A common misconception is that rocks grow in the same way living things do. The answer is no, not in the biological sense. A rock is an inorganic, naturally occurring solid aggregate of one or more minerals, and its increase in size is a purely physical and chemical process. The mechanisms by which rocks and mineral structures increase their mass or size are fundamentally different from biological growth. This difference lies in the absence of life processes like metabolism, cellular division, and genetic replication.
Defining Growth Biological Versus Geological
Biological growth, known scientifically as intussusception, involves increasing mass by adding new material from within the structure. This process requires a living organism to metabolize nutrients, replicate its DNA, and divide its cells, resulting in the organized enlargement of tissues and organs. A growing child, constantly building new cells from the food they consume, is a classic example of this internal, life-driven expansion.
Geological change, in contrast, involves accretion or deposition, which is the addition of material externally. This is a non-living process where new matter is physically or chemically added onto the surface of an existing structure. Rocks increase in size through these external mechanisms, which do not involve any internal living processes or metabolism.
Mechanisms of Geological Increase
The primary ways rock masses increase their size are through crystallization and accretion. Crystallization occurs when mineral crystals form and enlarge, such as within a cooling body of magma. As the magma cools, atoms arrange themselves into a crystal lattice, with new atoms attaching to the exterior of the existing crystal structure. A single mineral grain in an igneous or metamorphic rock can thus become larger, but this is purely chemical precipitation.
Accretion and deposition are responsible for the increase in size of entire rock formations. Sedimentary rocks grow by deposition, where layers of weathered material like sand, silt, or mud settle out of water or air and accumulate. Continental landmasses also increase through tectonic accretion, where large volumes of sediment and crustal fragments are scraped off a subducting oceanic plate and welded onto the edge of a continental plate.
Phenomena That Look Like Growth
Certain geological formations visually suggest growth, which often leads to the initial question of whether rocks can grow. Speleothems, such as stalactites and stalagmites found in caves, are prime examples. These formations are built slowly by the chemical precipitation of minerals from water. As mineral-rich water drips or flows, carbon dioxide is released, causing calcium carbonate to precipitate and deposit an incremental layer of mineral material onto the surface.
Concretions
Concretions form in sedimentary rock when a cementing material, like iron oxide or silica, precipitates from groundwater around a central nucleus, such as a shell or a grain of sand. This process builds the structure outward layer by layer.
Geodes
Geodes begin as hollow cavities, often formed by gas bubbles in volcanic rock or by the decay of organic material in sedimentary rock. Over thousands to millions of years, mineral-rich groundwater seeps into the hollow space, depositing crystals like quartz or calcite that grow inward from the walls.