Spec Tech: The Whole Tuff and Nothing but the Tuff

Local geology has a profound effect on on culture. Geology determines what sort of agriculture is possible, for example, and will influence economy with the presence (or absence) of gemstones and metals. Geology can also determine what sort of building materials are present for humans (or non-humans!) to use in construction of dwellings.

We’re all familiar with the idea of quarried stone, or muds used to make bricks. People can also carve their dwellings in to rocks, creating artificial caves. Two examples of this can be found in the Bandelier cliff dwellings in New Mexico, US, and in Kandovan, Iran. These dwellings have a fantastic, organic appearance that trends toward curves.

In Bandelier and Kandovan, people have created their homes by carving into a specific type of rock called a tuff. (Technically, tuff is pronounced “toof,” but Americans tend to just pronounce it like “tough.”) Tuffs are remarkably soft igneous rocks, which makes them accessible for carving even by cultures that lack sophisticated tools.

The Making of Tuff

Tuff is a volcanic rock, which means in order for there to a tuff formation in the area, there must be volcanoes. More specifically, there must be volcanoes that erupt with significant quantities of airborne debris, called tephra.

In order to get tephra, you need explosive eruptions. Massive, violent belches of gases (primarily water vapor) escaping from lava cause the liquid to fragment, and also pulverize solid rocks within the volcano’s cone. Lava that’s been splattered into the air can solidify very quickly, flash-cooling to bits of glass that make up tephra. Tephra comes in three sizes – ash (which is not at all soft), lapilli (pea-sized), and volcanic bombs (anything bigger).

Technically, any volcano can create tephra (and therefore ultimately make tuff); it just takes an explosive eruption. However, these sorts of explosive eruptions are much more common in volcanoes with intermediate to felsic (feldspar and quartz-rich) magma. Those tend to be stratovolcanoes, which have the classic “volcano shape” we see from Mt. Fuji and Mt. St. Helens. So-called “super volcanoes” that form calderas will also have explosive eruptions, and will put out massive amounts of debris during eruptions. The Valles Caldera in New Mexico is the source of the Bandelier Tuff, for example.

Once the tephra is out of the volcano, what goes up must come down. Bigger tephra, like volcanic bombs, tend to fall nearer to the volcano. The tephra piles up on the ground in thick layers, which will cause some compression. Subsequent eruptions, lava flow, or regular sedimentation can bury the layer of debris and compress it further, consolidating it into tuff.

How thick a tuff will be really depends upon how much debris the volcano put into the air – and how far you are from the volcano. A tuff layer will be thin at the edges and become progressively thicker toward the volcano. To make a tuff thick enough to people to carve homes in to it, you would probably want a layer around 20-50m thick. Considering that the upper Bandelier Tuff was laid down by an eruption that put out 300 km³ of debris, this is not unreasonable. Near the Valles Caldera, that tuff layer gets up to 200m thick.

How Tuff Looks

Since tuff can be made by an explosive eruption from any sort of volcano, it comes in a wide variety of colors. Tuff from intermediate/felsic volcanoes, like those that formed the tuffs at Bandelier and Kandovan, will tend to be lighter in color, ranging from white to gray to pinkish to orangish. Tuff from mafic (magnesium and iron rich magma) will be dark gray or black or even green. Basaltic tuff was used to make many of the moais at Easter Island, as one example.

How tuff looks up close really depends on how far you are from the volcano. Close to the volcano, there can be large chunks of pumice and rock, surrounded by ash. Volcanic bombs in close to a volcano can be fairly impressive in size. An eruption from Mt. Asama in Japan shot out volcanic bombs that were 5-6m in diameter – not the sort of thing you’d want landing on you or your car. Generally bombs will be a bit smaller than that, but you can easily find them football or human-head-sized. Volcanic bombs will also ted to stand out from the rest of the tuff, since they tend to solidify into solid, much darker rock. Farther from the volcano, tuff will be mostly ash, with possibly some small bits of pumice.

Generally, tuff will have a “ground mass” of fine ash, which will appear smooth until you get fairly close to it.  Bits of rock and pumice float in the ash and will likely be darker in color. Because ash tends to be glassy, a fresh surface in the tuff may have a little sparkle here in there if the light catches it at the right angle.

Tuff can also come mixed with other types of rocks. This is most common when the ash layer ends up in a body of water, such as a lake or sea. This can cause the ash to mix with other sediments, such as sand or mud. Also, if the tuff is made from multiple eruptions, thin layers of sediment can be put down between eruptions. There are several examples of tuffs that have layers of limestone in them.

How Tuff Feels

There are actually two general types of tuff: “regular” and welded. A regular tuff is formed by tephra piling up, compressing, and then sort of consolidating into a rock. I say only sort of in this case, because regular tuff tends to be fragile since it’s not really cemented together. Welded tuff forms when the tephra is still extremely hot as it piles up. The various particles will weld together from the heat, which makes that sort of tuff significantly tougher. Non-welded tuff can be picked up and crumbled in one hand. Welded tuff feels more like stiff foam of the sort used for carving or mountain flower arrangements.

Picking up a piece of tuff for the first time is a surprising experience, as if you’ve come across a joke rock made out of styrofoam. It’s far too light for its size. It’s also very rough; even tuff that’s mostly ash will feel very much like handling a pumice stone like you might have in your bath.

Even with a welded tuff, you can dig into it with your fingernails and scrape the ash away. Larger debris (lapilli and bombs) will be dragged out of the surrounding ash whole. Doing that is a good way to wear down or break fingernails, though, and it’s very possible to cut yourself on a shard of pumice or get small, exceptionally irritating glass fragments stuck in your skin. Metal and stone (as long as the stone is sterner stuff than tuff) are easy to use on tuff.

Tuff Over Time

Obviously, people aren’t going to be carving their houses in to a welded tuff layer when it’s still fresh and hot. (Presumably, anyone living that close to the volcano didn’t make it, and everyone else has fled in terror.) So the tuff will probably have been buried, and then later unearthed erosion. Rivers eroding down into a tuff layer can produce dramatic cliffs.

Erosion from rainfall and other smaller processes will produce a lot of the organic curves you see in pictures from Bandelier and Kandovan. As a note, the site for Kandovan says that the cone shapes are how the ash was laid down. After examining the pictures, I actually rather doubt that; to me it appears that those conical shapes were probably made by a much more massive and continuous layer being eroded. However, I haven’t been able to find any papers to conclusively say one way or another; useful research might exist, but my inability to read Arabic leaves the answer out of my grasp.

The Pueblo Tribe made its home in Bandelier for thousands of years, cutting houses into the tuff. People in Kandovan still dwell in these man-made caves in modern times, keeping cool in the rocks. Tuff has historically been used as a building material, and for sculpting because it’s so soft and relatively light. No matter the level of your created society, they could still find this natural resource useful.

First, we adapt to our surroundings; then we adapt our surroundings to us.