Thursday, April 21, 2011

Springtime on the California Prairielands: Violence in a Gentle Landscape (and a gratuitous picture of a duck)

The gentle wind caresses the waving blades of grass...meadowlarks sing, frogs's a serene day in the springtime of the Sierra foothills. Of course the science of geology holds no such sense of peace. Almost every aspect of this peaceful view speaks of violence, both in the recent past, and in the depths of time.

Take the layers revealed in the gentle slopes of the first picture above: a few terraces of dark rock on the distant skyline and some white exposures near the creek. These are two of the distinctive strata that make up much of the Sierra foothill slopes near the Central Valley, the Mehrten Formation and the Valley Springs Formation. The Mehrten layers resulted from numerous volcanic mudflows (lahars) and floods caused by eruptions of andesitic volcanoes in the vicinity of the Sierra Nevada crest in Mio-Pliocene time. Events that form layers like these are no picnic. Volcanoes of the 20th century like Mt. St. Helens and Mt. Pinatubo of the Philippines produced destructive mudflows that caused damage dozens of miles away from the volcanoes.

Likewise, the Valley Springs Formation speaks of volcanic violence. In Miocene time, around 20 to 30 million years ago, huge rhyolite caldera eruptions rocked the American west. A single eruption was capable of producing hundreds of cubic miles of volcanic ash that buried thousands of square miles under a stiflingly hot layer of what amounted to microscopic glass shards. The layers of the Valley Springs Formation record a number of violent eruptions, some relatively close, such as at the Sierra Crest, but some of them originated from ash explosions in eastern Nevada. Today, the differentially eroded layers provide shelter for burrowing owls and swifts along the small creek above.

Roadside runoff provides a bit of extra moisture supporting a thicket of monkeyflowers along Willms Road near Knights Ferry (above). But again, the landscape reveals a bit of geologic mayhem with the black boulders of the Table Mountain lava flow in the distance.

A favorite stop of ours on our seasonal treks through the foothills along Willms Road is a small stock pond a few miles south of Knights Ferry. At this time of year the flowers and birds are plentiful. The cattail marsh on the far shore was filled with chattering birds of all kinds. A peaceful, if raucous scene, but once again the present-day scene reveals a history of violence. The bits of rock sticking out on the lakeshore and far hillside are exposures of the Jurassic (yes, that Jurassic, the dinosaur period) Gopher Ridge Volcanics. These are andesitic and rhyolitic rocks that formed volcanic islands along subduction zones in the Pacific Ocean many miles away. Not only was there the violence of the original eruption, there were also the tectonic movements that carried the island arc into the western edge of the North American continent. The rocks were jammed into the continent, deformed, and metamorphosed into greenstone and slate.

The timing doesn't have to be measured in the millions of years. A look at the grass stuck in the fence (above) reveals that the road was inundated three and four feet deep during one of our intense rainstorms a month or two ago. The flooding obviously didn't do the road a whole lotta good either.

The frogs in the creek seemed fine, but unfortunately they reveal a certain level of violence, this time to the ecosystem of the foothills. Bullfrogs are an introduced species. They are voracious predators of other frogs and the native species have not been able to compete. They have driven the native species into a few refugia where the bullfrogs can't survive (even there, the native frogs have problems; they are eaten by the introduced species of fish as well).
I would almost be depressed by all the violence, but geologic processes are the very essence of drama, and the earth would be a boring (and probably unliveable) place if we didn't have earthquakes, volcanoes, plate movements and mountain-building. Just the same, I am ending with a picture of a duck that I saw hanging out along the water. As far as I know, the duck is not a terrifying animal unless you happen to be a snail in the creek....


Anonymous said...

Great post. We still have a few more weeks, maybe more, before the wild flowers will poke through the snow pack. The way I look at geologic processes is not as violence, but as a system trying to achieve equilibrium.

As for that duck, I wonder which Jurassic dinosaur is her ancestor?

Garry Hayes said...

I guess there was also a terrifying fossil they found in Australia that they named the "Demon Duck of Doom"