Fossils are not sedimentary rocks themselves, but are primarily found preserved within them. A fossil is any preserved remains, impression, or trace of a once-living thing from a past geological age, ranging from bones and shells to imprints, tracks, or even preserved DNA.
Sedimentary Rocks: The Ideal Environment
Sedimentary rocks form from the accumulation and cementation of sediments, particles derived from pre-existing rocks or organic matter. Sediments originate through weathering, where rocks break down, and erosion, where fragments are transported by water, wind, or ice. Over time, transported sediments are deposited in layers, often in bodies of water like lakes, rivers, or oceans.
As more layers of sediment accumulate, the weight from above compacts the lower layers. Minerals dissolved in water seep through compacted sediments, acting as a natural cement to bind particles together in a process called lithification, forming solid sedimentary rock. This layered formation, coupled with low temperatures and pressures, makes sedimentary environments uniquely suited for preserving delicate organic remains.
The Fossilization Process Within Sedimentary Rocks
The preservation of an organism as a fossil within sedimentary rock typically begins with rapid burial by sediment. This swift covering protects the remains from scavengers, physical disturbance, and decomposition by limiting exposure to oxygen and bacteria, preventing decay.
Several common types of fossilization occur within this sedimentary setting. Permineralization, a frequent method, happens when mineral-rich groundwater seeps into the porous spaces of buried remains, such as bone or wood. Minerals then precipitate and fill these empty spaces, effectively turning the remains to stone while preserving their internal structure. Molds and casts form when an organism’s body creates an impression (a mold) in soft sediment, and this mold is later filled with minerals, creating a replica (a cast) of the original organism. Carbonization occurs when organic material, like plants or soft tissues, is compressed under sediment, squeezing out liquids and gases and leaving behind a thin film of carbon that outlines the organism.
Why Fossils Are Absent in Other Rock Types
Fossils are rarely, if ever, found in igneous or metamorphic rocks due to the extreme conditions under which these rock types form. Igneous rocks originate from the cooling and solidification of molten rock, known as magma or lava. The temperatures involved in this process typically range from 600 to 1,300 °C (1,100 to 2,400 °F), which would incinerate any organic material, preventing fossil formation.
Metamorphic rocks form when existing rocks undergo significant transformation due to intense heat, pressure, or chemical alteration, without completely melting. These conditions, occurring deep within the Earth’s crust, can severely damage or destroy any organic remains that might have been present in the original rock. While some low-grade metamorphic rocks might rarely retain altered fossil traces, heat and pressure generally obliterate recognizable organic structures.
Exceptions to the Sedimentary Rule
While sedimentary rocks are the primary hosts for fossils, rare instances of preservation occur outside this typical environment. Some organisms are preserved in amber, which is fossilized tree resin. Insects, spiders, and even small vertebrates can become trapped in the sticky resin, which then hardens, encasing them in an airtight, watertight environment that prevents decomposition.
Another rare form of preservation involves organisms trapped in ice or tar pits. For example, woolly mammoths have been discovered remarkably preserved in permafrost, and ancient animals have been found in tar pits, where the sticky asphalt prevented decay and scavenging. These striking instances represent unusual circumstances where specific environmental conditions allowed exceptional preservation, contrasting with the widespread fossilization in sedimentary rocks.