Karst Landform

 A Karst landform is a feature created on the Earth's surface by the drainage of water into the ground or by its discharge at springs. The term derives from Kras, a region in western Slovenia where typical karst forms such as sinkholes, caves, natural bridges and sinking streams were first studied in detail. Karst landforms are an important variant of landforms created by flowing water. In the former, water is routed underground via solutional cave systems, instead of flowing at the surface in normal river channels (see RIVER LANDFORM). The development of karst landforms is limited to areas where comparatively soluble rocks, principally limestones and dolomites (carbonate rocks), and gypsum and anhydrite (sulphate rocks), occur. Approximately 8% of the Earth's land surface is karst terrain. There are 1.2 million km2 of karst rock outcroppings in Canada. They are common in all geological regions except the Canadian SHIELD.

Karst Processes

There are 2 solution processes important to karst terrain formation. Gypsum, anhydrite and very soluble molecular substances, such as natural salt, dissociate in the presence of water. For example, gypsum dissociates until there are 2500 mg dissolved per litre of water (at 25°C). The solution is then saturated and may precipitate gypsum crusts, eg, at mineral springs. Calcite (the limestone mineral) and dolomite are dissolved chiefly by carbonic acid produced by the solution of atmospheric carbon dioxide (CO2) in water. Global rates of limestone solution are determined, therefore, by the amounts of water and CO2 available in an environment. CO2 may build up to high concentrations in soils, where it is dissolved by seeping water. At saturation (thermodynamic equilibrium), concentrations of dissolved calcite will range from about 50 mg per litre in warm surface waters to 250 mg per litre in deep, cool subsoil waters. Rates of limestone solution range from less than 5 m3 per km2 per year in deserts and very cold regions to more than 100 m3 per km2 in rain forests.

Karren

The most widespread surface karst landforms are small solution pits, grooves and runnels, termed karren. Individual features are rarely longer or deeper than 10 m, but frequently they are densely clustered to dissect larger areas, termed limestone pavement. Solution pavement is particularly well developed in Île d'Anticosti, Qué, and the Bruce Peninsula and Manitoulin Island, Ont. Small patches may be seen within the city limits of Hamilton, Montréal, Ottawa, etc. In metropolitan Winnipeg, approximately 3500 km2 of dissolutional pavement is preserved beneath later glacial-lake clays (Lake AGASSIZ) and serves as an important industrial water store. There are spectacular subarctic pavements in the Carcajou Range west of Norman Wells. Further north, ice penetration into dissolutional cracks in the bedrocks becomes dominant, shattering them into rubble fields that are widespread on the extensive carbonate rock terrains of arctic Canada.

The diagnostic karst landform is the sinkhole (doline). This is a bowl-, funnel- or cylinder-shaped depression feeding water underground. There may be a periodic or permanent pond in the base. The length or diameter of sinkholes ranges from 10 to 1000 m. Most are formed by solution focused in the funnel or by collapse of the roof of an underlying cave. In southern Saskatchewan, collapse-solution cavities in salt have extended through as much as 1000 m of overlying insoluble rocks to produce shallow sinkholes at the surface.

Sinkholes often occur in lines or clusters. In some karsts their frequency exceeds 500 per km2, giving the terrain a shell-pitted appearance. Thousands are known in southern Canada, from gypsum terrains in western Newfoundland to limestones on Vancouver Island. There are many large, spectacular examples in WOOD BUFFALO NATIONAL PARK, in the Franklin Mountains, and west of Great Bear Lake where limestones and dolomites have collapsed into cavities in gypsum. Some new collapses occur each year, the hole appearing in a matter of seconds. These are a hazard to settlement on gypsum terrains in parts of Newfoundland and Nova Scotia.

Dry Valleys and Poljes

Larger karst landforms include dry valleys and gorges, carved by past rivers which now flow underground, and poljes, which are major sinkholes with alluviated, flat floors. Medicine Lake in JASPER NATIONAL PARK is a polje measuring 6 km by 1-2 km. The Maligne River drains into it and floods it to a depth of 25 m during the summer melt season. In winter, the lake reduces to small pond sinkholes in the polje floor. The sinking water is discharged 16 km northwest at some 60 springs in the floor of Maligne Canyon. With an aggregate discharge that may exceed 65 m3 of water each second, these are the largest karst springs known in Canada. Many other springs are known with peak discharges exceeding 10 m3 per second.

Karst and Glaciation

Karst landform development is rather limited in Canada when compared to countries that have not undergone repeated GLACIATION. Glacier action has eroded or infilled much karst. Our finest karstland, Nahanni Karst, is found in a region of the Mackenzie Mountains, NWT, which has escaped glaciation for the past several hundred thousand years. Major karst forms have developed without interruption or destruction and include hundreds of sinkholes up to 150 m deep, giant solution grooves intersecting to form a natural labyrinth, several poljes and dry canyons. Parts of the karst have reached a very advanced stage, displaying residual rock towers and natural bridges, features rarely seen in northern latitudes.

A dense karstland of karren, small sinkholes and innumerable caves has developed on steep limestone tracts clad in Douglas fir in northern Vancouver Island. The landforms pose some hazard to forestry practices, while clear-cutting may severely damage both surface and underground karst.

Alpine karst, comprising fields of karren and shaft sinkholes above the treeline that drain into deep caves, is well developed in parts of the Rocky Mountains and Vancouver Island. CROWSNEST PASS offers very good examples of classical alpine karst forms: major springs issue from active water caves in the floor of the pass while fragments of drained, relict caves are scattered at higher elevations up to the mountain summits. Some of the modern spring water is believed to have travelled as much as 70 km underground. The largest icefield in the Rockies, COLUMBIA ICEFIELD, is mostly drained by sinkholes surviving in the limestone and dolostone beneath it. The waters flow in great caves through Mount Castleguard, to emerge as spectacular springs in the valley of Castleguard River (a headwater of the North Saskatchewan River). It is the world's pre-eminent example of modern subglacial karst.