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asphalt - teaching hand/display specimen of natural asphalt from seeps on Sulphur Mountain, CA

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Oil seeps on the south flank of Sulphur Mountain were noticed by geologists in the 1800s. In 1863, Prof. Benjamin Silliman of Yale University remarked that, "California will be found to have more oil in its soil than all the whales in the Pacific Ocean. The oil is struggling to the surface at every available point and is running down the rivers for miles." 

The slopes of Sulphur Mountain were too steep for derricks, so the oil was extracted by mining, with tunnels dug into organic-rich Miocene shales of the Monterey Formation. In 1861, Josiah Stanford, brother of the founder of Stanford University, completed an 80-foot long tunnel. The tunnel floor was sloped toward its mouth and oil flowed by gravity out of the excavation and into pits. In the next 30 years, over 54 tunnels were dug by Stanford and others, mostly with Chinese labor, some as long as 1,600 feet and producing as much as 60 barrels of oil a day.

Clearing dangerous fumes from the tunnels was not easy. A "water blast" system was invented, using a funnel-shaped weathervane connected to a large pipe. A water mist was added and the air blast was directed into the tunnel mouth. This effectively cleared the air in tunnels just over 500 feet in about half an hour, but longer tunnels gave the engineers a headache. Tunnels were illuminated by a mirror which directed sunlight into the tunnel entrance. The sun beam guided the tunnel alignment as well as lighting the work face.

The remaining tunnels were abandoned and closed off by Union Oil in 1998 when they became tired of annual fines levied by revenue hungry state agencies and the Department of Fish and Game.

Oil continues to seep from Sulphur Mountain. Bacterial action, evaporation and oxidation convert it into tars and asphalts. These specimens contain shale fragments, smell tarry, and will hold together, though a specimen placed on a top shelf in an unventilated store room during the summer flowed over the edge of the shelf and developed a neat 90 degree bend before it was noticed weeks later. Under normal room temperatures, these specimens are stable. The asphalt occasionally glues together fragments of siliceous shale that make it possible to break these pieces free.

Students should understand that asphalt is derived from petroleum with a biological origin in the fatty of oil contained in diatoms floating in the sea. There is a constant rain of the shells of these organisms to the sea floor. Covered in sediments, compressed and heated, the oil droplets become either kerogen, natural gas or petroleum, depending on the temperature, migrating upward until either trapped by a geological structure or an impervious rock layer, or seeping to the surface.

The field photos show petroleum seeping out of the slopes of Sulfur Mountain and flowing down a roadside ditch, not quite the flow that Benjamin Silliman described in 1863, but interesting nonetheless and a benefit to the whales.



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