Geological Events, Dating Methods, and Fossilization
Geological Events and Dating
Sequence of Events
To order past geological events chronologically, we use the principles of horizontality and superposition of strata. Another approach is the principle of cross-cutting relationships. This principle states that anything that affects geological materials and structures is post-dated to them. Because it establishes the order in which events unfold, it’s also called the principle of succession of events.
Geological Dating Methods
Radioactive Isotopes
Each chemical element has a constant number of protons in its nucleus, which is its atomic number. The nucleus of every atom also contains neutrons. The sum of protons and neutrons is called the mass number. Atoms of the same element with different mass numbers are called isotopes. For example, hydrogen has three isotopes: protium, deuterium, and tritium. Some isotopes are unstable and spontaneously become stable. This transition occurs with the release of radioactivity. Radioactive decay releases energy, making these elements an important source of heat for our planet. The initial radioactive element is called the parent element, and the final stable element is the daughter element. The half-life or decay period is the time it takes for the radioactive isotope to reduce its number by half.
Radiometric Dating
Radiometric dating is the calculation of absolute ages based on the decay of radioactive elements. Major radioactive isotopes include:
- Parent element: potassium-40, uranium-235, carbon-14
- Daughter element: argon-40, lead-207, nitrogen-14
- Half-life: 1300 Ma, 713 Ma, 5570 years
Other Methods of Dating
- Glacial Varves: Lakes are covered by a layer of ice in winter that melts in summer. Each year, two layers of sediment are deposited: a light and a dark layer. The light layer forms in summer and consists of thicker materials. Very fine materials remain in suspension and are deposited in winter. Counting the number of pairs of bands indicates the number of years during which the sediment has been forming.
- Growth Rings of Corals: Some ancient bell-shaped corals deposit a thin layer of calcium carbonate daily. A small constriction marks the separation between each year, making it possible to count the number of layers (days) each year. Counting the rings of coral indicates the number of days in that year and, hence, its age.
Fossils and the Information They Provide
Fossilization
The process of fossilization:
- An organism, such as a mollusk, dies and is buried by sediments.
- Soft parts rot, but the shell remains long enough to leave its outer surface imprinted in the sediment. Sometimes, small mineralogical transformations occur; for example, the aragonite of the shell more frequently becomes calcite.
- The shell dissolves, and the gap left is filled by minerals contributed by circulating water. This forms the outer mold of the organism. On other occasions, the sediment covering the shell is also introduced into its interior, resulting in an internal mold.
Other Fossilization Processes
- Preservation in Amber: Amber is the fossilized resin of conifers. Sometimes, organisms, especially insects, are trapped in the resin, preserving them from bacterial decomposition and predation.
- Conservation in Asphalt: Oil, found in the subsoil under strong pressure, sometimes escapes to the surface. After oxidation and evaporation, a residue of asphalt remains. Any animal that falls into a pool of asphalt will be preserved because the asphalt prevents bacterial action.
- Icing: (Preservation by freezing)
Information Provided by Fossils
Fossils provide information about:
- Life in the past
- The environment in which the rock containing the fossil formed
- When the rock containing the fossil was formed
If we know when a particular organism lived, we can determine the age of the rock in which it is fossilized, or, in other words, date it. Not all fossils are equally useful for dating a rock. The best fossils for dating are called index fossils or characteristic fossils and must possess three characteristics:
- Having lived for a geologically short period of time, which makes the age of the rock clear.
- Having had a wide geographical distribution, making them useful beyond a local context.
- Being abundant in sedimentary rocks, making them frequently useful.
Facies
Facies are a set of lithological and paleontological features that help determine how a rock originated. When referring only to lithological features, we speak of lithofacies, and when referring to paleontological features, we speak of biofacies.