Thursday, July 7, 2011

Wallowa Lake

For the 4th of  July weekend, we decided to get out and see another of Oregon's geologic gems, Wallowa Lake.

Aside from being a rare large lake in eastern Oregon, Wallowa Lake is one of the best examples of a glacial morain damming a valley to form a lake. From a view point off the Mt. Howard Tram, the moraines forming the boundary of the lake are easily visible.

While the moraines are easily visible, it is hard to convey a sense of disproportionate scale. The moraines rise over 200 meters from the lake surface and almost 300m from the surrounding area. A road cut through the moraine along the edge of the lake reveals a poorly sorted mix of granitic and basaltic rocks, cobbles and smaller sediments.


Tuesday, July 5, 2011

Pacific Northwest NAGT #3 Diamond Craters

The Pacific Northwest NAGT meeting was supposed to have 4 days of field trips, and one day (Saturday) of meetings. This years meeting only had about a 1/2 day of meetings so Saturday afternoon we headed out to Diamond Craters,, BLM designated "Outstanding Natural Area". Diamond craters began erupting about 20 ka and according to the the Global Volcanism Program website was last active about 7 ka. But according to our guide, there may be indications of eruptive activity in archeological sites that are less than 500 years old. 
From the ORE BIN vol 26 No 2, Feb 1964 
Unfortunately time was limited so we were only able to make three stops in the area, but each stop represented a different kind of volcanism so it was a great overview of the eruptive history: The first stop was a pair of Maars at the western end of the road. The first was smaller and dry, but the second (Malheur Maar) was deep with an active mosquito breeding farm at the bottom.
Maars/craters are generally caused by the ejection of hot steam and other volcanic gasses. The pressure of these gasses blow a hole in the surface that becomes the crater, but the eruption containes little to know magmatic material. So at our second stop we saw an eruptive area that showed indications of fountaines of eruptive matieral. Red Crater was a lapilli crater built up to about a hundred feet on the eastern side. The rim was comprised of lapilli sized tephra that had fused while still molten. Scattered around the site were a number of "Breadcrust Bombs". Volcanic Bombs that were beginning to cool and harden on the outside while still molten and outgassing on the inside. The release of the internal gasses cause the bombs to continue to expand, stretching the outercrust creating cracks like a crust of baked bread. This bomb was found about 50 feet from the crater rim.

Our last stop was Lava Pit Craters. Actually three craters formed by drained Lava lakes. Eminating from each lake are a sereies of tubes and caves of highly basaltic lava. 

One of the pits has a large cave/tube with lavacicles hanging down and a hornito on top.
Unfortunately because our time was limited, we did not make the hike into the central vent area with more than 30 vents and some "dribblet spires" where smalll amounts of molten lava is forced to the surface with just enough energy to run down the sides as it cools. Sort of like the magma version of a drip sand castle.  Oh well, it gives me a reason to make the drive back. 

Friday, July 1, 2011

Pacific Northwest NAGT Field Trip #2

I began writing about the 2011 Pacific Northwest NAGT field meeting here. On Friday we circumnavigated Steens Mountain. The final stop was in Colony Canyon south of Fields, Oregon near the Oregon Nevada Boarder. The area was postulated to be accreted terrain that was originally near the continental craton, but has been moved hundereds of kilometers east by the extension of the Basin and Range Province. There were numerous outcrops of highly metamorphic rock, and not enough time to thoroughly explore.
Andy and Turk debate the Metamorphic grade of the outcrop
I am not strong on metamorphic rocks, but I know a nicely folded schist when I see one, and it makes a nice folded structure for a Friday.
The schist was actually folded by three separate events, the first forming the shist planes, the second altering the foliated planes into wavey folds, and finally a large tilting of the whole block to dip towards the north east. The layer on the right that appears to be intruding into the schist was interpreted by Turk and Andy to be Ultra-mylonite. The area was also full of granitic intrusions with lots of trace minerals, many of which I could not identify.

A great stop, but it would take a field season to really get a feel for all of the geology.