Depressions caused by solution of near surface rock, by groundwater action at greater depths, and by other collapse mechanisms (such as is caused when subterranean mine openings induce failure at the surface [coal mines are particularly vulnerable]) produce distinctive landforms including those collected under the term "karst". This solution occurs mainly in regions where carbonate rocks (mainly limestones) make up much of the strata in the top 1000 meters or so (where groundwater can circulate), but collapse can also occur where salt beds (evaporites) are dissolved. This map shows the potential for carbonate/evaporite solution in the United States:
Surficial landforms developed from subsurface solution of halite and gypsum are fairly common worldwide. A search of the Internet failed to find good examples to show on this page. The one exception is this satellite image of the Isachsen salt dome in the Canadian Arctic. Note the rough surface which is solution sculpturing of the salt itself as exposed at the surface:
Landforms resulting from carbonate solution are even more widespread and numerous examples on the Web were found, as now described. Carbonate 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). The "type locality" for karst terrain is in the Dinaric Alps of Croatia, where Mesozoic limestones are notably pitted by solution, as seen in this astronaut photo:
Expansion of dissolved volumes may leave the carbonate 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 at the top and a Landsat image of a region west of Guilin, a popular tourist center in China, at the bottom.
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
The next pair of images are from an area next to the above scene. The first new image is an enlarged subscene containing mostly Karst topography. Below it is a further enlargement which shows the individual hills making up this distinctive landform.
The island of Jamaica in the Caribbean is composed mostly of limestones. As such, it is subject to intensive karst development in response to high annual rainfall. This Seasat radar image shows the typical karst topography developed under subtropical conditions.
The image shows a variety of landforms: some of mountains that are foreshortened, areas of land use, and roughened offshore waters. Limestone beds govern much of the island's underlying geology. In this area of heavy rainfall, these beds readily dissolve to form typical karst topography. Doline depressions (solution pits) occupy large sections of the scene. These pits are obvious in the aerial photo below, but in the Seasat image, they form a fine texture that may not show well on a monitor. If it does, look also for a faint but visible lineation caused by fractures that have a preferred orientation because of the look direction.
This next image is a perspective view of Jamaican cockpit karst using IKONOS and DEM inputs:
Another landform found almost entirely in terrains made up of limestone at the surface is the sinkhole. There are three types: Solution; Collapse; and Subsidence. Sinkholes are more common in humid climates because of the increased rainfall. But they can develop over time in arid climates. Here are sinkholes in the Kaibab limestone strata in northern Arizona; note their morphology:
Sinkhole like depressions are common on the surfaces of large areas in states making up the Interior Lowlands. Carbonate rocks (unfolded; not involved in tectonic mountain-building) are prevalent in these supracrustal rocks on the central craton. Here is a view of a landscape in Kentucky that shows distinct pockmarkings from sinkhole-related solution:
Large volumes of carbonate rocks can be dissolved away to form caves. In Kentucky the most famous cave is Mammoth Caverns, which is a National Park. Its main entrance is not particularly impressive but once inside there are miles of passages, some open to the public but other parts less familiar. The first figure shows that entrance.
Claims are made that Mammoth is the largest cave in the world. More than 570 km (350 miles) of openings have been mapped. Some of the rooms are quite large:
The aerial photo of the surface above part of Mammoth Caverns shows topography similar to the cockpit terrain of Jamaica:
As we learned earlier, Florida's surficial bedrock is almost entirely limestones of Cenozoic age. Hence sinkholes are in abundance, especially in the central part of the state. They can form slowly over time but some appear in days if collapse follows long term solution. Here is a map by the Florida Geological Survey that shows sinkhole distribution in the state:
Sinkholes can occur anywhere in the state, sometimes in populated areas with calamitous results:
Many sinkholes fill with water to become lakes. Here is one such feature:
Satellite images of central Florida show the widespread distribution of sinkhole lakes, seen first in this Landsat subscene around Orlando and then in an aerial photo:
These last scenes transition us to lacustrine (lake) landforms. Lakes come in many shapes, sizes, depths, and origins. Most are freshwater but some may be saline because of evaporation that concentrates dissolved salts. Glacial activity is a particularly common cause of lake development:
The first image shows elongated, thermokarst lakes in the coastal plains ending at Point Barrow in northwest Alaska; the second is an ASTER image of similar lakes on Alaska's northern slope. 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 vadose water in upper layers above the (ground)water table freezes permanently to some depth). The upper layers in the permafrost will experience some pore water melting when 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.
Lakes are a widespread feature on the Earth's land surfaces. They develop usually in depressions that collect water from runoff or, more commonly, from streams that empty into them. Generally, lakes survive for only a few million years at most, as they tend to disappear by evaporation or erosion.
The thermokarst lakes shown above are aligned, owing to directional movement of the ice. Lakes that have a similar appearance and are commonly aligned occur on the coastal plains of North America. They have been named the Carolina Bays but have been found from New Jersey to Texas. Ones like those shown below are shallow and usually only a kilometer or so in length. Their origin is still uncertain: the impact crater explanation is totally unsupported; processes associated with the influence of Pleistocene climatic conditions are proffered as a plausible cause but the exact mechanism remains uncertain (something akin to solution collapse is one hypothesis).
Here is an example of a fast-forming lake. An earthquake in the Sichuan province of China caused a landslide that dammed up a river. Water quickly accumulated, as seen in these Chinese satellite images:
Lakes can undergo significant area and volume changes in short time spans. A drought in the first decade of the 21st century in the desert Southwest of the U.S. has produced the changes observed at Lake Meade (formed by the Hoover dam) in Arizona:
Lakes forming by manmade dams are common. One is Lake Nasser in Egypt, formed by the Aswan Dam. In this unusual photo taken by an astronaut, the lake appears white instead of blue. This is actually sunglint, white provides stark contrast with the rock at the shoreline:
Lakes can form as an intermediate body of water along streams. Lake St. Clair lies just northeast of Detroit, Michigan. The St. Clair River drains out of Lake Huron to the north and into Lake St. Clair; the Detroit River empties from Lake St. Clair into Lake Ontario. Lake St. Clair was produced by glacial scouring along with the larger Great Lakes (some geographers include it among these lakes).
The continental glaciers of the Pleistocene Epoch reached into today's northern states east of the Great Plains. Deep gouging of the relatively soft sedimentary rocks produced big and variably deep depressions that have filled with water since the last glacial retreat some 8000 years ago. These produced today's Great Lakes, which collectively are the largest grouping of fresh-water lakes in the world. Here they are in this Landsat mosaic (lakes identified in the caption):
Lakes are formed by mountain glaciers as well. They may result from accumulation of meltwater behind a terminal moraine. The term "alpine lakes" applies. Here are a group of such lakes in the Italian Alps; they include Lake Como, Lake Lugano, and Lake Maggiore:
The Finger Lakes of Alaska owe their origin to glacial action in mountains and to damming by moraines (see next page):
Lakes can be directly or indirectly related to tectonic activity. The deepest lake in the world is Lake Baikal in southern Siberia. Formed in a rift valley, it bottoms at 1637 m (5370 ft) and is also the largest (volumewise) freshwater lake in the world.
Smaller lakes abound in the African Rift Valley such as seen in this Envisat SAR image:
Some of the African lakes associated with volcanism can hold poisonous (toxic) gases such as methane, or CO2, that escape into the air when thermal overturning occurs. This has resulted in deaths among villagers living on the lake shores. One such lake is Lake Kivu in the African Rift on the border between the Democratic Republic of the Congo and Rwanda:
Lakes are commonly formed within volcanoes whose peaks have collapsed or were blown off. Among the best examples is Crater Lake in Oregon which was formed during an eruption of Mount Mazama that resulted in the loss of its upper structure. Here are space and ground views:
As will become evident in the next Section, on impact craters, lakes often form in the depressions made when such craters are formed. The best example are the two Clearwater lakes in Canada: