Why are wines from volcanic soils so good?

Hello there! I'm Dr. Vinifera, but you can call me Vinny. Ask me your toughest wine questions, from the fine points of etiquette to the science of winemaking. And don't worry, I'm no wine snob—you can also ask me those "dumb questions" you're too embarrased to ask your wine geek friends! I hope you find my answers educational, empowering and even amusing. And don't forget to check out my most asked questions and my full archives for all my Q&A classics.
Dear Dr. Vinny,
If science has yet to prove that soil affects wine flavor, why are wines from volcanic soils so consistently good?
—Silvestro, Lecce, Italy
Dear Silvestro,
Soil absolutely affects a wine’s flavor, but it’s usually thought to do so indirectly. The way a soil retains water, reflects or absorbs sunlight and so forth affects how vines develop and, therefore, the resulting wine’s flavor. The only thing that is not proven or universally understood is whether you can taste the minerals or other flavors of the soil in the wine directly. I know I can’t taste clay or limestone in my wine, but it somehow makes sense when I learn that’s where the grapes were grown.
You’re right that volcanic soil is considered among some of the best for growing winegrapes. It’s found in Napa Valley, Willamette Valley and Mount Etna in Sicily, among many other places. But other types of soils are found in some other terrific wine regions too. Plus, there’s a huge variety of types of volcanic soil, as well as the vineyards they are located in. But in general, volcanic soils encourage the development of desirable flavor and texture compounds in winegrapes; such soils are porous and provide good drainage, which causes grapevines to grow deep to seek nutrients.
—Dr. Vinny