is an evaporative sedimentary rock composed primarily of the mineral
(sodium chloride). Halite beds form when seas and playas dry, precipitating
their mineral content. Some halite beds are more than 1000 feet thick.
Some of the more interesting
halite beds reveal the history of the
Mediterranean basin, as some of these salt beds lie beneath the sea, and must
have formed when the Strait of Gibraltar closed and the Mediterranean Sea dried
out. This event (or series of events) is called the Messinian Salinity Crisis
and occurred nearly 6 million years ago. Evidence exists that the dry basin may
have been 3 to 5 kilometers below sea level (far deeper than any current dry
land), enough to experience very high pressures (50% greater than sea level air
pressure today), and consequent very high temperatures due to adiabatic heating:
up to 80°C (176 °F) at the greatest depths.