Not to mention the rise of life and the record of mass extinctions, including the Cretaceous-Paleogene extinction event 66 million years ago, infamously known for ending the age of non-avian dinosaurs.
These changes usually occur so slowly that they are totally undetectable throughout the life of a human being. So how do we know all this then? How can we date, for example, a newly discovered fossil, a geological event, or, as highlighted above, the age of the Earth?
Well, earth scientists, or more specifically, “geochronologists,” have come up with their own clock using various dating techniques to accomplish this goal.
Interesting Engineering (IE) explains everything you need to know about the three main approaches.
How do earth scientists date rocks and fossils?
Geologists use a wide variety of different techniques to address the question, “How old is this fossil/rock?” These tend to fall into one of three general approaches that allow geologists to date rocks and fossils, including the use of radioactive isotopes (absolute geochronology) and methods for relative dating, such as the measurement of stable isotope ratios and paleomagnetism.
In a nutshell, relative dating arranges events or rocks according to their chronological order of occurrence. Absolute dating is more specific: it assigns more precise dates and times to fossils, rocks, or events. For example, lining up family members from oldest to youngest or guessing their ages based on their appearance or other information is relative dating, while finding each person’s exact age is absolute dating.
What methods does relative dating use?
A common method for relative dating is stratigraphy, or the study of rock strata. This involves applying a set of principles to the volcanic and sedimentary rocks that are exposed on the Earth’s surface to determine the relative ages of geologic events preserved in the rock record.
Overlap
Sedimentary rock layers at Zabriskie Point, Death Valley, USA
One of the commonly used principles in stratigraphy is superposition. Generally, the upper layers of a group of rocks (‘formation’) are younger than those below. Naturally, this will only be accurate if the sedimentary layers have maintained their chronological order (in order). That is, they have not been interrupted.
If sedimentary rocks are disturbed by events, such as fault movements, that traverse layers after the rocks have been deposited, then the principle of cross-sectional relationships is used. This establishes that any geologic features that cut through the strata must have formed after the rocks they passed through.
Examining these relationships allows us to order the geological events in a place. However, it cannot be used to determine the relative ages of rocks in different areas. In this case, fossils can be useful tools for understanding the relative ages of rocks.