Dry lake beds in low-lying flat areas of desert environments
that may only have water in them once every few
years are known as playas or hardpans in the United States,
sabkhas in Africa and the Middle East, and salinas in South
America. When there is water in these basins, they are
known as playa lakes. Playas may form 5–10 percent of
desert basin areas in many mountainous desert regions of the
world, where water may drain from mountains and occasionally
fill intermountain basins with water that quickly
evaporates, leaving salts, silts, and evaporite minerals
behind. Playas typically have deposits of white salts mixed
with lake bottom silt and clay that form when water from
storms flows into the dry lake and then evaporates, leaving
the lakes dry. Coarser grained material is deposited between
the mountains and the playa in large alluvial fan systems in
which the grain size of clasts increases toward the mountains.
Sedimentation rates vary from a few tenths of an inch
to three feet (2–3 cm–1 m) or more per year, depending on
rainfall, erosion rates, and tectonic subsidence rates. Playas
tend to become more level with time since water and sediment
flows into the center of the basin and preferentially fills
in the lowest parts of the basin as the water evaporates, erasing
topographic variations.
There are more than 100 playas in the American southwest,
including Lake Bonneville, which formed during the last
ice age and now covers parts of Utah, Nevada, and Idaho.
Large playas are also visible in parts of Death Valley, California,
and some of these are fed by springs that keep water in
them most of the time. Playas form the flattest geomorphological
surfaces on Earth and make excellent racetracks and runways.
The U.S. Space Shuttles commonly land on Rogers Lake
playa at Edwards Air Force Base in California.
See also ALLUVIAL FANS; DESERT; EVAPORITE.
Tidak ada komentar:
Posting Komentar
Catatan: Hanya anggota dari blog ini yang dapat mengirim komentar.