Rumblings in Ecuador

Rumblings in Ecuador

In geological time, Ecuador is a relatively young country formed as a result of the oceanic Nazca Plate subducting under the continental South American plate and the resulting active continental margin is part of the 6500km Ecuador/Chile/Peru Andean Cordillera with the ~6000m deep Chile trench offshore.

In very basic geological terms Ecuador is in 4 sections; the unrelated Galapagos island hot spot, the low lying coastal sedimentary Quaternary/Tertiary deposits, the remote eastern Amazonian (Oriente) Quaternary/Tertiary sedimentary deposits. These Amazonian deposits are separated by a band of pre-Cambrian undifferentiated material, the central dividing sierra where the mountains, volcanoes, cloud forests and the 3000+ m high plateaus (the paramos) are located. The distinct lowland/highland topography is also a cause of infrastructure problems (such as the Devil’s nose railway) and political/social/ethnic divisions.

The central sierra is generally formed from Cretaceous/Tertiary (145 -65Ma) volcanics, Mesozoic (250 -60 Ma volcanics) and Cenozoic/Mesozoic (65ma to present) intrusives, andesite, dacite, rhyolite are the most common rocks with some basaltic andesite and basalt.

Central sierra

One of the most notable features is the Avenue of the Volcanoes (Pan-American Highway or Pana). This north-south road runs in the inter-Andean valley between the Eastern and Western Andes and features some of the more significant, and active, volcanoes of South America (see figure 1). One thing I noticed (apart from heavy breathing; Quito is at 2800m and is low down compared to some places) is the lack of “solid” volcanic activity; no lava flows, granitic formations etc although the volcanic topography is evident. Figure 2 was taken about 100m from the equator museum and one interesting story associated with the museum is the 1735 Condamine/Bouguer expedition to establish a degree of longitude.

What is very noticeable is the enormous volume (100s of km2 you can travel all day seeing it) of semi-consolidated yellow/grey ash fall along with (relatively) lesser amounts of tuffs, ignimbrites, pumice and/or pyroclastic flow tephra beds. This ash is often incorporated into concrete breeze blocks in many of the roadside “bloquerias” or factories.

/ Figure 1 Central Sierra Volcanoes
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Figure 2 local view at the equator / Figure 3. Deformed road side deposits

The unconsolidated yellow/grey ash is often seen in road works or road cuttings and in the huge scale of the mass wasting on the steep slopes. A typical bed (near Cotopaxi) can be seen in Figure 3. Note the deformed strata however I could not see if the bed colour change was different material or had been weathered. Note also the upper horizon slumps, the piles of fallen ash and the breaking up by water of the surface at the bottom.

The pyroclastic flows build classic strato-volcanoes like Cotopaxi. However Cotopaxi was cloud shrouded when I went past it so this picture is from Wikipedia.

Figure 4 Cotopaxi from

Symmetrical, glacier-clad Cotopaxi (shining peak in Quechua) is a 5911m Andesitic strato-volcano on the eastern, and geologically older, side of the inter-Andean valley and is Ecuador's most well-known and most active volcano. The steep-sided cone is capped by nested summit craters, the largest of which is about 550 x 800 m in diameter. Deep valleys scoured by lahars radiate from the summit and large andesitic lava flows extend as far as the base of Cotopaxi. The modern conical volcano was constructed about 5000 years ago. Pyroclastic flows have accompanied many explosive eruptions of Cotopaxi, and lahars have frequently devastated adjacent valleys. The most violent historical eruptions took place in 1744, 1768, and 1877. Pyroclastic flows descended all sides of the volcano in 1877, and lahars travelled more than 100 km into the Pacific Ocean and eastward into the western Amazon basin. The last significant eruption of Cotopaxi took place in 1904.

From the map you can see that a big eruption would cause problems in Quito (population 2.2 millions) and Latacunga (~56,000) which was destroyed in the 1744, 1768 and 1877 eruptions. One danger of a huge eruption of Cotopaxi would be the flow of ice and melt water from its glacier. Recently fumarole activities and sulfuric emissions have increased and ice around the inside and on the southeastern side of the cone has started to melt.

Other volcanoes along the road include the dormant Corazon (4790m), the dormant but heavily eroded 4721m Rumiñahui stratovolcano and the Andesite-Dacite North and South Iilizanas (5248m on the western side of the inter-Andean valley). According to some guide books people train on these prior to climbing Cotopaxi and volcano walking holidays are popular! The roadside cutting in figure 5 also shows the ash fall beds.

Turning off the Pan-American near Lasso and heading into the hills to the west you are on the Quilotoa loop. Most noticeable are the steep hill sides (mainly of the unconsolidated yellow/grey ash fall), the deeply incised rivers, the mass wasting/land slides and the shear drops (one bend had about 10 crosses where drivers failed to make the sharp turn). Figure 6 gives a view with the river right at the bottom and a landslide with the road to the left centre.

Figure 5 North and South Iilizanas / Figure 6 Quilotoa Loop view

After a spectacular but very rough ride from Chugchilan; the road is eroded away in places with gullies, rivulets, mudslides, rills, bends and landslips (part of the road is a wadi and it is occasionally a get out and push ride) but still fine dust, scoria, tephra but now with the occasional boulder- you arrive at Quilotoa village and the caldera of Quilotoa volcano at 3850 m. (figure 7)

Figure 7 Composite photo of the Quilotoa Caldera looking east

Quilotoa is a water-filled caldera and the westernmost volcano in the Ecuadorian Andes. The 3kilometer wide caldera was formed by the collapse of this truncated Dacite volcano following a catastrophic VEI-6 eruption about 800years ago, which produced pyroclastic flows and lahars that reached the Pacific Ocean (~80 km distant), and spread an airborne deposit of volcanic ash throughout the northern Andes. The steep-sided walls rise 400 m above the surface of 250m deep caldera lake, which has a greenish colour as a result of dissolved minerals (surprisingly it is alkaline not acidic). Fumaroles are found on the lake floor and hot springs occur on the eastern outer flank of the volcano. More than a half dozen lava domes form a circular array along the caldera's perimeter. This small volcano has produced eight major explosive eruptions during the past 200,000 years.

Here you have a choice; you can walk down to a very small beach/hostel by the water’s edge some 1.5km and 45 minutes down but 1.5 hours up. Or with a guide there’s a 6-7 hour walk around the rim although the track often disappears. I was time limited so I walked down to a mirador (viewpoint) about 750m down but with great view.

Of note were the very distinctive light/dark strata at the lake edge all around the far edges of the crater. Without a boat it would not be possible to check whether they are “tide” marks but I thought them to be the strato-volcano layers (figure 8).

Figure 8 Crater strata

Figure 9 shows the path down the internal crater wall with mostly fine friable ash, dust, scoria, and tephra as the main visible rock (no major blocks or lava flows) but as figure 9 shows the block to the left has many in-bedded clasts and some matrix texture changes appear in places. The ash beds are shown in figure 11 and the clast matrix texture changes are shown in figure 12.

I was somewhat limited in the number of samples I could take or even indentify but just within a metre or so of the spot where I was sitting I found andesite, ignimbrite, dacite, rhyolite, tuff and what I thought was basaltic- andesite. One dacite sample was encrusted in sulphur (figure 10) but as shown in figure 12 there was a large number of different clasts to be seen. An expert would have a field day and this is a staggering site but I was time limited and had to move on to another spectacular location Baños.

Figure 9 Path inside crater / Figure 10 Sulphur encrusted Dacite clast
Figure 11 Fine friable ash cross bedding / Figure 12 Clasts in-bedded in the coarser matrix material

Travelling south and east towards the town of Baños the smoking cone of Tungurahua appears, fortunately, at present, Baños is on the safe (r) north eastern flank but the town map has all the emergency exit routes marked and local exercises take place. Also of note as you approach the town is a new Bailey bridge which replaced the old bridge washed away by a lahar in the, already high energy, river Pastaza.

Tungurahua, a steep-sided andesitic-dacitic stratovolcano at 5,023 metres is one of Ecuador's east Andean valley’s most active volcanoes. Three major volcanic edifices have been sequentially constructed since the mid-Pleistocene over a basement of metamorphic rocks. Tungurahua II was built within the past 14,000 years following the collapse of the initial edifice. Tungurahua II itself collapsed about 3000 years ago and produced a large debris-avalanche deposit and a horseshoe-shaped caldera open to the west, inside which the modern glacier-capped stratovolcano (Tungurahua III) was constructed. Historical eruptions have all originated from the summit crater. They have been accompanied by strong explosions and sometimes by pyroclastic flows and lava flows that reached populated areas at the volcano's base. An eruption beginning in 1999 caused the temporary evacuation of the city of Baños at the foot of the volcano however the last major eruption occurred from 1916 to 1918. A VEI 3 event started in November 2010 with a central vent explosive eruption with pyroclastic and lava flows, causing land and property damage along with lahars and local area evacuation.

One feature of Baños, as its name suggests, are the thermal baths. Another feature is a waterfall which feeds the communal washing sinks. Hotel Luna Runtun is situated at the base of Tungurahua (figure 13) and one the hotel’s pools (perched on the edge of a shear cliff over looking Baños) uses hot geo-thermal water. As can be seen from figure 13 the minor activity is continuing but Tungurahua has an approximate 100 year major eruption pattern. Most tourist web sites recommend checking the activity level before visiting this very interesting town.

Figure 13 Tungurahua from the hotel / Figure 14 View of the Cajas Park with the road to right centre

Later after a short stay in the lovely town of Cuenca a short drive, with increasingly wilder volcanic scenery, takes you into the stunning Cajas National Park with its peaks, lakes, swamps, tundra vegetation and the Pacific-Amazon watershed of the western Andean range. A major start point for viewing and trekking is the Tres Cruces pass at 4166m and the view from here is spectacular (see Figures 14, 15, 16) and much of the landscape has been re-worked by ice action.

Figure 15 Cajas park View / Figure 16 Cajas inhabitants

The Cajas area consists mainly of rocks belonging to the Tarqui formation covered by tillite. The tillite is found around the depressions which make up the Cajas lakes or swamps and was formed in the last ice age. The most recent glacial events of Ecuador can be divided three groups. The oldest is dated around 13000 to 16000 years. The second between 10000 to 12000 years and the last between 9000 and 4000 years B.P

The Tarqui formation (more than eighty percent of the park’s area) consists of various types of parent material which can roughly be divided into three stratified units. A dark, fine-grained andesitic layer overlaid with a sequence of tuff of a Rhyolitic to dacitic composition. The uppermost layer consists of a uniform Rhyolitic, medium grained, material that appears in the upper part in the park and forms the visible hills. The majority of the rocks show alteration, which is a result of hydrothermal processes (Figure 17).

After the last ice age the area has been covered by a large amount of uniformly distributed ash layer. The ashes are thought to originate from Tungurahua and/or the 5230m Sangay volcano. These volcanoes are, respectively, 110 and 130 kilometres northeast from Cajas and this distance is thought to explain why the ashes are relatively equally sorted in composition and depth across the whole of Cajas. The most important recent eruption of Tungurahua was in 1916/1918 and the volcano has an eruption interval of about a hundred years. Sangay is constantly active.

However do not let this stop you. Ecuador is not only spectacular for its volcanoes but the flora, fauna and bio-diversity is virtually un-surpassed anywhere in the world; and I’ve not yet covered the Galapagos its volcanic landscapes and the flora and fauna. The only problem being time to do justice to each facet and space to show the 100s of photos I took.

/ REFERENCES


Condamine/Bouguer expedition. See
Cotopaxi details. See Volcano Number: 1502-05
Iiilizanas. See
Rumiñahui. See
Quilotoa .See
Quilotoa.
Tungurahua. See
Sangay. See
Cajas Park. See parque-nacional-cajas.org/sp.html
Buytaert W, Sevink J, De Leeuw B, Deckers J. 2004. Clay mineralogy of the soils in the south Ecuadorian paramo region. Geoderma 127 (2005) 114-129
Cuenca. See
Colin Mould To see the photos in colour visit the web site
Figure 17 Hydrothermal alteration in a rock.