Metamorphic Rocks

We come now to the metamorphic rocks, the result of modifications to already existing rock. I’m well aware that this can all seem a bit mysterious. After all, no one has ever seen the changes take place; no one has ever witnessed a metamorphic rock form—the processes are imperceptibly slow, and they happen deep in the Earth’s crust, way out of sight. Why should these changes happen? Well, they are primarily driven by increases in pressure and temperature, so we begin with a look at these two factors. There are sites in the Earth’s crust where material becomes progressively buried. It happens, for example, where a tectonic plate is driving underneath another one, taking rocks ever deeper as it descends. It can happen in the central area of a plate that is stretching and sagging, allowing thick accumulations of sediment. It’s pretty self-evident that as buried material gets deeper, because of the growing weight of rocks above bearing down due to gravity, it becomes subjected to increasing burial pressure. Less intuitive, though, is the fact that this pressure acts on a volume of rock equally in all directions. Imagine a small volume of rock at depth. It’s bearing the weight of the rocks above it, and so it responds by trying to move downward and to spread out laterally. Of course, it can’t because it’s constrained all around by other volumes of rock that are trying to do exactly the same thing. And so the downward gravity is translated into an all-around pressure. It’s the same effect as diving down to the bottom of a swimming pool. You feel the increased pressure owing to the weight of water above, but you feel it equally in all directions. All-round pressure like this can cause things to change in volume, through changing their density, but it can’t change their shape. However, there can be another kind of pressure as well, and this does have direction, and it can cause change of shape. In the Earth, we call it tectonic stress. It comes about through heat-driven motions in the Earth, including the movement of tectonic plates.

The Rocks Change Shape: Folds, Faults, and Joints

You may have looked at some rocky cliff and noticed sedimentary strata bent into huge curves, the shapes that geologists call folds. You may even have heard of terms like anticline and syncline. Almost certainly you will have heard of geological faults: the San Andreas Fault in California must be one of the best-known geological features there is. They are all examples of what geologists call geological structures. They can affect vineyards, and the names of examples appear around the world on wine labels. So how do these structures come about, and what decides whether rocks make folds or faults? We introduced the concept of tectonic stresses in the previous chapter. We learned that because they act in a particular direction they can induce foliations within metamorphic rocks, but of relevance here is that they can also cause rocks to change their overall shape. That is, the rocks deform, which gives rise to various geological structures. Any solid matter (unlike a liquid) that feels stresses, of whatever origin, will resist them up to a point before it starts to change shape. That point is what defines the strength of the material. The same principles apply when stresses are applied to a sediment or a soil, though rocks, with their constituent minerals firmly bonded together, resist much greater levels of stress before they deform. As one wag put it, the difference between a rock and a soil is that when you kick them a rock hurts your foot . . . So, focusing in on rocks, we see two ways in which they can deform: by flow and by fracture. Looking ahead to where this is going to lead, it’s flow that gives rise to folds, and faults result from fracture. A good analogy for the flow of rocks is glacial ice. The ice is solid to us, but given time, it can flow, to give the “river of ice” that is a glacier. If you leave a ball of silicone putty on a table top, after a few days it will have flowed, while still being a solid, to make a pool.

Igneous Rocks

Igneous rocks were once molten. This is a simple statement, but it’s exactly what sets them apart from the other two great divisions of rocks: sedimentary and metamorphic. So, deriving from the Latin word for fire—ignis, the same word that gives us ignition—igneous rocks are associated with heat. Some are simply solidified lava, but most originated by slowly cooling below the Earth’s surface. Thanks to erosion through time of the overlying material, such rocks are now widespread at the Earth’s surface and consequently underlie many of the world’s vineyard regions, from Washington State to the mountains of Hungary, from Lodi, California, to the Cape Peninsula of South Africa. Although it gets warmer with depth everywhere across the Earth, generally the weight of the overlying rocks makes the pressure too great to allow melting, so as a rule the rocks below our feet are solid. In some places, however, the heat increases so rapidly that temperatures can reach over 600°C at just a few kilometers below the ground surface, a temperature at which some rocks are molten, even under pressure. The initial melting usually takes place in and below the lower part of the Earth’s crust, but the molten rock then rises, typically to reside tens of kilometers or so below the surface, though less under volcanically active areas. Such depths may seem large to us, but seeing as its well over 6000 kilometers to the center of the Earth, geologically they are pretty close to the surface. In other words, the igneous rocks we now see at the surface did not form incredibly deep in the Earth’s interior; they were nowhere near Earth’s core, as some writings claim. We call this underground molten material magma. People seem to like the word. Not only does it appear on wine labels, but it is also the name of a number of wine shops, bistros, and various drinks. It exists in the Earth in magma chambers. It would be simplistic to picture these as some sort of enormous underground caves filled with liquid rock: there may be patches that are wholly liquid, but almost certainly there will be plenty of solid matter, minerals that are below their melting point.

How Minerals Work

We might expect the ground of vineyards to consist of bewildering permutations of elements, but because its composition is dominated by just eight of them and there are chemical restrictions on how they can combine, the number of common minerals is not huge. Even so, their names are not particularly well known, even those of the very minerals that make the ground we live on and the soils that vines grow in. Mineral names that might spring to mind are more likely to be those used in jewelry or that are commercially mined. Such gemstones and ore minerals are not widespread, but geological processes have concentrated them in certain parts of the Earth, and if we can locate these accumulations, it may be worthwhile to extract them for profit. The rocks and soils that mainly concern us here are composed of silicate minerals, and Chapter 3 is devoted to these workhorses. They are sometimes called the “rock-forming minerals,” though there is an outstanding exception to this term: calcium carbonate, which makes the calcareous rocks. So in this chapter’s survey of the kinds of nonsilicate minerals we may come across in vineyards, we will pay particular attention to the carbonates. But first, let’s examine some fundamental concepts concerning the nature of minerals. As we saw in Chapter 1, minerals are made of ions bonded together through giving or sharing electrons. But to achieve this linkage, the ions cannot combine in some higgledy-piggledy fashion; rather, they have to organize themselves in a particular, symmetrical physical arrangement. It’s a bit like the sight of soldiers on formal parade. We call the three-dimensional framework of ions a lattice, and it’s this regular pattern that makes the material crystalline; it is a crystal. In other words, the pieces of mineral in a vineyard are crystalline. We may think of crystals as having the attractive, light- catching facets seen in gem shops and museums. Although this is a manifestation of the crystalline structure of the constituent ions, it is not what defines them as crystals. Consequently, minerals lying in a vineyard may be dull, shapeless chunks, but they are still crystals.

What Are Vineyards Made Of ?

What strikes you first when looking at a vineyard? Perhaps the vines themselves? Your eye may be caught by random scatterings of gnarly old bushes or by the military neatness of rows of trained vines, luxuriant in foliage in summer and little more than gaunt woody skeletons in winter. But possibly more striking might be the land itself—the geology, or at least manifestations of it. The vines may extend across a vast, flat plain, or they may be perched on a vertiginous slope, or anywhere in between—it depends on the bedrock geology. How well the vines grow will be influenced by how that bedrock weathers into soil and how the vine roots respond. The soil may have an eye-catching color or may be astonishingly stony, consisting of little more than rock debris. This quality, too, depends on the geology. But what exactly is this vineyard ground? What are such things as bedrock, soil, and stone made of? Where do they come from? How did they get this way? The answers form the basis of understanding vineyard geology, so let’s begin here, with a few fundamentals. We can think about what the ground in a vineyard is made of in three ways. The first way is that, like all matter, it consists of atoms of chemical elements. And remarkably, although there are nearly a hundred different chemical elements in nature, the ground is dominated by just eight of them (Figure 1.1a). You could even say that it’s pretty much made up of only four of these elements, as the first four on the list account for nearly 88% of the composition. Preponderant among them are oxygen, at no less than 46%, and silicon, at 28%. So there’s a lot of these two elements in most vineyards! As an aside, it’s the same kind of story with living organisms: about 95% of their composition consists of just three elements—carbon, oxygen, and hydrogen—and that includes grapevines and grapes (Figure 1.1b).

Epilogue

We have seen in previous chapters how grapevines interact with rocks and soils, and in Chapter 10 I discussed the role of geology in terroir. But a question remains, one that is probably uppermost in the mind of many a wine lover: to what extent does geology affect the taste of the wine in your glass? I argued in Chapter 9 that the perception of a mineral taste in wine can’t have a literal meaning, but what about other tastes ascribed to geology? We might reasonably expect that the geological influences on vine growth have at least some role in wine flavor, but what? Many populist wine writings imply that the answers to such questions are clear-cut, but unfortunately they aren’t. Claims are routinely made in wine descriptions that sound fine but that don’t easily tally with scientific understanding. In other words, there’s some divergence between popular beliefs and scientific understanding of the geology—wine flavor connection. Part of the explanation may be that many of the populist assertions seem to be based on custom and on anecdote—narratives passed on enthusiastically but unquestioningly between wine fans. Two situations are common. First, a description of a wine casually mentions the kind of geology where it originated, implying a significance but without any justification or indication of how it might come about. I give illustrations of this in the following section. Second, some character of a wine is ascribed to particular rocks and soils but without providing any rationale. For example, a Riesling from Kamptal’s Gaisberg vineyard (Austria) is said to have “complexity because of the slaty para-gneiss, amphibolite, and mica” soils. But there’s no indication of how these two very specific rock types together with this particular mineral bring this complexity about.

Vineyards and the Mists of Geological Time

Geological time is much mentioned in the wine world. Many a label proclaims the geological age of the rocks and soils in which the vines were growing; many a vineyard description enthuses about just how old its bedrock is. The age may be expressed as a fine-sounding technical term or as a quantity, typically, some unimaginably large number of millions of years: “The area’s best vineyards are on Turonian soils”; “Cretaceous limestone is best for our vines”; “the wine’s secret is the Devonian slate”; “our Shiraz grows in soils 500 million years old.” It’s almost as though the older the geology can be made to appear, somehow the finer the wine. I must declare my own position in all this: surely the geological age of the bed­rock has little to do with viticulture? The age of the soil is certainly relevant, as it is continually changing on a human timescale, but these geological time words almost always are referring to the age of the vineyard bedrock. And almost invariably the age of the soil will be unrelated and vastly younger than the bedrock. Surely the vine doesn’t care, so to speak, how long ago the bedrock happened to form. Nevertheless, the fact is that geological time pervades wine literature, so this chapter explains how geologists work with the ages of rocks. The thinking is nicely explained by outlining how geological time was “discovered.” Modern geology began two or three centuries ago, essentially when it dawned that answers to questions about the physical world were better answered by going out and observing nature rather than poring over ancient scriptures. We saw in Chapter 1 how James Hutton peered into the “abyss of time.” Soon after, other founders of the science began to compare features preserved in rocks with processes they could see happening all around them, and they were able to establish rules (see the accompanying box) that enabled them to disentangle past geological time and to work out the geological history of a particular place. Using these kinds of principles, the early geologists were soon able to recognize past intervals of geological time and give them names.

Soil, Water, Sunshine, and the Concept of Terroir

If we look at a vineyard, it’s very tempting to assume that what we see at the surface simply continues on downward. Maybe it does, but most soils vary with depth, and the surface can be quite unrepresentative of down where the work is done, of the materials that surround the vine roots. That’s why these days vineyards are peppered with soil pits. Normally, immediately below the surface of the ground is the topsoil, the most fertile part, from which vines get most of their water and nutrients. Below this is increasingly compact, commonly clayey material, subsoil, in which relatively little grows. If we continue downward, sooner or later we hit bedrock, for every vineyard sits on bedrock, at some depth or other. Unlike many plants, vine roots can probe many meters downward into the subsoil and even penetrate fissures in the bedrock, particularly if there’s a need to seek out supplementary water. The way soil varies with depth is called its profile. The variations in physical and chemical properties may be gradual, or in discrete layers, referred to as soil horizons, an arrangement sometimes called a duplex soil. A hypothetical example of a layered soil profile is shown in Figure 10.1, and Figure 10.2 gives an example of how a property can vary with depth. The overall depth of a soil above bedrock is termed its thickness. In vineyards, this can be anywhere from as little as 20 centimeters, such as at Auxey-Duresses in the Côte d’Or, to alluvium on plains such as California’s Central Valley that is measured in hundreds of meters. Even where bedrock has weathered in place to yield the overlying soil, its effects (Figure 10.3) can only be very generalized, because of all the permutations of climate, landform, biology, history, and so on, that influence soil profiles. Granite, with its coarse grains and high content of feldspar and quartz, both of which are fairly stable minerals physically, tends to yield sandy, well-drained soils. They are often pale colored, like the parent rock, though in places with a higher manganese content, such as parts of Barolo and Beaujolais, they can have a bluish tone.

Weathering, Soil, and the Minerals in Wine

Weathering of rocks is the crucial first step in making vineyards possible. For where the debris produced by weathering—the sediment we met in Chapter 5—becomes mixed with moist humus, it will be capable of supporting higher plant life. And thus we have soil, that fundamental prerequisite of all vineyards, indeed of the world’s agriculture. So how does this essential process of weathering come about? Any bare rock at the Earth’s surface is continually under attack. Be it a rocky cliff, a stone cathedral, or a tombstone, there will always be chemical weathering—chemical reactions between its surface and the atmosphere A freshly hewn block of building stone may look indestructible, but before long it will start to look a bit discolored and its surface a little crumbly. We are all familiar with an analogy of this: a fresh surface of iron or steel reacting with moisture and oxygen in the air to form the coating we call rust. In his “Guide to the Lakes” of England, William Wordsworth put the effects of weathering far more picturesquely: “elementary particles crumbling down, over-spread with an intermixture of colors, like the compound hues of a dove’s neck.” A weathered rock is one that is being weakened, broken down. The rock fragments themselves are further attacked, which is why stones in a vineyard often show an outer coating of discolored material, sometimes referred to as a weathering rind (Figure 9.1; see Plate 22). If the stone is broken open, it may show multiple zones of differing colors paralleling the outer surface of the fragment and enclosing a core of fresh rock. Iron minerals soon weather to a powdery combination of hematite, goethite, and limonite, and the rock takes on a reddish-brown, rusty-looking color. The great example of such weathering in viticulture is the celebrated terra rossa, but the rosy soils in parts of Western Australia and places further east such as McLaren Vale and the Barossa Valley are also due to iron minerals. Several Australian wines take their names from this “ironstone.”

Sediments and Sedimentary Rocks

We are on more familiar ground in this chapter, looking at processes and materials found in the world all around us. Even the names of sedimentary rocks are well known—sandstone, shale, limestone, and so on. Clearly, these materials are highly relevant to vineyard geology because more than three-quarters of the land surface is sedimentary in origin: most of the world’s vineyard areas are underlain by sedimentary rocks. Sediment is the detritus produced from the weathering of already existing rocks. (I explore the process in Chapter 9.) Usually, wind, ice, or water soon moves the debris away, eventually to be deposited and then buried beneath further sediment and with time hardened into sedimentary rock. Weathering can also dissolve material, later to be precipitated. And, needless to say, all the sediment in question here is of geological origin; it has nothing to do with the organic sediment that is thrown, say, in a bottle of vintage port! Wind and flowing water may be able to pick up sediment and move it, depending on the size of the fragments. Faster-moving currents can carry bigger particles: it’s to do with energy, as discussed in the context of rivers in Chapter 8 (see Figure 8.8). The result is sediment sorting. We can easily see the results on a beach—a sandy spot here, a pebbly patch there—because the tides and shore currents have moved the sediment around and sorted it. Thus, most detrital sediments have a characteristic grain size, and we use this to classify the material. The terms for the different sizes are pretty much in line with everyday language: sand, silt, clay, and so on (Figure 5.1). Clay is the finest sediment. It’s composed mainly of the tiny clay minerals that we met in Chapter 3 and has the smooth, slippery feel and handling properties we’re all familiar with; the individual constituent particles are far too fine to see, even with a powerful hand lens. Imagine: if we scaled up a grain of sand to the size of a wine cask, then an individual clay flake would be smaller than a coin.