Oregon Geology: Sixth Edition

Life on the Edge

9 Life on the Edge Geology is the story of how the earth has evolved through time. Some processes such as volcanism, landslides, or earthquakes are sudden, highly visible , and catastrophic while erosion, deposition, or changing climates and sea levels are more subtle and lengthy. Because of Oregon’s position on the leading edge of a moving crustal plate, a striking diversity of geologic events have gone into molding its topography. Before this complex picture could be deciphered, over a century of field work collecting data on stratigraphic relationships, faulting patterns , volcanic episodes, environments, and dates of rocks had to be compiled. Only then could there be an understanding of the tectonic overprint that drives most of the state’s geologic episodes. As the details of Oregon’s past emerged, it became apparent that the underlying geology was not completely in concert with the physiographic boundaries. Landforms are more obvious and can readily be drawn, in contrast to the outlines for geologic phenomena, which tend to overlap, be buried, or obscured. In spite of this, the geologic content has been adapted to the individual geographic provinces, defined in 1950 by University of Oregon geographer Samuel Dicken. He imposed them on a base map of the state drawn by Erwin Raisz. Geologists such as Thomas Condon, Joseph Diller, Israel Russell, and Howel Williams took the first steps at interpreting Oregon’s geologic terrain. (In this unusual 1898 photo, Diller is sitting beneath a large schist outcrop of Otter Point Formation near Winston, Oregon; courtesy Douglas County Museum, Roseburg) 10 Oregon Geology Drifting tectonic plates The premise of moving continents or plate tectonics is central to all aspects of Oregon’s geology. The notion that continents move was first published in 1915 by its chief advocate Alfred Wegener when he reconstructed the supercontinent Pangaea by matching the shapes of continental margins. Continental drift as a workable theory soon came up short in light of what was known at the time about the structure of the earth’s crust. The theory stalled for almost 50 years until marine geologists recognized evidence from ocean floors to support the hypothesis that deep-seated plates, with continents imbedded on their surfaces, are in motion. The new idea of global plate tectonics emerged after studies of rock magnetics and the realization that the plates are not drifting but are spreading along mid-ocean ridges. The adoption of this idea ushered in an entirely new way of looking at the prehistory of the earth as well as at the beginnings of the Pacific Northwest. Interaction of plates The advent of plate tectonics was a milestone in geologic thinking. Moving slabs of crust and upper mantle may separate, collide, or grind past one another . Where plates rift and divide, new crust forms. Currently, lengthy continuous chains of undersea volcanic mountains can be found at rifting zones life on the edge 11 along the floors of every major ocean. Subduction takes place when two plates collide, and one descends beneath another. Once ocean crust ages and cools, its density increases, and the older, heavier crust is thrust below or subducted by the overriding slab. Where plates slide past each other, transform faults of epic size and length develop at the boundary between the two, accompanied by destructive earthquakes. Among the most visible by-products of plate collision and subduction are the build-up of an­ accretionary wedge, the emplacement of volcanic archipelagos (arcs), and the formation of sedimentary basins. During the subduction process, sediments atop the descending slab are peeled off to accumulate as a jumbled prism or mélange at the outer margin of the upper plate. Associated with this, magma, rising from the lower descending plate, penetrates the upper slab to emerge at the surface as a volcanic chain. Between the accretionary prism and the volcanic archipelago, a forearc basin or depression may develop with a similar backarc basin between the arc and the larger continental mass (craton). Over time, erosion of the volcanic highlands sheds copious amounts of sediment into both basins. Arrival of terranes Almost a half billion years in the past, the oldest rocks that would make up Oregon were being Going back 400 million years, Oregon has had a fascinating history of piecemeal construction, volcanism, and sedimentation even as it was populated by an array of plants and animals. 12 Oregon Geology The configuration of the Pacific Northwest adapts easily to global...