We got a comment the first week in July from a reader living on the flanks of El Estribo (The Stirrup) volcano at Patzucaro Lake in Mexico. The reader asked when Estribo last erupted and what the volcanoes visible on the other side of the lake were. The comment was appended to our 2018 Michoacan – Guanajuato Volcanic Field (MGVF) post.
As it turns out, El Estribo and Patzucaro Lake are in the north-central part of Michoacan State within the MGVF, so it is back to this volcanic field for another post. Thanks to the reader for the suggestion.
Patzucaro Lake and city are tourist attractions in the state of Michoacan, visited by over 100,000 people a year. Average precipitation is 94 cm/year with July being the wettest month. Total 2015 population of neighboring three towns of Tzintzuntzan, Erongaricuaro and Patzcuaro is around 123,000.
The economy of the region is generally agricultural with farming and livestock. Crafts are manufactured. There is some fishing left in the lake, though it is polluted by runoff from many of the neighboring towns and villages. Tourism is centered mostly in Patzcuaro and archeological sites in Ihuatzio and Tzintzuntzan. Volcanic eruptions, flank collapses, earthquakes and lahars in the basin represent a significant daily risk. The lake basin was likely formed by lava damming associated with the fracture system in the region.
Patzcuaro Lake contains the island of Janitzio, the second most famous cinder cone in Mexico, after Paricutin. This is the main site of the “day of the dead” celebrations that started with the Purepecha culture. The island receives thousands of pilgrims yearly. The best Day of the Dead celebration in Michoacan is said to be in Patzcuaro.
Patzcuaro Basin has a long history of human occupation, with first evidence of maize farming 1690 – 940 BC. Population before 350 AD was small villages. These grew 400 – 900 AD to include ceremonial centers. Increased population 900 – 1350 AD. The basin became the geopolitical core of the Tarascan Empire 1350 – 1520 AD. The royal dynasty was located in the capital of the empire, Tzintzuntzan. This place has multiple pyramids and a sacred religious center.
The Tarascan Empire operated until the Spanish Conquistadors showed up. Diseases brought from Europe decimated the indigenous population. The Spanish operated as conquerors rather than neighbors. After the Spanish conquest, Patzcuaro was founded as the capital of the State of Michoacan in 1524. The capital was transferred to Valladolid, known as City of Morelia today.
Region
Patzucaro Lake is located in the central part of the MGVF. There are at least 45 individual volcanic landforms in the region over the last 3.9 Ma. These include five medium sized shield volcanoes, 25 scoria cones, 10 lava domes, two lava flows associated with scoria cones, a fissure lava flow, and two volcanic complexes. These erupted a minimum volume of 20 km3 of mainly andesites over the period. In the midterm, these were basaltic to dacitic, and basaltic – andesitic to andesites during the Holocene. Cones are generally aligned along local faults NE-SW, N-S and E-W with the youngest being associated with the E-W Morelia-Acambay Fault system. Volcanic complexes El Estribo and La Muella have cone collapses to the N that correspond with regional extension associated with the E-W fault system.
The lake covers around 79 km2 at an elevation of 2,040 m. There are a total of 25 cinder cones, including the island of Janitzio in Patzcuaro Lake. This is the most common volcanic structure in the area. These cones are either isolated or on the flanks of medium-sized shield volcanoes. The diameter of these vary from 1.3 – 0.4 km and their heights vary from 228 – 24 m.
The five shields are the most voluminous landforms, with wide bases and mild slopes. They vary from 8.4 – 3.9 km in diameter and heights of 941 – 291 m. El Bosque NW of the lake is the largest. El Estribo is a medium sized shield S of the lake. It is disrupted by an E-W fault scar. In the eastern part of the area is Tariaqueri, with a NE-SW collapse scar. It is surrounded with debris fans. El Frijol to the S of El Estribo has an E-W elongated crater.
Elongated domes include Huracan, Carichuato and Colorado. Round domes include Tecuena, Vado, Chato, Chendanas and La Muela. Flat domes include Chapultepec and Buena Cista. These range 3.2 – 0.4 km in diameter and top out 240 – 57 m. There are two exposed landforms in the southern part of the area that are partly eroded and covered with paleosols up to 20 m thick. Lava flows cover around 30% of their area. These were erupted from either fissures or older volcanoes and are buried under younger deposits. The structures are responsible for around 17 km3 and underlying lavas around 3 km3.
El Estribo and La Muela volcanic complexes were built by multiple eruptions shifting through multiple eruption styles over time. They are not monogenetic volcanoes. El Estribo was originally a shield that was reactivated around 28 ka building a cinder cone on its summit. La Muela has a horseshoe shaped crater open to the NE containing 3 domes.
The MGVF has several volcanoes with a base shield topped by a cinder cone or dome. There is not sufficient study of these as yet to understand the relationship between magma supply and evolution of volcanic activity in these. El Estribo is a good example of this type of volcanism.
The oldest volcanic unit includes the two oldest structures in the Lake Patzucaro region are the Carichuato and Chapultepec domes. They date 3.9 and 2.6 Ma respectively. Both are constructed of massive blocky lava flows and block and ash flow deposits.
The following unit includes the oldest volcanoes of Chendanas Dome (2.2 Ma), Vado Dome (1.8 Ma) and Tariqueri medium shield (1.3 Ma). The Vado Dome and Tarigueri are built of lava flows. Tariqueri is cut by a fault that triggered a flank collapse NW toward the lake. It was built by at least three volcanic periods of activity. There are four islands in the southcentral part of the lake. These include the Pacanda lava flow (837 ka), Yunuen lava flow, Tecuena dome and Janitzio cinder cone, all three dating around 896 ka. Buena Vista lava dome (880 ka) and El Huracan lava domes (1.0 Ma) were emplaced along a NW-SE fault. Cerro Blanco, Domo Colorado, Tziranga and Tren cinder cones all date sometime in the period 2.2 – 0.8 Ma.
The next volcanic unit had medium sized shield volcanoes of Yahuarato (540 ka), El Bosque (394 ka) and El Estribo (126 ka). It also includes multiple cinder cones and lava flows (El Olvido cinder cone, Ihuatzio fissure lava flow, Axasjuata lavas, Ajuno cinder cone, Loma San Jose dome, Loma Charahuen medium shield, La Tacuana lava, Napizaro lava, LaSeccion lava, Chato Lava, Zarzamora cinder cone and lava flow) dating 502 – 36 ka.
The second most recent unit includes 12 lava flows and cinder cones dating 30 – 20 ka. La Muela volcanic complex dates 33 – 18 ka. The El Frijol medium shield is built of massive blocky lavas that date around 3.2 ka.
The youngest and most recent volcanic landforms are four cinder cones, La Taza (9.3 – 8.4 ka), Cerro Amarillo (7.0 ka), Los Lobos (6.0 ka) and the undated El Borrego.
Volcanism around the lake shifted in style and frequency over time. It started around 3.9 Ma and was dominated by effusive eruptions until 100 ka, forming medium sized shields, lava flows, domes and a few cinder cones. Since 100 ka, activity has been more intense forming cinder cones, lava domes, and medium shields including El Estribo and La Muella. Both of these volcanoes have phreatomagmatic episodes, the only ones in the area. Magmas are thought to have ascended as packets in multiple discrete paths which in turn led to varied compositions of lavas even in individual pulses. These magmas were likely sourced from different depths and erupted through discrete paths opened by regional extension.
El Estribo is located south of the southern part of Patzucaro Lake. The region is tectonically active, with multiple earthquakes. Sedimentation into the lake has been disrupted by tectonic and volcanic processes. Past earthquakes triggered landslides into the lake from El Metate, Isla de Janitzio (scoria cone) and El Estribo volcanoes. These likely created tsunamis of unknown size in the lake. Large historical earthquakes hit the region in 1845 (estimated at M8 Richter) and 1858.
Volcano
El Estribo Volcanic Complex is notable for three features. First it is a shield volcano built somewhere before 126 ka ago. After a 100 ka year hiatus, it suffered a pair of flank collapses to the north and was topped with a cinder cone. Timing of the oldest flank collapse and construction of the cinder cone is very tight, but the cinder cone is thought to have been built first.
There are three main units of the volcano – older than 30 ka, 30 – 10 ka, and less than 3 ka. The oldest of these are the basaltic – andesitic lava flows of the shield. These are covered with stratified scoria falls, reworked beds, and scoria falls interbedded with pyroclastic surge deposits covering a paleosol dated around 28 ka. The shield was built quickly, as there is no appreciable erosion or paleosols between andesitic lava flows on its flanks.
The volcano is cut by an E-W normal fault exposing the base and 200 m of lava flows. The new cinder cone was not cut by the fault. The debris avalanche exposed the internal structure of the shield. Quarrying exposed the internal structure of the cinder cone. The cinder cone topping El Estribo is 2,451 m above sea level.
The shape of the El Estribo volcanic complex has been cut by the E-W fault, producing a vertical offset of 200 m. The base shield has a slope around 10°; the cinder cone around 30°. Both volcanoes are separated by a paleosol layer widely exposed in the area. The shield has a base diameter of 3.6 km and a volume of 0.38 km3. The cinder cone has a base diameter of 1.2 km and a volume of 0.05 km3. Unlike neighboring cinder cones, the one topping El Estribo has a wider crater, with less steep walls. This is likely due to phreatomagmatic explosions.
One of the questions surrounding monogenetic volcanic systems is their ability to resume activity. This is not uncommon, with Cerro Negro in Nicaragua previously considered monogenetic reactivating multiple times during its life span. Successive reactivations may be due to changes in underlying tectonics and the volume of the underlying magma reservoir. El Estribo was formed by at least two eruptive magmatic episodes with two different magma batches, the first for the shield and the second for the cinder cone.
Monogenetic volcanic fields are usually located in regions with strongly deformed and fractured upper crusts. The neighboring Tzirate volcanic field 25 km N demonstrates a clear relationship between tectonics and magmatic activity. Magmas are tapped from different depths and reach the surface through multiple faults. The fault density around the El Estribo volcanic complex is obscured by multiple neighboring volcanoes and their eruptive products. The differing batches of magma that built El Estribo erupted through reactivated E-W faults that control the magma feed. The E-W fault scarp is the source of the two flank collapses. E-W dikes are also present.
After the shield building stopped some 120 ka, tectonic and volcanic activity continued through the region, building 8 volcanoes in the Patzucaro Lake area as new batches of magma reached the surface. One of these new batches of magma reactivated the old feeder system under El Estribo shield volcano, vented through the top in a series of Strombolian type events that built a cinder cone. This was a short, small volume event likely measured in a few weeks to months.
There was some erosion by wind and rain, though not long enough to cut gullies in the flanks of the new cinder cone. The next magma batch encountered water, triggered a phreatomagmatic eruption with a wet base surge. This enlarged the original diameter of the cinder cone. Water during this eruptive sequence was used up, with the eruption switching over to lava emission toward the end. Water must have come from rainfall or from the neighboring lake via faults.
The two debris avalanche deposits date as follows: The oldest one dated 28 ka, is just a bit more recent than the cinder cone deposits. The second one dates around 14 ka. The first debris avalanche ran out 3.2 km. The second some 2 km. Causes of these avalanches are still being studied, though may be related to destabilization of the shield during emplacement of the cinder cone, varying rock strengths between emplaced rocks and paleosols, seismic events, and general slope instability. Neither of these flank collapses were hot. They did not involve volcanic activity.
The first debris avalanche covers around 4 km2 with an average thickness of 10 m, with an estimated volume of 0.026 km3. It has a maximum runout of 3.2 km, well into the southern portion of the neighboring lake to the N. There are at least 14 hummocks in the debris field. The flow can be traced back to a U-shaped scarp that extends to the E-W shield escarpment. It took place after activity that built the cinder cone on top of the shield. Runout of this debris avalanche was helped by flow over wet ground, mud and water in the southern part of the lake. It likely created a tsunami of an unknown size in the lake. The first avalanche is dated between 28.4 – 28.1 ka or about the same time or shortly after the cinder cone was built. Displacement of lake sediments is dated around 28 ka, which correlates nicely with the dated paleosol covering the first avalanche deposit.
The second, more recent debris avalanche deposit covers 3.5 km2, with an average thickness of 25 m, and a maximum runout of 2.3 km. It flowed over the previous deposit, which limited its coverage. The second avalanche is dated somewhere between 28.4 – 14.4 ka, likely. These are the ages of the paleosol layers covering the first collapse and the second. It has several recent pyroclastic deposits from neighboring volcanic eruptions at La Taza (8.4 ka), Los Lobos, El Frijol and Cerro Amarillo (9.2 and 7.0 ka).
Today, at least 1,500 people live in villages on top of the debris avalanche deposits.
Tectonics
We have focused on the larger tectonic picture of the Trans Mexican Volcano Belt (TMVB) in multiple posts over the years. Our post on Michoacan – Guanajuato Volcanic Field, Popocatepetl and Nevado de Toluca would be good places to start with a review. This section will focus more on the regional tectonics driving volcanic activity near the lake.
The TMVB contains nearly 8,000 volcanoes stretching from the Gulf of California to the Gulf of Mexico. Volcanic activity in the TMVB migrated southward over time. Volcanism began in the MGVF around 5 Ma and continued into recent times. This field has some 1,040 volcanoes over 40,000 km2. Several cones are aligned along axes of older fault systems.
The MGVF is located in the central part of the TMVB and is constrained by three regional fault systems. The youngest of these at 9 – 7 Ma is an E-W normal fault system associated with the Morelia-Acambay Fault system that has been active since the Miocene. It is the most seismically active system and produced the M 6.9 Acambay earthquake in 1912. It is also associated with the Jarcuaro – Patzucaro graben. An older NW-SE to N-S normal fault system is the Taxco-San Miguel de Allende Fault system perpendicular to the TMVB axis, driven by Basin and Range Province extension. The oldest is a NE-SW left lateral strike slip fault system active as long as 15 Ma. It was reactivated during the early Pliocene.
Conclusions
The region around El Estribo and Patzucaro Lake is complex and volcanically active. It has ongoing extension, a magma supply active for the last 100 ka, and at least 45 volcanic structures, many of them as recent as 100 ka. As none of these volcanoes are actively monitored, which is expected in a predominately monogenetic volcanic field, announcement of future volcanic activity may be via a future earthquake swarm.
Additional information
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Granyia
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