.

The circular Aorounga Impact Crater lies approximately 110 kilometers (68 miles) to the southeast of Emi Koussi and has its origins in forces from above rather than below. (Note that the image is rotated so that north is at the bottom.) The Aorounga structure is thought to record a meteor impact from approximately 345 to 370 million years ago. The crater in the image may be but one of three impact craters formed by the same event; the other two are buried by sand deposits. The linear features (image lower left) that arc around Emi Koussi and overprint Aorounga and the surrounding bedrock are known as yardangs—rock ridges formed by wind erosion.

Astronaut photograph ISS030-E-5456 was acquired on November 26, 2011, with a Nikon D2Xs digital camera using a 48 mm lens, and is provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center. The image was taken by the Expedition 30 crew. The image has been cropped and enhanced to improve contrast. Lens artifacts have been removed. The International Space Station Program supports the laboratory as part of the ISS National Lab to help astronauts take pictures of Earth that will be of the greatest value to scientists and the public, and to make those images freely available on the Internet. Additional images taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by William L. Stefanov, Jacobs Technology/ESCG at NASA-JSC.

Instrument(s): 

ISS - Digital Camera

The Ouarkziz Impact Crater is located in northwestern Algeria, close to the border with Morocco. The crater was formed by a meteor impact less than 70 million years ago, during the late Cretaceous Period of the Mesozoic Era, or “Age of Dinosaurs.”

Originally called Tindouf, the 3.5-kilometer wide crater (image center) has been heavily eroded since its formation; however, its circular morphology is highlighted by exposures of older sedimentary rock layers that form roughly northwest to southeast-trending ridgelines. From the vantage point of an astronaut on the International Space Station, the impact crater is clearly visible with a magnifying camera lens.

A geologist interpreting this image to build a geological history of the region would conclude that the Ouarkziz crater is younger than the sedimentary rocks, as the rock layers had to be already present for the meteor to hit them. Likewise, a stream channel is visible cutting across the center of the structure, indicating that the channel formed after the impact had occurred. This Principal of Cross-Cutting Relationships, usually attributed to the 19th century geologist Charles Lyell, is a basic logic tool used by geologists to build relative sequence and history of events when investigating a region.

Astronaut photograph ISS030-E-254011 was acquired on April 21, 2012, with a Nikon D3X digital camera using a 400 mm lens, and is provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center. The image was taken by the Expedition 30 crew. It has been cropped and enhanced to improve contrast, and lens artifacts have been removed. The International Space Station Program supports the laboratory as part of the ISS National Lab to help astronauts take pictures of Earth that will be of the greatest value to scientists and the public, and to make those images freely available on the Internet. Additional images taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by William L. Stefanov, Jacobs/ESCG at NASA-JSC.

Instrument(s): 

ISS - Digital Camera

The views from the top of Mount Kilimanjaro—a 5,895-meter (19,341-foot) dormant stratovolcano in Tanzania—are as surreal as they are spectacular. After ascending through multiple ecosystems—including cropland, lush rainforest, alpine desert, and a virtual dead zone near the summit—climbers can find themselves peering down on a thick blanket of clouds below that seems to stretch endlessly in the distance.

But in the immediate foreground, ice dominates the view. Looking north, a shelf-like block of ice with a sharp vertical cliff sits on an otherwise featureless, sand-covered plateau. In the other direction, a second ice field spills off the edge of the plateau, down the mountain’s southern face.

Kimberly Casey, a glaciologist based at NASA’s Goddard Space Flight Center, was savoring the views from Kilimanjaro’s summit and caldera when she snapped these panoramic images of Kilimanjaro’s northern (middle) and southern (bottom) ice fields. The Advanced Land Imager on NASA’s Earth Observing-1 satellite acquired the top image, which shows some of the same ice fields from above on October 26, 2012.

Casey was taking part in a September 2012 research expedition to Kilimanjaro to study the ice at the summit. For scale, bright tents that were part of the scientists' base camp are visible in the lower left of the northern ice field image.

Despite Mount Kilimanjaro’s location in the tropics, the dry and cold air at the top of the mountain has sustained large quantities of ice for more than 10,000 years. At points, ice has completely surrounded the crater. Studies of ice core samples show that Kilimanjaro’s ice has persisted through multiple warm spells, droughts, and periods of abrupt climate change.

But trends beginning more than a century ago suggest Kilimanjaro’s peaks may soon be ice-free. Between 1912 and 2011, the mass of ice on the summit decreased by more than 85 percent. Researchers say it’s no longer a question of whether the ice will disappear but when. Estimates vary, but several scientists predict it will be gone by 2060.

Rising air temperatures due to global warming could be contributing to the ice loss, but a number of other factors are just as important, if not more so. An increasingly dry regional atmosphere, for example, is starving the mountain of the fresh snow needed to sustain the ice fields. Drier air is also reducing cloud cover and allowing more solar energy to warm the ice surfaces.

Casey and colleagues noticed yet another ominous sign during their 2012 expedition. The northern ice field, which had been developing a hole since the 1970s, has separated. “This was the first year that the northern ice field completely divided into two,” said Casey. “We were able to walk on land—or we could have even ridden a bicycle—directly through the rift.”

See more images of Casey’s Kilimanjaro expedition on Flickr.

• References

• Cullen, N. (2012, October 1) A century of ice retreat on Kilimanjaro: the mapping reloaded.The Cryosphere Discuss.
• Earth Science Picture of the Day. (2012, October 18) Kilimanjaro Icefield in September. Accessed November 7, 2012.
• Hardy, D. (n.d.) Kilimanjaro climate and glaciers. Accessed November 7, 2012.
• NASA. (2012, April 2) Meet Kimberly Casey: Studying how debris influences glaciers. Accessed November 7, 2012.
• NOVA. (n.d.) Volcano above the clouds. Accessed November 7, 2012.
• Thompson, L. (2009, September 29) Glacier loss on Kilimanjaro continues unabated. Proceedings of the National Academy of Sciences.
• UNEP. (2012, August) Africa without ice and snow. Accessed November 7, 2012.

NASA Earth Observatory image by Jesse Allen and Robert Simmon, using ALI data from the NASA EO-1 team. Photos by Kimberly Casey. Caption by Adam Voiland.

Instrument(s): 

EO-1 - ALI

Up to 3.3 million people are facing starvation after severe drought stunted crops in parts of Kenya. The rainy season that supplies Kenya with the water needed to grow crops typically runs from March to June. This year, the rains fell from the second week of April through the first week of May, and though the rains were heavy in places, much of the country remained dry during May and June. The severe drought has led to food shortages in Kenya's Eastern, Coastal and Central provinces, and the situation may not improve soon. On July 14, Kenya's President, Mwai Kibaki, announced that poor rainfall caused at least a 60-percent crop failure in five of Kenya's seven provinces. In the same statement,which was reported in a number of international news sources, he declared a national disaster and requested nearly $100 million in international aid. Subsequently, Kenya's Ministry of Agriculture has once again reduced its national long-rains season maize projection to 1.97 million metric tons, 13 percent lower than the 2.27 million metric tons originally anticipated. While normal to above-normal maize output is anticipated in western Kenya and the North Rift, well-below-normal output is expected in the lowlands of the Coast, Eastern, Nyanza and Central Provinces.

The effects of the drought are visible in the above Moderate Resolution Imaging Spectroradiometer (MODIS) composite image. The image shows the Normalized Difference Vegetation Index (NDVI), which is a measure of how dense and green plant leaves are--an indicator of plant health. By comparing current NDVI values with the long-term average for the region at a particular time of year, scientists can determine the condition of vegetation in a region. The above image shows the NDVI anomaly (the deviation from the average) for June 9 to June 24, 2004. The region of drought, shown in brown, extends from Lake Turkana in the north southwards through central and eastern Kenya to the coastal region, where spots of brown are visible through the clouds (gray) in the south. Green regions in the southwest mark areas where above-normal maize production is expected.

The above MODIS image and story were produced by the joint Global Agricultural Monitoring Project between NASA, USDA's Foreign Agricultural Service (FAS), and the University of Maryland. More data and information about this joint project is available at Satellite Information for Agricultural Monitoring.

Instrument(s): 

Terra - MODIS

Zambesi River Delta - http://earthobservatory.nasa.gov/IOTD/view.php?id=82361

It drains a watershed that spans eight countries and nearly 1.6 million square kilometers (600,000 square miles). The Zambezi (also Zambeze) is the fourth largest river in Africa, and the largest east-flowing waterway. From headwaters in Zambia, it rolls across 2,574 kilometers (1,599 miles) of the south-central African plateau before pouring water and sediment into the Indian Ocean through a vast delta in Mozambique.

The Operational Land Imager on the Landsat 8 satellite acquired this natural-color image of the Zambezi Delta on August 29, 2013. Sandbars and barrier spits stretch across the mouths of the delta, and suspended sediment extends tens of kilometers out into the sea. The sandy outflow turns the coastal waters to a milky blue-green compared to the deep blue of open water in the Indian Ocean.

The Zambezi Delta includes 230 kilometers of coastline fronting 18,000 square kilometers (7,00 square miles) of swamps, floodplains, and even savannahs (inland). The area has long been prized by subsistence fishermen and farmers, who find fertile ground for crops like sugar and fertile waters for prawns and fish. Two species of endangered cranes and one of the largest concentration of buffalo in Africa—among many other species of wildlife—have found a haven in this internationally recognized wetland.

However, the past six decades have brought great changes to the Zambezi Delta, which used to pour more water and sediment off of the continent. Hydropower dams upstream—most prominently, the Kariba and the Cahora Bassa—greatly reduce river flows during the wet season; they also trap sediments that would otherwise flow downstream. The result has been less water reaching the delta and the floodplains, which rely on pulses of nutrients and sediments from annual (and mostly benign) natural flooding.

The change in the flow of the river affects freshwater availability and quality in the delta. Strong flows push fresh water further out into the sea and naturally keep most of a delta full of fresh (or mostly fresh) water. When that fresh flow eases, the wetlands become drier and more prone to fire. Salt water from the Indian Ocean also can penetrate further into the marsh, upsetting the ecological balance for aquatic plant and animal species. Researchers have found that the freshwater table in the delta has dropped as much as five meters in the 50 years since dams were placed on the river.

Less river flow also affects the shape and extent of the delta. Today there is less sediment replenishing the marshes and beaches as they are scoured by ocean waves and tides. “What strikes me in this image is the suspended sediment offshore,” said Liviu Giosan, a delta geologist at the Woods Hole Oceanographic Institution. “Sediment appears to be transferred from the delta offshore in plumes that not only originate in active river mouths but also from deactivated former mouths, now tidal channels. This shows the power of tidal scouring contributing to the slow but relentless erosion of the delta.”

1. Further Reading

2. World Rivers Review, via International Rivers (2006) Restoring the Zambezi: Can Dams Play a Role? Accessed December 10, 2013.
3. NASA Earth Observatory (2001, March 11) Flooded Zambeze River.
4. Richard Beilfuss, International Crane Foundation (2013) Environmental Flows and Restoration of the Zambezi Delta, Mozambique (PDF) World Delta Dialogues. Accessed December 10, 2013.
5. The Travel Guide to Victoria Falls The Zambezi River. Accessed December 10, 2013.
6. The Ramsar Convention on Wetlands The Annotated Ramsar List: Mozambique. Accessed December 10, 2013.

NASA Earth Observatory images by Robert Simmon, using Landsat 8 data from the USGS Earth Explorer. Caption by Mike Carlowicz.

Instrument(s): 

Landsat 8 - OLI

Nabro Volcano - http://earthobservatory.nasa.gov/IOTD/view.php?id=51253

Since the beginning of the recent eruption, a dense plume of water vapor, gas, and ash has concealed the summit of the Nabro volcano. New images from June 29 finally provided a nearly unimpeded view of the summit, where lava flowed out of the erupting vent and down the slope of the volcano.

Located in the East African nation of Eritrea, Nabro began its eruption explosively on June 12, 2011. The powerful eruption sent plumes of ash streaming over North Africa and the Middle East, and pumped vast quantities of sulfur dioxide into the atmosphere. The ash halted flights in East Africa for a time. The eruption killed seven people, said the Eritrean government, and other reports indicate that thousands were affected in both Eritrea and Ethiopia, though news from the region is sparse.

More recently, the volcano has eased into a quieter, lava-oozing phase, as shown in these images from the Advanced Land Imager (ALI) on the Earth Observing-1 (EO-1) satellite. The top image shows the volcano in visible and infrared light (shortwave infrared, near infrared, and green). The hot lava glows orange-red, fading to black as it cools. The long flow on the west side of the volcano is mottled with black, a sign that the surface is cooling. The lava to the east and south of the vent appears to be newer, since little of it has cooled. It is possible that the cooling lava in the western flow diverted the fresh lava to the south and east.

The lower image provides a natural color view of the volcano. A small, slightly brown plume rises from the vent, and ash blackens the ground to the west and south.

Throughout the eruption, satellite images have been nearly the only source of new information about activity at the volcano. Detailed images like this one provide insight into how erupting lava is behaving. For example, volcanologist Erik Klemetti used previous images from ALI to estimate how quickly the lava is moving and to guess at how thick (viscous) the lava is.

The Nabro volcano has not erupted in recorded human history, but lava flows near the volcano are relatively recent geologically. Nabro is part of the very active East African Rift, where three tectonic plates are pulling away from each other. As the Earth’s crust thins in the region, volcanoes rise in weak spots.

1. References

2. BBC News. (2011, June 15). Eritrea volcano: Ash disrupts air travel in East Africa. Accessed June 30, 2011.
3. BBC News. (2011, June 20). Eitrea volcano ash hits Ethiopia villages. Accessed June 30, 2011.
4. Eruptions. (2011, June 29). Nabro. Accessed June 30, 2011.
5. Global Volcanism Program. (2011, June 28). Nabro weekly reports. Smithsonian and U.S. Geological Survey. Accessed June 30, 2011.
6. Sudan Tribune. (2011, June 23). Eritrean opposition asks for international support as volcano kills seven. Accessed June 30, 2011.

NASA Earth Observatory image by Robert Simmon, using EO-1 ALI data. Caption by Holli Riebeek.

Instrument(s): 

EO-1 - ALI

Okavango - http://earthobservatory.nasa.gov/IOTD/view.php?id=51190

This short focal-length astronaut photograph shows the entire Okavango “delta,” a swampland known in southern Africa as the “Jewel of the Kalahari Desert.” This enormous, pristine wetland almost miraculously appears in a desert where surface water is typically non-existent. The water comes from the Okavango River, which rises in the high-rainfall zone of southern Angola, hundreds of kilometers to the northwest.

The dark-green forested floodplain is about 10 kilometers (6 miles) wide where it enters the view (image left). The Okavango then enters a rift basin, which allows the river to spread out and form the wetland. The width of the rift determines the dimensions of the delta—150 kilometers (90 miles) from the apex to the downstream margin (image right). The apex fault is difficult to discern, but two fault lines define the downstream margin; the faults appear as linear stream channels and vegetation patterns oriented at right angles to the southeast-trending channels at image center.

The channels carry sediment from the Okavango River that is deposited within the rift basin. Over time, a fan-shaped morphology of deposits has developed, leading to characterization of the wetland as the Okavango “delta.”

The greens of denser savanna vegetation in the north give way to browns of the open “thornscrub” savanna to the south, matching the precipitation patterns of higher rainfall in the north and less rainfall in central Botswana. More subtle distinctions also appear: the arms of the delta include tall, permanent riverine forest and seasonal forest (dark green), with grasses and other savanna vegetation (light green) on floodplains.

Linear dunes, built up by constant winds from the east during drier climates, appear as straight lines at image left. The dunes are 10 meters high, and their sands hold enough moisture for some trees to grow on them. Counter-intuitively, the low “streets” between the dunes are treeless because they are dominated by dense, dry white soils known as calcretes.

Only 2 to 5 percent of the water that enters the Okavango delta flows out of it. (Compare the small Boteti River (image right), where water flows out of the delta, with the wide Okavango floodplain at image left.) In wetter years, some water reaches Lake Ngami (lower right), where it evaporates. Over the decades, various groups have argued that the 95 percent reduction in water from apex to toe of the delta is a “loss,” and that water from the Okavango might be better used for local, irrigated agriculture. Others have called for moving it via long canals to maintain the diamond mines to the south. Various cities also have proposed to use the water, including Pretoria (South Africa), Gaborone (Botswana), or Windhoek (Namibia).

Such plans have been vigorously fought by conservationists, who have argued that the water is critical for the pristine Okavango wetland. This protected wildlife zone attracts tourists from around the world.

Another feature in the image also suggests modern globalization. The curved line in the southwest part of the delta is the Southern Buffalo Fence, a major installation that separates wild buffalo herds within the wetland from cattle herds, which occupy more populated areas surrounding the delta (image bottom, image right). The fence divides lighter-toned and darker grassland; suggesting that vegetation growth is stronger (greener) on the populated southwest side than within the delta. The fence was erected to control the spread of foot-and-mouth disease from buffalo populations to the domestic cattle herds that are the basis of an expanding beef industry. Wildlife proponents argue that fences have affected the size of wild herds by disrupting migration routes. They also cause deaths by entanglement in the fence cables and by preventing animals from reaching water.

Astronaut photograph ISS028-E-6830 was acquired on June 2, 2011, with a Nikon D2Xs digital camera using a 28 mm lens, and is provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center. The image was taken by the Expedition 28 crew. The image has been cropped and enhanced to improve contrast. Lens artifacts have been removed. The International Space Station Program supports the laboratory as part of the ISS National Lab to help astronauts take pictures of Earth that will be of the greatest value to scientists and the public, and to make those images freely available on the Internet. Additional images taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by M. Justin Wilkinson, Jacobs/ESCG at NASA-JSC.

Instrument(s): 

ISS - Digital Camera

The great Okavango Delta in the Kalahari Desert is illuminated in the Sun’s reflection point in this panorama taken from the International Space Station (ISS). Using this sunglint technique, astronauts can capture the fine detail of water bodies.

In this image, the bright line of the Okavango River shows the annual summer flood advancing from the well-watered Angolan Highlands (upper image margin). The flood water slowly seeps across the 150 kilometer-long (100 mile) delta—supplying forests and wetlands—and finally reaches the fault-bounded lower margin of the delta in the middle of winter. The wetlands support a highly diverse number of plant and animals species in the middle of the otherwise semiarid Kalahari Desert. For this reason, the Okavango Delta is now one of the most famous tourist sites in Africa.

Most of the water from the Okavango River is consumed by forests or evaporates in the dry air. Only 2 percent of the river’s water actually exits the delta. This photograph shows the small quantity of water exiting through the Boteti River. Okavango water only reaches the dry lake floors (visible on the lower edge of the large image) in the wettest years.

Part of one of the ISS solar arrays is visible on the right.

Astronaut photograph ISS040-E-8209 was acquired on June 6, 2014, with a Nikon D3S digital camera using a 42 millimeter lens, and is provided by the ISS Crew Earth Observations Facility and the Earth Science and Remote Sensing Unit, Johnson Space Center. The image was taken by the Expedition 40 crew. It has been cropped and enhanced to improve contrast, and lens artifacts have been removed. The International Space Station Program supports the laboratory as part of the ISS National Lab to help astronauts take pictures of Earth that will be of the greatest value to scientists and the public, and to make those images freely available on the Internet. Additional images taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by M. Justin Wilkinson, Jacobs at NASA-JSC.

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ISS - Digital Camera