Which media type is primarily mineral-based volcanic rock?

The correct answer is perlite, which is a type of media that is primarily made from volcanic rock. Perlite is formed from obsidian, a natural volcanic glass, that is heated to high temperatures causing it to expand and form lightweight, porous particles. This unique structure provides excellent aeration, drainage, and moisture retention, making it an ideal growing medium for a variety of plants, particularly in container gardening and hydroponic systems.

The other options listed do not share this mineral-based volcanic characteristic. For example, pine bark is organic, derived from the bark of pine trees, and is commonly used for its ability to improve aeration while retaining moisture. Sawdust and rice hulls are also organic materials, with sawdust coming from various types of wood used in milling, while rice hulls are the protective coverings of grains of rice. These organic materials provide different benefits like moisture retention and nutrient content but do not have the mineral properties associated with volcanic rock that are characteristic of perlite.