Density, defined as mass per unit volume, is a fundamental property of matter that changes predictably as rock material cools from a molten state. Magma, or molten rock, is a high-temperature liquid mixture. As it loses heat, its volume generally decreases, causing its density to increase as it transitions from a liquid to a solid. This density increase is a multistage process, beginning with a gradual change in the liquid state and culminating in a significant jump upon crystallization.
The Initial Change: Thermal Contraction
The initial phase of cooling occurs while the rock is still in its liquid state as magma or lava. In this molten phase, intense heat gives constituent atoms high kinetic energy, causing them to move rapidly and occupy a large volume. This disorganized arrangement contributes to the melt’s lower density compared to the final solid rock.
As the temperature of the magma drops, the kinetic energy of the particles decreases. This reduction in energy causes the particles to slow down, allowing the material to pack together more closely. This results in a measurable decrease in volume.
This process is known as thermal contraction and occurs gradually throughout the liquid phase. The density of the molten rock steadily increases, even before any crystals begin to form, because the same mass occupies a smaller space.
Density Gain During Solidification
The most substantial density increase occurs when the molten rock transitions to a solid crystalline structure. Molten rock has a random, disorganized arrangement of atoms, which is less volume-efficient and contains more empty space between particles.
As the temperature falls to the crystallization point, atoms bond and arrange themselves into orderly, repeating three-dimensional patterns called crystal lattices. This crystalline structure is intrinsically more volume-efficient than the chaotic liquid arrangement. The organized packing minimizes empty space, resulting in significant compaction of the material.
The rate of cooling plays a significant role in the efficiency of this packing. Intrusive rocks cool slowly deep beneath the surface, allowing time to form large, well-ordered crystals, which leads to higher intrinsic density. Conversely, extrusive rocks cool rapidly on the surface and may form volcanic glass, an amorphous solid that is less densely packed than a fully crystalline rock. Slowly cooled igneous rocks generally exhibit greater density than their rapidly cooled counterparts of similar chemical composition.
Compositional Factors Modifying Bulk Density
While the intrinsic material density increases upon solidification, the final bulk density of the rock can be significantly modified by volatile components. Magma contains dissolved gases, such as water vapor and carbon dioxide, which remain in solution under high pressure deep within the Earth. As the magma rises toward the surface, the confining pressure drops, decreasing the solubility of these gases.
The dissolved gases exsolve, or come out of solution, forming bubbles within the melt, a process called vesiculation. If the magma is erupted quickly, these gas bubbles expand rapidly and become trapped within the solidifying rock. The resulting air-filled voids, known as vesicles, can make up a large fraction of the rock’s total volume.
The inclusion of these gas-filled spaces drastically lowers the overall bulk density of the final rock. For example, pumice is a rock formed from rapidly cooled magma that is so highly vesicular it can float on water. This effect highlights the distinction between the intrinsic density of the solid rock material and the overall bulk density of the rock mass, which is heavily influenced by its porosity.