The Volcano Behind Oakland (redux)

Round Top crater, tilted 90° to right, and quarried.

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The Volcano Behind Oakland (redux)

I think I have settled on the epitaph for my tombstone: Have Fun and Be Kind. Actually, while I concede the certainty of mortality, I don’t really want a tombstone. I would prefer that my lifeless body be cremated and my bones crushed, so my powdery remains could be scattered at sea, perhaps in Bodega Bay following my father (or further out if the charter boat captain isn’t too stingy about using fuel). Then the gritty grey plume of my exiting ghost would settle out as a thin dusting on the muddy surface of the sea floor, and in time my ashes would get pressed into a thin-banded shale or mudstone of future millennia.

The only uncertainty about human extinction is its timing: Will it occur in the distant future when the sun expands into a Red Giant and consumes the inner planets, or will it be an unnecessarily premature self-inflicted demise by some combination of war (whether radioactive or merely with firearms), environmental exhaustion and climate change? I suspect humanity will select option B, but — through the magical power of denial — conceptualize the experience as victimization by unanticipated natural catastrophes. In any event, I do not find the inevitability of human extinction to be regrettable. Humanity is but one of many simultaneous and transitory expressions of Life On Earth, and this earth is only a minute speck of an unimaginably vast universe that can obviously sprout life. So relax, the earth and the universe will carry on majestically without us; we are as unnecessary to them as the dinosaurs were.

Like the animals and plants at the time of the dinosaurs, the life forms of today may become the coal and petroleum fueling the pottery kilns of future primitives, like the Anasazi, or the industries of future technological societies, if any.

I find the contemplation of geologic history, and the learning about its particulars on those parts of earth’s surface where I circulate, to impart a mental serenity like that gained by viewing vistas from mountaintops, or out in the expanses of the Great Basin and the Colorado Plateau of the American West.

The North American continent moves west across the surface of the globe because of the widening of the Atlantic Ocean caused by the magmatic extrusion of new rock from a linear spreading center that runs down the middle of that ocean basin from the Arctic to the Antarctic.

The western edges of the American continents ride over the eastward-moving heavier tectonic plates supporting the Pacific Ocean, and which lie east of a series of north-south trending spreading centers. The eastern edges of the Pacific Ocean plates colliding into the American continents plunge into the earth, or subduct, to melt under the weight of those continents’ westward advance.

A Pacific spreading center, whose northern end was tilted to the east relative to its southern end, began to subduct under North America 30 million years ago (30 Mya), in what is today Southern California. Faults perpendicular to this spreading center extended westward (with a tilt to the north) from its northern and southern extremities. As this spreading center was overridden by the western edge of North America, it caused that portion of the overlying continent that was west of the buried spreading center to break off and begin sliding to the north, and west. The line of this fracture lengthened as the subduction of the spreading center continued, and this was the origin of the San Andreas Fault (SAF).

The north-south trending SAF lies near the California coast. By 20 Mya, the northern end of the SAF had extended north into Central California. By 10 Mya, the northern end of the SAF was near Monterey Bay, and today it is in Northern California at Cape Mendocino, which is the westernmost extent of the California coast.

The northern end of the SAF is a triple point from which emerges the Mendocino Fault, which runs perpendicular to the SAF and westward out to sea. Slippage along both the SAF and the Mendocino Fault is such that earth on the opposite side of the fault is seen to move to the right. The land just north of the triple point is colliding eastward into North America, and the land (ocean floor) just south of the triple point and west of the SAF is colliding northward into the subduction occurring north of the triple point.

The confluence of tectonic motions at the triple point generates sufficient geologic pressure and heat to cause vigorous seismic activity and volcanism in its vicinity. For over 20 million years, volcanoes have preceded the northward advance of the northern triple point of the SAF. The Neenach Volcano, in Los Angeles County, erupted 23 Mya and was subsequently split by the SAF so that its western half now lies 314 km (195 miles) to the northwest in present day Pinnacles National Park (which is east of the Salinas Valley and 130 km, or 80 miles, south of San Jose). From 10 Mya to 9 Mya, volcanoes erupted in what are today the hills east of the cities of Oakland and Berkeley. About 2.4 Mya, numerous volcanoes were active in the Clear Lake Volcanic Field, which lies north of the counties of Napa and Sonoma. The most recent eruptions in the Clear Lake Volcanic Field occurred about 11,000 years ago around Mount Konocti. Today, the magma chamber beneath the Clear Lake Volcanic Field is exploited to extract geothermal energy by the largest complex of geothermal power plants in the world.

The volcano in my neighborhood is Round Top, which is uphill from the city of Oakland in what is today a public park, the Robert Sibley Volcanic Preserve.

Between 16 Mya and 14 Mya, the San Francisco Bay Area was part of a marine basin a mile deep (1.6 km). The marine sediments of this basin are today lithified in the rocks of the Claremont Formation. The ocean floor of this basin was uplifted, between 14 Mya to 12 Mya, and the sedimentation of this period occurred in shallow water. This seafloor was lifted above sea level some time after 12 Mya, and for 2 million years the land was an alluvial plain accumulating sediments from streams running east from a chain of coastal mountains, which were situated where the San Francisco Peninsula and the Golden Gate Bridge (the entrance to San Francisco Bay) are today. This plain dipped toward the east to meet the shore of an inland sea near the present day towns of Orinda and Lafayette. These terrestrial sediments, from 12 Mya to 10 Mya, were lithified into the rocks of the Orinda Formation.

The Round Top volcano erupted many times between 10 Mya and 9 Mya, spreading thick layers of basalt lava over the layered stream sediments and gravels of the Orinda Formation. The surrounding countryside was a low plain with lakes, so volcanic ash ejected during eruptions would fall into water to settle out in thin, uniform layers, which were subsequently compacted into finely-banded rock. Ash and cinders also fell on dry land, and this too was eventually compacted into rock. Between periods of volcanic activity, sedimentation occurred by the action of streams or in lake bottoms, as in the earlier time of the Orinda Formation. The combination of rocks formed during the million-year period of Round Top’s volcanic activity is called the Moraga Formation.

You can see an impressive cross section of the alternating layers of lava and sedimentary rocks of the Moraga Formation on either side of Highway 24 on the eastern side of the Caldecott Tunnel, which passes through the north-south trending range of low mountains defining the eastern boundary of the San Francisco Bay lowland. These layers have been tilted up nearly to the vertical because of the flexing and faulting of the land due to the tectonic pressure of continental collision. (1)

Since its extinction, the Round Top volcano has been tilted over about ninety degrees, along with the rest of the Moraga Formation, and its interior has been exposed by the combined effects of erosion and quarrying. Today, one can walk up to a cross section of a basalt lava feeder tube (exposed by excavation), which was once deep inside the volcanic cone;

Cross-section view of now horizontal Round Top basalt lava tube.

and one can walk into the tilted crater of the volcano to see lithified layered ash and massive basalt.

basalt lava flow, and ash

A video about Round Top, which also shows scenes of the countryside in and about Robert Sibley Volcanic Preserve, is posted on the Internet at

http://youtu.be/ZWk47VrRbKk.

This article is not what the English teachers call a “persuasive essay,” as I have no argument to advance, nor any moral to conclude with. I was just rambling on my own, following trails in my mind. But, I will leave you with this, from one future rock to another: have fun and be kind.

Note

(1) For photos of the road-cut at the Caldecott Tunnel (Moraga Formation), see the following web page (and the web links therein):

The route 24 cut, south side
11 August 2013
http://oaklandgeology.wordpress.com/2013/08/11/the-route-24-cut-south-side/

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Originally published at:

The Volcano Behind Oakland
12 August 2013
http://www.swans.com/library/art19/mgarci69.html

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