Remote Sensing Tutorial Page 17-5

Karst topography encompasses a variety of landforms, such as solution valleys, sinkholes, subterranean caves, and towers, which develop largely by chemical dissolution of limestone rocks. Solution often begins in and extends from structural joints. As these enlarge, they can turn into valleys that may widen and coalesce to leave the terrain pockmarked with depressions (sometimes called "cockpits", a term applied to Jamaican karsts). Expansion of dissolved volumes may leave the rock units between them as residual peaks or towers. Nowhere in the world is this more spectacularly developed than in the Guangxi Province of Southern China. In the image pair below, a group of towers appears on the left and a Landsat image of a region west of Guilin, a popular tourist center in China, on the right.

The karst topography in this scene appears in the darker-toned surfaces. These karsts are a thick series of carbonate rocks that elsewhere in the image have their outer rock removed to expose older non-carbonates underneath. Two main joint sets criss-cross the carbonate sequence and enlarge into intersecting valleys. Drainage across these plateaus is now mostly internal, so that the region also contains numerous caves. This scene offers a remarkable illustration of how dissimilar rock types (which host different joint spacings) give rise to strikingly varied topographic expressions and resulting landforms

17-11: What common feature of internal drainage (both groundwater and underground streams), quite popular to visit, is frequently associated karst topography? ANSWER

Lakes come in many shapes, sizes, depths, and origins. Most are freshwater but some may be saline because of evaporation that concentrates dissolved salts. We show two extremes here:

The first image shows elongated, thermokarst lakes in the coastal plains ending at Point Barrow in northwest Alaska. The lakes tend to be ovoid and elliptical and are generally less than 6 m (20 ft) deep. They result from collapse, slumping, and caving during the summer thaw of soils and sediments affected by permafrost (a condition in which groundwater in upper layers freezes permanently to some depth, except for upper layers, heated in the warm season. The cause of elongation is uncertain but may be due to paleowind control at a time when winds came from the northwest, rather than today's northeast.

17-12: Much of Alaska and Canada were glaciated in the Pleistocene. Suggest another way in which glacially-related action can cause elongate, aligned lakes. ANSWER

The huge lake (more than 9,000 km² [3,475 mi²]) in the image above is the Salar de Uyuni in Bolivia, the largest salt playa in the world. A playa is a lake bed formed in an arid climate by rapid evaporation during dry season of wet-season stream inflow causing sediment and salt deposits (evaporites) to accumulate over time. In this March 1973 image, the salar (salt pan) has about a meter of briny water (blue). Some of the surface is uncovered salt (white), which, in addition to sodium chloride, NaCl (rock salt) and calcium sulfate, CaSO₄, also contains lithium chloride, LiCl, making this the biggest source of lithium in the world. This, and the Salar de Coipasa (upper left), were once part of a much bigger lake that had formed in this part of the Altiplano province of the Andes, during a rainier climate in the past.

Many landforms characterize arid regions and we would need many images to show them. We elect here to show two examples of what many people visualize as a desert–sand dunes, usually in the Sahara Desert.

The largest active sand sea on Earth (560,000 km² [216,237 mi²]) is the Rub'al Khali or Empty Quarter, shown above, in the southern Arabian Peninsula. Loose sand, up to several hundred meters thick, covers most of the solid rock landscape in this region. In the Landsat scene on the left, six distinct sand fields, each characterized by particular types of dunes (distinguished mainly by shape), occupy the entire area. Of these, we mention four. Most of the center consists of Complex Crescentic Dunes on the left and Complex Linear Dunes on the right. In the lower right corner are Star Dunes, and along the left is a separate sea consisting of small Linear and Complex dunes.

The above image is part of the Dasht-E-Lut, or stony desert, located in southeast Iran. Dominating the scene is a large, but isolated, sand sea containing linear or elongated star dunes with a prevailing northeast trend. The surrounding lowlands have gravelly surfaces. Along the lower left edge of this Landsat image are dark streaks that orient towards the southeast. These streaks are the eastern end of a big complex of long, narrow ridges called yardangs, left behind when winds blew out soft clay/sand sediments, leaving trenches. As the trenches elongated, they controlled the eventual scooping out of the linear valleys that defined the residual yardangs. In this scene, exposed bedrock comprise the low mountains to the east of the sand sea, from which streams carry and deposit dark rocks in a series of distributaries and fans.

Glaciation is commonplace in cool regions of the globe, at high, mountain elevations and in high (polar) latitudes. Ice can cover nearly an entire continent, such as Antarctica, or a huge island such as Greenland. When ice begins to flow by gravity to lower elevations, usually along distinct, often narrow paths, it becomes a glacier. Here are two examples of ice accumulation and glacial movement:

The first image is a special, false-color image, using yellow, red, and blue filters, to enhance various types of ice and glacial zones. It shows the thick ice cap (defined as a dome shaped ice mass with radial flow) known as Vatnajökull, Greenland which has 43 outlet glaciers (light blue), many with lobate termini. Areas in yellow-orange are vegetated, while reds associate with basaltic rocks and sparse vegetation. Green in the ocean is sediment, and black around the ice cap is a zone of ground soaking from glacial meltwater.

The image above shows typical alpine or mountain glaciers developed from snow fields covering the higher elevations of part of the Wrangell mountains of southeast Alaska. The range rises to 5,043 m (16,541 ft) at Mount Kennedy. Among the large glaciers are Nabesna (to the north), Kennecott and Rohn sending meltwater into the Chitina River, and Russell and Barnard coming off the eastern segment of the Wrangell group. Note the dark streaks in some of these piedmont glaciers. These are medial and lateral moraines (glacial debris that may become rock deposits). The reds in the scene are mostly tundra vegetation.

17-15: What would be the appearance of the terrain if all the glaciers in the Wrangell Mountains were to melt and disappear? ANSWER

This ends our tour of highlighted, thematic images from "Geomorphology from Space." You can see more, with detailed captions, on the Internet Site mentioned above or in the CD-ROM prepared by JPL. If you study the book, the Web Site, or the CD-ROM, please look at Chapter 11 that presents an effort by Dr. Robert S. Hayden to demonstrate the value of Landsat imagery in geomorphic mapping. Examine particularly his Figures 11-7 and 11-8, his geomorphic maps of the Imperial Valley, California, and Tucson, Arizona, using Landsat. We cannot reproduce these here because of their small print. Although he was not the first to produce this type of map solely from space imagery (C.F. Pain's 1985 map of an area in New South Wales, Australia is the earliest known to the writer), he has put together excellent examples of this capability.

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