Wednesday, July 13, 2016

Landscapes and soils of the Mt Etna grape-growing region

"All Etna soil rests on, or directly derives from, lava that flowed and hardened for thousands of years, along with ejected pumice, lapilli, and windblown volcanic ash" (Nesto and di Savino). I explore the origins, composition and deployment of the region's soils in this post.

Historically, eruptive events at Mt Etna have been of the Strombolian style but occasional Hawaiian-style eruptions generate considerable lapilli fall on the flanks (The Strombolian and Hawaiian styles are described in the table below.). Large active volcanoes with the Etna eruptive style present some of the most complex soil-forming environments on earth (James, et al.).

Factors such as diversity in age and characteristics of volcanic materials, land surface morphology, local climate, vegetation, and land-use history all contribute to complex soil spatial patterns. In the profile dimension, complex soils result from intermittent tephra deposition, anthropogenic disturbance (in the case of Mt Etna, over 70% of the vineyards are terraced), erosion and subsequent deposition. According to James, et al., "soil profiles may reflect the amount and frequency of tephra deposition as much as 'normal' profile-forming soil processes operating on stable surfaces."

Landscape Formation
Volcanism in the Etna region began during the middle Pleistocene, at around 600 ka. The peak today stands at 3350 m elevation and the base is 40 km across. At elevations below 1100 m, lava varies in age from the 2014/2015 flow to the 500,000-year-old tholeitic basalts of a small area on the lower part of the southern flank (James, et al.). The terrain of historical (12th century to today) flows, as well as some pre-historic flows, is dominated by aa lava (basaltic lava with a rough surface, pahoehoe (basaltic lava with a smooth or billowy surface), and toothpaste (transition between aa and pahoehoe) morphology (James, et al.).

On Etna, depositive explosive activity from the summit crater is frequent with less frequent eruptions, often with higher effusive rates, from the flank vents and Strombolian activity from vents high on the volcano. The tephra varies in deposition rate and particle size with distance and direction from the source and accumulates unevenly on rugged lava surfaces. As an example, areas on the western and northwestern slopes of Mt Etna are barren rockscapes due to insufficient topsoil for significant vineyard development.

Soil Formation
As described above, volcanic activity of Mt Etna is both effusive (lava flows) and explosive (airborne ejection of pyroclastics). According to Nesto and di Savino, the lava flows create a patchwork of terroirs that is pertinent to any discussion of Etna contradas. Initial flows are barren rock pasteurized by heat which, after cooling, require hundreds of years to erode into soil and develop hummus, and, in so doing, become suitable for vines. The erosion product is sand rich in potassium and other minerals. Organic matter, created initially by the growth of micro-organisms (and later by plants and animals), results in rich, fertile soil.

But, according to Marco Perciabobco (Department of Agriculture, Sicily Region), soil parent material in the Etna environment is primarily pyroclastic material (My prior post on volcanic soils detail the weathering of these materials). Weathering of this coarse-textured parent material, according to Marco, produces soils with an "aerated hypogeal (underground) environment and the following characteristics:
  • Extremely well suited for the growth and development of vine roots
  • Soil water stagnations are rare
  • They warm easily (this generates stable conditions for the occurrence of the chemical reactions required for the weathering of the finest materials.

Soil Distribution
According to Perciabobco, the Department of Agriculture's soil survey dataset shows five different landscape systems in Etna: northern; northwestern; eastern; southeastern; and southern. The soils of these environments differ in the degree of weathering of the primary clay minerals. From north to south wetness decreases and so does weathering of the volcanic constituents. The soils of the northern landscape, when compared to the soils of the south, are finer textured, have a higher organic matter content, and a have a higher value of cation-exchange capability.

Bill Nesto MW and Frances di Savino, The World of Sicilian Wine.
James, et al., Development and spatial distribution of soils on an active volcano: Mt Etna, Sicily.
Sonia Calvari and Harry Pinkerton, Lava Tube Morphology on Etna and evidence for lava flow emplacement mechanisms, J Volcanol Geotherm Res 90 (3-4) 1999.

©Wine -- Mise en abyme

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