In November of 2009, the Volcano World team and colleagues travelled to the high desert region of Chile, known as the Altiplano, to research and sample the volcanoes, domes, ignimbrites, and ancient lava flows that are abundant across the area.
After getting supplies in the port town of Antofagasta and the mining hub of Calama, our team climbed onto the Altiplano Plateau and worked our way along one of the most dramatic and beautiful volcanic arcs in the world.
The tour is split into four sections and a photo gallery.
Click the links to the right to start your trip!
Providing insight and unparalleled technical support was UCLA Adjunct Associate Professor, Dr. Axel K. Schmitt
Research Students on the Expedition Included:
For the past 200 Ma, subduction-related volcanism has defined the volcanic activity in the Central Andes. Around 26 Ma, shallowing of the subducting slab as a result of fast, nearly orthogonal convergence resulted in changes in convection and magma supply rates in the mantle wedge. These changes, accompanied by erosion of the lower part of the upper plate by stoping or delamination effectively increased the heat flux in the upper mantle and created a zone of weakness within the crust. Compressive failure of the weakened lithosphere later led to extensive crustal thickening from ~26-14 Ma (Isacks, 1988). Subsequently, the earliest magmatism in the Altiplana-Puna region occurred over a much broader area than the current Andean arc, consistent with a shallow angle of subduction. Steepening of the subducting slab to it’s present dip of 25 to 30° in the mid-Miocene (16-12 Ma) increased the volume of the mantle wedge and promoted melting of the hydrated lithosphere. The steepening of the dip was accompanied by delamination of the continental lithosphere (e.g. Kay et al. 1999; Allmendinger et al., 1997), evidence for lithospheric delamination includes high crustal elevations with lower crustal thicknesses suggesting the crust is supported by hotter more buoyant mantle that has risen into the lower crust and an apparent temporal relationship between the changes in subduction angle and an increased mantle power input that led to crustal melting and the APVC ignimbrite flare-up (Isacks, 1988; de Silva and Gosnold, 2007).
The APVC is a barren volcanic plateau located between 21° and 24° S with an average elevation of ~4000 m. From 10-1 Ma an ignimbrite flare-up deposited at least 10,000 km3 (one estimate said 30,000 km?) of material over an area of about 70,000 km2 in the APVC (de Silva and Gosnold, 2007). Volcanic activity continued from 10 Ma to present, but was punctuated by distinct pulses of activity at 8, 6, and 4 Ma. Prior to 6 Ma activity was widespread across the plateau, however, after 6 Ma eruptions became more focused in the central APVC. The pulse at 4 Ma was the climactic stage of the APVC, and activity since 4 Ma has been comparatively minor, the largest eruptions being those from the Purico and Laguna Colorado centers at ~ 1 Ma (de Silva et al., 2006b).
The ignimbrites from the APVC are all very similar in composition, and are typically of the large “monotonous intermediates” described by Hildreth (1981). They are high K dacites to rhyodacites with very volumetrically minor rhyolites. Andesites and basaltic-andesites are rare and occur as bands and inclusions in the pumices from some ignimbrites, and although volumetrically insignificant they may represent the thermal input into the upper-crustal Antiplano Puna Magma Body (APMB).
Prior to the ignimbrite flare-up, the APVC crust must have been thermally prepared. Delamination events discussed above led to a large increase in thermal input from the mantle. The increased thermal input along with with ongoing arc magmatism and crustal thickening thermally prepared APVC crust (elevated geotherms), resulting in extensive crustal melting. The impact of this increased thermal input was a period of intense generation, transport, and storage of thousands of km3 of magma in the mid crust. The elevated heat flux thermally softened the continental crust causing rapid elevation of the brittle-ductile transition zone. Increasing sizes of eruptions from 10-4 Ma indicate that progressively larger batches of magmas were being stored within the crust as the flare-up proceeded. Storage of increasingly large magma bodies may have been facilitated by changes in the mechanical properties of the wall rock surrounding the magma bodies due to the increased geotherms (i.e. wall rocks are thermally softened and more ductile). The elevated brittle-ductile transition zone coupled with regional tectonic stresses eventually resulted in major roof failures and the eruptions of the voluminous ignimbrites (e.g. de Silva et al., 2006a).
Travelling in the Altiplano required the team to get used to the elevation. Over the course of the trip, we climbed from sea level to over 5100 m (17,000 feet)! For reference, the peak of the highest mountain in the entire lower 48 states (Mount Whitney, California) is only 4,421 m (14,505 ft).
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We have split the trip into four sections - primarily to keep the navigation organized - and some pages will represent multiple waypoint stops at the same general location (aka La Poruna, San Pedro, etc.). This first section covers from the City of Antofagasta to the Divasaco Ignimbrite seen on the road to Chao.
The first thing you notice when the plane lands in Antofagasta is that there is no vegetation on the landscape that hasn't been planted there. This is because rain in Antofagasta is extremely rare, averaging less than 4mm a year with recorded rain free droughts of over forty years!! (Vargas, et.al., 2000) Instead, it is the brightly colored houses and shops that bring color to the landscape.
Antofagasta is a prominant port town and has a population of over a quarter of a million people - the fourth largest in Chile. Because of it's proximity to the mines, much of the economy in town is in support of the mining industry.
The City of Antofagasta is underlain by Jurassic era basaltic andesite lava's known as the La Negra Formation. (Ferraris and Di Biase, 1978) These lava's are overlain by sandstone that is often embedded with or covered by dense layers of shells (see photo below) and local pebbles formed from a Pleistocene marine terrace. (Ortlieb, 1995)
(Figure from Hartley, et al., Rev. geol. Chile v.28 n.1 Santiago jul. 2001)
Eighteen kilometers north of Antofagasta is the famous La Portada (which means The Portal or Gateway in Spanish). La Portada is a 140 foot tall archway of sandstone (resting on top of the La Negra) formed from continuous marine erosion of the shorline cliffs.
The sandstone cliff walls are layered with an ancient densely packed layer of shells.
Our goal in Antofagasta was to get supplies and rent reliable transportation. Hertz was in town and had two four-wheel drive trucks (a MUST) ready to go and proved to be an excellent choice because of their customer service. (We had a truck burn a clutch halfway through the trip and they drove a replacement out to meet us in less than a day!) Renting two cars ensures that you will be able to drive out should something go wrong... because on the Altiplano, you do not want to break down without another way out. Also, there are no gas stations past Calama on our journey. So, we packed in all the gasoline, water, food, cooking equipment, field gear, and camping gear into the two trucks.
Speaking of water... there is none. What naturally occuring fresh water there is has filtered through run-off from the mines and has been known to contain high levels of arsenic. (Pérez-Carrera et.al, 2010; Borgoño et.al, 1976) Today, the city gets much of its water from sea water desalinization. Either way, bottled water is a good idea in the city and an absolute must beyond this point on during the trip. With the change in elevation and the amount of running around we did each day, staying hydrated was difficult at best. You cannot have too much water! (and remember... Con Gas = with bubbles, Sin Gas = without)
Finally, while we were here, we had the opportunitty to meet with researchers from the Universidad Católica del Norte. These experts, who regularly work on the Altiplano, included our good friend - the one and only (and all around great guy) Dr (c) Benigno Godoy Neira.
The City of Calama is just south of the mega-copper mine Chuquicamata and primarily serves as a bedroom community for the workers and their families. This is the prime copper producing province in the world mainly due to the hyper arid environment. (Reich et al, 2009)
From Antofagasta to Calama runs the Antofagasta–Calama Lineament (ACL), a series of Northeast trending strike slip faults which is thought to have controlled the emplacement of the extensive porphyry copper deposits. (Palacios et al, 2007) The town has consequently had its share of earthquakes including a 7.8 in 2005. (The town was also completely destroyed in 1870 by an earthquake!)
The stratigraphic succession in the Calama Basin (from Moreno and Gibbons, 2007)
Renowned photographer Paula Allen did a photographic story on the Courageous Women of Calama.
With a population of almost 150,000, the community has the basic services you might expect in any major city. This was our last chance to stock up on gasoline, water, and food for some time!
Latorre C, Betancourt JL, Rylander KA, Quade J. 2002. Vegetation invasions into Absolute Desert: A 45,000-yr rodent midden record from the Calama-Salar de Atacama Basins, northern Chile (22–248 S). Geological Society of America Bulletin 114: 349–366
Moreno, T. and Gibbons, W. The Geology of Chile. Geological Society of London. ISBN 186239220X. p. 84.
Palacios, C. et.al. (2007) The role of the Antofagasta–Calama Lineament in ore deposit deformation in the Andes of northern Chile. Mineralium Deposita; Volume 42, Number 3: 301-308.
Reich, M., et al. 2009. Supergene enrichment of copper deposits since the onset of modern hyperaridity in the Atacama Desert, Chile. Mineralium Deposita; Volume 44, Number 5: 497-504.
THE UPPER RIO SAN PEDRO IGNIMBRITE
From Calama, we took the 21 North towards San Pedro which was now clearly visible to the North. We got off at the Village of Chui Chui and headed north along the Rio Loa river valley.
This stop is a tourist attraction as well as a geological stop that is located along the banks of the Loa River on the road from Calama toward San Pedro. There are petroglyphs carved into the Sifon Ignimbrite (8.1 Ma) also referred to as the Upper Rio San Pedro Ignimbrite.
This outcrop represents the most western part of the ignimbrite that continues north to San Pedro. In certain areas of the APVC the deposit is 800 m thick, while here is roughly 300 m thick, indicating that it is relatively proximal to the source or the valley fill. The Sifon is a massive crystal-rich deposit that lacks pumice clasts, and contains abundant biotite and lithics. The deposits underlying the Sifon are Pleistocene lake deposits.
There are also ruins on the opposite side of the petroglyphs that are most likely “Pukará de Lasanna” from Quechua Pukará, signifying “fortress”. Lasanna is a small village located 40 km northeast of the city of Calama and 8 km north of San Francisco de Chiu Chiu in the Antofagasta Region of northern Chile. The main cultural attraction of the village is a pre-Columbian fortress built in the 12th century that was declared a national monument in 1982. Peppercorn trees and desert sage characterize the landscape in the area and camping is possible in the campground adjacent to the ruins.
A nice outcrop of the Lower Rio San Pedro Ignimbrite is on the road between the San Pedro area and Turi/Chao.
The Lower Rio San Pedro Ignimbrite, 9.66 Ma is a dense-micro vesicular dacite with abundant lithics and juvenile pumice. The ignimbrite contains quartz, plagioclase, copper-colored biotite, titanite/sphene, but the pumice is crystal-poor (~ 25%) in relation to the other ignimbrites in the area. The Lower Rio San Pedro is a simple cooling unit of two distinct flow units with an average thickness of 20 m along the Rio San Pedro.
Just about 30 miles north of Chui Chui, lie the San Pedro and San Pablo Volcanoes on the right and the La Poruna cone directly ahead.
La Poruna is a scoria cone on the western flank of San Pedro Volcano. Several lava flows from La Poruna extend westerward for almost eight kilometers! These were mapped in O'Callaghan and Francis, 1986 as part of their work on the San Pedro/Pablo Volcanoes.
La Poruna is one of the primary targets we have been researching as part of a project exploring lava flow morphology in satellite imagery.
The San Pedro Volcano with the La Poruna Cone and lava flow in the foreground. Dark material in the closer foreground is from a debris avalance caused when the older edifice collapsed leaving the horseshoe shaped crater we see today.
The San Pedro Volcano is a composite volcano and one of the highest active volcanoes in the world. As we drove up, it was clearly puffing away from a fumerole at the top and continued to do so the entire time we were in view. (Perhaps a puff around every minute or so)
The San Pedro Volcano at Sunset, taken from our camp in the La Poruna lava's (seen in the foreground). Click the image for Hi-Rez.
The lava flows at San Pedro are significantly larger than the flows across the street at La Poruna. (Note the tractor trailer in the forground with the lava front just behind.)
The Carcoté Ignimbrite, 5.6 Ma is a high silica rhyolite that is visible from the road. The ignimbrite flowed from the Aucanquilcha area and overlies the Sifon Ignimbrite, 8.1 Ma. The Carcoté is a pumice-rich rhyolite, typically 5-6 m thick and has a white-gray color.
The pumice clasts show a sillar texture, indicative of vapor phase alteration. The post-depositional texture is created when the matrix recrystallizes and welds together as bubbles are stretched as they ascend up the conduit. This process results in silky pumice clasts. Lithic fragments are small (~ 2 cm) and rare and the matrix is fine-grained, crystal-poor (10-15%) and contains abundant plagioclase and biotite
Baker and Francis, 1978
THE SIFON IGNIMBRITE
The view of Rio Loa shows exposed ignimbrite and lake deposits. The lowest unit is Sifon Ignimbrite, a crystal-rich dacite deposited 8.1 Ma. In this northern area, the Sifon is a single, non-welded, homogenous deposit with a thickness of roughly 8-10 m (de Silva, 1989). Abundant plagioclase and biotite exist within the Sifon and lithics are rare. Small pumice clasts (~3 cm) have silky textures and contain plagioclase and biotite (de Silva, 1989).
The Sifon is overlain by Carcoté Ignimbrite, ~5.6 Ma along the Rio Loa. The uppermost units are younger lake deposits from Chiu Chiu. In some areas, the Lower Rio San Pedro Ignimbrite, 9.66 Ma outcrops below the Sifon. As the Andes were being uplifted the river was downcutting to form the valley that we see today.
After two days of running around lava flows, we decided to go for a drive. Our destination was a fall deposit between the Salar de Ascotan and the Salar de Carcote, but we also wanted to go to get a glimpse of the volcanoes Ollagüe and Aucanquilcha.
The Salar is home to several Boron and Lithium mines as well as the first wildlife we had seen since leaving Antofagasta! Both Flamingos and Vicuna call the Salar home drinking from the water that remains in the trapped lakes. These lakes also contain a endangered and very rare species of fish known as Orestias ascotanensis.
Astronauts on board the International Space Station took this picture of the Salar de Ascotan in October 2007. (Image Science and Analysis Laboratory, NASA-Johnson Space Center. "The Gateway to Astronaut Photography of Earth.")
21°25'11"S, 68°23'3"WElevation: 3694m (13096 ft)
About 60 km North of San Pedro, we arrived at our destination, the Salar de Carcote. The Carcote is a salt flat that covers over 100 square kilometers and all that remains of an ancient lake!
Looking out across Carcote towards Aucanquilcha
We came there to examine an amazing layered fall deposit from either Aucanquilcha or Ollagüe.
While there, we ran into the two most masochistic bikers on the planet!
Returning to the San Pedro/San Pablo area after visiting the salars, we came across the first, of many, major lava dome we would see on our trip. These enormous lava domes dot the APVC and represent a style of volcanism different than we had seen previously on our trip. The large eruptions that created the ignimbrites of the region, seem to have given way to the more gentle, effusive style of volcanism which created these domes. It is possible that the formation of many of these domes may represent the waning stages of the magmatic system that gives rise to the APVC.
Right off the main road, Chanka was formed on the northwest slopes of Azufre Volcano, and just north of San Pedro. Like most of the lava domes, Chanka is primarily dacitic in composition, and is extremely crystal rich (30%-40% crystals). The crystal rich nature of the lavas that formed Chanka, along with its dacitic composition, lead to incredibly viscous lavas. Due to this incredible viscosity, many of the domes take on a standard “torta” or cake shape, having steep sides and flat top.
While the exact date of Chanka is unknown, it is probably post glacial in age.
Azufre Satellite image with Chanka to the west (Cc)
Chanka is of dacitic composition and high crystal content, generally over 30% crystals. The phenocryts seen are typically Quartz, Plagioclase, biotite and hornblende. There are frequent, finer grained, mafic enclaves present that show phenocrysts of Plagioclase, hornblende and some Olivine. The host lava also has a friable texture, due to some microvesiculation of the matrix.
This location is noteworthy due to the exposure of the Divisoco Ignimbrite, 10.6 Ma that underlies the Sifon Ignimbrite, 8.1 Ma. The Divisoco is the lowermost unit in the area north of the Rio Salado, and it lies directly above the basement rocks. The deposit has been described as a massive, crystal-rich (~ 60%) dacite containing quartz, plagioclase, biotite and hornblende. The Divisoco has a fine-grained matrix, abundant lithics and a pink color.
When we arrived at the Village of Turi, it was the first time we had seen naturally occurring water since Antofagasta - and we were anxious to jump in and wash the dust off!
Looking west, we could still see the smelting plume from the Chuquicamata mine over sixty kilometers away.
Continuing our voyage through the APVC we traveled south from the San Pedro/San Pablo area, through the wonderful village of Turi, to the slopes of mighty Chao. Located between the composite cones of Leon and Paniri volcanoes, Chao is the second major lava dome we have come across, and the largest we will see in our time in the APVC. At over 14km long and 53km2 in total area Chao is the largest silicic lava flow known on earth - roughly 26km3 of erupted volume.
Dating of Chao has proven difficult, but due to deposits from recent glaciations it is certain that Chao is at least older than 11,000 years. Ar-Ar dating of Chao has put its age at 420,000 years +/- 100,000, however this is probably an overestimate. What is known is that the formation of Chao was not one discrete event but rather Chao was constructed in at least three stages.
Stage 1 was a mildly explosive stage, which produced coarse non-welded pumice and some block and ash flows later. Produced by this activity were also two overlapping cones on the northern flanks of Chao. Overall this activity resulted in roughly 1km3 in erupted material.
Built on top of stage I pyroclastics, stage II was the major phase in the construction of Chao. In contrast to stage I, stage II was primarily effusive and resulted in over 22.5km3 of erupted material. This enormous volume and the steep slope that Chao is formed on resulted in stage II stretching 14km with flow fronts of over 400m. As the lava flowed, it also formed impressive ogives, reaching heights of 30m, seen clearly in the satellite image.
Stage III was the final stage in the formation of Chao, built on top of the stage II flow its 6km long and 3km wide. The total volume is roughly 3km3.
All three stages of Chao are mineralogically similar, with minor variations in mafic enclave content. Phenocrysts of Plagioclase, Quartz, hornblende, biotite, and sphene are seen in a friable glassy matrix. The friable nature of the matrix is due to microvesiculation that took place in the lava.
A few things to note, getting to Chao can be difficult. To get to the base of Chao, as with everything on the APVC, 4WD is required. The road that runs to the south of Chao, between Chao and Leon, is washed out in a few places. However, great camping does exist along this road with some amazing views.
Beware of Vescatchas!... Ok, not really.
Driving along the Rio Salado Norte leads to another wonderfully exposed section of the Sifon Ignimbrite. Although this stop is along the main road, it is slightly inconvenient because just beyond the exposure cars are instructed to turn around due to land mines. The ignimbrite, however, is roughly 80 m thick in this part of the APVC.
The Sifon is crystal-rich, including large quartz crystals, plagioclase, biotite, hornblende, abundant pumice clasts and few lithics. The Sifon within the San Bartolo Group is a generally at is well to moderately indurated and has a fine-grained, crystal-rich (~65-70%) matrix.
The pumice rafts are not all connected throughout the outcrop, suggesting there may be a fault in the area resulting in differential flow movement between the blocks of flow.
The Village of Tocance sits along the south end of the Tocance river canyon.
The United Nations World Heritage entry for Tocance reads as follows:
It follows an agglutinated pattern of typical Andean architecture based on stone masonry and roofs of Cactaceae wood covered by vegetable fibers. It is worth mentioning that the space is divided according to some characteristic Andean criteria based on the principles of duality and tripartition, which are a manifestation of typically Andean cultural features. In fact, the villagers distinguish three sectors: the "Toconce town", the highest area near the church, "Katunmarca", to the center and West of town, and "the Chaco town" in the lowest access area. Toconce is part of, and associated with, important archeological sites, bearing witness to the presence and continuity of the Andean people's culture. Toconce is a highly traditional community with regard to its economy, architecture and craftsmanship, cultivating a close relation with neighbor communities which intensifies on the occasion of both religious and economic ritual festivities, such as the ceremonies of the "Cleaning of Irrigation Channels", and the "Cattle Adornment".
We were also able to buy a Coke for the first time here... there was great rejoicing!
For more information on the history and culture of the area see:
THE CASPANA IGNIMBRITE
This location reveals a great exposure of the Caspana Ignimbrite, the uppermost unit of the Toconce Formation that is unique from the other ignimbrites that comprise the APVC. It has a distinctive orange color that makes it extremely identifiable and explains its name as the “Orange Sillar”. The Caspana contains rhyolitic and andesitic pumice clasts as well as mixed clasts, making it compositionally heterogeneous.
The Caspana plinian deposit contains little pumice and only minor amounts of crystals, specifically quartz and biotite. The plinian and the ignimbrite have the same source, which is likely an ancestral volcano that is now covered by Leon or Tocanao. At this location, lake sediments are exposed underlying the ignimbrite.
The Caspana lies between the Linzor tuff, 5.77 Ma and the Puripicar ignimbrite, 4.18 Ma. The best age estimate of the Caspana is somewhere between these dates.
CASTLE ROCK AND THE TOCONCE FORMATION
Castle Rock is a formation comprised of layers of ignimbrite that have been eroded into the shape of a castle. Most ignimbrites are units of the Toconce Formation and the identifiable pink layer at the bottom is Sifon Ignimbrite, 8.1 Ma.
Just over the pass near Linsor dome and to the east of the Chao complex and Cerro Lean, we come across Chillahuita, yet another large silicic lava dome. Like Chanka and many of the domes yet to come, Chillahuita has the “torta” shape often exhibited by these large domes - flat tops and steep sides with some talus around the flanks.
Along with close proximity, Chillahuita has many similar characteristics to Chao, it is thought to be nearly contemporaneous in age and contains similar texture and mineralogy. It does, however, differ to Chao in that it was build on a much more gentle slope, and thus is smaller in size – 5km3 in volume.
Click for a high resolution image
Like most of the domes we have seen, and will see, Chillahuita is dacitic in composition and extremely rich in crystals (50%+). Mineralogically, the phenocrysts consist of plagioclase, quartz, hornblende, and sphene. The texture, like Chao, is friable due to microvesiculation of the matrix.
As an interesting note, many boulders that have fallen down the steep sides of Chillahuita show evidence of the primary wind direction through the products of wind erosion. Saltating sand grains hit these large boulders and overtime have formed many small holes that almost resemble a honeycomb texture.
Traveling south from Chillahuita, near the Tatio geothermal fields, we come across Copacoya, yet another large silicic dome of the APVC. Like its brethren, it takes on the typical “torta” shape with steep sides and a flat top.
Also like its brethren, Copacoya is dacitic in composition and crystal rich (40%+). Mineralogically it contains phenocrysts of plagioclase, quartz, hornblende and some devitrified glass in a glassy matrix. The matrix is much denser than seen at previous domes, due to the lack of microvesiculation within the lava. Mafic enclaves are also seen frequently – more andesitic, finer grained.
There are lines of evidence pointing to this dome being around the same age and close to the source of the Sifon Ignimbrite as the Sifon is greater than 300m think in this area.
To the southwest of the Tatio geothermal fields lies the Tocopuri volcanic complex. The name Tocopuri refers to the large composite cone that sits on the border of Chile and Bolivia. To the west of this cone lies La Torta, which was the focus of our stop here. True to its name, La Torta is in fact another “torta” shaped lava dome with steep sides, a flat top and small talus field surrounding the base. La Torta is moderately sized at 5km3 in volume. An age estimate of less than 1 million years has been reported to the Tocopuri system with La Torta being slightly younger.
La Torta is dacite to rhyolite in composition and like all the lava domes of the area is extremely crystal rich, generally over 60%. Mineralogically, La Torta contains phenocrysts of Quartz, Plagioclase, hornblende and biotite in a glassy matrix.
The third leg of our trip launched out of San Pedro de Atacama and climbed up into the land of calderas and ancient supervolcanoes.
From El Tatio, take the B-245 South all the way to San Pedro de Atacama.
While there we stayed at the Paso Tres Campground
Which has a swimming pool!
The town had a great marketplace with intersting merchants...
and good food!!
The town also features a significant archeological museum, the R. P. Gustavo Le Paige Archaeological Museum, with a large collection of relics and artifacts from the region as well as the the Church of San Pedro which is a National Monument.
THE TOCANAO QUARRY
There is a 30-40 m thick exposure of the Tocanao ignimbrite, 4.49 Ma the first phase of the Atana Formation at the Toconao quebrada. The quarry has become a national monument, and they now charge 1500/person and do not welcome geologists without permission. It is necessary to go to the Tocanao city official’s office with passports and verification of employment (business card) showing that you are a geologist in order to receive permission. All communication is done in Spanish.
The Tocanao ignimbrite within the quebrada contains a relatively crystal poor (< 10%) and lithic-poor matrix. Devitrified pumice clasts are randomly distributed throughout roughly 30% of the deposit and show distinctive elongated, tube-like vesicles and spherulites as evidence for devitrification. The pumice and the matrix contain copper-colored biotites and plagioclase as well as lithic enclaves from an older ignimbrite unit, possibly the Pujsa dated at 5.6 Ma.
The top of the flow has a cemented sillar texture that is quarried as construction material known locally as “Piedra Blanca”. This vapor-phase altered texture was created by fusion processes that thermally indurated pieces of ash together and by the escape of hot gases through the conduit that form the elongated vesicles.
At the Toconao quebrada the ignimbrite is in contact with the underlying plinian fall deposit. The fall is mantling the surface of the underlying Pujsa ignimbrite, 5.6 Ma and has also incorporated some lithics from the Pujsa. The flow deposit as well as the fall layer are related to the La Pacana caldera that is roughly 20-30 km from this location. It is possible that both the Tocanao member and the subsequent Atana ignimbrite were the result of the eruption from the La Pacana caldera.
You can drive down from the quarry itself and into the national monument, which is essentially a canyon comprised of ignimbrite. There is a nice trail that leads you along the edge the canyon along the deposit. In the distance you are able to see the Purico ignimbrite directly overlying the Toconao. The Atana/Talabre ignimbrites are not exposed here, which makes this the most distal part of the Purico deposit. Also, as you continue up the trail, you can see the Pujsa exposed beneath the Tocanao. If you take the trail down the canyon, it leads you to a river and beautifully lush vegetation.
S22 55.792; W68 04.438
Recharged from our stay in San Pedro de Atacama, we headed east toward the Purico complex. With Dr. Schmitt navigating we found our way to the Corrale Blancos outcrop. The Corrales Blancos outcrop is a large exposure of the Lower Purico ignimbrites. This may have been the most dangerous location of the trip due to potentially live land mines that have been transported down the wash. The lower Purico ignimbrite has been divided into two sections that will be discussed seperately in the following paragraphs.
Figure from Schmitt et al. (2001). Note that
only section A is exposed at Corrales Blancos.
LPI contains crystal-rich dacitic pumice, crystal-rich inclusions, and banded pumice. The crystal-rich dacite represents the juevunile material from the eruption, the crystal-rich inclusions have been speculated to represent side-wall crystal cumulates, and the banded pumice appear to be an incomplete mixture of the crystal-rich dacite and more mafic material (andesite), possibly recharging the system. Systematic size and distribution changes up section in this unit are problematic, however, mafic material seems to increase up section. Dacitic pumices from LPI and LPII are indistinguishable and contain plagioclase, hornblende, quartz, biotite, and possible orthoyroxene in a glassy vesicular groundmass. Crystal-rich inclusions are also similar in LPI and LPII and contain plagioclase, hornblende, quartz, and biotite in a crystalline/glassy groundmass. It has been proposed that the crystal-rich inclusions represent side-wall cumulates (de Silva et al., 1989c)
Image of previously retired land mine.
Image of LPI.
LPII displays greater compositional variability than LPI due to increased proportions of mafic material. LPII contains crystal-rich dacite pumice, a yellow pumice, crystal-rich inclusions, banded pumice, andesitic pumice, and rhyolite pumice. The base of LPII differs from LPI physically due to the presense of pyroclastic surge deposits and a crystal-poor rhyolite fall deposit.
Geochemical studies of the LPII ignimbrite show that the rhyolite fall deposit can not be explained as a fractionation product of the crystal-rich dacite, but instead appears genetically related to the andesitic pumice (Schmitt et al., 2001). The dacite pumice and crystal-rich inclusions are indistinguishable from those in LPI, see LPI for modes. The rhyolite pumice at the base of LPII is crystal-poor with the only significant crystals being plagioclase (some hornblende and biotite may also be present in trace amounts). Detailed geochemical analyses show that rhyolite is not a fractionation product of the dacite, but instead a liquid fractionated from the andesite (Schmitt et al., 2001; Grove and Nolin, 1986). The andesitic pumice in LPII contains plagioclase, clinopyroxene, and orthopyroxene in a glassy groundmass. Groundmass in the andesite pumice is ~50% less vesicular than the dacite pumice (Schmitt et al., 2001).
Image of LPII. Note the plinian fall deposit at the bottom of the section.
Image of different populations of pumice collected at Corrales Blancos.
Although the upper Purico ignimbrite (UPI) doesn't crop out here, it's an oppurtune time to discuss the differences between the lower and upper flow units. The UPI is white, well to moderately sorted ignimbrite containing both flattened white pumice and fiamme in a glass and crystal-rich groundmass. The UPI is easily distinguishable from the lower Purico ignimbrites by the fact that it's welded and contains fiamme.
23° 9'43.38"S, 67°57'22.38"W
On our drive to the Silapeti Group located to the northeast of Tocanao, Lazcar Volcano (#0353) is visible in the distance. Fumaroles were seen actively degassing on Lazcar in November of 2009. Licancabur, a stratovolcano is also visible from the road (#0354).
The southern section of the ignimbrites is most complete in the gorges or quebrada around Silapeti. Five ignimbrite units are exposed that comprise the Silapeti Group:
Purico (1.35 Ma)
Talabre (2.17 Ma)
Atana (4.09 Ma)
Tocanao (4.49 Ma)
Pujsa (5.87 Ma)
Once you reach Silapeti, there is a noticeable, clean trail that leads you down the canyon and provides a great opportunity to the see the complete section of ignimbrites. It is also a great idea to begin the hike in the morning in order to arrive at the base of the canyon by midday for some shade and a nice lunch break by the river.
The top layer of the quebrada is the uppermost member of the Silapeti Group, the Purico ignimbrite, 1.35 Ma. The Purico is comprised of Lower Purico 1, Lower Purico II and the Upper Purico (see following descriptions of Purico). The main volume of the ignimbrite is the typical non-welded, crystal-rich ignimbrite. The Purico contains large pumice clasts that are often concentrated in “rafts” or lenses. Minerals include course-grained hornblende, quartz, small copper-colored biotite and plagioclase. The matrix is white/pinkish, very ashy and has a devitrified glassy exterior. The Purico Ignimbrite was deposited during the late-stages of the APVC formation.
The Talabre ignimbrite, 2.17 Ma is a rhyolite, homogenous, well-indurated unit. The matrix is fine-grained, crystal-poor (10-15%) and contains plagioclase and copper-colored biotite. Tubular pumice and lithic clasts are common. The deposit is extremely thin (<5 m) and is only exposed as wedges within the canyon. The unit has been subjected to intense vapor phase alteration. When the flow was deposited, the material was very gas-rich and as the gases escaped, the crystals were altered. The gas rich nature of the ignimbrite gives it a sillar texture. The source area for the Talabre ignimbrite is a small caldera now occupied by the Salar de Aguas Calientes in the southern La Pacana caldera.
The Atana formation is directly beneath the Tocanao ignimbrite and consists of two units: the younger Atana ignimbrite and the older Tocanao ignimbrite. Both members were likely the result of the eruption from the La Pacana caldera. The Tocanao is thought to represent the volatile-rich cap of the magma chamber, which when erupted, actually caused the dome to collapse. When the caldera forms, Plinian eruptions are less likely, which explains why the Atana pyroclastic flow followed the Tocanao with no fall layer deposited between them.
The Atana ignimbrite, 4.09 Ma is the next layer of rocks that are visible on the walk into the quebrada. The unit is indurated due to vapor phase alteration. The ignimbrite is dacitic and contains abundant crystals including cristobalite or quartz, plagioclase, altered biotite and hornblende. The Atana lacks abundant pumice clasts and has relatively small lithics (~5 cm). The Atana was likely erupted from the bottom of the La Pacana magma chamber, which explains its crystal-rich nature.
The Tocanao, 4.49 Ma is very similar to the exposure in the Tocanao Quarry (Stop 22). It is a crystal-poor rhyolite that sounds flat when it is hammered into. Little silky pumices are distinctive, elongated clasts that have been formed by vapor-phase alteration. The pumices are easily removed by erosion, leaving holes in the deposit and giving the ignimbrite a spongy texture. They contain plagioclase and biotite crystals. Abundant tubed pumices are found at this location. The Tocanao was likely erupted from the top of the La Pacana magma chamber, which explains its crystal-poor nature.
The Pujsa ignimbrite, 5.87 Ma is the lowermost unit of the Silapeti Group.The deposit is a purple-gray, crystal-rich (55-60%) dacite that is over 40 m thick in this area. The base of the Pujsa has not yet been found. The unit is less indurated and less matrix supported than the other ignimbrites in the canyon. It contains large (~20 cm) crystal-rich pumices that are welded to the matrix and are often found in concentrated lenses near the tops of the flows. The matrix contains plagioclase, quartz, biotite and hornblende crystals. The Pujsa sits above the basement, which explains the abundant angular lithic clasts of basement lithologies. Lithics of ignimbrite material are also apparent, which may be from older ignimbrites that are not exposed. The Pujsa ignimbrite may represent an early eruption from the La Pacana caldera.
The 1.3 My Purico complex is a ~circular apron of ignimbrites and summit domes overlying older ignimbrites from the La Pacana caldera (4.5-4.1 My). The summit domes overly the ignimbrites and can be characterized as flat "torta" domes (Dome D) or conical Pelean domes (Cerros Purico, Negro, Putas, Chascon, Aspero, and El Cerrillo). The ages of the domes are not available. However, Chascon, Aspero, and El Cerrillo are all post-glacial and therefor less than 0.5 M.
Figures from Schmitt et al. (2001).
Dome D is compositionally similar to the Purico ignimbrite and probably represents the same magma degassed, whereas, post-glacial domes commonly contain mafic enclaves and show signs of magma mixing. These post-glacial domes are of particular interest because the magmatic components (i.e. mafic enclaves) within the domes may be the components responsible for the formation of the APVC. See following stops for more information on the individual domes.
Image is the Cerro Purico dome
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Leaving Cerro Purico we headed West to Cerro Aspero. Cerro Aspero is a post-glacial (<0.5 My) dome erupted after the Purico ignimbrites (1.3-1.0 My). Hawkesworth et al. (1982) found that mafic enclaves at Cerro Aspero were basaltic-andesite in composition, and therefore represent the most primitive compositions found in the Purico Complex.
Image of the Cerro Aspero dome.
The host rock at Cerro Aspero, similar to the other domes of the Purico Complex is a crystal-rich dacite. Multiple mafic enclaves were collected at this location. The enclaves are fine grained, dark gray, and porphyritic. Crystals include quartz, plagioclase, and hornblende. Quartz crystals are relatively large (~10 mm) and have green reaction rims of clinopyroxene. The quartz crystals along with some of the plagioclase may not be primary. The crystals are sitting in a groundmass of glass, plagioclase, and hornblende.
Mafic enclave in Cerro Aspero dacite.
Rocks from Aspero also have large megacrysts of plagioclase that may represent the granitic basement beneath the Purico Complex.
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The Chascon dome is another post-glacial dome (<0.5 My) erupted after the Purico ignimbrites (1.3-1.0 My). The bulk of Chascon is crystal-rich dacite. However, mafic enclaves are very prevalent (up to 20%).
The Chascon dome is dominated by porphyritc crystal-rich (30-35% xls) dacite containing plagioclase (<1-15 mm), quartz ( <1-4 mm), hornblende (2-15 mm), and biotite (<1-5 mm) in a glassy groundmass. Rocks from Chascon also contain plagioclase megacrysts (>25 mm), andesitic enclaves, crystal-rich enclaves, and rhyolite. Plagioclase megacrysts may represent basement rocks which have interacted with the APMB. Samples CH09013-DHB (mafic enclave) and CH014-DHB (host) collected at this location.
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Leaving the post-glacial domes behind we then headed back east to Dome D. Dome D is a pre-glacial dome that yields similar ages and compositions to the Purico ignimbrites, and is thought to be the same magma that formed the Purico ignimbrites just degassed. Dome D lavas are crystal-rich containg plagioclase, hornblende, quartz, and biotite in a glassy groundmass, and are ~60% less vesicular than the purico ignimbrite (Schmitt et al., 2001). Mafic enclaves were found in Dome D lavas, which would be expected if Dome D is genetically related to the Purico ignimbrite.
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The La Pacana caldera is one of the largest calderas on earth and has been the source of two major ignimbrites in the past 5.5 My. The first activity at La Pacana was the eruption of the ~5 My Toconao ignimbrite, followed the eruption of the caldera-forming Atana ignimbrite (Lindsay et al., 2001). Following the emplacement of the ignimbrites, continuing volcanic activity created a large resurgent dome. Activity continued until ~1.6 My, producing a series of silicic domes on the margin of the resurgent block, and within the caldera moat. The silicic domes are crystal-rich dacites, and are similar to the Atana ignimbrite in composition. The silicic domes likely represent degassed Atana magma (Lindsay et al., 2001).
Figures from Lindsay et al. (2001).
The Toconao a crystal-poor rhyolitic ignimbrite with an estimated volume of ~180 Km3. The ignimbrite can be sub-divided into two facies, a lower non-welded and non-indutrated facies containing abundant tube pumice, and an upper phase that is clearly indurated. See stop 22 for more details.
The Atana ignimbrite crystal-rich dacite that occur as both outflow and intracaldera fill and has a total volume of ~2500 Km3. The Atana ignimbrite is typically welded and show various degrees of devitrification. The Atana outflow is typically ~30-40 m thicks and sits atop pyroclastic surge and soft ash deposits. See stops 31 and 33 for more details.
Taking a break from the Purico complex, we travel into the La Pacana Caldera. Build on top of the resurgent dome of the caldera, Cerro Bola is the last large silicic dome we will see on our tour. Like most of the previous lava domes we have seen in our time on the APVC, Cerro Bola takes on the class “torta” shape. Around the flanks of the dome the fallen blocks seem to take quite a beating from the intense winds that dominate the area. Incredible yardangs and hoodoos can be seen all over the area, many taking on amazing and complex shapes, be sure to check it out.
Cerro Bola seems to have undergone a few different stages in its formation. The first stage was a series of pyroclastic flows that can now be seen at the base of the dome and in some places around its flanks. These flows are fine grained, ash rich and matrix supported. Pinkish in color, abundant lithics are commonly found with phenocrysts on quartz, plagioclase and hornblende. Obsidian can be found weathering out of this deposit, some pieces quite remarkable.
The second stage is the effusive dome building stage, where lavas of dacitic composition we erupted onto the floor of the La Pacana caldera. Like all the other lava domes of the area, Bola is extremely crystal rich (generally over 60% crystals). Mineralogically, the lava displays phenocrysts of plagioclase, quartz, and hornblende is a glassy, microvesicular matrix, which also leads to a somewhat friable texture.
|Casey on top of Bola Dome||Bob looking down from Bola Dome|
After all that climbing... we were a bit tired!
As you continue to drive east on the international road toward Argentina, the Atana ignimbrite changes its dip direction. Its dominant dip direction was west and it suddenly it dips to the east, suggesting that the road crosses over the La Pacana caldera.
There is an obvious exposure along the international road that is a great introduction to the deposits that dominate the northeast region of Chile. When looking to the south, the lowermost unit is the Atana ignimbrite, 4.09 Ma. Above the Atana is a fall deposit that is overlain by the Tara ignimbrite, 3.68 Ma. The Filo Delgado ignimbrite is the uppermost unit exposed.
The Filo Delgado ignimbrite is younger than Purico (1.35 Ma) and is the youngest unit of the La Pacana caldera. It is exposed on the east side of the La Pacana caldera and is stratigraphically related to the Upper Purico that is exposed on the west side of the caldera.
The Filo Delgado is a thin crystal-poor unit that contains welded pumices and distinctive crystal-poor, obsidian fiamme. The ignimbrite is characterized as lenticulite, because the matrix is crystal rich but the obsidian-like lenses have minimal crystal content. Also the entire ignimbrite is welded and has a blocky texture.
Just past the hairpin turn where we first encountered the Filo del Gado, we turned off of the 27 to the dirt road heading East/Northeast into the Valley of the Spires.
Here, the winds blow hard and consistently in the same direction. Millions of years of erosion have worn away the Tara (?) Ignimbrite that once filled this valley leaving only these giant columns.
From there we we headed north on the road to Salar de Tara.
On the way, we stopped at the famous WMF outcrop.
The Salar de Tara is a beautiful place to camp with spectacular views of flamingos on the lake and the Tara ignimbrite in the distance. While you will be tempted to set up tents under shelter in the little cabins by the lake, beware because they are infested with rats! Camping outside may be the better option.
The Tara ignimbrite consists of two major units: lower Tara and Tara. The lower Tara is known as the Guacha because its source area is the Guacha caldera. It was originally mapped as the Atana ignimbrite but was re-labeled as a Guacha-sourced ignimbrite that ponded in the La Pacana caldera. The Tara lies above the Guacha and is comprised of 4 sub-units labeled 2-5, 5 being the youngest. The Tara is one cooling unit with huge breaks in time.
The Guacha, 5.6 Ma is a crystal-rich, lithic-rich andesite that contains large pumice clasts and bimodal quartz phenocrysts. The Tara, 3.68 Ma contains less pumice and lithics and has a more ashy matrix. Between the Guacha and the Tara is a Plinian deposit that is very crystal-rich and clast supported.
We had some visitors in our camp the next morning!!!
Did we mention the flamingos...?!
THE FILO DELGADO IGNIMBRITE & THE TARA IGNIMBRITE
This location shows a wonderful section of the Filo Delgado overlying the Tara ignimbrite. The Filo Delgado Ignimbrite is the youngest unit and contains those distinguishable fiamme of obsidian. The middle unit is the Filo Delgado fall deposit that contains banded rhyolites, obsidian clasts and is dominated by lithics. The oldest deposit, underlying the Filo Delgado is the Tara ignimbrite that has an ashy matrix, lithics, and no obsidian and/or rhyolite.
It was questionable whether the fall deposit should be associated with the overlying Filo Deglado or with the underlying Tara ignimbrite. Because the plinian contains obsidian and rhyolite, it has been suggested to be part of the Filo Delgado deposit.
GUACHA/LOWER TARA IGNIMBRITE/BIG YELLOW
This is a great location for seeing the Guacha of the Lower Tara ignimbrite, also known as “Big Yellow” because of its yellow color. The Lower Tara fills in the depressions in the area and outcrops horizontally as a flat plane. In this location units 4 & 5 of the Tara lie directly above the Guacha.
TARA PLINIAN DEPOSIT
After driving around the area searching for the Plinian deposit of the Tara, we finally found a great exposure of the airfall at this location! The Tara Plinian fall deposit contains abundant pumices with quartz, plagioclase, biotite and magnetite crystals. While the crystals may not appear immediately apparent, with a closer look they are actually found in clusters and are quite coarse. The pumice shows variations in vesicularity, some appear silky and others more gas rich.
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A co-ignimbrite ash is a cloud of fine ash particles that overrides a pyroclastic density current.The process responsible for the formation of these clouds is call elutriation. The co-ignimbrite deposit overlies the Tara ignimbrite and is capped with dilute flow, surge, and possibly flow deposits. This is the only co-ignimbrite ash deposit ever identified in the central Andes.
Image of the co-ignimbrite ash deposit with Bob for scale.
Close up image of the co-ignimbrite ash deposit with hot sauce for scale.
See Sparks and Walker (1977) for more details
One of the final stops on our Chilean voyage was Jarellon. Located on the Chile/Boliva border near the La Pacana Caldera, Jarellon is the surface expression of a large ring dike, estimated at less than 5.6 million years old. Notable about the ring dike is the extremely abundant obsidian clasts that are weathering out. Red, black, and snowflake obsidian can be found, much to our delight.
Note: while breaking open obsidian can be fun, we would recommend wearing gloves and glasses as the shards can be very very sharp.
Panorama from the Lascar Southern pyroclastic flow
Lascar is the most active volcano in the central Andes. It is an andesitic to dacitic stratovolcano with six overlapping craters, trending roughly northeast, with the active, fuming crater located near the center. The largest historical eruption of Lascar took place in 1993, producing pyroclastic flows to 8.5 km NW of the summit. Grey pumice deposits from the 1993 eruption are visible in the photo above and below.
Light, dark, and banded pumice are mixed in this flow.
And we made a few friends!
This expedition could not have happened without the generous support of the National Science Foundation and the Tower Geosciences Graduate Student Research Grant.
We would also like to thank:
Everyone at the Universidad Católica del Norte for all of their help and support. Thanks for letting us borrow a corner in the bodega!!
The staff at hotel San Marcos in Antofagasta.
The wait staff at the Bundes Shop for putting up with us!