What is Volkanix Terra Lava Rock? Lava rock is a form of igneous rock called Scoria that is created by the eruption of volcanoes. Scoria is formed when the molten lava solidifies, in that process air is trapped creating the porous, brownish red lava rock. The Volkanix Terra range is produced and graded specifically for the horticultural market and each rock is sized between 8–32mm How to use Volkanix Terra Lava Rock? Volkanix lava rock can be used a top dressing or mixed into your existing soil. All plants will benefit from its amazing properties. Plants that do not tolerate water-logging and over watering, will really thrive including Cacti, Succulents and Bonsai.