WorldWide Drilling Resource

Alaska Miners Association Phone: 907-563-9229 Fax: 907-563-9225 ama@alaskaminers.org www.alaskaminers.org American Exploration and Mining Assn. Phone: 509-624-1158 Fax: 509-623-1241 info@miningamerica.org www.miningamerica.org WorldWide Drilling Resource ® is proud to be a member of these mining associations. Women in Mining 866-537-9694 wim@womeninmining.org www.womeninmining.org 57 WorldWide Drilling Resource ® OCTOBER 2014 Amboy & Pisgah Craters by Harry W. Short Engineering Geologist The Southern California Mojave Desert features a treasure chest of phenomenal parks and geological sites of colorful beauty for all to view and photograph. You can’t help but marvel at the beauty of Mother Nature’s hand- iwork of rock formations she left for us to enjoy. Geologists reconstruct the struc- tures formed, identify the rocks present, what they are made of, and how they got there. Two of these desert masterpieces are the Amboy and Pisgah Craters which are recent lava flows dated to be 100,000 years old, with craters in their summits. Cinder cones volcanoes eject spongy basalt fragments an inch or two in diameter. Sometimes, blocks the size of automobiles are blown from the crater. Aristotle coined the word “tephra” from the Greek word for ashes. Tephra refers to any fragment (ash and cinders) blown into the atmosphere. Volcanic bombs were sticky globs of lava that were pear shaped while flying through the air in their soft stage. The Amboy and Pisgah Craters dis- charged lava from their craters. The resulting surface is known as “Phoehoe” (Pa-hoe, hoe) from the Hawaiian Islands. They display ropelike coils which are usu- ally jagged, making walking difficult. The term “AA” (pronounced ah-ah) is lava flow with a rough, fragmental blocky sur- face consisting of clinkers and scoria. The surface temperature of the lava when it’s ejected, is over 1000ºF (538ºC) and may flow for miles. The hot lava begins cooling on reaching the surface, and lava flowing underneath the main body creates expanding pressure ridges. Pressure ridges form when the under- flow is moving - ridges rise up and an obvious elongated surface feature ap- pears. If the ridge collapses, it is called a collapse depression. You will find many of these in the Pisgah lava flows. If the ridge remains open, it’s called a lava tube (tunnel). There are some of these in Hawaii you can walk through. The young basaltic volcanoes like the Amboy and Pisgah volcanoes are common in the desert regions of west- ern North America. Geologists wonder why they only occur in this environment. Some may be connected to faults in the local region, but the r eason r ema i ns unsolved. G e o l o g i s t s have mapped three periods of flow vol- canic eruptions at the Pisgah Crater which have been differentiated by their mineral con- tent. The first flow has small crystals w h i c h c a n b e observed by the human eye. The overlying second flow contains crys- tals 1/8- to 1/4-inch of plagioclase and green olivine. Flow three, the youngest, has more and larg- er crystals than the first two eruptions. You can drive to the Pisgah Crater, but be careful, if any mining opera- tions are in progress, the road may be closed. You can park at the entrance to the Amboy Crater and view the display. Hikers, allow three hours for a round trip to the crater. Be sure to carry water, sun lotion, sunglasses, and snacks. Note, this is not a place to hike with small children. Harry Harry Short may be contacted via e-mail at admin@ worldwidedrillingresource.com Amboy Crater blocky pressure ridge. Pisgah Crater cinder cone. Harry on a lava flow in March 2004.