Wine aficionados love to talk about geologic aspects of vineyard sites that produce their favorite wines. Whether it’s the Jurassic limestone of Burgundy, the granite of Beaujolais or the schist of the Douro Valley, the geology is approached with reverence. The study of geology revolves around understanding how the rocks we observe on the surface of the earth are formed. By understanding their genesis, we can develop far-reaching theories on the earth’s formative processes.
Granite … Visible Crystals in an Intrusive Igneous Rock
Rocks are generally divided into three types: Igneous, metamorphic and sedimentary. Igneous rocks are crystalline solids which form directly from the cooling of molten rock or magma. The earth is made of igneous rock because the interior of the planet is hot enough to melt rocks. The igneous family includes rocks that cool slowly deep within the earth – these are plutonic, or intrusive igneous rocks that can develop large, easily visible crystals. Also included are rocks that are erupted to the surface in liquid form and cool quickly. These are volcanic, or extrusive igneous rocks and their crystals are quite small. Granite is an intrusive, plutonic igneous rock that forms some famous vineyard soils. Basalt is an extrusive, volcanic igneous rock that is equally noted as a vineyard substrate.
The name metamorphic is derived from "meta" (change) and "morph" (form). Metamorphic rock can be formed from any other rock. The base rock is subjected to enough heat and pressure that the minerals which make up the rock become unstable and out of equilibrium with the new environmental conditions. This results in a change of crystalline structure of the rock. Slate and schist are the two metamorphic rocks most commonly mentioned in relation to vineyards.
Sedimentary rocks are sometimes called secondary, because they are often the result of the accumulation of small pieces broken off of pre-existing rocks. Plutonic igneous rocks on the earth’s surface are unstable because their crystals were formed under completely different conditions than those on the surface. Thus, they are subject to chemical weathering that breaks down their components into clay minerals or very fine granules. There are three main types of sedimentary rocks:
Clastic Sedimentary Rocks: Clastic sedimentary rocks are accumulations of clasts: Pieces of weathered and broken up rocks, which can range from cobbles to pebbles to sand, silt and clay particles. These clasts are generally moved by water and piled up to a point where they can be lithified by compaction and cementation.
Clastic rock formation is happening now in a slow but inexorable process. Sand grains and clay particles washed from a northern Minnesota hillside may slowly work their way into the Mississippi River. Then, borne for more than two thousand miles by the churning waters of the great river, they will be deposited at the mouth of the Mississippi delta, far out in the Gulf of Mexico. It’s a slow process, but it’s the relentless nature of the action that creates new rocks. If only one millimeter of sediment per day -- a thin dusting -- is deposited, that will accumulate to over a foot of sediment in a year. Over a one-thousand year time span -- a blink of an eye in geologic time -- twelve hundred feet will be deposited. In ten thousand years, over two miles of sediment will accumulate. As the pile grows, the increasing weight forces water out, compresses and ultimately turns the sediments to rock. That’s a simple overview of the clastic lithification process.
Chemical Sedimentary Rocks: These rocks form when mineral-laden water evaporates, leaving the formerly dissolved minerals behind. This is most common in arid environments, where seasonal playa lakes occur in closed depressions. Thick deposits of salt and gypsum can form due to repeated seasonal cycles of flooding and evaporation over thousands of years. The Great Salt Lake is a modern example of chemical sedimentation. There are some vineyards in the world that are prized for their gypsum content -- a result of chemical sedimentation.
Fossiliferous Limestone -- an Organic Sedimentary Rock
Organic Sedimentary Rocks: While the other two sedimentary rock categories involve inorganic clasts and minerals, organic sedimentary rocks are an accumulation of sedimentary debris caused by living organisms. Many animals use calcium carbonate to create shells, bones, and teeth. These bits of calcium detritus accumulate on the seafloor and, much in the same way as clastic deposits, and become thick enough to lithify into an organic sedimentary rock. Most rocks that contain fossils are organic sedimentary rocks. For vineyards, limestone is the most lauded organic sedimentary rock. Limestone is being formed today in areas where coral reefs exist and in shallow marine areas like the Bahamas.
The Rock Cycle embraces all three types of rocks. Igneous rocks are created, brought to the surface by tectonic forces, then subjected to slow weathering and decomposition. The broken up pieces of rock then become the clasts that are the basis of clastic sedimentary rocks. Similarly, calcium-secreting sea life -- mollusks, corals, some microscopic organisms are living, dying and accumulating on the sea floor as the raw material of future limestones.
The Rock Cycle Illustrated
As the sedimentary rocks are buried and lithified, some may be heated and compressed to the point where they begin to crystallize and form metamorphic rocks. As heat and pressure increases, so does the degree of metamorphism.
Let’s take our example of Mississippi mud. Over millions of years of accumulation, the overburden pressures squeeze out the water and compress the mud into rocks, which we call shales. As these rocks get buried more deeply, they are subjected to ever-greater pressures and temperatures and they slowly begin to recrystallize become a metamorphic rock known as slate. Slate has the property of fissility or slaty cleavage -- it splits easily into large plates. That makes it an excellent source for chalkboards, roofing materials, and decorative stones for flooring, sidewalks and the like.
As slate undergoes further increases in temperature and pressure, the re-crystallization continues. The crystals form a visible sheen on the rock, which is now classified as another metamorphic rock called phyllite. Phyllite is a foliated rock. The term comes from the Latin folia
, meaning “leaf” (think of phyllo dough) and refers to the sheetlike character of the rock. When we increase the pressure and temperature even more, the re-crystallization continues and the rock has now metamorphosed into our famous schist. Further increases in heat and pressure result in coarser crystals and more visible bands in the rock, which is now called gneiss. Ultimately, with enough heat and pressure, it may melt entirely and form a crystalline igneous rock known as plagiogranite with no trace of its foliated sedimentary origins. Our Rock Cycle is now complete -- the sediments have been metamorphosed then finally melted. They will then be forced to the earth’s surface and begin the cycle of sedimentation anew.
The Granite Slopes of the Sommerberg Grand Cru in Alsace
What, then, does all this mean to a wine lover? It can inform one’s understanding of the unique combination of factors that separate great vineyard sites from merely good ones. The granite of a Cru Beaujolais creates a different soil composition and drainage characteristic than the limestones of southern Beaujolais. The empiric evidence is in the greater intensity of aroma and flavor in a Fleurie or Moulin-à-Vent than in a simple Beaujolais.
Too, the vineyards in Alsace with the evaporite gypsum in their bedrock have abosrbent soils that yield weighty wines. The Grands Crus of Kanzlerberg, Schoenenberg and Altenberg de Bergbieten all have significant gypsum components. The resultant Pinot Gris, Gewurztraminer and Riesling wines are rich and weighty as well. You can do a bedrock comparison by sampling the gypsiferous Grands Crus and comparing them with granite-based Grands Crus like Schlossberg and Sommerberg. Part of the distinction among these wines can be the differences in the rocks and the stucture and composition of their resultant soils.
When next enjoying a favorite Grand Cru Chablis or Alsace wine, give a thought to the eons of formative geologic processes that set the stage for that unique terroir. It will add to the enjoyment and appreciation of the wine world’s truly great vineyard sites.