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The sedimentary layers at Raven Ridge were originally deposited as flat-lying beds in an ancient lake known as Paleolake Uinta, and later they were tilted by tectonic forces. The tilted beds are visible in this astronaut photograph as hard, erosion-resistant ridges of tan, buff, and white rocks (freshwater limestones), with less resistant layers (mudstones and shales) forming valleys between ridges.
A prominent topographic break in the ridgeline, Mormon Gap, provides road access across the ridge. To the northwest of the gap, dark brown alluvial deposits (sand and gravel carried by moving water) extend southward from the crest of the ridge. These deposits likely cover a pediment—a flat, sloping surface of eroded bedrock that extends outward from a mountain front.
The rocks and fossils at Raven Ridge indicate that during the time when they were created, the area was covered in tropical forests that were filled with the early ancestors of modern primates, horses, and deer (among other animals).
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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.
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This detailed astronaut photograph is centered on the older city of Fort William, in the southern portion of Thunder Bay. Winter snows outline the street grid of the city, while parks appear as roughly rectangular areas of unbroken white snow. Built materials (buildings, streets) appear light gray, while vegetated areas and the rock outcrop near Mount McKay are dark green to dark gray. The Kam River to the south of Fort William is ice-covered, and has an even covering of snow that traces the river channel.
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Karst topography occurs when naturally occurring acids in surface and ground water dissolve the carbonate rocks. As the rock dissolves, underground networks of drainages and caves form. As more empty space develops underground, the overlaying rock and soil collapse to form a variety of landforms including sinkholes, blind valleys, and towers. In the Biokovo Range, much of the karst surface has a pitted appearance, easily visible in the early morning light in this astronaut photograph. The pitted appearance is produced by numerous circular or semi-circular collapse valleys known locally as vrtace. While this image captures Sveti Jure covered with snow, there are no glaciers or ice fields in the Biokovo Range.
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The dramatic landscape of the city is the result of tectonic forces; the Santa Barbara Channel is part of the boundary between the oceanic Pacific Plate and the continental North American Plate. Movement along the San Andreas Fault—the actual zone of contact between the two plates—over geologic time both raised the Santa Ynez range and lowered the seafloor, forming the deep Santa Barbara Channel. The city has experienced two earthquakes, one in 1812 and another in 1925, that caused significant damage.
The urban street grid is defined by white and red rooftops at image top center; to the southeast lie beaches and the boat slips of a large marina (image top right). Two large golf courses, characterized by expanses of green grass, are visible at image center. Low, east-west-trending hills that parallel the coastline are almost completely covered by residential and commercial development, lending a speckled appearance to the hillsides. Immediately offshore, giant kelp beds are the focus of the Santa Barbara Coastal Long Term Ecological Research site, part of the National Science Foundation’s Long Term Ecological Research Network.
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The ring of islands—covered in green vegetation and white-to-tan sandy beaches—develops on coral reefs, which originally form around a volcanic island. As the volcanic island gradually disappears due to subsidence and erosion, the coral reefs continue to grow upwards. Over time, the central volcanic island is completely submerged, leaving a ring of coral reefs and islands that surround a lagoon. The shallow lagoon waters appear blue-green in this image, and contrast with darker and deeper Pacific Ocean waters surrounding the atoll.
One of the motus in the southern portion of the atoll, Tahuna Rahi, is a protected bird sanctuary, and it is the nesting site of red- and brown-footed boobies, frigatebirds, and terns (among other species). Access to the atoll is via boat, as the airstrip was closed in 2004 due to safety and security concerns.
Tetiaroa Island is also known as “Marlon Brando’s Island.” The late film star purchased the atoll from the French Polynesian government between 1966 and 1967. While the motus were his property, the government retained the rights to the coral reefs and lagoons to preserve control of marine resources. Following Brando’s death in 2004, ownership of the approximately 8-kilometer-wide atoll passed into other private hands, and there are now plans to build a luxury resort among the islets.
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This section of the canal requires constant maintenance. Approximately 68,000 acre-feet of water per year are lost by seepage from the All American Canal, especially where the canal crosses the great Algodones Dune Field, a portion of which is visible extending from top to bottom in the center of the image. Additionally, dune sand is constantly blown southeast into the canal. As part of California's Colorado River Water Use Plan, 37 kilometers (23 miles) of the canal is being lined to prevent water loss by seepage. A recently opened sector parallels the old canal (image left), and new lined sectors are under construction (bright lines, center). Engineers have sited new sections of the canal to avoid the worst areas of dune-sand invasion, so that the new configuration will be significantly cheaper to maintain and operate.
A new road-unseen in prior imagery-crosses the dunes and marks the U.S.-Mexico border, part of border fence construction efforts. The margin of the Colorado River floodplain in Mexico is just visible 2 kilometers (1.2 miles) south of the border (image upper right corner). This floodplain is Mexico's equivalent of the Imperial Valley in terms of its enormous irrigated agricultural production.
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Weather conditions on the Kerguélen Islands are typical for the latitude; at 49 degrees South, the islands sit at the crossroads of the latitude zones referred to as the “roaring forties” and the “furious fifties.” This astronaut photograph was captured on January 6, 2009—early summer in the Southern Hemisphere. That day, the mean daily temperature was 4.5 degrees Celsius (40.1 degrees Fahrenheit), with mean westerly winds of 9 meters per second (about 20 miles per hour).
The coastlines of many sub-Antarctic islands, like the Kerguélen Islands, are occupied by highly productive giant kelp beds (Macrocystis pyrifera). One of the largest marine macroalgaes (seaweeds), the species can grow to lengths of 50 meters (164 feet), forming undersea forests in hard-bottom, subtidal areas (nearshore areas that remain underwater at low tide). Fronds can spread out to form a canopy that totally covers the water surface; we interpret the black patches surrounding coastal areas in this astronaut photograph as offshore kelp beds. These kelp forests are habitat for marine animals, and due to their large biomass and relatively long survival, they are an efficient sink (storage location) for atmospheric carbon dioxide.
The surface wave pattern that travels southeastward along the gray-blue ocean surface and through the kelp beds is visible due to sunglint, the mirror-like reflection of sunlight off the water. The sunglint also improves the identification of the kelp beds by creating a different water texture (and therefore a contrast) between the dark vegetation and the reflective ocean surface.
Kerguélen Archipelago hosts thousands of marine birds (penguins, albatrosses, and petrels among others) and seals (elephant and Antarctic fur species). Whales (humpback) and dolphins (killer whales and Commerson’s dolphin) are very common in the area. Fishing boats also frequent the Archipelago—including unlicensed, so-called “pirate,” fishing vessels.
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This astronaut photograph highlights two volcanoes located near the southern boundary of the Nazca–South America subduction zone in southern Chile. Dominating the scene is the massive Minchinmávida Volcano (image upper right). Charles Darwin observed an eruption of this glaciated volcano during his Galapagos Islands voyage in 1834; the last recorded eruption took place the following year. When this photo was taken, the white, snow-covered summit of Minchinmávida was blanketed by gray ash erupted from its much smaller but now-active neighbor to the west, Chaitén Volcano.
Chaitén Volcano is dominated by a large lava dome within a caldera (an emptied and collapsed magma chamber beneath a volcano). With no recorded history of eruptions, Chaitén roared back to life unexpectedly on May 2, 2008, generating dense ash plumes and forcing the evacuation of the nearby town of Chaitén. Volcanic activity continued at Chaitén in early 2009; several days before this astronaut photograph was taken, a new lava dome partially collapsed and generated a pyroclastic flow (a scalding avalanche of gas, ash, and rock debris). A steam and ash plume extended northeast from the eruptive center of the volcano at the time of this image.
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Recent volcanic activity at Heard Island has occurred at 2,745-meter (9,006-foot) Mawson Peak, which sits within a breached caldera on the southwestern side of the Big Ben Volcano. Calderas form when a magma chamber beneath a volcano empties and collapses. The shadow cast by Mawson Peak points directly to the crescent-shaped caldera rim. Detailed geologic study of Big Ben Volcano is complicated by the presence of several glaciers, including Gotley and Lied Glaciers on the southwestern slopes. This image, taken during Southern Hemisphere summer, also reveals non-glaciated volcanic rock (light to dark brown) on either side of Gotley Glacier.
