Reference Work Entry

Geomorphology

Part of the series Encyclopedia of Earth Science pp 231-233

Cryptodepressions

  • Rhodes W. Fairbridge

A cryptodepression is part of the earth's crust that lies below mean sea level, the floor of which is hidden (hence, the Greek prefix crypto-) below the waters of a lake. Since the depths of such water bodies are not obtained by ordinary land survey methods and can only be obtained by soundings from a boat, there are numbers of such depressions with only imperfect data, although most lakes are moderately well known. (Note: each of the world's major lakes is treated in detail in separate articles in this volume; see also pr Vol. VI: Hydrology and Limnology). The presence of deep cryptodepressions of great antiquity in the continental crust is an answer to those geologists and geophysicists who claim that the crust has little strength and will everywhere flow (“like wax”). These narrow depressions, forming clefts 2–3 km deep, have clearly remained gaping open for many millions of years. At the temperatures prevailing in the crystalline rocks of the upper crust flowage is rarely possible. It is primarily in the soft superficial sediments of geosynclines that major flowage occurs. Certain very shallow lakes covering cryptodepressions in the continental interiors like Lake Eyre (South Australia) and the northern Caspian Sea are relatively stable autogeosynclinal depressions.

The causes of such depressions in the continental crust are structural, i.e., they are tectonic depressions, due to block faulting, grabens or general downwarping. Lake Baikal with its floor at — 1279 meters not only is the world's deepest continental depression, but also has the greatest volume of any freshwater lake, comparable indeed in volume with that of all the Great Lakes of the United States and Canada put together. It is a graben depression which is believed to go back to the Cretaceous in age—one might say about 100 million years old. The Dead Sea depression is also a graben, the downwarping of which is equally old, but the actual fracturing and lake formation is younger, perhaps 20 million years old. In contrast, the Caspian Sea has a vastly greater area and is actually a land-locked arm of the sea, a geosynclinal depression of Mesozoic age, well over 100 million years old, that became cut off from the ocean in the Quaternary period; it is still an active but slowly subsiding geosyncline, partly block-faulted in the south, in the foredeep of the Elburz Mountains of Iran.
TABLE 1.

MAJOR CRYPTODEPRESSIONS OF THE WORLD (DATA MOSTLY FROM MURRAY, 1910; HUTCHINSON, 1957, AND AVAILABLE MAPS)

 

Floor (m)

Depth (m)

Surface (m)

(a) Tectonic Depressions (grabens, downwarps)

   

Lake Baikal (U.S.S.R.)

−1279

1741

+462

Caspian Sea (U.S.S.R./Iran)

−972

945

−27

Dead Sea (Jordan/Israel)

−793

394

−399

Sea of Galilee (Lake Tiberius, Israel/Syria)

−260

50

−210

Lake Matano (Celebes or Sulawesi, Indonesia)

−208

457

+249

Lake Asale or Assal, swamp (Danakil Depression, Ethiopia)

−119

ca. 3

−116

Lake Enriquillo (Dominican Rep.)

−48

ca. 2

−46

Lake Eyre (South Australia)

−13

1

−12

(b) Glacial Scours

   

Great Slave Lake (Canada)

−464

614

+150

Hornindalsvatn (Norway)

−461

514

+53

Salsvatn (near Fosnes, Norway)

−432

448

+16

Lake Mjøsa (Mjösen; Norway)

−325

720

+395

Lake Fagnano, Tierra del Fuego (Chile/Argentina)

−309

449

+140

Loch Morar (Scotland)

−310

310

Sea level

Lake Garda (Italy)

−281

342

+61

Lake Manapouri (New Zealand)

−262

444

+182

Lake Como (Italy)

−212

411

+199

Lake Maggiore (Italy)

−176

371

+195

Lake Ontario (U.S./Canada)

−151

226

+75

Lake Chelan (Washington, U.S.)

−129

419

+290

Lake Superior (U.S./Canada)

−124

315

+191

Lake Elena (Chile)

−122

124

ca. 2

Lake Michigan (U.S.)

−88

265

+177

Lake Iseo (Italy)

−65

251

+186

Lake Seneca (New York, U.S.)

−54

190

+136

Lake Huron (U.S./Canada)

−46

223

+177

Lake Cayuga (New York, U.S.)

−17

137

+120

(c) Volcanic, crater lakes (incl. maare , calderas )

   

Lake Tazawa (ko) (Japan)

−175

427

+252

Lake Apoyo (Nicaragua)

−110

ca. 200

+90

Lake Toya (ko) (Japan)

−96

96

Sea level

Lake Nicaragua (Nicaragua)

−13

46

+33

(d) Karst ( ponors , sink-holes )

   

Lake Scutari (or Skader, Yugoslavia/Albania)

−38

44

+6

(e) Deflation Hollows

   

Lake (Birket) Qarun (El Faiyum, Egypt)

−48

3

−45

Qattara Depression, swamp (Egypt)

−133

ca. 1

−132

Wadi el Natrun, with 9 salt lakes (Egypt)

−80

ca. 1

−79

Note: The surface levels of most closed lakes are subject to marked variations; those with outlets are fairly constant.

By far the greatest number of lakes in the world, and this holds for cryptodepressions too, are glacial scour depressions , due to deep excavation by tongues of ice. Some of these are due to continental ice sheets, others to mountain ice caps and valley glaciers. In cases where a continental ice sheet has withdrawn into mountainous areas such as Scandinavia and Scotland, it is not immediately clear if the major scour was due to the continental ice or to valley glaciation either in the early or late stages of the glacial epoch. Similar deep-scoured depressions (of the order of — 1000 meters) were gouged out by the Permo-Carboniferous ice (in Western Australia and South Australia).

Other cryptodepressions are rather special types (what Davis and Cotton call geomorphologic “accidents”): volcanic depressions (crater lakes, q.v.; maar, q.v.; calderas, q.v.); karst depressions (q.v.; ponors, sinkholes, q.v.); and deflation hollows (wind-scoured depressions).

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© Reinhold Book Corporation 1968
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