Excursion protocol by: Rafaël Mercier
The Teide-Pico Viejo complex is the product of the most recent phase of central volcanism on Tenerife. Constructed inside the Las Cañadas caldera (Figure 1), this complex (Figure 2) lies above the youngest pre-Las Cañadas Caldera rocks of the Diego Hernandez formation, and partially fills the caldera, extending north and westwards where the caldera walls are absent.
Pico Viejo and Teide stratovolcanoes overlap to form an elongate double edifice with a shallow saddle at about 3000 m altitude.
Figure 1: Structural map of the Las Cañadas Caldera, showing the three Las Cañadas depressions. Partly redraw after Araña (1971) using data of Martí et al. (1994).
Figure 2: Geological map of the Teide-Pico Viejo complex.
This day begins with the ascent of the Teide-Pico Viejo edifice by cable-car from the car-park. The ascent is about 1700 m high on the southern flank of the Teide and leads to the near-summit of the edifice.
Teide summit:
Teide rises from the Las Cañadas caldera floor at about 2000 m. It is composed of 3 distinct cone-crater systems (Figure 3):
The oldest cone (C1) is approximately circular, with a basal diameter of 5-5.5 km. It gives to the edifice its external shape and comprises overlapping, sheet-like flows with intact levees and tops of phonolitic tephrite and tephritic phonolite. It is interpreted as a collapse crater, formed in response to lateral withdrawal of phonolitic magma to satellite vents, located beneath, at the break in slope, and also visible during the ascent. C1 is now mostly filled with younger lavas. Its top is truncated by a northward dipping surface, upon which successive younger summit cones associated with nested craters, are built.
The inner crater (C2) is about 80 m above C1 and much smaller. A major proportion of the walls are now buried by the younger lavas. The products of this cone are crystal-rich phonolites and a small volume of poorly vesiculated pumices.
The present summit (3718m) is formed by a third cone (C3) named El Piton, constructed on the western margins of C2, suggesting that C2 collapsed along steeply deeping fractures which acted as conduits. The products of C3 are voluminous vitric phonolites, containing alkali-feldspar, Na-augite, magnetite and apatite. This crater also hosts a weak fumarolic activity (not seen).
Figure 3: Simplified geological map of the Teide summit.
Mirador del Pico Viejo:
From this point, the summit of Pico Viejo can be seen approximately 2.5 km to the WSW. The walking path is marked out through dark lavas. These are young blocky-aa phonolites erupted from C3. Some fumarolic activity is present, giving small sulphur deposits. To the west, a wide area of young basaltic to basanitic vents are visible. They are located on regional NE-SW fractures which inflect close to Pico Viejo to become radial to the summit. The western margin of Las Cañadas is not visible due to coverage by Pico Viejo products.
Saddle between Pico Viejo and Teide:
The saddle between these two volcanoes, at the NE of Pico Viejo is mantled by a phonolitic pumice fallout deposit, which is interpreted on the basis of its physical and compositional similarity as the stratigraphic equivalent of the 2020 y B.P. subplinian eruption of Montaña Blanca: it is a non welded, well-sorted bed of angular pumice lapilli, with obsidian and massive lava lithics. The deposit is poorly exposed and generally only its top is seen. However, some studies show that the thickness of the fallout is generally lower than 1.5 m, indicating that the deposit is not larger than strombolian. This deposit is overlain by the Teide's late black phonolite and postdates all activity on Pico Viejo except for the 1798 eruption of phonolitic tephrite on the SW flank.
Pico Viejo caldera:
Ascent of the eastern flank of the Pico Viejo summit yields a view of the summit caldera some 800-1000 m in diameter (Figure 4). The walls are steep, up to 150 m high. The north, west, and east walls of the caldera cuts mainly outward dipping tephritic phonolite lavas interbedded with basanitic tephra and lava flows, and invaded by dykes.
In contrast, the southern wall is formed by a block of sub-horizontal grey pahoehoe flows of poorly but coarsely vesicular, massive, light-grey phonolitic tephrite, which is unconformable with the outward dipping lavas of the Pico Viejo cone. This block is interpreted to be the in situ remains of a thick lava pile pounded in the summit caldera. This material also occurs beneath the caldera floor, which is flat, except for a 76 m, deep, funnel shaped pit in the SW part.
At least three generations of dykes are visible in the caldera walls. Most are radial to the summit and cut some or all of the outward dipping flow.
Figure 4: Geological map of the Pico Viejo summit region.
Lunch stop:
Non welded but differentiated layers of bread, cheese and vitrified thunafish interbedded with fruit-cake-water related phreato-stomachal activity.
SE caldera rim:
The isolated southern block is overlain by poorly-lithified basanitic surge deposits and ultimately by a widespread grey deposit of variable (0.1-1 m) thickness. It is an unlithified, poorly sorted, heterolithic volcaniclastic unit containing large (up to 1 m) blocks of basanitic, phonolitic and tephritic lava, gabbroic and syenitic plutonic xenoliths, and basaltic clasts. Similarity between the stratigraphy of the southern block and the caldera floor suggest that the two were once continuous, and filled the whole caldera to the rim.
The interpretation is that there was two episodes of caldera formation during the evolution of Pico Viejo. First, steeply dipping walls and the sharp bisection of pre-existent structures suggest the mechanism was vertical collapse. The presence of concentric dykes utilised by the caldera filling support this. An explosion or sector collapse is discounted because a stratigraphically correlated explosion deposit is absent and the crater wall is intact. It is still unclear which products represent the displaced content of the magma reservoir, though similarity in volume and age is found between the first caldera and Roques Blancos, a compound satellitic phonolitic flow on the NW side of Pico Viejo.
The second caldera forming event was unequivocally collapse, because the caldera fill is down-faulted to produce the present caldera floor. It is constrained to have occurred after the 2020 y BP eruptions and before the formation of the upper explosion deposit, interpreted as the result of phreatic explosions triggered by the "1798 magma". A relation between the second collapse and the 2020 y BP eruptions (Montaña Blanca pumice) is supported by their similar volumes (0.25 km3) and appropriate age. The presence of xenoliths and the absence of juvenile material suggest that the widespread grey heterolithic volcaniclastic unit on Pico Viejo is the result of deep phreatic explosions interpreted to reflect vent opening before the 2020 y BP eruptions.
Descent of Pico Viejo - 1798 Narices del Teide eruptions:
The descent of Pico-Viejo is made by the SW flank. This is the location of the youngest eruptive activity on Pico Viejo which occurred in 1798 (no activity is seen between the second collapse and 1798). The eruption produced about 0.05 km3 of phonolitic tephritic magma and scoria from a linear array of vents between 2700 and 2800 m altitude. The initial eruptions occurred from the highest vents and constructed several large scoria cones. The lowest of these was breached and lava eruptions started.
The linear array of the 1798 vents imply the intersection of magma filled fractures with the surface. Within 3 km of Pico Viejo, these fractures have a radial disposition to the summit but inflect at greater distances to adopt the regional NE-SW trend of western Tenerife. Radial vents arrays and radial dykes exposed in the summit caldera indicate episodes of tumescence of the Pico Viejo edifice. Three generations of dykes may be recognized, of which the 1798 fractures are the youngest, suggesting that tumescence usually precedes collapses.
The explosion pit in the SW of the summit caldera floor lies on an extension of the main 1798 eruptive fissure, suggesting that the first magmas to ascend the fracture interacted with the Pico Viejo aquifer to generate phreatic explosions which excavated the pit. The violence of these explosions is confirmed by the existence of large blocks of caldera-fill tephrites above the 2020 y BP pumice, 8 km away, on Montaña Blanca.
Further down the Pico Viejo flank , older Pico Viejo products are exposed, representing the first-cone building phase in the construction of the edifice. These include phonolitic tephrites and basanitic rocks rich in plagioclase phenocrysts.
Conclusion:
The development of the Teide-Pico Viejo complex implies the eruption of increasingly felsic magmas with time. First eruptions of basalts occurred from vents probably distributed along a NE-SW array, succeeded by basanites and intermediates as activity focused at the sites of the stratovolcanoes. Growth of these volcanoes resulted from persistent volcanism controlled by the intersection of regional faults with the Las Cañadas Caldera ring faults. The appearance of the first phonolitic magma is stratigraphically close to the first summit collapses of the two cones. From then, constructive phases involved essentially phonolites and were punctuated by further summit collapses.
The eruption of felsic magmas is a common feature of the Las Cañadas Caldera vents and is interpreted to reflect an area underlain by felsic intrusives, where more mafic magmas cannot reach the surface due to their high density. The size of this area (200 km2) suggest that the intrusive complex beneath the caldera is of a substantial size, compared with the plutonic complexes revealed in the older Canary Islands (Fuerteventura, Gran Canaria). The presence of long-lived extra caldera regional fractures which acted as conduits for primitive mafic magmas throughout the development of the Teide-Pico Viejo complex suggests that mafic magma may be intruded into this system both laterally and from depth.
Tumescence and radial fracturing associated with caldera collapses is focused on both central vents, indicating that the shallowest parts of the reservoir system occur beneath Teide, Pico Viejo and Montaña Blanca. Petrological differences between contemporaneous magmas erupted at Teide and Pico Viejo suggest that two independent reservoirs have coexisted for much of the history of the complex. Mixing and mingling have also occurred, showing that these reservoirs have both developed chemical heterogeneities, probably by stratification. Lateral communication between the systems is also demonstrated, by the simultaneous eruption of phonolites from Montaña Blanca and Pico Viejo at 2020 y BP. Previous eruptive phases occurred on Pico Viejo while Teide was dormant, implying that the shallow reservoirs need not be at different depths.