Question

Why can't we date sedimentary rocks directly? How do we assign numerical ages to intervals on the geologic column, to produce a geologic time scale?

Short answer

We cannot date sedimentary rocks directly because they are formed from accumulated and cemented particles, making their age different from the particles they contain. Radiometric dating, which works for igneous and metamorphic rocks, is not suitable for sedimentary rocks due to the lack of useful isotopes. Instead, we assign numerical ages to intervals on the geologic column by dating interbedded igneous rocks using radiometric dating methods. This provides a minimum or maximum age for the sedimentary layers, allowing us to estimate the numerical age of intervals on the geologic time scale. The geologic time scale is developed through the correlation of rock layers, fossils, and radiometric dating methods, providing a quantitative record of Earth's history and insights into the evolution of organisms, environments, and geologic events.

Step-by-step solution

Understand Sedimentary Rocks

Sedimentary rocks are formed from the accumulation and cementation of mineral and organic particles. These rocks form layers (strata) that can provide information about Earth's history. However, the age of the sedimentary rock itself can be different from the age of the particles it contains, as the particles may come from much older rocks. Time spent between erosion of a source rock, deposition, and accumulation should be considered when assigning ages to sedimentary materials.

Radiometric Dating Limitations

One common method for obtaining numerical ages of rocks is radiometric dating, which is based on the decay of radioactive isotopes. This method is suitable for dating igneous and metamorphic rocks, as the composition and age of those rocks can be determined by analyzing the radioactive decay of the isotopic elements found in them. Unfortunately, sedimentary rocks lack the isotopic materials useful in radiometric dating.

Dating Using Fossils

Instead of directly dating the sedimentary rocks, scientists can turn to fossils found within the layers of sedimentary rocks. Fossils help in correlating the age of sedimentary rock layers deposited across regions, and they represent the approximate time when the organisms lived. However, this method only provides relative ages, not the absolute ages needed for a geologic time scale.

Radiometric Dating of Igneous Rocks

To assign numerical ages to intervals on the geologic column, we can use radiometric dating of igneous rocks that are found interbedded with, or cutting through, sedimentary rock layers. By dating the age of an igneous rock that is in contact with a sedimentary layer, we can determine a minimum or maximum age for that sedimentary layer. The age of the igneous rock will provide a boundary for the age of the sedimentary layer, making it possible to estimate the numerical age of the interval on the geologic time scale.

Developing the Geologic Time Scale

The correlation of rock layers, fossils, and radiometric dating methods allows scientists to create a geologic time scale. This scale divides Earth's history into intervals such as eons, eras, periods, and epochs. This geologic time scale represents a composite, quantitative record of Earth's history and helps grasp the evolution and interrelation between organisms, environments, and geologic events.

Key concepts

Sedimentary Rocks

Sedimentary rocks are fascinating records of Earth's history. They are formed by the accumulation of mineral and organic particles that settle in layers over long periods. Think of them as a time capsule, each layer holding clues about the environment at that time.
But here's where it gets tricky: the age of a sedimentary rock isn't the same as the age of its components. The particles within may have come from eroded older rocks, making direct age determination difficult.
Understanding this distinction is crucial because it affects how we use sedimentary rocks to map and interpret Earth's past.

Radiometric Dating

Radiometric dating is a powerful tool for determining the age of rocks and fossils. It relies on the decay of radioactive isotopes within rocks. When these isotopes break down, they transform into other elements, giving us a "clock" to measure the time elapsed since the rock formed.
This method is particularly valuable for igneous and metamorphic rocks, which are rich in radioactive materials. However, sedimentary rocks pose a challenge since they are made of sediment from different sources, often lacking in the needed isotopes for radiometric dating.
Despite this limitation, radiometric dating remains critical in anchoring the relative dates we obtain from fossil records.

Fossil Correlation

Fossil correlation is like connecting the dots across different regions. Fossils found within sedimentary layers serve as time indicators, showing us the relative ages of these layers.
As certain organisms inhabited Earth only during specific time periods, their fossils can tell us when the layers they are found in were likely formed. This works well to match up sedimentary layers across vast distances.
Yet, this method alone doesn't give us exact numbers. It’s more about determining relative ages, which still plays a key part in piecing together Earth's dynamic history.

Igneous Rocks

Igneous rocks form from the cooling and solidification of molten magma or lava. They are like the Earth's internal heartbeat, marking episodes of intense geological activity.
These rocks are particularly useful for dating because they contain radioactive isotopes necessary for radiometric dating. Here's the clever part: by dating igneous rocks that intersect with sedimentary rocks, scientists can set minimum or maximum age boundaries for the sedimentary layers.
This method helps convert the relative timelines inferred from fossils into numerical ages, offering a way to build a comprehensive geologic time scale.