Roadside Geology (and some Ecology) of Oregon
One thing that struck me, driving back and forth through western Oregon, was how young the landscape was. The Cascades are still volcanically active (witness Mt. St. Helen’s), and the geologically-recent eruptions throughout that range mean that the soil and vegetation are still relatively simple. In the last post, I showed you the Belknap Crater, near McKinley Pass, and its surrounding lava flows.
That volcano erupted over 4,000 years ago, and very little has colonized the area since then. Bright green lichen has gained a toe-hold on much of the rock, but pioneer grass and pines are a long time in following, and only a portion of the area has enough vegetative history to have developed a thin layer of soil.
This same story repeats itself throughout the state, as the overlapping volcanic and glacial histories define the landscape and its ecology. Glaciers of the last Ice Age defined the wide valley of western Oregon between the coastal mountains and the Cascade range, and the eastern second set of mountains,
however, and the eastern portion of the state is considered High Desert, barren and sparsely populated, despite the glacial deposits found there. In between, the Cascades rise,formed by millennia of volcanic eruptions. Despite having more rainfall than the eastern desert, disturbance by rock-fall, erosion, unchecked winds, and of course molten lava create a harsh life there for trees and groundcover.
In addition to affecting the vegetation, volcanic activity changed the terrain. Driving through the state, one can still see evidence
of these eruptions as though they had just ended. In geological terms, I guess they really have! Where rivers have carved
through softer layers, the hardened lava almost looks like it is still moving: separate layers of distinct lava flows can be seen in places like the Upper Rogue River’s gorge. Along the banks of this Wild and Scenic River there is also a very clear example of the lava tube formation. In this case, lava flows continuously out from a volcano (sometimes for periods of several months), and eventually the upper crust of the flow hardens after prolonged air contact. The continuing eruption, however, keeps the flow moving between that hardened crust and the bedrock below. Once the volume of lava has decreased, and the flow finally stops altogether, the crust
remains, but the tube through which the molten lava has been flowing is empty. These tubes can (apparently) be miles long. Most times the crust eventually collapses, leaving deep holes at the surface, or shallow caves when encountered from the side. In some cases, water, always looking for the easiest way to the sea, breaks through softer rock into these tubes. This is the case on the Upper Rogue, where the river, diverted by the eruptions, worked its way through both collapsed and entire lava tubes to create the gorge seen today.
I certainly am not a geologist (if you couldn’t tell from the preceding paragraphs), and the volcanic landscape still seems eerily barren to me, but I learned to identify what I was viewing with a basic understanding, and witness these patterns repeated as I went along. Crater Lake (initially formed by an eruption 7,000 or so years ago) may be one of the more famous locations
for viewing the aftermath of volcanic activity, and I spent a day hiking and driving around its rim. Check out the next post for the stunning detail of that trip!