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ESCI 321 &322 Field Trip: Tales of Mud Volcanoes, Evaporite Deposits and Lava Tubes.

 

This blog describes the 5-day field trip for the Fall 2017 ESCI 321 and 322 classes to Southern California from October 6 and 10, 2017. The trip was led by professors Cin-Ty Lee and Rajdeep Dasgupta, and reported by undergraduate students Kendra Baldwin, Kyle Bartsch, Simon Chan, Aitash Deepak, Jennifer Kroeger, Jason Mendez, Jared Nirenberg, Sarah Silberman, and Jackson Stiles. The group was assisted by graduate students James Eguchi, Damanveer Grewal, Hehe Jiang, Sriparna Saha, Chenliang Wu, and Dr. Ming Tang.

The Southern California region has a complex geologic past with subduction, rifting and strike-slip motion, occurring along the major plates underlying the North American Continent. Most of California’s Paleozoic geologic history is evident in regions of the Mojave Desert and Death Valley. The Paleozoic era saw deposition of limestones, sandstones, and shales that were later eroded, displaced or uplifted during the tectonic activity that followed in the Mesozoic and Cenozoic. The rocks are primarily limestone with an abundance of fossils that indicate the evolution of early marine life forms. The region saw multiple periods of sea level fluctuation that led to the deposition of carbonate reefs along what is today’s Death Valley.

The Mesozoic was a period of increased geologic activity. During the middle to late Jurassic period, the Farallon Plate began subducting beneath the North American Plate, leading to the development of an extensive volcanic-arc system along the western coast of North America, and uplift of the Rocky Mountains in the late Cretaceous. Subduction also led to formation of the Southern California Batholith and other plutonic and extrusive volcanic regions in the Mojave Desert Region. An example of Mesozoic plutonic rocks can be seen in the Joshua Tree National Park, where the extensively exposed rocks exhibit uniformity in texture and grain size. The peak of volcanic activity occurred during the late Cretaceous in the southern Sierra Nevada region. The region was also covered by sedimentary deposits, with uplifting and metamorphism combined to create intertwined sedimentary and volcanic deposits.

During the early Paleogene period, while Farallon Plate subduction continued, volcanic activity in the peninsular region ceased. Uplift and erosion rates were continuous, which led to the deposition of vast amounts of sediments along the western continental shelf. Moving into the early Neogene period, the Farallon Plate disappeared into the subduction zone forming the San Andreas Fault; at an angle oblique to the western margin of the continent. As mountains began to rise along the coastline, marine sedimentary deposits became exposed. Eastward, crustal thinning and tectonic uplift along the Mojave desert region led to the development of the basin and range landscape. Additionally, Baja California started to move westward, away from Mexico.

In the quaternary period, the major driver of tectonic activity is the strike-slip San Andreas Fault, resulting in transverse ranges and extensional regions. Modern Rivers flowed through the basins while there was still volcanic activity in the Mojave region.

Day 1

The first stop of our trip was at Tourmaline Beach in La Jolla, California. We looked at the Point Loma Formation, which consists of marine deposits of sandstone and siltstone, formed originally in the Sierra Nevada- Peninsular Ranges batholith during the late Cretaceous. These were eroded from the roof of the magmatic arc that rose quickly due to compression and magmatism. Rapid erosion and transport of the deposits can be implied from the poorly rounded igneous sand grains. The marine deposits of forearc sandstones are tilted; graded-bedding and small areas of soft-sediment deformation are visible here.

Day 2

Marine depositional layers at Tourmaline Beach Photo Credit: Jackson Stiles

San Diego River Upper Estuary

 After a breakfast of bagels we reached an overlook above the San Diego River. The river is slow moving, and while the water level was fairly low when we were there, we could tell that the water level is often much higher. The water here is estuarine and thus brackish, but does not have strong enough sediment discharge rate to create a full delta. The sediment load is mostly comprised of fine sands and silt.

Hospitality Point-San Diego River Mouth and Lower Estuary

 Following the San Diego river overlook, we made our way to where the river meets the sea. While the San Diego River has migrated along the coast over time, it is now contained by a jetty, cementing its path to the ocean at one spot. The engineered jetty also prevents beach sediments carried by currents along the shore from piling up in mission bay in favor of generating a large sandbar at the river channel. This piled up sediment is rarely breached by floods and suffers minimal erosion.

Sunset Cliffs Next, our group stopped at the cliffs overlooking the Pacific Ocean at Point Loma. These cliffs are an example of late Cretaceous sandstones (75ma) that have a high sand to mud ratio, making them fairly resistant to erosion. These sandstones belong to a forearc setting, exposed due to tectonic uplift, that enabled preservation and exposure of deep water sediments. Finely laminated layers at the bottom of the exposed strata are most likely the result of deposition in a calm depositional environment with high clay content. Overlying the thin laminated layers was a thicker layer of sandstone (indicating a much higher energy environment) and the white markings are erosional surfaces along the cliffs. Within this layer  large rip-up mud clasts were deposited. Further up in the outcrop we could see an old soil horizon and erosional unconformity that represents a 70 million year gap between the Cretaceous sediments and the more recently deposited sediments. We also identified sections of soft sediment deformation and local slumping from differential compaction of the overlying layers on the soft sediments. The entire outcrop was interlaced with faults.

Soft sediment deformation overlaying clay layers.  Photo Credit: Jackson Stiles

Coronado Overlook

 Our next stop was at the top of the Cabrillo National monument overlooking the San Diego Bay. The bay itself is a graben bound by faults on the west, and the Cretaceous mountains in the East. Coronado Island is a sand spit created by long-shore sediment pulses from the ocean. The sediment deposited on the coastal plains from the eastern mountains is manifest into turbidites and deep water deposits.

Peninsular Ranges Batholith-Cretaceous Plutonic Rocks

Outside of San Diego, we parked along the roadside to look at a large outcropping in the Peninsular Ranges batholith region, which is around 90 million years old. At the road cut the majority of the rocks were tonalite (a type of felsic granodiorite mostly made of plagioclase). This tonalitic outcrop was crosscut by large dark colored (mafic) and light colored (silica-rich) dikes. The dikes intruded after the tonalite batholith had formed and the brittle nature of the tonalite led to the development of extensive fractures.

Cuyamaca Lake

Cuyamaca Lake is underlain by Cretaceous gabbro composed mostly of plagioclase and orthopyroxene that are associated with the Cretaceous volcanic arc. Olivine was also present, but it was difficult to identify. The rocks around Lake Cuyamaca are also about 90 million years old, but is unique given it is one of the only sites in all of Southern California with layered gabbro deposits.

Jurassic Migmatites

 To reach this location we took a quick hike through the bush.  Exposed on the plain beyond were Paleozoic aged sedimentary rocks that had been melted and deformed during the Mesozoic era by through ductile underthrust in a subduction zone. During this process, many felsic (white colored) dikes formed through the rock, cross-cutting each other in several places. Within these dikes, we observed clusters of black coarse-grained tourmaline crystals; these are described as tourmaline pegmatites. Throughout the surface of the exposure, there were small, unnatural depressions which interestingly served as evidence that the outcrop was used by Native Americans to grind nuts.

View of Alluvial Fan
After leaving Julian (without having any of their famous apple pie), we stopped along the side of the road in a vast plain surrounded by mountains. The region showed a high quantity of brittle shear within fault gouges, and ductile shear visible in mylonite metamorphism resulting from high shear strain. Protomylonites from this intense ductile shear were quite prominent. The tonalite in this area was highly foliated, and there were few dikes present. The alluvial fans observed here had a high inclined gradient and contained many boulders and cobbles. The vast plain was actually a large coalescence of alluvial fans known as a bajada.

Two cross-cutting dikes in the Jurassic Migmatites.  Photo Credit: Chenliang Wu

 At the next stop, we were greeted by a small shack with the words “dead body inside” crudely painted on it. The stop was located at the base of the mountains near Anzo Borrego, with a view of a range across a playa. The mountains in the distance had reddish black sediment deposits along their hillsides. These were representative of fluvial sediments about 3 to 5 million years. The reddish black coating was likely a layer of iron-manganese oxide around 10 microns thick, and it indicates they were older than the incision. The rocks along the hillside we had stopped at were proto-mylonites. The shear stress during the mylonitization process caused various minerals in the rock to recrystallize that resulted in presence of clusters of garnet crystals forming throughout the matrix. The mylonitization was an inter-arc shear zone and most likely occurred during the late Cretaceous arc magmatism, and suggests a connection between the arc magmatism and a great amount of deformation of the upper plate.

Day 3

Salton Sea Area

 Our first stop on the third day was at the Salton Sea Pliocene deposits between the Salton Sea and the Gulf of California Basin. The deposits were fluvial, lake, and deltaic associated with the initiation of the rifting of the Gulf. We saw different surfaces, including tilted mudstones with sandstones on top of them. The canyon had large cobbles around the rim, likely transported there by large floods. Many of these cobbles were mylonites similar the ones we saw at the end of Day Two.

Our next stop was at the Salton Sea which is a landlocked water-body, formed as a result of northward extension of the Gulf of California, to accommodate the developing faults in the area. While the base of the Salton Sea is oceanic crust, the water itself is not that deep (~10s of meters). Interestingly, the Salton Sea is about 234 feet below sea level. Over the last 100,000 years or so, the water level has been decreasing continually. The restricted nature of sea lead to conditions with no upwelling, and therefore there is no thermohaline circulation that caused the sea to stratify and there is a good amount of bioactivity in the top layers. Every once in awhile this circulation will start up, and many of the fish die. The sea is now fed by agricultural runoff.

Obsidian Butte Outcrop

 Here we had the chance to see what was left of a rhyolitic, Pleistocene lava flow that quenched to obsidian. Rocks layered with obsidian and pumice and xenoliths were interesting features at this outcrop. These features indicate that an initially volatile-rich magma was subjected to decrease in pressure which led to the formation of these layered obsidian-pumice flows. As the volatiles began to escape, the magma was left with little/no gas and it quenched eventually to form glassy obsidian. The escape of bubbles was in fact facilitated by their size. The smaller bubbles that could not escape eventually formed the pumice layers. We also evidence of shearing and slow stirring of the magma at the surface. The rhyolites indicate that this area tends to have a high heat flow and a thin crust.

Presence of mud-volcanoes in the area, further establish the dominance of a high geothermal gradient in this area. These mud volcanoes are interesting because they are constantly migrating.The mud-volcano flows are essentially like basaltic lava flows, that flow slowly, traveling over larger distances owing to their low viscosity. The tops of the volcanoes were spurting out mud and bubbling out CO2.

Box Canyon

Tilted sandstone layers.  Photo Credit: Jennifer Kroeger

At Box Canyon, we observed tilted sandstone layers from the Pliocene with some conglomerates in the layers. These sandstones were tilted as the Gulf of California subsided during rifting. We saw evidence of cross-bedding and several channels indicating that most likely they formed in a braided river system. There was high grain size variability is the sandstones and conglomerates, so the depositional environment was likely close to the source. Also, we noted that gypsum was later precipitated in the fractures in the sandstone after deposition. Rip-up mud clasts were also found in both the sandstone and conglomerate indicating a high energy environment.

Mecca Hills

 At Mecca Hills, we stopped on the side of the road to observe a large anticline on the side of one of the hills. The sediment layers were most likely around 3 million years old, while the anticline was due to younger, active tectonics.

Pelona schist

 We stopped along the side of a highway dodging speeding vehicles, to observe the Pelona Schist that formed as part of the accretionary complex as the Farallon plate was subducting during the late Cretaceous period. The schist here underwent greenschist-facies metamorphism as is evidenced by the abundance of chlorite, biotite, and muscovite minerals. The rocks sections were well foliated and weathered on the top.

Joshua Tree – Jumbo Rocks

  At Joshua Tree National Park, we observed the Jumbo Rocks that are coarse-grained granites formed in the Cretaceous and Jurassic eras. Interestingly, the rocks are extremely homogeneous in composition and grain size without much texture. The rocks are jointed, that likely formed while the pluton was cooling to release stress. The joints were filled by dikes that had a very similar composition to the surrounding rocks, though we observed less biotite in the dikes. These dikes are possibly the remains of the magma that formed the pluton that came back up later to fill the joints after the joints formed during cooling.

Palms Canyon Oasis The last stop of the day was at Palms Canyon Oasis, where we observed Jurassic quartz monzonites with grey feldspar megacrysts. Here we observed large, single crystal alkali feldspars that were prominently larger than the surrounding grains. While it is not well-established how these megacrysts form, it has been proposed that fluid transport aided in the growth of these large phases.

Day 4

Salt Deposits
At this stop, we walked out on evaporite deposits. The evaporites were seemingly most NaCl but presumably had some amount of lithium and calcium chloride as well. The deposition of these evaporites is aided by the topography of the area.

 

Wind created evaporite formation. Photo Credit: Jackson Stiles

The area is a local low: structurally a small graben bounded by faults and mountains on either side drain into it. Water draining from the surrounding mountains enrich the water in minerals that then form evaporite beds as the water accumulates and evaporates in the graben. We also saw several interesting structures in the evaporite deposits including hair like structures formed by wind, beautiful halite crystals, and thin layers of evaporite formed on the surface of bubbles of briny water.

 

Vesicular rocks. Photo Credit: Jackson Stiles

Amboy Crater
At Amboy Crater, we looked at a cinder cone volcano and the associated extensive basalt lava flow. The cinder cone and basalt lava flow formed in the Pleistocene and their young age is reflected by the characteristic shape of the cinder cone as weathering has not changed its form much yet.  The extensive basalt flows seen here flowed over much larger areas than the rhyolite flows we previously saw due to the basalt having less silicate in it which lowers the viscosity allowing the lava to flow further. We examined bombs and other debris shot out from the cinder cone during its formation, although we did not actually walk up to the cinder cone itself.

The flow and pyroclasts were quite vesiculated and several flow indicators like ridges and folds visible. The cinder cone itself formed as lava was spit out of the vent as opposed to the usual flowing lava that would create other types of volcanoes. The cinder cone is formed as lava cools quickly after being launched into the air and falls back to the ground creating the characteristic conical shape made up of pyroclastics.  

Dish Hill

 Dish Hill was another volcanic cinder cone nearby; however, it was a few million years older. Due to weathering, and from mining for road fill, faces of this cinder cone had been dissected which allowed us to examine the interior structure of a cinder cone. Extensive weathering and oxidation also gave Dish Hill a reddish color rather than Amboy Crater’s black color.

Olivine and pyroxene xenoliths from the mantle. Photo Credit: Chenliang Wu (L: peridotite xenolith) and Jackson Stiles (R: pyroxenes)

At this stop, we saw more bombs and pyroclasts, both of which were larger for the most part. The dissected side of the cinder cone gave us access deeper into the cone where we found many peridotite xenoliths brought up from the mantle. We also saw granitic xenoliths from the crust that were brought up from the walls of the magma chamber and vent. Common in the rocks were olivine and pyroxene xenoliths.

Kelso Dunes

The Kelso dunes make up the largest field of windblown (aeolian) sand deposits in the Mojave Desert. The field is still active and includes migrating dunes, vegetation-stabilized dunes, sand sheets, and sand ramps. The grains are light-colored which leads to the conclusion that the protoliths are quartz and feldspar. A closer look at the grains themselves shows that they are fairly round and around 100 microns in the maximum dimension. These sand grains most likely originate from the Mojave River sink (west of the dune’s location). We also observed that the ripples on the dune face indicated wind direction, with the shadowed ripple portions on the lee face towards the wind. The south side of the dunes also held more vegetation than the lee side.

Cima Volcanic Field

Interior of a lava tube. Photo Credit: Chenliang

This volcanic field is made up of alkali basalt cinder cones and lava flows. These are associated with the development of the  Basin and Range. Most of the lavas in this area are younger than one million years.  The lack of erosion of the cinder cones and unweathered lava flows show the youth of these volcanoes. The lava tubes present in this field would have formed when a low-viscosity lava flow developed a hard crust (due to cooling of surface lava) that acted as a roof allowing the lava beneath to sustain high temperatures and continue flowing. Within the tube, marks, called flow ledges/lines, on the wall indicate different levels at which the lava flowed. Additionally, lavacicles (stalactites) and drip stalagmites are clearly visible inside of the tube. These could form when lava splashes onto the roof of the tube and slowly oozes down.

Mountain Pass Syenites

At this mountain pass, a carbonatite-syenite complex is approximately one billion years old. There is a mine nearby that extracts rare-earth elements from the carbonatite. Carbonatites are rare igneous rocks but have three models to explain their formation. Carbonatites are usually associated with undersaturated (low silica) igneous rocks; instead of silica tetrahedra, the anions are carbonate complexes. This rock can form directly by low-degree partial melts in the mantle, and melt differentiation. Another way is through liquid immiscibility between a carbonate melt and a silicate melt. Finally, extreme crystal fractionation may saturate CO2 levels in the magma, leading to the formation of carbonatites. Fun fact!: Mountain Pass used to be the largest rare-earth element mine in the world until China discovered large deposits which can produce up to 105,000 MT of rare earth minerals a year.

At this pass, we observed syenite (intrusive plutonic rock) with high amounts of orthoclase, along with minor amounts of hornblende and biotite.

Day 5
Red Rock Canyon State Park

The reddish rocks are Jurassic age sandstones that represent fluvial, lacustrine, and Aeolian environments, most likely having formed in a shallow, continental sea. The sandstones are overlain by gray Cambrian limestones. A closer look revealed that below the Jurassic sandstones lay Permian sandstones. These actually formed from large dunes, resulting in cross-cutting beds and dipping in a massive scale.

Aeolian Jurassic dunes. Photo Credit: Jackson Stiles

Late Cretaceous-early Tertiary back arc thrusting resulted in an inverted sedimentary sequence where older rocks are underlain by younger rocks during a retro-arc thrust event where the Farallon oceanic plate was being subducted beneath the North American plate.

 

ESCI 322 Field Trip: Mysterious Visitors from the Deep

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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):

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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):

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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.

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Professor Dasgupta discussing pillow basalts with the students

 

 

 

 

 

 

 

 

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

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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.

 

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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:

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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.

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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.

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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.

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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.

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

Owen’s Gorge:

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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.

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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:

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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.

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

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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:

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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!

Lunch

Lunch

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.