the metabolism of methyl bromide by the lake’s planktonic bacteria (Connell et al.
1997). We were floundering to some extent, casting about for a new Mono discovery
to sink our teeth into, when we came to the realization it was staring us right in our
faces all along: Arsenic!
31.2.4 Tufa and Tephra and Tuff! Oh My!!!!
Some brief mention of basic pertinent geological information is required here. Mono
Lake, although it is circular and lies at the bottom of a closed basin (water flows in
but not out), is not in itself the remnant of a giant volcanic caldera. It looks like one,
but it isn’t. It is a subsided region that sank about one million years ago, as the
mountains to its west were still rising. Back in the Pleistocene (10,000–20,000 years
ago) the lake occupied five-times the area and contained eighteen-times the water
than it does in modern times. It even over-filled its basin, spilling out from its SE
corner and establishing fluvial transport south, extending even to Searles Lake in the
Panamint Valley. One of the advantages of the Mono Basin is, owing to the lack of
trees, to easily discern past geological features, be they the glacier-carved U-shaped
valleys of the neighboring Sierras or the former lake levels, presented as obvious
striations on the hillsides circling the basin. Indeed, while driving along Hwy
167, looking down at the lake gives one the impression of a draining bathtub, with
its prominent islands as exposed rubber duckies and loofa sponges in the residual
water. The surrounding striations are reminiscent of the need to scrub the tub clean of
its accumulated soap rings. It must have been quite spectacular back in its heyday,
not only was the lake greater in size (Lake Russell) but it was infringed upon by giant
glaciers (1000 meters high) descending from the Sierras. One only needed to await
the coming of the volcanoes for some further excitement.
Yet because of the grinding and subduction of the Pacific plate’s peripatetic
boundaries, the region itself is seismically active and has its own past history of
eruptive volcanoes, the most prominent that of the mega-sized Long Valley Caldera.
This pressure cooker blew open about 700,000 years ago with a force estimated to
have been 300-fold greater than that of the Mount Saint Helens 1980 showstopper.
The Long Valley eruption spread pyroclastic flow throughout the region that solidified into a long series of tan-colored block-like rocks that abound and have served
well as backdrop for many a Hollywood Western movie or TV series. The subsequent active volcanism moved northwards toward the lake, and a number of rhyolitetype volcanoes (e.g., the Mono-Inyo Craters) line either side of US-395 when one
drives north from Mammoth Lakes to Lee Vining, including the most recent eruption
of Panum Crater on the lake’s south shore (erupted about 600–700 years ago; you
can hike into this one and see all the obsidian piles too), and extended through the
lake (Paoha and Negit Islands, the latter a black cinder cone) and onto the northern
shore (Black Point).
674
R. S. Oremland
1997). We were floundering to some extent, casting about for a new Mono discovery
to sink our teeth into, when we came to the realization it was staring us right in our
faces all along: Arsenic!
31.2.4 Tufa and Tephra and Tuff! Oh My!!!!
Some brief mention of basic pertinent geological information is required here. Mono
Lake, although it is circular and lies at the bottom of a closed basin (water flows in
but not out), is not in itself the remnant of a giant volcanic caldera. It looks like one,
but it isn’t. It is a subsided region that sank about one million years ago, as the
mountains to its west were still rising. Back in the Pleistocene (10,000–20,000 years
ago) the lake occupied five-times the area and contained eighteen-times the water
than it does in modern times. It even over-filled its basin, spilling out from its SE
corner and establishing fluvial transport south, extending even to Searles Lake in the
Panamint Valley. One of the advantages of the Mono Basin is, owing to the lack of
trees, to easily discern past geological features, be they the glacier-carved U-shaped
valleys of the neighboring Sierras or the former lake levels, presented as obvious
striations on the hillsides circling the basin. Indeed, while driving along Hwy
167, looking down at the lake gives one the impression of a draining bathtub, with
its prominent islands as exposed rubber duckies and loofa sponges in the residual
water. The surrounding striations are reminiscent of the need to scrub the tub clean of
its accumulated soap rings. It must have been quite spectacular back in its heyday,
not only was the lake greater in size (Lake Russell) but it was infringed upon by giant
glaciers (1000 meters high) descending from the Sierras. One only needed to await
the coming of the volcanoes for some further excitement.
Yet because of the grinding and subduction of the Pacific plate’s peripatetic
boundaries, the region itself is seismically active and has its own past history of
eruptive volcanoes, the most prominent that of the mega-sized Long Valley Caldera.
This pressure cooker blew open about 700,000 years ago with a force estimated to
have been 300-fold greater than that of the Mount Saint Helens 1980 showstopper.
The Long Valley eruption spread pyroclastic flow throughout the region that solidified into a long series of tan-colored block-like rocks that abound and have served
well as backdrop for many a Hollywood Western movie or TV series. The subsequent active volcanism moved northwards toward the lake, and a number of rhyolitetype volcanoes (e.g., the Mono-Inyo Craters) line either side of US-395 when one
drives north from Mammoth Lakes to Lee Vining, including the most recent eruption
of Panum Crater on the lake’s south shore (erupted about 600–700 years ago; you
can hike into this one and see all the obsidian piles too), and extended through the
lake (Paoha and Negit Islands, the latter a black cinder cone) and onto the northern
shore (Black Point).
674
R. S. Oremland