Lava rock is an informal term for igneous rock that forms when molten material (lava) solidifies after being extruded onto the Earth’s surface during a volcanic eruption. As the lava cools rapidly, often upon contact with the atmosphere or water, it transforms into a solid, fine-grained rock. This resulting volcanic rock, frequently basaltic in composition, is a common feature in areas of volcanic activity worldwide.
Is Lava Rock Heavy? The Direct Answer
The short answer to whether lava rock is heavy is generally no, especially when compared to most common stones. Lava rock used commercially, such as for landscaping or grilling, is surprisingly lightweight due to its internal structure. This material is significantly less dense than typical igneous rocks like granite (around 2.7 grams per cubic centimeter) or standard concrete aggregate. Some extreme forms of lava rock, like pumice, are so light that they float on water. The common variety, often called scoria, can weigh approximately 1,300 to 1,500 pounds per cubic yard, making it a much lighter option than denser alternatives.
Understanding Porosity and Density
The lightness of lava rock is a direct result of its porosity, which refers to the volume of open space within the rock structure. Since density is mass per unit volume, increasing the amount of empty space significantly lowers the overall density. As molten lava erupts, it contains dissolved volcanic gases, such as water vapor and carbon dioxide, trapped under immense pressure. When the lava reaches the surface, the pressure drops quickly, causing these gases to bubble out rapidly, similar to opening a carbonated drink. If the lava cools and solidifies before these gas bubbles fully escape, they become trapped, forming small pockets called vesicles, which creates a highly vesicular, foam-like structure.
Comparing Common Volcanic Rock Types
The term “lava rock” encompasses several types of volcanic rock that vary widely in density and porosity. Scoria is a dark-colored, extrusive igneous rock characterized by a moderate level of porosity, with typically larger and interconnected vesicles. Its density generally falls in the range of 1.28 to 1.60 grams per cubic centimeter, causing it to sink in water. Pumice represents the extreme end of lightness, boasting an extremely high porosity, sometimes exceeding 90% by volume, and a density often less than 1 gram per cubic centimeter. This high volume of fine, isolated gas pockets enables pumice to float on water.
Basalt, the foundational rock of the oceanic crust, is the denser form of volcanic rock. When basalt cools more slowly or with less trapped gas, it forms a fine-grained, solid rock with low porosity, resulting in a significantly heavier material. When people refer to lightweight “lava rock,” they are generally referring to the vesicular types like scoria or pumice, rather than the denser, solid basalt.
How Low Density Impacts Practical Uses
The low density of vesicular lava rock translates directly into several functional benefits across various industries. In landscaping, the lightness of scoria makes it easier to transport and spread, reducing labor costs and allowing a single ton to cover a far greater area than standard gravel. The porous structure also provides superior drainage and aeration in garden beds, preventing soil compaction and promoting healthier root systems. This light weight makes lava rock a desirable aggregate for use in construction, specifically for creating lightweight concrete that reduces the overall weight of a structure without significant strength loss. The ability of the rock to absorb and evenly radiate heat is also leveraged in gas grills, where the low-density stones help distribute heat across the cooking surface efficiently.