The method of reading the order is called stratigraphy layers of rock are called strata. Relative dating does not provide actual numerical dates for the rocks. Next time you find a cliff or road cutting with lots of rock strata, try working out the age order using some simple principles: Sedimentary rocks are normally laid down in order, one on top of another. In a sequence, the oldest is at the bottom, the youngest is at the top. Most sedimentary rocks are laid down in flat horizontal layers, although these can later tilt and fold.
Layers of sedimentary rock extend sideways in the same order. A later event, such as a river cutting, may form a gap, but you can still connect the strata.
Fossils and relative dating Fossils are important for working out the relative ages of sedimentary rocks. Throughout the history of life, different organisms have appeared, flourished and become extinct. Many of these organisms have left their remains as fossils in sedimentary rocks. Geologists have studied the order in which fossils appeared and disappeared through time and rocks.
This study is called biostratigraphy. Fossils can help to match rocks of the same age, even when you find those rocks a long way apart.
This matching process is called correlation, which has been an important process in constructing geological timescales. Some fossils, called index fossils, are particularly useful in correlating rocks.
For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places.
For example, ammonites lived in the Mesozoic era. If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic. Different species of ammonites lived at different times within the Mesozoic, so identifying a fossil species can help narrow down when a rock was formed. Correlation can involve matching an undated rock with a dated one at another location.
Suppose you find a fossil at one place that cannot be dated using absolute methods. That fossil species may have been dated somewhere else, so you can match them and say that your fossil has a similar age. Some of the most useful fossils for dating purposes are very small ones.
For example, microscopic dinoflagellates have been studied and dated in great detail around the world. Correlation with them has helped geologists date many New Zealand rocks, including those containing dinosaurs.
Activity idea Bring relative dating principles to life with the activity Rock layers and relative dating. The activity offers literacy opportunities as well as practice using the science capability 'Interpret representations'.
Two broad categories of classification methods are relative dating and absolute dating. In a sequence, the oldest is at the bottom, the youngest is at the top. The table below shows characteristics of some common radiometric dating methods. This means that the oldest are the strata that are lying at the bottom. Craters are very useful in relative dating; as a general rule, What is relative dating in biology, the younger a planetary surface is, the fewer craters it has. This is possible because properties of rock formations are closely associated with the What is relative dating in biology of the artifacts found trapped within them. Scientists from the former Soviet Union lead the study of melt inclusions in the decades after World War II Sobolev and Kostyuk,and developed methods for heating melt inclusions under a microscope, so changes could be directly observed. As organisms exist at the same time period throughout the world, their presence or sometimes absence may be used to provide a relative age of the formations in which they are found. A similar situation with igneous rocks occurs when xenoliths are found.
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