I'm personally not a big fan of lava rock in most situations - it kind of looks out of place. Other types of rock tend to look more natural. I'm not an expert, but based on my DIY knowledge mulch is no worse than rock against your foundation, as water actually seeps through rock faster than mulch. The problems arise when you have plants or grass that require watering too close to foundation. The most important thing to ensure is that the ground UNDER the rocks or mulch is compacted & sloped away from the house to keep water from pooling against foundation. Any homebuilder or civil engineer should be able to tell you recommended slope for your area/soils, but genrally +/- 1 foot of slope in first 10 feet away from house is sufficient.

I know this is an old thread but I wanted to update what I did over time to get "rid" of the lava rock problem that we had. This last week, I finally scraped alot of the remaining rock out of the large bed and spaded topsoil, compost and some organic fertilizer into the soil. However quite a bit of the lava rock remained and I spaded that in along with the amendments. I simply could not get all the rocks out with a flat type of coal shovel. I double dug the areas adding amendments each time along with organic fertilzer, soy pellets, etc. (Not alot of fertilizer but a sprinkling to get things cooking along with the compost in there.) Of course this was heavy, sweaty work but it is finally finished off. I wish I had been able to get almost all of the lava rock out of there but too much of it had already been mixed in with soil over the years and I simply couldn't get it all out without removing ALOT of the soil along with it. But I was able to spade the vast majority of it back into the amended soil before adding a final layer of topsoil, compost, soy pellets and wood mulch. (In the end I probably added back 4 inches of amendments to make up for the removed lava rock and put another 3 inches of wood mulch on top of all of that.) So now most of the remaining lava rock has finally been worked into the soil where it can hopefully decompose more quickly and become part of the soil. I NEVER recommend lava rock or landscape fabric to anyone but if you move into a house that has them you can probably get away with leaving some to dig into the soil. I say this because after all the amendments and digging in of the remaining lava rock (which was still a fair amount of lava rock) the final "soil" drains well. (But it took ALOT of work that wouldn't have been necessary if I had never had the lava rock in the first place.) The plantings are doing very well but I'll know more this summer when the wife plants flowers into this newly amended soil with some lava rock spaded in. Time will tell. In the end though, I WILL NEVER use lava rock or weed barrier fabric on my garden ever again. These were the two biggest mistakes I have ever made in all my years of gardening. Just my personal opinion. (The reason I got rid of mine and/or incorporated the remaining lava rock into the soil was it had become unsightly over time and some parts stayed too damp and a small bit of moss was growing on some of them. This situation had to be corrected because it was really bugging me. LOL!) Finally, I was surprised to find quite a few earthworms in this old "lava rock" soil when spading in all the new stuff. That did really surprise me, I didn't expect to see earthworms in this old stuff. Now the earthworms have a good soil to live in!! (I hope!!)

There are any number of articles about this online and the consensus seems to be it is a moisture issue. Lava rock is very porous and will absorb and retain water in all those little nooks and crannies indefinitely. Or until the heat of the gas flame turns that water to steam and the rocks explode and go flying. You say you 'let them dry'....what exacty was involved with this drying? Just letting them set until they looked dry was probably insufficient. You might need to manually dry them in the oven (at low heat). Or one source suggested turning on the gas to a very low flame to let the water slowly but fully evaporate out of the rocks without triggering an explosive response. Or you can replace the lava rocks with fire glass, which is completely nonporous, tempered to withstand high heat and has a much higher degree of heat reflectivity.

I just saw this, sorry it's been a year!! I did this exact thing, using red lava rocks. After a year or two, though, they had all turned brown and looked very ugly! Worse, my cats took it as a challenge and tried to use it as a litter box. And my beagle thought it would be awesome to pull the lava rocks out and try to eat them!! I ended up having to dig them all out. So I would not recommend doing it.