Ancient Seas, Modern Images
SeaWiFS image of the western United
States. The features of interest that that will be discussed in this Science
Focus! article are labeled on the large image that can be viewed by
clicking on the image. (Other features and landmarks are also
labeled.)
It should be no surprise to be informed that the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) was designed to observe the oceans. Other
articles in the Science Focus series have discussed various oceanographic applications of SeaWiFS data. However,
this article discusses geological features that indicate the presence of
seas that existed in Earth's paleohistory which can be discerned in
SeaWiFS imagery.
Great Salt Lake and Lake Bonneville
Image excerpt showing the Great Salt Lake and
Bonneville Salt Flats, the greyish-white areas slightly southwest of Great
Salt Lake. The area covered by ancient Lake Bonneville encompasses
all of the greyish-white areas seen in this image.
The Great Salt Lake is the remnant of ancient Lake Bonneville, which gave
the Bonneville Salt Flats their name. Geologists estimate that Lake Bonneville
existed between 23,000 and 12,000 years ago, during the last glacial period.
Lake Bonneville's existence ended abruptly when the waters of the lake began to
drain rapidly through Red Rock Pass in southern Idaho into the Snake River
system (see "Lake Bonneville's Flood" link below). As the Earth's climate
warmed and became drier, the remaining water in Lake Bonneville evaporated,
leaving the highly saline waters of the Great Salt Lake. The reason for the
high concentration of dissolved minerals in the Great Salt Lake is due to the
fact that it is a "terminal basin" lake; water than enters the lake from
streams and rivers can only leave by evaporation. As the process occurs over
time, the dissolved substances in the river water become increasingly
concentrated.
The reason that Great Salt Lake exhibits two distinctly different
colors is due to both evaporation and the presence of a causeway that
divides the lake and restricts circulation. Less water flows into
the northern part of the lake than the southern part, making the northern
part more saline than the southern part. Due to this difference in salinity, there are distinct populations of phytoplankton on either side of the causeway which have different colors.
Lake Bonneville / Great Salt Lake links:
The "Channeled Scablands"
Image excerpt showing the "Channeled Scabland" region of eastern Washington State.
Lake Bonneville was not the only ancient lake whose existence ended rather
abruptly. Another ancient lake that experienced an abrupt and
catastrophic end was Lake Missoula, located in the vicinity of the modern city of Missoula, Montana. Unlike Lake Bonneville, however, Lake Missoula occurred due to glacial meltwaters, and was formed due to a dam of ice which formed
and broke repeatedly. Each time the dam broke, a huge flood of water burst from Lake Missoula and drained to the Pacific Ocean through eastern Washington
State and the Columbia River gorge.
The evidence of the immense floods from Lake Missoula is clearly visible
in the region known as the "Channeled Scablands", shown in the image
above. The drainage pattern is distinct and indicative of the forces that caused it. However, the origin of the "Channeled Scablands" was a geological
controversy for many years, due to the reluctance of the geological community
to accept a "catastrophic" explanation for the origin of the erosive features
in this region. Geologist J Harlen Bretz (he didn't use a period after the "J") argued persuasively for this explanation of the geology in the "Channeled
Scabland" region. The last piece of the puzzle, which convinced numerous
geologists, was the discovery of the many shorelines of ancient Lake
Missoula.
In an interesting twist, NASA scientists have used the geology of the
"Channeled Scablands" to understand many of the erosive geological features
observed on Mars.
Channeled Scabland links:
Uyuni Salt Plain (Salar de Uyuni)
SeaWiFS image of western South America. Lake
Titicaca is the body of water at the top of the image. The Uyuni salt
plain is the large white area south of Lake Titicaca. The snow-capped
peaks of the northern Andes mountains are visible in the southern
part of the image.
The final destination in this tour of ancient lakes and seas is
the Uyuni Salt Plain (Salar de Uyuni) in the altiplano of Bolivia.
The Uyuni salt plain is thought to be the largest salt playa in the
world, covering nearly 9000 square kilometers. Unless you know what it
is, it's easy to mistake the Uyuni salt plain for something else, such
as the snow on the peaks of the Andes mountains. But once you know
where it is, it's hard to miss.
The origin of the Uyuni salt plain is similar to the origin of
the Bonneville Salt Flats and the Great Salt Lake. During the late
Pleistocene era of Earth's geological history, there were two lakes
on the altiplano -- Lago Ballivian (which covered the area of modern Lake
Titicaca) and Lago Minchin, which contained the area of the Uyuni salt
plain. However, unlike Lake Bonneville, Lago Minchin apparently just
slowly dried up from 15,000 to 10,000 years ago, leaving the dissolved
minerals within its waters to form the salt plain. This slow drying
period can be determined by geological examination of the surrounding
rocks for an "evaporite sequence" -- the minerals that form as water
containing dissolved ions evaporates and becomes more concentrated. The
first mineral to form is limestone (calcium carbonate), which is
found on the margins of the salt plain and indicates that the lake
was at least 100 meters deep at one time. The next stage of
evaporation produces gypsum (calcium sulfate) and then halite (sodium
chloride), also known as salt. The surface of the Uyuni salt plain
contains gypsum and halite.
The Uyuni salt plain today is now a unique eco-tourist destination. One of
its most unique aspects is a hotel that lies within it, the Hotel Play Blanca,
offering vistas of the vast white plain in every direction, and one of the
quietest places anywhere on Earth. One suggestion: bring your own water, as
you might get thirsty.
Landsat image of the Uyuni salt plain (Salar de Uyuni).
This picture of a group of tourists and their
bus on the Uyuni salt plain gives an idea of the vast size and sheer white
vistas of this remarkable place.
Uyuni Salt Plain links:
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