Clark Larsen Answers Students FAQs

1. Question: What are the two major classes of geologic dating and how do they differ?

Response: The two major types of dating include relative methods and absolute methods. Relative methods focus on comparison of fossils, strata, or other artifacts to determine which is older and which is younger. They do not provide “exact” dates for the materials. Relative methods include stratigraphic correlation, which matches strata from different sites that are temporally comparable. Other relative methods include chemical dating, biostratigraphic or faunal dating, and cultural dating. Chemical dating, such as fluorine dating, compares the relative amounts of absorbed fluorine in bones from the same site. Biostratigraphic or faunal dating utilizes known dates of appearance or extinction for specific fauna to provide approximate dates of the strata in which they are found. Cultural dating utilizes changes in form of various cultural objects, such as tools or pottery, to provide a general date for a stratum or site. Absolute dating uses chemical and physical processes to provide a date range for an object, fossil, or site. Absolute methods include radiometric and non-radiometric methods. Radiometric methods, such as radiocarbon dating and radiopotassium dating utilize the regular radioactive decay of isotopes from one form to another. Measuring the ratio of the two forms provides date information. Non-radiometric methods utilize other processes to assess a date. Non-radiometric methods include dendrochronology, or tree-ring dating, amino acid dating, fission track, paleomagnetic dating, electron spin resonance, and luminescence.

See Figure 8.13 Fluorine Dating


2. Question: What is radiocarbon dating?

Response: Radiocarbon dating or carbon-14 dating is based on isotopes, or alternate, forms of carbon. An organism takes in 14C, a radioactive or unstable isotope, and 12C, a stable isotope, during life. Both carbon isotopes are found in plant material, that acquires carbon through the use of carbon dioxide in photosynthesis. The ratio of 12C:14C in an organism remains constant throughout life. Once an organism dies, however, the unstable 14C begins to decay into 14N, and the ratio of 12C:14C alters. By measuring this ratio, researchers are able to calculate how long the organism has been dead. The time it takes for half of the original amount of 14C to decay into 14N is referred to as a half-life. The half-life for 14C is 5730 years. This relatively short half-life means radiocarbon dating can only be used for sites within the past 50,000-70,000 years.

See Figure 8.22 radiocarbon dating


3. Question: What are fossils and fossilization?

Response: Fossils are the remains of deceased organisms that have become rock, enabling them to be preserved for thousands or millions of years. Inorganic materials, such as iron and silica slowly and steadily replace the organic materials, like calcium and phosphorus, in the organism. Fossils are found in a variety of rock, but most often in sedimentary rock, which forms from the accumulation of sediments. The replacement of organic materials with inorganic materials is not always complete, as DNA is sometimes preserved within the fossil. The DNA can then be analyzed to learn more about the organism. Any living organism can become fossilized, including bacteria, plants, and animals. The chances of fossilization are very slim, because it requires that the remains be quickly buried and be kept in an oxygen-free environment. Scavenger and bacterial activity must be prevented so that decomposition is limited. These specific required conditions explain why fossils are so rare.

See Figure 8.4 What Is in a Fossil?: The Making of the Biological Past


4. Question: What are the limitations of the fossil record?

Response: Since fossilization requires very unique circumstances, fossils are very rare. As a result, fossils do not necessarily provide a complete representation of the past. The more fossilized remains available for study, the more information can be obtained and a more complete understanding achieved. The incomplete fossil record is due in part to lack of discovery, as only some areas are the focus of excavation and investigation. In addition, local climatic and environmental conditions determine whether fossilization will occur, so that there is differential preservation among different locations. Moreover, the rock sequences are not always complete in all places, so certain periods cannot be investigated. Paleoanthropologists focus much of their research in East and South Africa, because these areas have the most ideal conditions for fossilization and access to the deep layers containing the fossils. While early hominids likely lived throughout the entire continent, it is in the specific regions where paleoanthropologists have had the most success at discovering fossils.


5. Question: What is Steno’s law of superposition and how does it apply to stratigraphic dating?

Response: Niels Stensen created the Law of Superposition, which is the foundation for relative dating methods. This law states that the youngest objects or layers are on the top, while the older objects and layers are deeper. As the earth builds up layers, one on top of the other, the newest and youngest layers are on the surface. As you move down from the surface, the layers get progressively older. This law is the important for stratigraphic dating, because fossils, bones, or other objects incorporated into these layers can be compared to each other to determine relative ages. Those materials in the lowest layers are the oldest, while those that are in the highest levels are the youngest. This law allows a general age sequence to be established for layers or strata at a site or location.