Nevado de Toluca (Xinantecatl) volcano is 23 km SW of Toluca and 80 km W of Mexico City.
It is an andesitic – dacitic stratovolcano built on a basement crisscrossed by a complex set of three fault systems. The crater is E – W elongated 1 – 1.5 km with a horseshoe opening to the east. The central crater is built on the remains of two older amphitheater-shaped craters still visible on the SE and NE flanks of the volcanoes. There are remains of glacial erosion on the volcano.
There are two lakes in the crater, Lago de la Luna and Lago del Sol separated by two dacitic dome intrusions. Their areas are around 0.02 km2 apiece. The volcano tops out at 4,680 m above sea level, 2,210 m above the surrounding plains
Neighboring Toluca, Mexico’s fifth largest city has 2.1 million in its greater metropolitan are. Mexico City to the ENE has over 21 million in its greater metropolitan area. Climate of the region is classified as subtropical highland. The high altitude contributes to relatively cool temperatures. They rarely get below -3 C in the winter or above 27 C in the summer.
Toluca is in the economic center of Mexico. Agriculture, livestock and some tourism predominate outside the immediate metropolitan area. The city was industrialized starting in the 1950s and exerts strong economic influence in the surrounding valley.
The volcano is surrounded by Nevado de Toluca National Park, created in 1936. It was initially created for conservation purposes but has come under increasing resource development pressure by the growth of neighboring Toluca, illegal logging by neighboring communities, and mining. Vehicular traffic has also done damage to the volcano, particularly around the crater. The crater and crater lakes are popular with visitors, particularly because the crater it accessible by dirt road. There are a number of archeological sites in the park. There are hiking, mountain biking, horseback riding and limited skiing in the park. It was redesignated in 2013 as a protected area to legitimize unregulated mining activity.
Activity at Nevado de Toluca volcano began more than 2 Ma. One paper describes its start as far back as 2.6 Ma. It sits near the intersection three major fault systems one of which underlies the edifice. These are the Taxco – Queretaro, San Antonio and Tenango fault systems.
The volcano is the highest, most prominent volcanic structure in the area. It is composed of domes, short viscous lava flows, and voluminous pyroclastic, fall and epiclastic deposits (sediments generated by reworking pre-existing deposits by weathering and transported to their current location by gravity, air, water or ice). It has been glacier covered during the ice ages. While there is snow during the winter, there is no permanent ice cap at present.
The oldest volcanic deposits are andesitic with dacite lavas. Activity was originally thought to have started 1.6 – 1.5 Ma. Recently collected samples on the southern flank are dated back to 2.6 Ma. The growth of the volcano ended 1.2 Ma and was followed by long period of quiet. This period was dominated by significant erosion and flank collapses, mainly to the south. The earliest activity produced mainly andesitic domes and lava flows, generally in the SW flanks of the current volcano. Middle activity produced a widespread block and ash deposit. It is overlain by thick dacitic lava flows emplaced around 1.31 Ma. The lava flows were followed by large andesitic domes mainly in the southern sector of the current volcano.
The paleo-Nevado was destroyed at least twice by flank collapse, producing debris avalanche deposits mainly to the south. The preferred southern direction of collapse seems to be related to the general tectonic stress patterns in the TMVB. The first of these has a maximum runout distance of 45 km. It is undated and underlies the block and ash flow deposit dated 37,000 years ago. The second of these covers around 500 km2 with an estimated volume of 5 km3. It is also older than 37,000 years.
The quiet period ended 100,000 years ago with the growth of the current Nevado de Toluca cone. This cone was built by emplacement of numerous dacitic domes and a complex sequence of pyroclastic and epiclastic deposits. The sequence includes of pyroclastic flows, surge and fall deposits, with debris avalanches and debris flow units. This activity began with pyroclastic deposits related to emplacement of dacitic domes at the summit. Explosive activity began some 42,000 years ago producing thick sequences of pumice fall, pumice flow, block and ash flows, and debris avalanche deposits. At least five major Plinian eruptions and at least five dome collapses with major block and ash flows at 37,000 and 28,000 years ago deposited a thick sequence of pyroclastic deposits around the volcano.
The largest of these eruptions was a VEI 6 Plinian eruption 10,500 years ago that put pumice and ash as far away as the Mexico City basin 80 km to the ENE.
The western sector of the neighboring Chichinautzin Volcanic Field is the youngest volcanic activity in the Toluca area. It is a series of basaltic – andesitic scoria cones and fissure-fed lava flows. Age of this field is 38,000 – 1,500 years.
In the past 50,000 years, Toluca underwent at least three smallish sector collapses on the E, SE and W flanks. These were caused by faulting and destabilization of young dacite domes on the summit. The collapses strongly correlate to the presence and activity of the E – W striking active Tenango fault system. This means that future sector collapse directions are expected to be generally E and W from the summit. As such, the volcano presents a future threat to its 25 million neighbors.
In the last 42,000 years, at least 14 individual dacitic domes were emplaced in the vicinity of the crater. The youngest dome dates around 9,100 years ago. Some of the domes are somewhat farther from the crater. The initial stage of dome emplacement put domes on the southern and northern crater rims. The second stage emplaced at least two major domes along the western wall of the crater rim. The final stage emplaced at least two domes in the central portion of the crater. Total volume of the dome complex is in the vicinity of 5 – 6 km3. These domes are directly to sector and flank collapses and debris avalanche deposits younger than 50,000 years. There are three debris avalanche deposits funneled debris 5 – 9 km away from the source.
The oldest of these is the Arroyo Grande unit, a 0.35 km3 deposit of hummocky debris with some volcanoclastic products on top of the deposit. It dates around 41,00 years ago and is covered by a block and ash deposit around 13,000 years ago. It was generated by an ESE sector collapse of the El Fraile dome in the crater area.
The El Zaguan is mainly on the eastern flank with an estimated volume of 0.4 km3. It has at least three discrete flow units. The unit overlies the 37,000-year-old block and ash pyroclastic flow deposit and is covered by the 28,000-year pyroclastic block and ash deposit and the 21,700-year-old Lower Toluca Pumice Plinian ash fall deposit. Its source appears to be an eastward sector collapse of the Cienaga summit dome immediately after emplacement of the 28,000 block and ash pyroclastic flow deposit.
The final unit is the 0.3 – 0.4 km3 Nopal debris avalanche deposit. It is massive, poorly sorted and well consolidated. It took place after the 28 or 37,000 block and ash pyroclastic deposit. It is covered by the 10,500 Upper Toluca Pumice. Its source is thought to be a WNW sector collapse of the Nieve summit dome.
Deformation along the Tenango fault system is thought to have driven volcanic activity on Toluca.
There were at least two sector collapses that took place before 50,000 years ago that emplaced deposits on the southern flanks of the volcano. Explosive activity deposited pumice flows and blocky and ash flows around 42,000, 36,000 and 28,000 years ago. A third sector collapse destroyed the eastern wall of the crater creating the current horseshoe shape and deposited a debris avalanche on the east and NE flanks on the volcano. This deposit underlies the Lower Toluca Pumice, a product of the Plinian eruptions 24,500 years ago. There were at least four other explosive events <14,000, 12,100, 10,500 and 3,300 years ago. The 10,500-year event is represented as the Upper Toluca Pumice.
Two Plinian eruptions that produced the lower Toluca Pumice 24,000 and the Upper Toluca Pumice, 10,500 years ago. A third known Plinian eruption took place 36,000 years ago. There is speculation in at least one paper that the recurrence period between Plinian events is 14 – 11,000 years.
The Lower Toluca Pumice was erupted around 21,700 years ago. This sequence was originally dated 24,500 years ago and then the dates adjusted over time. The sequence produced a 24 km high Plinian column that lasted 11 – 13 hours. It deposited 2.3 km3 of tephras to the NE. Subsequent eruption pulses were sub-Plinian, phreatomagmatic, and emplaced pyroclastic surge deposits. The column collapsed to the NE and emplaced a pyroclastic flow deposit. The eruption took place after a quiet period of 4,800 years and appears to have been triggered by an injection of andesitic magma into a dacitic magma chamber. As such, this means that Plinian activity from Toluca can resume after an extended period of quiet (currently 3,300 years and counting) and present a hazard for the entire neighboring Toluca Basin, home to over 1,000,000 people today.
The 5 cm isopach for this eruption covers around 520 km2. There were at least four other pulses following the collapse of the initial Plinian plume. These all produced plumes less than 20 km and were likely accompanied by phreatomagmatic components. Eruptive activity at Toluca changed from effusive to explosive around 42,000 years ago. This is about the same time as the neighboring Chichinautzin volcanic field started erupting juvenile magmas creating scoria and fissure lavas. This suggests a mixing process between two different magma sources.
The other oddity from this eruption is the presence of fragments of basement rock in the deposit, not found in other pyroclastic deposits. These are thought to be caused by explosions in the conduit and erupting rock wall fragments.
The Plinian eruption sequence that created the Upper Toluca Pumice has been variously dated 12,800 – 12,100 years ago. Initial dating was 11,600 years. Final dating of charcoal gives the 10,500-year date. For a long time, this was thought to be the last major eruption from Toluca and for a while, the volcano classified as extinct. That changed with the discovery and dating of eruption debris 3,300 years old. This was the most violent and voluminous known eruption from Toluca. It blanketed the area now occupied by Mexico City 80 km ENE with over 40 cm of pumice.
This eruption produced a series of tephra deposits on the ESE flanks of the volcano. It ended with a column collapse that produced two massive pumice rich pyroclastic flows that traveled along the Arroyo Grande up to 14 km from the summit (14 k white pumice unit). The flows burned the vegetation along its path. The presence of the vegetation suggests that the volcano was covered with vegetation up to at least 3,200 m or higher. By the time the Upper Toluca Pumice eruption took place, climatic conditions (Younger Dryas) were sufficiently cold and dry to keep aboral vegetation to grow above 3,000 m.
The Upper Toluca Pumice has two fall members separated by a thin brown ash layer and capped by associated deposits named the “pink valley-fill lahars.” This sequence covered over 2,000 km2 and is estimated to consist of over 14 km3 of tephras.
This sequence began with a phreatomagmatic phase that emplaced a hot pyroclastic flow on the northern and eastern flanks of the volcano. The eruption decompressed the volcanic system, allowing a 25 km (perhaps as high as 39 km) Plinian column dispersed by winds generally ENE. This column was followed by three other Plinian columns dispersed to the NE and E reaching heights up to 39 km and surges mainly on the E and N flanks of the volcano. The third and most voluminous phase of the eruption began with a 42 km column dispersed by winds to the E. This eruption ended with extrusion of the El Ombligo dacitie dome in the center of the crater. Deposits from this eruption were removed by lahars with a wide age span after the eruption sequence completed.
The most recent eruption from Toluca took place around 3,300 years ago. It was a minor event. Since that time, the volcano has been mostly quiet with some fumarolic activity reported in the nineteenth century.
The Trans-Mexican Volcanic Belt (TMVB) is an andesitic volcanic arc about 1,200 km long lying roughly W – E from the Gulf of California across central Mexico to Veracruz. The major volcanoes of the TMVB are generally subduction volcanoes related to the subduction of the Cocos and Rivera Plates beneath the North American Plate along the Middle American Trench. Initiation of volcanic activity along the TMVB is still controversial, with dates ranging between 10 – 4.5 Ma in the western edge. Volcanism in the central portion began some 30 Ma and 20 Ma in the eastern portion.
The region is dominated by huge andesitic stratovolcanoes that generally form N – S chains becoming younger to the south, toward the trench. Trenchward migration of the andesitic volcanoes may be related to differing subduction rates between the Cocos and Rivera Plates.
The highest features near Toluca are the San Antonio volcano (3,680 m), Nevado de Toluca (4,680) and the Sierra Las Cruces Volcanic Range. The Miguel Graben is located between the two main volcanoes, and two more volcanoes south of Toluca. The two volcanoes are located 20 km SW and S of the city of Toluca.
The Toluca area is located near the boundary between the central and eastern TMVB on the Guerrero Block, confined to the north by the Chapala – Tula fault system and to the south by the Ouxaca – Chapala fault system. In this, Nevado de Toluca overlies a series of NNW – SSE trending normal faults and a series of E – W trending normal faults. The pattern of the Taxco – Queretaro fault system is similar to the Basin and Range province in the western US and Northern Mexico with a NNW – SSE alignment of horsts and grabens mainly to the south of Nevado de Toluca. It has been active since the Miocene. The San Antonio fault system is part of the Tenochtitlan Shear Zone. It is located in a wide graben generally to the W of Toluca. It strikes generally NE – SW and has been active in the Pilocene. The Tenango fault system is generally oriented E – W and underlies Toluca itself stretching to the E. It appears to have had two motion episodes. The first a generally oblique slip followed by a more recent extensional phase.
Nevado de Toluca is a massive andesitic – dacitic volcano that transitioned from effusive eruptions to violent, explosive eruptions some 43,000 years ago. While its eruptions are separated by thousands of years, when they do occur, they are powerful. Recent activity has been dome extrusion, collapse, and associated phreatomagmatic, Plinian and sub-Plinian eruptions. Given the relatively close proximity of millions of residents, this volcano should be viewed as a quite dangerous system.