ESCI 322 Field Trip: Mysterious Visitors from the Deep


Photo credit: Hehe Jiang

This blog describes the 3-day field trip that the Fall 2015 ESCI 322 class took to northern California with Professors Cin-Ty Lee & Rajdeep Dasgupta and TA Laura Carter, as told by the undergraduate students: Sofia Avendano, Anthony Foster, Alexandra Homes, Garrett Lynch, Oliver Lucier, Ian Mellor-Crummey, Leila Wahab, Shannon Wang, and Sam Zapp.

Day 1:

Marin Headlands (chert):


Garrett Lynch in front of deformed cherts

This site overlooked San Francisco Bay directly across the Golden Gate Bridge. During the Jurassic period, following Pangaea, the Farallon plate subducted underneath the western North American plate.

At this stop we saw silica-rich cherts on top of ancient oceanic crust. The cherts were once laminated sedimentary deposits formed from diatoms and during subduction were scraped off as part of the accretionary prism. These specific cherts formed near the equator, probably during the Jurassic era. In composition, these cherts are 99% silica, but also contain some iron and manganese. This composition arose as iron and manganese were dispelled from the mid-oceanic ridges and deposited within the silicic ooze that becomes chert. We also noticed fractures filled with quartz in the form of white veins. Over to the right we saw darker, older, green bands, which had not been oxidized.

Marin Headlands (pillow basalts):


Students Oliver Lucier and Sam Zapp looking at a pillow basalt outcrop with Garrett Lynch and Alexandra Holmes looking on from above and TA Laura Carter from below.

The pillow basalts here formed from magma that upwelled due to the divergent boundary at the mid-ocean ridge and was erupted underwater. Zoned phenocrysts of plagioclase feldspar were visible, indicating that they grew intrusively before eruption. The surrounding fine grained matrix was extruded on the seafloor where it cooled quickly. The edge of each pillow featured a glassy rind where the surface of the lava body cooled even quicker with direct contact with the water. Layers of chert were deposited on top of these pillows in a marine setting.


Professor Dasgupta discussing pillow basalts with the students









Tiburon Peninsula (Eclogite, blueschist, and peridotite, boulder outcrops):


Eclogite. Photo credit: Prof Cin-Ty Lee

Here we saw a number of boulder outcrops. Eclogite needs high pressure and high temperature conditions to form, so its presence was indicative of subduction zone setting. Chemically, it is very similar to the pillow basalts we saw earlier at the Marin Headlands, suggesting this may be the protolith. However, the eclogite samples we saw were also enriched in sodium, probably from hydrothermal processes, giving a special form of pyroxene: jadeite. In eclogite, the aluminum in the protolith plagioclase is now held in garnet. As it moved back up through the crust, some of the eclogite retrograded into blueschist, greenschist and amphibolite. The heterogeneity of the metamorphic grade may have been caused by uneven distribution of fluids. The kinetics of retrograde metamorphism can be slow, but the presence of fluids makes retrograde metamorphism occur faster.



Students looking at a peridotite xenolith


The last outcrop we saw here was composed of peridotite. It contained orthopyroxene and olivine, so it would be classified as harzburgite. Peridotites with a much greater amount of orthopyroxene are lherzolite. These peridotite xenoliths had a slightly brown / tan coloring due to the weathering at the Earth’s surface. We also saw some serpentinite—altered peridotite—in the area.




Bolinas Lagoon (San Andreas fault, SEALS!):

We observed a rookery of seals accompanied by a lone sea lion as if he meant, through his presence, to elicit an ironical play upon a Danish children’s story. Abreast of these seals, two shovelers swam, feeding, and one merganser swooped through, low, tantalizing the water.

Point Reyes Visitor Center / Earthquake Museum:

Here we did a short nature walk travelling along the path of the San Andreas fault. We saw a fence that was displaced several feet due to previous movement along the fault, which is evidence of an ongoing right lateral strike slip fault.

The beach, the cliffs:


Students looking at an unconformity on the beach at Point Reyes. Photo credit: Laura Carter

Along Point Reyes on the Pacific Ocean there was a series of cliffs with igneous outcrops that were the roots of a magma chamber from the Cretaceous next to marine sediments from the Miocene, representing a large unconformity. Here, we could see cross cutting relations in the igneous outcrops where magmas of different compositions interacted. The parental magmas were more basaltic, while the more felsic magmas were much more evolved. The intruding felsic magmas were much more viscous, so they mixed in some places but stayed clearly separate in others, and then froze in place. Mixing of the magmas indicated that the surrounding rock was still magmatic mush when they mingled.

Sharper boundaries between two magmas is also visible. This was likely due to the fact that basalt has a higher melting point, so when it cooled, it contracted and pulled some of the surrounding liquid in. These intrusions occurred further inland from the subduction zone, around 10 km below the surface.

Day 2:

Sedimentary Road Cuts, Ione Formation:

In the foothills of the Sierra Nevada, we saw an outcrop on the exposed shoulder of a highway. Sediments had been collected from the Sierra Nevadas and compacted into layers. The layers originated from granodiorite which lost much of its magnesium and iron, leaving behind what became predominantly kaolinite (Al2SiO5). The lower layers of laminated deltaic sediment gave way to cross-bedding features as a fluvial system moved through the area. At the edges of the formation, there was evidence of soft sediment deformation.

Silver Lake:

We also stopped at Silver Lake for lunch. Beside the lake there were these interesting boulders which appeared to contain clasts of petrified wood. Across the lake we saw large lava flows of andesitic magma, now part of an imposing mountainside.


Andesitic lava flows overlooking Silver Lake. Photo credit: Prof Cin-Ty Lee

Mono Lake:

The edge of Mono Lake contains massive collections of tufa deposited in towers. We learned that these tufa deposits are a result of the very alkaline water from the lake mixing with calcium rich spring water from the Sierras, which precipitates calcium carbonate (CaCO3). The tufa deposits become visible and form large columns due to the water level of Mono Lake lowering over time. In the past, the springs feeding Mono Lake were drained to provide water for places like Los Angeles and other urban areas, which made more and more of the tufa deposits visible. Only in recent years has this been slowed and Mono Lake protected.


Tufa towers at Mono Lake. Photo credit: Ian Mellor-Crummey

Day 3:

Long Valley Caldera:

The Long Valley Caldera is in the Basin and Range region, which underwent extension. The Caldera was active 2 million years ago, but the last eruption occurred 700,000 years ago and erupted 700,000 m3 of rhyolite. The eruption style was Plinian, which is very explosive, caused by magma fragmentation, and characterized by fine ash. The ash of the eruption spread throughout the western US. There were also resurgent domes with the same magma composition, but with effusive style eruptions since the magmas had already been degassed.

The youngest lava flows were 7,000 years old.  Under the Basin and Range extension, decompression allows the mantle to cross the solidus, which leads to alkalic basaltic magmatism. Basalts that re-melt the crust fractionate and form rhyolites, so there are both extremes here: basalts and rhyolites. It is still unclear how basalts skip to rhyolites with no intermediate composition.


Glacial moraine in front of the Sierra Nevadas on the edge of Long Valley Caldera

The actual magma body was 10 km below the surface. The magma body builds up pressure, creating fractures, and, when the pressure exceeds the yield stress of the rock above, the system evacuates and the roof collapses. This has left a low, flat ~10 x 20 km area. Fumaroles, where magmatic gases were still escaping due to residual heat, were also visible.

Around the caldera, there were also moraines, large sediment mounds moved by glaciation, visible along the outskirts in front of the Sierras.


Long Valley ignimbrite deposit at Owen’s Gorge. Photo credit: Prof Cin-Ty Lee

Owen’s Gorge:


Long Valley ignimbrite containing pink pumice and granitic lithics

At the base of the gorge, we saw evidence of a huge rhyolitic ignimbrite, which must have erupted over an incredibly short period of time (estimated at around 6 days). There were three different layers denoting sequential eruptions. The lowest layer was more porous, with many lateral fractures and no evidence of compaction, which indicates that the lowest layer cooled before the weight of the next eruption could deform it. The texture of this tuff was friable, easily breakable, and it was more silicic than the upper layers. The first layer was probably an initial pulse of small eruptions right before the large eruption. Weak precursor eruptions tend to have ashfall rates slower than cooling rates.
The middle layer had a very clear fabric with small pumice ejecta elongated and oriented in the same direction, which shows that the layer was compacted. Thus it cooled slower than the layer below. The strain of the rock can be found from the aspect ratio of the elongated clasts, perpendicular to the bedding. This layer was welded ignimbrite, which contained fiamme, and was formed under high temperature and pressure. Jointing occurred after cooling due to contraction.


Long Valley ignimbrite containing pink pumice and granitic lithics


The upper layer was mostly ash with pre-existing metamorphic, Paleozoic clasts caught in the ash (lithics). There were also pink pumice clasts that also have an individual fabric, which probably came from a different eruption, and had elongated vesicles caused by compaction. There was no larger scale fabric because there was not enough overpressure to deform the material, so the last eruption must have cooled quickly.





Panum Crater:


Flow textures in banded obsidian layers

Panum Crater is what remains of a rhyolitic plug-dome volcano. Volcanic eruptions in this area began around 40,000 years ago. Panum Crater itself is approximately dated at 650 years old and is the youngest in the area. The explosion occurred as a magma bubble heated the water table into instant, high-pressure steam. Pumice was the first to be expelled from the volcano, and deposited around the outside of the crater. Following, breccia formed a dome over the crater, followed by lava flows that cooled to obsidian, which dominate the landscape.


Vesicles in the pumice at Panum Crater. Car keys for scale. Photo credit: Prof Raj Dasgupta


Highly zoomed image of the vesicle walls in the Panum Crater flows. Photo credit: Prof Cin-Ty Lee

The rhyolitic eruptions in the crater produced very silicic lava (roughly 76% SiO2), which is highly viscous and created the two main types of rock we saw: pumice and obsidian. The pumice was very low in density and highly vesicular. The vesicles formed from trapped air bubbles, indicating that the lava solidified quickly. Obsidian had little to no bubbles, but is the same composition as pumice. The obsidian cools quickly forming a glass, evidenced by the black rock’s conchoidal fracturing we saw.

A curious sample of rock we found looked like obsidian but had what looked like mud cracks on its surface. This was actually a breadcrust bomb formed from molten lava chunks flung into the air. The outer surface cooled rapidly, but the inside was still molten, so the gases inside continued expanding. This cracked the surface of the rock, causing it to look like the crust of a bread loaf.

Geologically speaking, obsidian never lasts very long. Devitrification causes glass to be more like rock and less like glass, so It is difficult to find glassy obsidian older than 10 thousand years old. The more vesicular the obsidian is, the faster the rate of devitrification since water gets in the vesicles. Thus, the rock samples at Panum were much younger than the samples at other formations we studied.

Basaltic Phreatic Deposit:


Deformation and bedded phreatic ash deposits with Alexandra Homes for scale

On our way from Panum Crater, we stopped to examine an omega-shaped formation on the side of the road. Upon close inspection, we found the rock to be very fine-grained and soft, with cross-bedding evident on the folds. This was formed when a basaltic flow erupted underwater. When basalt hits groundwater, it fragments and becomes very, very fine-grained, creating the soft texture that we observed instead of the texture we saw in previous basalt deposits. The deformation is very similar to soft-sediment deformation, and most likely occurred when the rock was still warm and ductile. Cross-bedding was likely a result of basaltic ash settling in bodies of water nearby, creating the sloped deformation, and cross-bedding typical of river systems.

ESCI 322 – Going Across Southern California

Mafic enclaves in the Bernasconi Hills Pluton in southern California. Emily Pain for scale.

Mafic enclaves in the Bernasconi Hills Pluton in southern California. Emily Paine for scale.

Students in 322 go on a geology trip across southern California led by their professor Cin-Ty Lee and graduate student Hehe Jiang.  This is a mid-term field trip that the department has used to give a students a chance to apply some of their classroom knowledge to the field.  When it comes to looking at rocks and minerals, there is nothing better than going to the field where one has context.  It makes learning about rocks much more meaningful and more memorable. 

So this year, the class did a transect across southern California.  We arrived Friday afternoon in Ontario, California.  Our first stop was in the northern Peninsular Ranges Batholith, part of a continental volcanic arc that extended from Mexico all the way up through Canada during the Cretaceous.  This volcanic arc would form the backbone for much of our trip, so-to-speak.  We visited the Bernasconi Hills pluton, where we examined outcrops showing extensive mafic-felsic mingling in the form of mafic xenolith swarms and highly attenuated schlieren.  We then made our way across the San Jacinto Valley, a late Miocene-Pliocene graben formed by extension in the vicinity of the San Jacinto Fault, a splay of the San Andreas Fault. Around us, rising above the valley floor were large knobs of Cretaceous granitoids.  We made our way down to Green Acres, where we had a chance to examine olivine-gabbros, examples of cumulates in a shallow mafic magma chamber. 

We found ourselves the next morning in San Diego.  After a quick breakfast at McDonald’s, we headed out to Point Loma to examine the Point Loma and Cabrillo formations.  We picked the perfect place to look at these outcrops because we were right along the ocean, with waves crashing, sea gulls squawking, pelicans diving and the cool breeze blowing against us.  These are late Cretaceous sediments. We examined them under our hand lens and discovered that they consisted of quartz, feldspars, and lots of fresh biotite and hornblende. Normally, biotite and hornblende don’t last long in the weathering regime, so their presence suggests a very juvenile sediment.  These sediments, it turns out, were being shed off the Cretaceous volcanic arc, most likely while the arc was still active, given its age.  Yesterday, we were looking at the eroded plutons and today, we are seeing their eroded tops in an ancient basin!

Cretaceous fore-arc sediments at Point Loma, California.

Cretaceous fore-arc sediments at Point Loma, California.

It was hard to pry ourselves from the ocean, but we had to because we a schedule to stay on.  Our goal was to head east.  On the way, we stopped by some Eocene forearc sediments exposed in a roadcut. These sediments were completely devoid of biotite and hornblende and were formed by the erosion of deeply weathered surfaces of the batholith, well after magmatism and mountain building had ended.  A few of the students found some shell fossils in the sediments, indicating a shallow marine origin.  After our brief foray into the Eocene, our drive along I-8 took us back into the Cretaceous plutons.  We turned north to Cuyama Valley, where we looked at more olivine-gabbro cumulates along the shore of Lake Cuyama. But our excitement quickly turned to a large area of exposed migmatites.  Here, the metasediments had been metamorphosed, deformed, and recrystallized to such a degree that they looked like a gneiss and in some cases looked like a highly foliated granite, but the tell-tale signs of migmatization were the abundant quartz- and feldspar-rich veins and dikes that appear to have formed during ductile deformation.  This was the birth place of some of the granites that contributed to the Cretaceous batholith.  There were a lot of mortar holes on the outcrop, left behind by Native Americans. How nice it must have been to grind food on migmatites!



Our next stop on Saturday was high up in the San Jacinto Mountains, just above Palm Springs. We stopped specifically at the top of Deep Canyon, where if one looked south, we could see a large dip slope composed of late Cretaceous mylontinized granites and tonalites.  We crawled all over the mylonites in search of two things: pseudotachylites and titanites.  Pseudotachylites are melts of the rock generated by intense frictional heating during an earthquake.  These melts were everywhere, some in foliation and many crosscutting.  The titanites, aka sphene, were large pistachio-green porphyroblasts formed during ductile deformation.   All of these deformational features are associated in intra-arc thrusting and exhumation during the late Cretaceous and early Paleogene.  Our day was soon coming to an end.  We descended into the Coachella Valley, but not before stopping at a vista point to look at the San Andreas Fault and the Salton Sea.  We ended our day in Palm Desert with a big Italian dinner!

We started Sunday morning with some unfortunate news. One of our students had his camera bag, tootbrush, and some clothes stolen out of his room, which had been inadvertently left slightly ajar in the chaos of organizing 18 students.  We spent several hours searching for the bag, looking at security videos, and calling the police, but deep down, we knew that we weren’t going to see his bag again. The good thing, if there was a good thing, was that he still had his wallet and identification.  So we moved on, but being a few hours behind, we decided to skip some of the stops.  We ended up driving down the west side of the Salton Sea, stopping briefly at North Shores to touch the sea and to talk about the opening of the Gulf of California.  The sea keeps receding each year as water supplies to agriculture have been steadily declining, so each year, we seem to have to walk further out across the barnacle beach to reach the water. 

After we got our fix of dead fish, we drove another hour south and found ourselves at the mud volcanoes.  The high water table and high heat flow here is what causes these mud volcanoes to form. Geothermal power plants have taken advantage of this combination.  The mud volcanoes gave us an analog for many of the different volcanoes and lava flows we talked about in class: spatter cones, pahoehoe, aa, etc.  It was great to see some of these mud volcano eruptions in action!  Our next stop was Obsidian Butte, a Pleistocene rhyolitic lava flowwhich generated a large obsidian dome.  We marveled at all the beautiful flow banding and we discussed why obsidian flows tend to be big blobs rather than long, thin lava flows. Viscosity!

Talking about mud volcanoes in the Salton Sea. Emily Paine, Detao He, Cin-Ty Lee, Josh Crozier.

Talking about mud volcanoes in the Salton Sea. Emily Paine, Detao He, Cin-Ty Lee, Josh Crozier.

The rest of the day was spent getting from the Salton Sea into the Mojave Desert.  We stopped briefly at Salt Creek, where we crossed over the railroad tracks to look at the San Andreas Fault up close and personal.  Unfortunately, while we were on the other side, the longest train in the world came up and decided to park itself there.  We decided it was too dangerous for us to climb through the train back to our cars as there was no telling when the train might start up again.  The only way was to go around or, in this case, we knew there was a bridge we could go under, but the bridge was quite a ways to the north. Should we wait or walk? We had no idea how long the train was going to stay, so we decided to do the long hike.  By the time we got back to our cars, delayed by an additional hour, the train still had not moved, so it was a good decision. 

Re-energizing with a good dose of fluids, we drove up out of the Salton trough, crossing the San Andreas Fault to the north and passing through Pliocene deltaic sediments, variably deformed and tilted.  We stopped here to look at a wonderful assortment of textbook sedimentary structures (sorting, cross-bedding, soles, soft-sediment deformation), and then further up the canyon, we found the contact between these sediments and the basement, only here the basement was a new rock for the trip – Pelona greenschists associated with subducted sediments or basaltic crust during the Cretaceous.  We ended our long Sunday trip in Joshua Tree National Park, with the students climbing up and down the giant quartz monzonite boulders.

On top of a boulder in Joshua Tree National Park. L to R: Detaho He, Michale Farner, Evan Neustater, Kate Nicholson, Elizabeth Finlay.

On top of a boulder in Joshua Tree National Park. L to R: Detaho He, Michale Farner, Evan Neustater, Kate Nicholson, Elizabeth Finlay.

students examining pebble lag deposits in the Mecca Hills. Yunong Xu, Beineng Zhang, Stephanie Zou, Farah Ashraf.

students examining pebble lag deposits in the Mecca Hills. Yunong Xu, Beineng Zhang, Stephanie Zou, Farah Ashraf.

Monday would be our last day out in the field.  Our first stop would be Amboy Crater in the Mojave Desert on the North American plate. This was our opportunity to discuss why basaltic lava flows can flow much further than rhyolitic flows. Close inspection of the lava flows revealed tiny euhedral crystals of olivine and lots of vesicles! Off in the distance, we could see a pristine cinder cone.  This must have been quite the sight when it was erupting.  After our Amboy Crater stop, our goals were to look for xenoliths at Dish Hill and then trilobites in the Marble Mountains, but due to all the rains and flash floods that had happened earlier in the season, the highways were all closed.  We had no choice but to change our plans on the fly.  Our professor, Cin-Ty, decided then to try a place he had never been to before.  Off in the distance, one could see limestones jutting up against some granitic basement, so we parked our cars and walked across the alluvial plain.  We had no idea what to expect, so we were told to fan out and just explore.  With 18 pairs of eyes, we were bound to see something, and soon, students were coming up with little fragments of Vesuvianite, a clear indicator mineral for skarns!  But where was the actual skarn?  So we fanned out again, falling the trail of bread crumbs and looking around for the contact.  And there it was, all the vesuvianite was forming right up against the contact!

Vesuvianite with Elizabeth Finlay and Sarah Gerenday

Vesuvianite with Elizabeth Finlay and Sarah Gerenday

With our pockets filled with new minerals, we continued north, deep into the Mojave.  We stopped briefly at the Kelso Sand Dunes and then had lunch at the Kelso train depot, a nice green spot in the middle of the desert.  We then headed to the Cima Volcanic field, where there are numerous cones, all erupted in the Late Miocene to present.  We stopped at well-known but difficult to find lava tubes, and then we drove a little further up a rough road to look for xenoliths. Normally, we don’t do this latter part because the road is so rough, but since our plans at Dish Hill were diverted, we thought it would be a nice way to end the trip.  Unfortunately, the road became far too rough to continue further, so we hiked the last stretch.  And xenoliths we did find!  Peridotites everywhere!

Hunting xenoliths in Cima. L to R: Cin-Ty Lee (hideen), Evan Neustater, Kate Nicholson, Yunong Xu, Farah Ashraf.

Hunting xenoliths in Cima. L to R: Cin-Ty Lee (hideen), Evan Neustater, Kate Nicholson, Yunong Xu, Farah Ashraf.

Our luck, however, would not hold out.  As we turned our cars around and headed back down the slope, one of our cars hit a rock hard, twisting its axle. Definitely not good because it made it hard to drive.  What to do now?  The sun had set, the students were tired and hungry, and we had early flights out the next morning, but all the way back in Ontario, a three hour drive with a good car.  We convened at the Mad Greek in Baker and it was decided that the everyone would pile into the other working cars and just go back to Ontario so they could get a good night’s sleep.  Our professor and a couple of visiting grad students would drive the disabled vehicle back, slowly.   It took the five hours and a tire blow out in the last 20 miles, but they arrived home safely.