Nevada's Own Devils Postpile EarthCache
Nevada's Own Devils Postpile
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This odd looking landscape may take you back to a time when you visited Devils Postpile at Mammoth National Park, in California. The looks are quite similar however, to a much smaller scale. To access the site, stay on I-80, between Fernley & Fallon exit
Geologic history:
An estimated 12.7 million years ago, during the Miocene Epoch period, a lava vent began spewing hot dacite lava. Dacite lava is most often light gray, but can be dark gray to black. Dacite lava consists of about 63 to 68 percent silica. Common minerals include plagioclase feldspar, pyroxene, and amphibole. Dacite generally erupts at temperatures between 800 and 1000 degrees Celsius. It is one of the most common rock types associated with enormous Plinian-style eruptions. When relatively gas-poor dacite erupts onto a volcano's surface, it typically forms thick rounded lava flow in the shape of a dome. As the molten rock began cooling into solid rock, it began to contract. This contraction caused stresses to develop because of the solid form of dacite which has a lesser volume than the hot liquid form. Those stresses operating in a vertical direction caused the mass to settle in response to gravity, but those oriented in a horizontal direction were relieved by cracking of the solidified rock.
Columnar joints, which are the cracks that you see, are formed as the lava flow cools and shrinks. This process occurs under certain conditions that allow the lava flow to crack into long columns. Jointing would have begun at the top, bottom, and all around the edges of the lava lake where the lava made direct contact with a cooler surface. It is ideal to see this in hexagonal cross sections. Consistent cooling conditions tend to promote the development of these columnar joints.
Regular hexagon columns are rare to find. This is due to the lava flows cooling process that is never completely uniformed. Therefore, the columns will form irregular-shaped polygons with a varying number of sides. The cooling settings for the lava flow are usually never ideal at the surface of the flow. As the flow cools, solidifies, and cracks begin to proceed from the surface into the interior, a point may be reached where the shrinkage forces may be consistent enough for irregular jointing to help form the columnar joints. Where there is more uniformed cooling you should find nearly perfect columns. The faster the cooling the smaller the columns will be.
Powerful erosive forces have been at work for thousands of years carving, shaping and destroying the evidence of this lava flow. Freezing and thawing cycles, as well as people, have broken apart the columns. Earthquakes and glaciers have also knocked columns over and repositioned them.
Ancient Lake Lahontan was a large lake that existed during the ice age and covered much of northwestern Nevada. It was formed by heavy rainfall during the time of the Pleistocene glaciers. At its peak existence 12,700 years ago, the lake had a surface area of over 8,500 square miles. Ancient Lake Lahontan disappeared shortly after the Pleistocene epoch. Pyramid Lake, Walker Lake and the Carson Sink are what is left behind from Ancient Lake Lahontan. In this geographical area, you will find evidence of the ancient shoreline by the tufa formations that were left behind. Some of these formations are located on the tops of the columns left behind from the ancient volcanic activity that once thrived in this area.
Questions to be answered to log this EarthCache site:
How large across is the average column?
a. 1-3 cm
b. 1 foot
c. 1 yard
How are the columns positioned?
a. vertical
b. horizontal
c. both, some are horizontal and some are vertical
History of the area:
In 1908, gold was discovered in Jessup which is located about 4 miles north of the present intersection of I-80. Within a few months of discovering gold, people came from all around to get in on the action. Nearly 300 people settled in this remote mining camp. However, its existence was short lived. People who had big dreams of finding their fortunes, did not come true. The boom was over by the end of 1909. This small desert town melted back into the hot desert and little is to show of its existence today.
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