Page 472 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

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part III The earth–Atmosphere Interface 80°
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ARCTIC OCEAN
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PACIFIC OCEAN
ARCTIC OCEAN
Arctic Circle
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▲Figure 14.15 Karst landscapes and limestone regions. Major karst regions exist on every continent except Antarctica. The outcrops of carbonate rocks or predominantly carbonate sequences are limestone and dolomite (calcium magnesium carbonate) but may contain other carbonate rocks. [Map adapted by Pam Schaus, after USGS sources, and D. C. Ford and P. Williams, Karst Geomorphology and Hydrology, p. 601. © 1989
by Kluwer Academic Publishers. Adapted by permission.]
Major karst regions
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Approximate form and extent of
outcrop of carbonate rocks or
predominantly carbonate sequences
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Carbonate outcrops
Carbonate outcrop patterns
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0 1000 2000 3000 KILOMETRES MODIFIED GOODE’S HOMOLOSINE EQUAL-AREA PROJECTION
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generalized in mountain ranges
Formation of Karst
For a limestone landscape to develop into karst topogra- phy, several conditions are necessary:
• The limestone formation must contain 80% or more calcium carbonate for dissolution processes to proceed effectively.
• Complex patterns of joints in the otherwise imperme- able limestone are needed for water to form routes to subsurface drainage channels.
• An aerated (air-containing) zone must exist between the ground surface and the water table.
• Vegetation cover is needed to supply varying amounts of organic acids that enhance the dissolution process.
The role of climate in providing optimum condi- tions for karst processes remains under debate, although the amount and distribution of rainfall appear impor- tant. The karst features found today in arid regions were formed during past climatic conditions of greater humidity. Karst is rare in the Arctic and Antarctic re- gions because subsurface water, although present there, is generally frozen.
As with all weathering processes, time is an im- portant factor. Early in the 20th century, scientists pro- posed that karst landscapes progress through identifiable stages of development, from youth to old age. Evidence has not supported this idea, and today karst landscapes
are thought to be locally unique, a result of site-specific conditions. Nonetheless, mature karst landscapes display certain characteristic forms.
Features of Karst Landscapes
Several landforms are typical of karst landscapes. Each form results to some extent from the interaction between surface weathering processes, underground water move- ment, and processes occurring in subterranean cave net- works, described just ahead.
sinkholes The weathering by dissolution of limestone landscapes creates sinkholes, or dolines, which are cir- cular depressions in the ground surface that may reach 600 m in depth. Two types of sinkholes are most prom- inent in karst terrain. A solution sinkhole forms by the slow subsidence of surface materials along joints or at an intersection between joints. These sinkholes typically have depths of 2–100 m and diameters of 10–1000 m (Figure 14.16).
A collapse sinkhole develops over a period of hours or days and forms when a solution sinkhole collapses through the roof of an underground cavern (Figure 14.17). These sinkholes can have dramatic features, not all of which are associated with karst processes. Human activi- ties cause many of these sinkhole subsidence events, as described in GeoReport 14.3.
Equator
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BAY OF BENGAL
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