Reported by: Ray Collins and Kathy Leber
What We Found at the South Shore of Mono Lake
What are these odd looking towers, spires, medieval castles and sunken cathedrals reflected in the surface of Mono Lake and rising from the ground near the shore?
The tallest of these are 6 or 7 meters high, about three or four times our height.
The color of the rock is grayish, and the surface is rough and irregular. The mineral is not very hard, and appears to be quite porous. This place is mysterious, fascinating, and very beautiful.
From reliable sources, we know that the mineral that makes up the tufa formations is called tufa (!), and is a form of calcium carbonate (chemical formula CaCO3 ). Tufa is an unusual form of calcium carbonate. Limestone (also CaCO3 ), for instance, is very common rock in the Earth's crust formed at the bottom of large bodies of water. As skeletons of microscopic organisms heavier than water pile up on the bottom over long periods of time, they may become cemented by pressure.
There are key pieces of evidence that point to a different process at work in the Mono Lake tufa: the hollow tube — or cavity — within the upright structure implies mineral being deposited at the top, adding to the height, and more irregularly to the outside. There is no evidence that the towers were made by animals — no fossils are observed. The recent history of Mono Lake completes what we are able to conclude about its fantastic formations.
What We Learned About the Mono Lake Tufa Formations
1. The formations are made under the water's surface: it has been with the falling of the lake's water level that the formations have been revealed. The tufa towers that we see around the edges of the lakes were created when the lake level's were higher. Tufa towers are forming today, under the water's surface.
2. The process is rapid: up to an inch can be added in 1 day (though this happens only occasionally) and all of the formations we saw are only a few centuries old.
3. Apparently underwater springs bring one component for the formation of tufa; the lake water provides the other:
- the calcium comes from the water emanating from the springs, dissolved in the form of its ion (Ca2+ ) derived from a deeper source of calcium carbonate; and
- the lake water provides an excess of the carbonate ion (CO32 ), as well as an alkaline environment in which the two oppositely charged ions come together, or precipitate : Ca2+ + CO32 > CaCO3. The precipitated CaCO3 remains largely insoluble in the alkaline lake water, and so is added to the tufa formation.
4. The fresh spring water is less dense than the lake water, and therefore rises — nearly straight up, toward the surface of the lake. The spring water emerges from the ground and proceeds through the pipe that it is forming. The chemical reaction deposits minerals at the top of the pipe, adding to it bit by bit. Small currents make the process irregular, building one spot more quickly than the rest, then another spot.
5. In many cases the springs are closely spaced in a fairly straight line — perhaps controlled by faults or fissures. The resulting formations contain many tubes, and these have deposited calcium carbonate in such a way as to produce one large, elongated, structure.