Page 214 - Fundamentals of Geomorphology

KARST LANDSCAPES 197

of calcium carbonate sediments (aragonite) to calcium–
magnesium carbonate rock (dolomite), the formation
of a terrace around the island, and the onset of chem-
ical weathering and the evolution of karst landforms,
particularly caves and karst holes. Similarly, Yoron-Jima,
2
a 21-km carbonate island located in the central Ryukyu
Island Arc of southern Japan, was raised above sea level in
the Quaternary period. Subsequent karst processes have
produced many closed depressions (Terry 2005).

Closed depressions
Plate 8.7 A limestone solution notch on a modern shore Dolines
platform on the east coast of Okinawa, Japan.
(Photograph by Derek C. Ford ) The word doline is derived from the Slovene word dolina,
meaning a depression in the landscape. It is applied to
the simpler forms of closed depressions in karst land-
scapes. Sinkhole, swallet, and swallow hole are English
terms with rather loose connotations. Dolines resem-
ble various shapes – dishes, bowls, cones, and cylinders.
They range in size from less than a metre wide and deep
to over hundreds of metres deep and several hundred
metres or even a kilometre wide. The large forms tend
to be complex and grade into other classes of closed
depressions.
Dolines are formed by several processes: surface solu-
tion, cave collapse, piping, subsidence, and stream
removal of superficial covers. Although these processes
Plate 8.8 Limestone coastal karren pitting (sometimes frequently occur in combination and most dolines
called phytokarst or biokarst) on the west coast of are polygenetic, they serve as a basis for a five-fold
Puerto Rico. classification of dolines (Jennings 1985, 107; Ford and
(Photograph by Derek C. Ford ) Williams 1989, 398) (Figure 8.7):

1 Solution dolines start where solution is concen-
wave action, wetting and drying, salt weathering, and trated around a favourable point such as joint inter-
hydration. sections. The solution lowers the bedrock surface, so
eating out a small depression (Figure 8.7a; Plate 8.9).
Coral island karst The depression traps water, encouraging more
solution and depression enlargement. Once begun,
Carbonate sediments are the building material of the doline formation is thus self-perpetuating. However,
world’s coral islands, all of which bear at least some karst insoluble residues and other debris may clog the
2
features. For instance, Navassa Island, a 5-km island in doline floor, sometimes forming swampy areas or
the Caribbean Sea between Haiti and Jamaica, may have pools to form pond dolines. Dolines are one of the
started life as a small coral atoll. Some 5 million years ago, few karst landforms that develop in soft limestones
these coral reefs began to emerge, leading the conversion such as chalk (e.g. Matthews et al. 2000).