Igneous rocks numerical dating
There are younger geologic deposits in Grand Canyon too, such as the Ice Age fossils found in caves, a 1000-year-old lava flow in the western canyon, and even the debris flow deposits that continue form each year.Yet, it is the canyon’s rock walls that allow people to develop their greatest perspective on geologic time, because of these rocks’ immense age, their fossil record, and because these rocks formed in environments far different than those found in northern Arizona today.By doing this repeatedly, for many locations around the world, the geologic time scale has been established.The geologic time scale is a composite vertical sequence representing all known rock units and their fossils, worldwide, placed into sequential order (oldest to youngest), based on relative dating and correlation.See Figure 1 for illustrations of other relative age relationships at Grand Canyon.Correlation determines if rocks are the same age, and is a key tool that geologists use to identify the relative ages of rocks where a rock layer is not continuously exposed.It is also how geologists relate rocks from one region to another.
You can click on the "golden spike" to learn about the location, see a diagram of the sequence of rock types (click to enlarge), with the ranges of the various fossil species plotted, and the EXACT location of the boundaries between geologic periods.
The absolute ages of the rocks have been determined through radiometric dating where possible.
Hence, the geologic time scale provides a calibrated "yardstick" for determining the ages of rocks worldwide through an examination of their fossils.
Igneous rocks can be used to provide absolute dates for sedimentary rocks.
In example A, the age of the shale is bracketed between two lava flows, one older than the shale, and one younger than the shale.