Lava Rock

Lava rock, geologically classified as scoria, forms when gas rich molten rock cools quickly at the surface, trapping air bubbles inside as it hardens. Those trapped bubbles are what give the finished stone its vesicular, sponge like texture and its low density compared to river rock, pea gravel, or crushed limestone, all of which form through very different, denser processes. After quarrying, the rock is crushed and screened into grades from fine 3/8 inch chips up to golf ball sized 1.5 to 2 inch nuggets. Color is determined by the mineral content of the source rock, with red and black being the two standard commercial colors and grey showing up as a secondary option. Red lava rock is generally more porous and more prone to fading, while black lava rock tends to hold its color and edge sharpness longer, which is one reason it is favored for fire features.

Start by removing existing vegetation and debris, then lay landscape fabric across the bed. Skipping the fabric is one of the most common mistakes, because without it lava rock gradually sinks into the soil and weeds root up through the gaps over time. Install a metal, plastic, or stone edging border before spreading rock, since lava rock is light enough to migrate or wash out of unedged beds during heavy rain, more so than denser river rock. Spread the rock 2 to 3 inches deep and rake it level, wearing gloves, since the sharp, angular edges are abrasive on bare hands. Do not use lava rock as a structural base under pavers, patios, or driveways, it has poor load bearing characteristics and will not compact into a firm surface the way crushed gravel or decomposed granite does.

Compared with river rock and pea gravel. Lava rock is 30 to 50 percent lighter per cubic yard, more porous, and sharply angular, while river rock and pea gravel are dense, smooth, and rounded. That weight difference means lava rock is easier to haul but also easier to displace, and its rough edges make it less comfortable underfoot.

Compared with organic mulch (bark, wood chips). Lava rock does not decompose, so it never needs replenishing like bark mulch does. It adds no nutrients to soil, retains more heat in direct sun, and is much harder to till under or remove if you later redesign the bed.

How much lava rock do I need. Measure your bed's square footage, then multiply by depth to get volume. At the standard 3 inch depth, one cubic yard covers about 108 square feet, at 2 inches it covers about 162 square feet, and at 4 inches about 81 square feet. This coverage math is based on volume, so it applies the same whether you choose lava rock, river rock, or gravel. Convert to tons only after you know your yardage, since lava rock's lighter weight per yard means a ton goes noticeably further than it would with denser stone.

How it is sold and delivered in bulk. Bulk suppliers price lava rock both by the cubic yard and by the ton, and because it is roughly 30 to 50 percent lighter than gravel or river rock per yard, it is worth confirming which unit you are quoted before ordering. A ton of lava rock typically covers noticeably more ground than a ton of denser rock would, so ordering by weight without accounting for that difference is a common way to under order for the coverage you actually want.