Geologic Age Dating Explained

The Re-Os isotopic system was first developed in the early 1960s, but recently has been improved for accurate age determinations. The main limitation is that it only works on certain igneous rocks as most rocks have insufficient Re and Os or lack evolution of the isotopes. This technique is good for iron meteorites and the mineral molybdenite. What’s more, if the whole rock is badly weathered, it will be hard to find an intact mineral grain containing radioactive isotopes.

The lower sandstone layer is disrupted by two faults, so we can infer that the faults are younger than this layer. But the faults don’t appear to continue into the coal seam, and they certainly don’t continue into the upper sandstone. The upper sandstone is youngest of all, because it lies on top of the coal seam. Thus, we have the concept of time vs. rock, in which time is an unbroken continuum but rocks may be missing and/or unavailable for study.


The decay rate is measured in terms of the half-life of the element, or the time it takes for half of the element to split into its daughter atoms. Some volcanic minerals and glasses, such as obsidian , contain uranium-238 . One of the most familiar applications of radioactive dating is determining the age of fossilized remains, such as dinosaur bones. Radioactive dating is also used to authenticate the age of rare archaeological artifacts. Because items such as paper documents and cotton garments are produced from plants, they can be dated using radiocarbon dating.

Principles of relative dating

For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when xenoliths are found. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them… The principle of inclusions and components explains that, with sedimentary rocks, if inclusions are found in a formation, then the inclusions must be older than the formation that contains them.

Evolution of life has been demonstrated by so many different means that to modern science it is a well-established Theory. Thus fossils of life that lived during specific times can be used to match up rocks of that age from one region to another. Image showing fossils that connect the continents of Gondwana . Wegener used correlation to help develop the idea of continental drift.Correlation is the process of establishing which sedimentary strata are of the same age but geographically separate. The earth is surrounded by a magnetic field generated by the magnetism in the core of the earth.

More commonly, the depositional age of a sedimentary rock cannot be determined directly. Instead, it may be narrowed down to a time range by dating igneous rocks that occur above, below, or within the sedimentary unit. For example, in Figure 2, the igneous intrusions numbered 2, 4, and 7 could be dated radiometrically. To determine the relative age of intrusive igneous rocks such as granite or gabbro, geologists rely on the principle of cross-cutting relationships. Intrusive rocks form when molten rock intrudes into and fills cracks in other rocks, then cools and crystallizes in place. The rock that was intruded (or “cut”) by the magma was there first and is therefore older than the intrusive rock (Fig. 2).

In recent years, a few of these methods have come under close scrutiny as scientists strive to develop the most accurate dating techniques possible. There are three types of unconformities based on the types of rocks present above and below the unconformity (Figure 6.4). A nonconformity is an unconformity where the rock type is different above and below the unconformity (Figure 6.4). For example, if uplifted intrusive igneous rocks are exposed at the surface and then covered with sedimentary rock, the boundary between the two rock types is a nonconformity. If the rocks above and below the erosion surface are both sedimentary, then the orientation of the layers is important. If the rocks below the erosion surface are not parallel with those above, the surface is called an angular unconformity (Figure 6.4).

Determining the numerical age of rocks and fossils

This method is based on the assumption that deeper layers of rock were deposited earlier in Earth’s history, and thus are older than more shallow layers. The successive layers of rock represent successive intervals of time. Both relative dating and absolute dating methods have their advantages and disadvantages. Relative dating is often used to make correlations between different sites or to compare materials from different areas. It is also useful when comparing material from different times, such as when studying a fossil. Relative dating cannot provide an exact date or year, only an approximate range.

The principle of original horizontality indicates that sediments are originally deposited as horizontal to nearly horizontal sheets. The exception happens on small scales, like cross-beds which form at an angle to the main bedding surfaces. Of the original rocks that have then metamorphosed and eroded creating the surface on which the sedimentary strata have accumulated.

To date past events, processes, formations, and fossil organisms, geologists employ a variety of techniques. These include some that establish a relative chronology in which occurrences can be placed in the correct sequence relative to one another or to some known succession of events. Radiometric dating and certain other approaches are used to provide absolute chronologies in terms of years before the present. The two approaches are often complementary, as when a sequence of occurrences in one context can be correlated with an absolute chronology elsewhere. Some minerals in rocks and organic matter (e.g., wood, bones, and shells) can contain radioactive isotopes. The abundances of parent and daughter isotopes in a sample can be measured and used to determine their age.