Arranging rocks around an outdoor fire is a common practice. However, placing wet rocks near a fire can introduce an unexpected and considerable hazard. This carries a risk of explosion, sending rock fragments flying with dangerous force. Understanding this potential danger is important for anyone building a fire.
The Science of Rock Explosions
Wet rocks can explode in a fire due to water’s behavior when subjected to intense heat. Rocks naturally contain small pores and crevices, trapping water within them. As the fire heats the rock, this trapped water begins to convert into steam. Water expands dramatically when it changes from liquid to gas, increasing its volume by approximately 1,700 times.
This rapid expansion creates immense pressure inside the rock. If the rock is not sufficiently porous or if the steam cannot escape quickly enough, the internal pressure can build until it exceeds the rock’s structural integrity. This sudden pressure release causes the rock to fracture violently, or “explode,” propelling rock fragments and embers outward at high velocity. Such an explosion can occur quickly and without warning, posing a threat to anyone nearby.
Which Rocks Pose a Risk
The likelihood of a rock exploding depends on its physical properties, particularly its porosity and moisture content. Porous rocks, with many internal spaces, are more susceptible to absorbing water. Examples of highly porous rocks that pose an increased risk include sandstone, limestone, shale, and pumice. These types of rocks can hold substantial amounts of moisture, even if they appear dry on the surface.
River rocks, or those collected from other water bodies like lakes or streams, are especially dangerous. Their smooth, rounded appearance often indicates prolonged water exposure, meaning they are likely saturated. In contrast, dense, non-porous rocks such as granite, basalt, or slate are generally safer choices because they absorb less water. However, even denser rocks can pose a risk if they contain cracks or fissures that trap water.
Ensuring Fire Safety
To prevent rock explosions and ensure fire safety, careful material selection and handling are essential. It is advisable to choose dry, dense rocks that are located away from water sources. Visually inspecting rocks for signs of moisture, cracks, or fissures can help identify those that might be problematic. Rocks found near rivers, lakes, or beaches should always be avoided due to their high potential for water saturation.
When constructing a fire pit, use materials designed for high heat, such as fire-rated bricks, steel, or cast iron fire rings. Lava rocks are another safe option because their porous nature allows them to release moisture without building pressure. Maintaining a safe distance from the fire is also important to minimize risk from flying debris. Additionally, never throw rocks into an active fire, as this can cause sudden temperature changes and increase the risk of an explosion.