My apologies for the dearth of posts lately- especially the last two missing meteorite Monday posts. I’ve been in a bit of a post-finals, I don’t-wanna-do-anything-but-veg, funk as of late. I’m slowly pulling myself out of it though. I changed my blog theme because I felt the old one didn’t handle my side bar as well. It started to look all cluttered as I added to the blog roll. The header image was taken by a friend during our petrology trip. I love the contrast between the flow banded rhyolite and the ubiquitous rock hammer.
So, for this post I wanted to share a few images from my petrology trip. The Google Earth image above shows all our stops during both days of the trip. The first eight stops are from day one and the next seven or so, are from day two. I tried to keep the image as zoomed out as possible so as to convey exactly where we were at in northeastern Oregon. However, the more I zoomed out, the less clear our stops became. Think of it like playing “Where on Google Earth”, but with a lot more detail to work with!
Oregon’s geology is fairly spectacular, but she really hits her stride in the Blue Mountains. It’s here that you get somewhat of a break from the Columbia River basalts and every other basalt group that dominates Oregon’s landscape. This part of the state is home to accreted terrain that was slowly stitched onto the west coast as the North American plate crawled its way over the Farallon Plate. Some of this stitching is marked by plutons that formed large reservoirs of granite and microgranite between the foreign terrains.
This was clearly illustrated by our instructor as we were leaving our sixth stop. We were in a broad valley that had been carved out by the Grande Ronde River. The rocks in the area where mostly rhyolites and welded tuffs of the John Day Formation (approximately 28 M.Y.A.). He kept stopping every minute or so to whack a rock, look at it with his hand lens, shake his head and throw the piece on the road. It was on the fourth or fifth try that he finally found what he was looking for: granite. He enthusiastically explained that the pluton marked the boundary between 140 M.Y.A terrain and 28 M.Y.A. terrain. Within less than a mile we traversed almost 120 million years of history!
The second day of the trip was spent mostly in the John Day Valley. It was here that we looked at outcrops of serpentinite, chert, and a few pillow basalts. It was pretty cool to think that we were in a rather dry part of the state, but surrounded by rocks that only form on the ocean floor. In fact, one of those pillow basalts can be seen in the picture with my professor. It’s in the center of picture, next to his right hand.
After hammering away and gawking at the seafloor rocks, we made our way into the John Day formation with all its rhyolites and ash-flow tuff. On one of the stops we looked at a columnar jointing pattern that formed not from basalt, but from a welded tuff. These were of the Rattlesnake Tuff formation and the topic of my instructors PhD dissertation while attending Oregon State University. What was really neat about these, were the pieces of obsidian and lithic fragments that were trapped in the super heated ash flow from the rhyolitic eruptions.
One of our final stops before making the four hour trip back to Portland, was this brecciated dacite outcrop. The vast majority of these rocks were oxidized, hence the pink color. However, there was a small, rounded part that for some reason, resisted the oxidation process and retained its grey color. In either case, the rocks contained some beautiful phenocrysts of plagioclase and some quartz. And that dark, vertical structure in the middle of the sea of pink? That my friends is one of the many dykes from which the vaunted Columbia River Basalts spewed forth.
There are some more pictures, but I really wanted to highlight some of the more interesting stops. In fact, my contribution to this month’s Accretionary Wedge will highlight an awesome rock that I got from one of the stops. Maybe a little later in the future the other pictures will crop up in some posts.