We will make four geological stops and a few pit stops.

We will be examining evidence for ancient lakes in western Nevada. At a diatomite mine, we will see fossils of fish that lived in a lake during the Miocene Epoch, approximately 7 or 8 million years ago. Diatomite is composed of tiny fossils of single-celled plants that thrived in the lake. The climate in Nevada was considerably wetter because the Sierra Nevada did not rise to our west and trap moisture from the Pacific Ocean as it does today.

Diatomite is a versatile mineral mined for use in acoustical tile and pipe insulation, as an absorbent, and to filter impurities from liquids in many industrial and environmental applications. On the way to the fossil fish locality, we will pass a diatomite processing plant and a nearby diatomite mine.

At Grimes Point, we will visit a site where prehistoric natives carved petroglyphs on the surfaces of boulders along what at the time was the shore of ancient Lake Lahontan. The lake reached its highest levels at least four times between 75,000 and 10,000 years ago. We will see abundant evidence for this lake, such as shorelines carved by wave action and sediments deposited in the lake. We will see pebbles that are highly polished by natural tumbling along the shores of the lake. Today, remnants of Pleistocene Lake Lahontan include Pyramid Lake and Walker Lake.

At Soda Lake and Little Soda Lake, we will visit two young volcanoes -- ones that are younger than the sediments that were deposited in Lake Lahontan. Geologists call these types of volcanoes maars (pronounced "Mars"), so after the last stop you can say that you "have been to maars."

Along the Truckee River on the way to Fernley and in the mountains around Fallon we will see volcanic rocks that range in age from about 20 million years old to younger than 1 million years. Many of the older volcanic rocks are part of the same geologic phenomenon that produces volcanoes of the Cascade Mountains today. They include andesite (a kind of volcanic rock named for the volcanoes of the Andes Mountains) and rhyolite (a silica-rich volcanic rock that commonly erupts violently). White tuffs are evidence of some of the violent eruptions.

Although there are some young rhyolites in the region (particularly near Mono Lake, California, and Steamboat Springs, just south of Reno), most of the younger volcanic rocks are basalts (relatively silica-poorer rocks that tend not to erupt as violently as rhyolites). These rocks are closely related to active extension (pulling apart) of the Earth's crust in the Great Basin.

You might notice that you rarely see balanced rocks or boulders that appear about to fall over and roll downhill. Geoscientists at UNR believe that such rocks have been toppled during past earthquakes in the region. Nevada is the third most seismically active state in the country. Breaks in the ground from the December 16, 1954 magnitude 7.2 earthquake near Fairview Peak are easily seen at a BLM interpretative site a few miles south of U.S. Highway 50, further east of Fallon than we will go on this trip.

The pebbles that we will collect along the shore of ancient Lake Lahontan were altered by hydrothermal fluids, much like waters in modern hot springs, which are common in Nevada. Such hot waters are used by geothermal power plants to produce electricity. Nevada annually produces about $110 million worth of electricity from geothermal power plants.

This type of hydrothermal alteration is commonly associated with the gold and silver deposits that have been so important to Nevada. Nevada leads the nation in gold and silver production and accounts for approximately 10% of current world production of gold. Nevada mines produce well over $2.5 billion in gold annually.

Along the route we will see gravel pits, aggregate mines, a cement plant, and a limestone mine -- all supplying the components needed to make the concrete used in buildings and roads. Aggregate currently rivals silver for annual production value in Nevada.

Thank you for helping us celebrate Earth Science Week, which has been officially designated as the second full week of October by Nevada Governor Bob Miller, the U.S. Congress, and the Association of American State Geologists to recognize the importance of geology and other Earth sciences to society. This field trip is sponsored by volunteers from the Nevada Bureau of Mines and Geology, Department of Geological Sciences, and W.M. Keck Museum at the Mackay School of Mines, University of Nevada, Reno; Desert Research Institute; Nevada Division of Minerals; Geological Society of Nevada; American Institute of Professional Geologists; Association of Engineering Geologists; Nevada Petroleum Society; Society for Mining, Metallurgy, and Exploration; U.S. Geological Survey; U.S. Bureau of Land Management; Eagle-Picher Minerals, Inc.; and Nevada Mining Association.

Road Log

From the Center Street on ramp in Reno, drive 35.7 miles east on Interstate 80, to the Fernley East exit. Turn right; we will make a pit stop at the gas stations or restaurants just south of the exit.

Continue southwest 1.1 mile on U.S. Alt-95 to the intersection with U.S. Alt-50. Turn left (east).

Drive 7.0 miles east on U.S. Alt-50. At the crest of the hill, turn right (south) onto the Farm District Road. Do not follow the paved road, but go south, across the narrow bridge over the Truckee diversion canal, along the gravel road for 1.9 miles, keeping left and not taking right turns at 0.6, 1.2, and 1.8 miles. At the truck tire in the middle of a triangular intersection, turn right (southwest). Continue 1.2 miles on the dirt road, passing under the power line, to the diatomite mine. Here we will collect fossils of fish (photo). You must get permission from the owner, Eagle-Picher Minerals, Inc. (343-1818), to visit this mine. Stay away from the high walls.

Return to the highway. Turn right (east) onto U.S. Alt-50 and drive 17.5 miles to the Burger King, Raley's, or McDonald's in Fallon for a rest stop.

Continue another 12.4 miles further east on U.S. 50, through Fallon, to Grimes Point. A short walking path lets you view many spectacular petroglyphs drawn by prehistoric natives (photo). Signs explain the different designs and likely ages. Eroded shorelines of Pleistocene Lake Lahontan are easy to see as horizontal benches on the hillsides in the Lahontan Mountains to the east.

Follow the gravel road uphill to the east of Grimes Point, 2.1 miles, to a fork in the road. Here, along one of the Lake Lahontan shorelines, you can find pebbles of naturally polished wonder stone (photo).

Return to Fallon (14.5 miles back to Burger King, Raley's, or McDonald's for another rest stop).

Continue west on U.S. 50, 2.3 miles past Burger King. Before the road narrows to two lanes, turn right (north) on Soda Lake Road. After 2.0 miles, turn left (west) onto Cox Road. After 0.8 mile, turn right (northwest) onto a dirt road, up the gentle slope to the rim of Soda Lake (photo). Here you can find bombs of basalt that contain crystals of olivine, plagioclase, clinopyroxene, and magnetite (photo). Some of the bombs have distinct ribs indicating flow of viscous lava. Some bombs are scoria, a type of volcanic rock with numerous holes that formed when gases trapped in the magma tried to escape.

You may also follow the dirt road 0.7 mile to the southwest to Little Soda Lake (photo).

The formal trip ends at Soda Lake. Return to U.S. 50 and find your way home.

We hope you enjoyed the trip. If you have any questions about the geology, natural hazards, or mineral, energy, or other natural resources of Nevada, please feel free to contact the Nevada Bureau of Mines and Geology by telephone (784-6691), e-mail (info@nbmg.unr.edu), or the Web (www.nbmg.unr.edu), or visit the offices on the UNR campus between 7:30 a.m. and 2:30 p.m., Monday through Friday.

Photos

Petroglyphs at Grimes Point. Prehistoric natives drew petroglyphs in boulders of andesite lava flows along the shores of the ancient Lake Lahontan. The boulders are coated with rock varnish, a thin layer of dark brown to black iron and manganese oxides. The U.S. Bureau of Land Management has constructed a path along which you can view the petroglyphs. Do not disturb the petroglyphs or collect rocks at this site. Click on image for enlargement.

Nevada wonder stone. This volcanic rock is a rhyolitic air-fall tuff, material ejected from a volcano about 12 million years ago. The rock was altered by hot waters that deposited pyrite (FeS₂) and quartz (SiO₂). Rainwater penetrated the rock and oxidized the pyrite to form liesegang bands of red hematite (Fe₂O₃) and orange and brown goethite (FeO(OH)). Erosion broke pieces of the rock from its outcrop, and streams carried pebbles to the shore of Lake Lahontan, which covered much of western Nevada during the Pleistocene Epoch. These pebbles, found along the road two miles east of Grimes Point, were tumbled and polished by the streams and by wave action along the shore of the lake about 13,000 years ago. Click on image for enlargement.

Soda Lake is a maar, a broad, low-relief, nearly circular volcanic crater. It probably formed when magma rose close to the surface, boiled the groundwater, and caused an explosive eruption. Click on image for enlargement.

Little Soda Lake, also a maar, and Soda Lake are probably the youngest volcanoes in Nevada. They erupted through sediments deposited in Lake Lahontan and are therefore less than 10,000 years old, perhaps even younger than 1,500 years. Click on image for enlargement.

Basalt bombs erupted from the Soda Lake volcano. These samples contain large (greater than 1 mm to as much as 1 cm) crystals of abundant clear and white plagioclase feldspar and green olivine (peridot, the gem variety) and less common, black, vitreous clinopyroxene. The rock contains small grains of magnetite and is slightly magnetic. Click on image for enlargement.

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