Lava is rock in a liquid state, heated deep within the Earth’s crust. When this molten material erupts onto the surface, a question arises: Can solid rocks float on their liquid counterpart? Given that most rocks are formed from solidified magma, it seems counterintuitive they could remain suspended. The mechanics governing whether an object sinks or floats rely entirely on the comparative physical characteristics of the solid object and the liquid medium it encounters.
The Science of Buoyancy in Molten Rock
The ability of any object to float on a fluid is governed by the principle of buoyancy. This principle depends on the relative densities of the solid object and the liquid medium it is immersed in. Density is a measure of how much mass is contained within a specific volume. A rock will float only if its average density is lower than the density of the surrounding lava it displaces.
If the rock is denser than the lava, the buoyant force exerted by the fluid will be insufficient to support the rock’s weight, causing it to sink. Conversely, if the object is less dense, it will remain at the surface. A rock must be less dense than the specific lava flow it encounters, regardless of the extreme temperature of the molten rock.
Lava itself is highly dense, often measuring between 2,500 and 2,700 kilograms per cubic meter. For a rock to float, its overall density must fall below this specific threshold of the molten material.
Factors Determining Lava’s Density
The density of lava is not constant and varies based on its chemical composition, primarily the silica content (\(\text{SiO}_2\)). Lavas high in silica, known as felsic lavas, tend to be less dense and often have a more viscous consistency. These flows are typically associated with continental crust.
Conversely, lavas low in silica and rich in magnesium and iron (mafic lavas) are significantly denser. Basaltic lavas, common in oceanic crust and shield volcanoes, are generally more fluid and have a higher density.
Temperature also plays a role in altering the density of a lava flow; hotter lava is less dense because increased thermal energy causes molecules to spread slightly apart. More significantly, the presence of dissolved or trapped gases within the liquid magma can lower its overall density. Gases create bubbles that effectively lighten the overall mass per unit volume of the molten rock. A fresh eruption will be less dense than the cooler, crusting edges of the same flow due to both higher temperature and higher gas content.
Why Certain Rocks Float (And Most Do Not)
The vast majority of common rocks will sink rapidly in a lava flow. Rocks like granite, basalt, and limestone have a mineral density that far exceeds the density of molten rock. These dense, crystalline structures contain very little empty space, ensuring they plummet through the liquid.
However, certain igneous rocks demonstrate the opposite behavior. The exceptions are highly vesicular rocks, such as pumice and scoria, formed when gas-rich magma rapidly cools and solidifies. These rocks are characterized by an extremely porous internal structure, riddled with gas bubbles frozen in place.
Pumice often has an effective density low enough to float on water. Floating occurs because trapped air pockets make up a large percentage of the rock’s volume. This drastically reduces the overall density of the rock-plus-air structure.
Scoria shares this vesicular nature, though its pores are larger and less numerous than pumice, making it denser and less likely to float. For these rocks to remain suspended, the gas bubbles must be sealed off. If the lava quickly fills the voids, the rock’s effective density increases instantly, causing it to sink.