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The Dark Side of Volcanic Rocks: Basaltic Basics
By Wayne Belding
Dec 22, 2015
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Oenophiles are often consumed with details about the soils of various vineyard sites around the wine world.  One term that is widely expressed with gravitas is that a vineyard has volcanic soils.  Let’s explore the meaning of the description.

Volcanic soils encompass a wide range of rock types and petrographic properties.  Volcanoes have long had a profound impact on the earth’s surface.  Dramatic photos of explosive eruptions -- Mount Etna’s recent explosions, for instance -- captivate the viewer.  So too do images of lava flows, moving inexorably forward and consuming everything in their path.

Deep beneath the earth’s surface, the heat and pressure is sufficient to begin a partial melting of the rocks.  Each mineral within a rock has a different melting point, so those with lower melting points (starting at around 1,300° F) begin the process.  After 20% of the rock mass melts, it becomes liquid and is capable of flowing.  Volcanoes are vents that allow magma to travel to the surface.  They form at points of weakness in the crust, often where continental plates collide and force one underneath the other, thus fracturing the earth’s surface and creating vents for the heated magma to rise.

There are two general categories of volcanoes, those that erupt with forceful explosive power, like the 1980 eruption of Mt. St. Helens in Washington State and those that erupt in a more subdued fashion, like Kilauea on the big island of Hawai’i.  The difference in eruptive style is closely tied to the properties of the magma beneath the volcanic vent.  Those with more silica in their composition – felsic in the geologic parlance - tend to be more viscous and erupt with greater explosive force.  Those with more magnesium and iron (ferric) components -- mafic to geologists -- will tend to be less viscous and flow rather than explode.

Basaltic Lava Flow on Hawai’i

We will deal with the less viscous, mafic rocks in this column.  These are the dark-colored basalts of the world.  Keep in mind that we are talking about rocks rather than minerals.  Minerals have a specific chemical composition and defined characteristics regarding their crystalline structure.  Rocks are not so precisely defined.  Rocks are composed of minerals in varying proportions.  Thus, a rock name can encompass a range of compositions and characteristics.  As we talk about basalt, we will find distinct differences depending on the specific location.

Close-Up of a Dark-Colored, Fine Grained Basalt Specimen

There are plenty of wine regions that have basalt in their underlying bedrock. In the USA, the best-known basalt-based area is the Columbia Plateau of eastern Washington State.  Here is one of the world’s largest expanses of basalt flows.  The dark cliffs that loom above the Columbia River are basalts, often displaying the columnar appearance that the rock is noted for.  The columns develop as a physical function of rapid cooling when the basaltic lava stops flowing and solidifies. 

Although much of the Columbia Valley AVA is overlain with a thick layer of loess, there are areas where the basaltic bedrock is exposed and valued for its heat retention and greater iron content.  A new AVA on the Oregon side Walla Walla Valley, The Rocks District of Milton-Freewater, is based on the presence of basalt cobbles in the soil. 

Vineyard in The Rocks of Milton Freewater

Other basaltic areas are found throughout the wine world.  The island of Madeira is basically a solid block of basalt in the middle of the Atlantic Ocean.  The basalt weathers more quickly here in a moist, subtropical climate to soils that are rich in iron and calcium.  The soil is thought to inhibit uptake of phosphorous and potassium by the vines which results in higher acidity levels in the grapes that make the famously acidic wines of Madeira.

The Dundee Hills, Chehalem Mountains and Eola-Amity Hills of Oregon are all noted for the basaltic underpinnings of their soils.  There are distinct differences among soil types throughout the Willamette Valley.  The famous Jory soil that is found in the Dundee hills is based on basalt.  The deep red color is identified with some of the most famous sites for Pinot Noir in Oregon.  Domaine Drouhin, Archery Summit and Sokol Blosser are only a few of the many famous wineries within the “Red Hills of Dundee”.  It can make for delicious comparison to try wines from basalt soils next to those grown nearby, but on lighter colored, marine sedimentary soils.

Vineyard in the Chehalem Mountains of Oregon

Italy has its famous active volcano in Sicily.  Mt. Etna has erupted many times in recorded history and a basalt flow will occasionally destroy a vineyard in its path.  The wines of Mt. Etna, however, have gained great popularity with the best versions of the Nerello Mascalese grape grown here extolled for their delicacy and complexity.  In Hungary, the vineyards of Somló and Badgacsony near Lake Balaton benefit from the warmth of basalt.

In Germany, there are some notable vineyards on basaltic subsoils.  The Forster Pechstein (pechstein means “pitch stone” in German -- a reference to the dark basalt cobbles in the soil) has long been noted as a top site in the Pfalz, yielding wines of greater richness than surrounding vineyards.  In recent years the dry wines of Pechstein have gained well-deserved accolades.  In the Ahr Valley, the Heimersheimer Landskrone vineyard offers a similar advantage to the Spätburgunder grapes grown there.

When you next enjoy a Madeira or Chehalem Mountains Pinot Noir, think about the extraordinary processes that created their distinctive growing sites.  It’s just one piece of the puzzle that makes wines so individual and interesting.