Published: March 14th 2012March 14th 2012
Sid pointing to 15.
Day 5! Wednesday, March 13, 2012 (I decided I’m going to start adding dates for posterity) As is becoming customary, I will close out my blog from the night before. We arrived at the Sierra Nevada Aquatic Research Lab (SNARL) just as the sun was starting to set. We were a little disappointed because of our failed attempts at seeing the petroglyphs and Owens River Valley, but on a trip like this, morale doesn’t stay low very long. The immediate remedy, in fact, came from the Coso Volcanic Field group that prepared an Italian meal which would’ve nearly made Grandma Aquilina proud. We listened to Dan Dawson, the Director at SNARL speak briefly on what the research lab is, how it plays into the bigger picture of California’s education system, the opportunities it holds for graduate study, and a few minor housekeeping things. Perhaps the best news he provided was that even though there were signs posted warning us about the presence of mountain lions, it wasn’t mountain lion season because of the migratory patterns of the local deer. I have to admit though; it would be pretty damn cool to see a mountain lion. With full bellies and a game
Base layer with 19.
plan formulated for the next day, it was time to relax and feed our modern day addiction to technology by jumping on our laptops and enjoying the free wi-fi. I used it to upload my blog (lucky you), but more importantly to fill out my March Madness brackets! LET’S GO ‘CUSE! LET’S GO DUKE! In other sporting news, I missed the Sabres game, but was able to watch the highlights of our latest victory and climb to the playoffs! VERY excited about how the boys are playing right now. It wasn’t long before my eyelids would no longer stay open and I enjoyed my first night on a real bed since last Wednesday.
After a peaceful night of sleep, it was time for some geology! Sid put the hard press on people to be packed and out the door by 8:30. You could tell by the conviction in her voice that she was especially excited for today. After all, today was Mono Lake, and Sid lives for Mono Lake. Today was going to go differently than yesterday, and boy, did it. I’ll start off with a little background on the basin. Like the rest, this basin was formed as
Entire outcrop with dark ash 2 near the top.
a result of crustal extension. The original lake, which reached 2155 meters at its maximum (and currently at 1946 meters) was the result of glacier melt running down from the surrounding mountains (which to the west is the Sierra Nevada and to the east is the Cow Track Mountains). Not overly impressive by numbers, but the 200+ meter difference between current and maximum elevation is overwhelmingly evident when we were on top of depository lake sediment far above the lake (pictures to follow for all of this). This basin is so geologically famous because of the surrounding volcanoes spewing ash into the once enormous lake. After the depletion of lake levels and an unnatural human intervention causing a creek (Wilson’s Creek) to flow through the now exposed sediment, it provides for an incredible outcropping (and subsequent geologic story) to study. The basics of studying the sediment deposits are centered on understanding the layering sequence of the volcanic ash. There are 19 numbered events visible in the outcropping that are broken into 5 packages, A-E. These denote groupings of events that appear within geochronological proximity and are visible in the rock record. Not to overly bog down people in details (I’m
OJ's glove with 15.
mostly saying that because I don’t know them), I’ll be throwing out numbers between 1 and 19 referring to ash layers. The takeaway from this is that 19 is the ash layer closest to the base (the oldest) and 1 is the newest layer closest to the top of the outcropping. Of the 19, 18 are rhyolitic and sourced from the Mono Craters which lie to the east. The one exception is layer 2 which is a distinctively basaltic layer sourced from Black Point (aptly named for the dark, mafic rocks it produced) to the northwest. For perspective purposes, I’ll attach a picture of the lake at its current level, Black Point and the Mono Craters. OK, now we have a foundation, it’s time to get into the fun stuff!
We finally took the time for a group photo when we first got out of the cars at the basin. The opportunity was right in front of a perfect outcropping of layer 2 (the dark, basaltic one), whose place in the basin would become much more clear over the following few hours (photo 1017). Our first stop with Sid was to get an idea of what we were looking
Thinolite tufa formed in near freezing water
for when she said “ash layer”. There was no brighter and more distinct layer than 15. Sid took the time to pose pointing to it! (photo 1004, 1010) From that point everything really seemed to come together for me as far as the stratigraphy of the outcrop goes. I’d actually love to talk more about this one than I’m willing to write right now, so if you’re interested in the layering, just ask me…. Actually, ask Sid, but if she’s not available then ask me. What you’re looking for in this picture is the brighter white stripes as ash and the dark layer almost at the very top is our favorite number 2 (photo 1009). The mound of eroding silt and clasts blocks a lot of the layers at the center, but the overall deposition is clear. Further up the creek, we were able to get a great view of the base layer. This is evidenced by the conglomerate being topped by ash layer 19 (photo 1008). Other interesting structures we saw were tufa structures. These were really cool looking icicle/mineral/lithified type of awesome. Tufas are formed when the water is just above freezing (not necessarily at 32 degree Fahrenheit
Drop stone with hand for scale
due to salt content of the lake) and Ikaite (a mineral) precipitates to form a crystalline structure that if buried quick enough (extremely susceptible to erosion) becomes a part of the rock record (photo 1011). One last note from this portion of the trip (even though there is much, much, much more to say) was the presence of drop stones, which are rocks rafted out into the lake by ice, and when it warmed, the rocks were dropped into the lake. These rocks are distinctive because of their angular appearance (from not being weathered or eroded) and erratic placement (photo 1016).
Our next stop was pretty brief, but worth mentioning. We stopped along the June Lake loop to see a polished Sierra Granite with distinctive striations and chatter marks. Both striations and chatter marks are made when glaciers drag rocks across the surface of other rocks leaving behind crescent shaped gouges on the base rock. The striations trend linearly in the direction the glacier is traveling (photo 1023). The polished appearance is the result of a sandpaper-type effect that has fine grained sediments at its base and rubs along the contact point of another rock. It effectively polishes the
Group shot by an ash 2 layer
base rock to a smooth, polished surface (photo 1025).
Another brief stop led us to a massive glacial erratic called “Perched Boulder” that was a cool, quick stop for pictures (photo 1026).
Next, we headed off to Convict Lake, which had plenty more geology and incredible views in store! I’m sure you all were hoping for that. This lake is bounded on its sides by lateral moraines (deposits left alongside of a moving glacier) and at its southern limit by a terminal moraine (maximum forward progress of glacial drift). Indicative of glacial activity is poor sorting of rocks, meaning boulders to pebbles were all mixed together (photo 1028). The lake had a looped trail that provided a view of metasedimentary outcrops, in this case meaning the country rock (or preexisiting rock) was metamorphosed by an igneous intrusion. This allowed us to see the banding of sedimentary rocks coupled with the bends and flows of metamorphic rocks. Pretty sweet, right? Generally, the glacial lake provided for incredible views of limestones, shales, slates in the country rock, granite from the igneous intrusion, and even fossilized stromatolites for you bio people (wink, wink) (photo 1037) . Other structures worth noting were
xenoliths (or rocks that were sucked up into the magma) found inside of granite boulders (photo 1029). As a side note, the lake got its name from 29 escaped convicts from a prison in Carson City, of which six tried to escape to this region. A posse tracked ‘em down and brought them to justice in a fashion that only the 1871 wild west would have it, hanging. Loved the history, loved the water, loved the geology, amazing stop.
Our final stop of the day landed us at the Owens River Gorge, which we tried in desperation to find yesterday. With some good recon work done by Rafael and Meg, we were able to see another great outcrop. Most notably was the columnar jointing, which in this case was the same rhyolite from Bishop’s Tuff that when cooling constricted and cracked into columns. This happens because rocks, like all other earthly things, like to take the most efficient way of doing anything (in this case, cooling) so the constriction causes joints in the shape of polygonal columns (photo 1042). More interesting columnar joints were found with bends which can be attributed to volcanic vents facilitating the cooling process of
Chatter marks with wallet for scale.
the rhyolite (photo 1043). After finally making it to the bottom of the gorge, I snapped one last great shot of Owens River flowing through the canyon before calling it a day and heading back to camp (photo 1045). What. A. Day. I would catch you all up on what happened back at camp, but the truth is, I spent the entire time writing this. It’s time for bed. More geological fun tomorrow!
There are more photos below