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The Green Scene in Watershapes, March 2011

Beware of Exploding Lava Rocks
By Scott Cohen

In selecting materials, most of us know enough to think about how our choices will work when exposed to water. Only rarely, however, do we think about how those materials will perform when exposed to fire — which is being featured in more and more projects these days — and how they will perform over time.

Obviously, I’m not talking about simply choosing materials that don’t burn. Rather, what needs to be discussed is finding materials that won’t explode. Indeed, what we’ve run into is that some of the materials we have commonly associated with fire features can pose significant problems under certain circumstances.

Scenario: We built a beautiful fire-and-water feature in a client’s side yard: a 12-foot-long fire trough backed by a six-foot-tall waterfall spilling over a stacked-stone wall. As the water trickled down that rough surface, it generated a wonderfully soothing, rain-like sound. And although the water appeared to spill into the fire, it actually flowed into a basin set behind and below the fire trough.

Fire and Water Feature

(A side note: I’m a big believer that side yards are too-often ignored. In this case, the dining room had a large pair of French doors that looked out over this area, where we also installed an arbor that tied into the fire-and-water feature. Truly beautiful both outside and from inside the house.)

Long story short, as we were working, I read an article that warned against using river rock with fire: It tends to be nonporous, so heat builds up inside, sometimes to explosive levels. The recommendation was to use lava rock instead, which neatly confirmed the decision we’d already made to fill our fire trough with lava rock.

In this case, however, we didn’t consider the fact that small amounts of water would be splashing onto the lava rock from the adjacent waterfall. As a result, the extremely porous lava rock immediately absorbed the water, which almost instantly turned into steam and created pressure that built up inside the rock.

Much to everyone’s chagrin, after we initiated the feature and went on our way, it wasn’t long before small bits of heated lava rock (pea-sized and smaller) began popping off and flying in all directions at considerable velocity. Before anyone noticed what was happening, however, the homeowners had developed so much affection for the feature that they’d gotten into the habit of leaving the doors open so they could hear as well as see the water.

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Scott Cohen is president and supervising designer of The Green Scene, an outdoor design and construction firm based in Northridge, CA. He provides consultation for clients nationwide and gives seminars on designing landscapes, swimming pools and outdoor kitchens.