Geology
Reading Teide's Lava Flows: Aa, Pahoehoe and the Black Rivers
Stand on the Montaña Blanca track and the ground underfoot is black rubble that clatters with each step. The edges are sharp enough to cut a boot sole, and walking any distance means picking your way around jagged blocks the size of a fist. This is ʻaʻā, one of two classic surface textures that Hawaiian words – adopted universally by geology – describe. The other, pāhoehoe, is smooth and ropy, and you can find both within a few hundred metres of each other in the Teide National Park. Learning to read these flows is a practical skill: it turns a walk across barren black into a walk through dates and processes.
Lava’s two faces: ʻaʻā and pāhoehoe
The difference between the two textures comes down to the lava’s viscosity and how it behaves as it cools and moves. ʻAʻā forms when the molten rock is thick, sluggish and already losing gas – the surface crust tears apart into sharp, angular fragments that tumble forward over the advancing flow front. The result is a chaotic, rubbly surface that is unmistakable once you have walked on it once.
Pāhoehoe, by contrast, forms from more fluid lava that retains enough gas to stay plastic. The surface skin wrinkles and folds like rope, often with a satiny sheen. Billows and lobes form as the inner lava pushes against the cooling crust. A single eruption can produce both: the front of a flow can be ʻaʻā while the inner core remains pāhoehoe, or the same vent can switch between the two as the eruption rate and gas content change.
| Texture | Surface appearance | Typical movement | How to recognise in the park |
|---|---|---|---|
| ʻAʻā | Rough, clinkery, broken into angular blocks | Front advances as a rubble pile tumbling over itself | The black scree slopes below Montaña Blanca and around the 1798 flow on Pico Viejo |
| Pāhoehoe | Smooth, ropy, billowy, sometimes glassy | Flows as a thin, fluid sheet that crusts and folds | Less common in the park; best examples on older flows south of the caldera rim |
| Transitional | ʻAʻā blocks with pāhoehoe lobes in lee | A mix of both within a single flow | Look for ropy patches clinging inside ʻaʻā channels |
Age written on the rock
Once the lava stops moving, the clock starts on a predictable sequence of weathering. The clues are on the surface, and with practice you can estimate relative age at a glance – without any chemical analysis.
A young flow – say, a few hundred years old – is bare and jet black. The fragments are sharp-edged, and the flow margins stand proud against the ground. You will see no lichens and no plants. The 1798 Narices del Teide flow is a textbook example. Walk onto it and you are walking on a surface that has barely changed since the last basalt solidified.
As the decades and centuries pass, the black begins to dull to a dark grey. Edges soften. White, grey or orange crustose lichens appear – first as scattered spots, then as patches. After centuries, a thin brown hue of oxidation spreads over the surface. Eventually, pioneer plants such as the broom-like retama del Teide (Spartocytisus supranubius) take root in cracks where fine dust has accumulated.
| Indicator | Young flow (few centuries) | Middle-aged flow (centuries to millennia) | Old flow (thousands of years) |
|---|---|---|---|
| Colour | Jet black | Grey-black, sometimes brown | Brown, ochre, dull grey |
| Surface sharpness | Jagged, sharp edges | Edges slightly rounded | Edges heavily rounded, blocks crumble |
| Lichen cover | None | Scattered spots | Extensive patches, sometimes covering half the surface |
| Vascular plants | None | Occasional isolated individuals | Scattered bushes, grasses |
The key point is that this is a relative scale, not an absolute one. A high-altitude flow in the freezing zone of the caldera weathers slower than a similar flow at 1,000 m. The sequence is reliable; the pace is not. You cannot date a flow to within a decade by eye, but you can always tell an old one from a young one.
Reading the 1798 Narices del Teide flow
The most instructive young flow in the national park is the one that issued from the Narices del Teide (Chahorra) vent on the south-west flank of Pico Viejo. It erupted from June to September 1798, making it the longest historical eruption on Tenerife. From the TF-38 road, the flow is a stark black tongue draped over the grey-brown pumice slopes of Pico Viejo. It stands out visually from kilometres away.
Walk to the edge of the flow – staying on the marked paths – and look closely. The surface is almost pure ʻaʻā, with blocks piled in chaotic heaps. Here and there you may spot a patch of pāhoehoe where the flow pooled briefly. The lack of any plant colonisation confirms its youth: nothing has had time to root into that black rubble. The only life is an occasional microbe or wind-blown dust.
This flow is also a good place to see oxidation colour variation within a single event. Parts of the flow appear reddish-brown, especially where gas escaped in the last stages. That is not a different eruption; it is the iron in the basalt reacting with air and steam at the time of eruption. The same flow can look black, grey, red and ochre all in a hundred-metre stretch.
The Chinyero flow: younger still
The most recent eruption on Tenerife happened in November 1909 at Chinyero, on the island’s north-west rift near Santiago del Teide. It lasted about ten days. The flow is even younger than Narices del Teide, and its surface is correspondingly even less weathered. The blocks are pitch-black and razor-sharp, and the lichen cover is still virtually zero.
Chinyero is not inside the Teide National Park – it lies on the west side of the island, just outside the park boundary – but it makes a useful comparison. If you visit both, you can see how little a century of weathering has done compared to two centuries on the Narices flow. The difference is subtle; both are unmistakably young, but the 1909 flow is marginally more reflective, the edges marginally more knife-like. It reinforces the point that relative dating works best when you have a reference point.
Channels, levees and tubes
Flowing lava is not a uniform sheet. From the road or from a high viewpoint like the Roques de García lookout, you can often trace linear grooves in the black surface. These are lava channels – open conduits that carried the molten rock downhill. As the fluid advanced, it built raised edges of cooled lava, called levées, along the sides of the channel. The levées are often lighter in colour because they oxidised while still hot.
More intriguing are lava tubes. When the surface of a channel crusts over while the interior remains liquid, the flow becomes a tunnel. Later, once the eruption ends and the molten interior drains away, an empty tube remains. The Teide volcanic system has many such tubes, though most are collapsed or filled. The best-known intact example is the Cueva del Hielo, a lava tube that runs below the summit cone and holds ice year-round. It is not open to casual visitors – access is restricted – but its existence reminds you that much of the mountain’s history is hidden underground.
Colour and alteration
Walk across an older flow – say, the ropy pāhoehoe surfaces on the caldera floor near Llano de Ucanca – and you will see that the original black has faded to a warm brown or ochre. This is not dirt; it is mineral alteration. Rainwater, snowmelt and atmospheric oxygen react with the iron-rich basalt, turning it into oxides and hydroxides. The same process gives the Canary Islands’ red soils their colour. On a single flow, the colour varies because gases and heat were not distributed evenly.
The skill of reading flows is not complicated. You look at the colour, the sharpness, the lichens, the vegetation. You compare what you see to a known reference such as the 1798 flow. Soon you can glance at a black outcrop from the TF-21 and say, without much thought, “that one is younger than that one over there”. It is the simplest kind of geology – and one of the most satisfying, because the evidence is right at your feet.