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PLATE TECTONICS AND GEOLOGIC HISTORY OF WESTERN NORTH AMERICA
PLATE SUBDUCTION

    Several years ago my wife and my self took a vacation to the North West part of the country. We took a flight from Salt Lake City to Portland Oregon. As we approached Portland our plane was heading due west along the general path of the Columbia River. Below me and to the left I could see a large strato volcano Mount Hood which was covered partly with snow. As I looked south I could see many more volcanoes Mount Jefferson, the Three Sisters and Broken Top one after the other in a straight line as far as you could see. I must have been lucky that day because the seats on the other side of the plane were empty. So I left my seat and glanced through the window to the north to see Mt. Saint Helens below me and to the right of the plane. You could plainly see all the devastation below were the rivers had been choked by huge amounts of sediment and the blast area were many square miles of trees had been blown over and the volcano its self were the top of the volcano was blasted away. Looking north east I could see more volcanoes Mount Adams, Mount Rainer and Mount Baker in a straight line as far as I could see. Looking to the west you could see the Pacific Ocean and all the volcanoes were approximately the same distance from the shore line. Later in the week we visited Mount Saint Helens by car and it was only then that I could see the cause and effect of the energy that had been released the day that Mount Saint Helens blew its top. For me this was a revelation it showed me how powerful the forces of plate tectonics are.

The processes work the same to day as they did millions of year ago. In my minds eye I envisioned these volcanoes starting to sprout up in Nevada and western Utah during early Mesozoic times at the end of the Permian period and the beginning of the Triassic period when the great continent pangea began to break apart with laurasia the north American plate moving west ward causing the farallon plate to the west of the continent to subduct under the north American plate forming similar terrain that I had seen flying over the Portland area. Not only volcanoes are created by plate tectonics but mountain ranges as well were pushed up andthrust faults pushed over younger rock scrambling up the geology of western Utah and eastern Nevada. When the farallon plate started subducting under the North American plate it acted like a huge bulldozer compressing the area between the subduction zone and eastern Utah distorting and bending the earth’s surface all through the Mesozoic period and into the Cenozoic period. In Utah you can see were giant folds




were formed in the surface rock and were old rock has been pushed up over younger rock during this time of mountain building. This mountain building distorted the earth’s surface creating mountains and Many large basins to the east later be filled by river and ocean sediment coming from the ancient western and southern mountains. This process would create the colorful type of sediments like conglomerate, sandstone and shale you see today at the San Rafael swell and eastern Utah and western Colorado


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Diagram of a strato volcano