Which is younger

A rock formation or surface that cuts across other rock layers is younger than the rock layers it disturbs. For example, if an igneous intrusion goes through a series of metamorphic rocks, the intrusion must be younger than the metamorphic rocks that it cuts through Figure This nicely illustrates the principle of original horizontality. The youngest rock layers are at the top of the canyon, while the oldest are at the bottom, which is described by the law of superposition. Distinctive rock layers, such as the Kaibab Limestone, can be matched across the broad expanse of the canyon.

We know these rock layers were once connected, which is described in the rule of lateral continuity. Finally, the Colorado River cuts through all the layers of sedimentary rock to form the canyon. Based on the principle of cross-cutting relationships, the river must be younger than all of the rock layers that it cuts through.

Determining the Relative Ages of Rocks The relative age of a rock is its age in comparison with other rocks. If you know the relative ages of two rock layers, you know which is older and which is younger, but you do not know how old the layers are in years. In some cases, it is very tricky to determine the sequence of events that leads to a certain formation.

Take the example, Figure The principle of cross-cutting relationships states that a fault or intrusion is younger than the rocks that it cuts through. So the fault must be the youngest formation that is seen.

The intrusion D cuts through the three sedimentary rock layers, so it must be younger than those layers. The principle of superposition states that the oldest sedimentary rock units are at the bottom, and the youngest are at the top.

Based on this, layer C is oldest, followed by B and A. So the full sequence of events is as follows:. Steno discovered the rules for determining the relative age of rock beds, but he did not have a good understanding of how long it would take for these rock formations to form. At the time, most Europeans believed that the Earth was around 6, years old, a figure that was based on the amount of time estimated for the events described in the Bible.

One of the first to question this time scale was a Scottish geologist named James Hutton Often described as the founder of modern geology, Hutton formulated a philosophy called uniformitarianism : The present is the key to the past. According to uniformitarianism, the same processes we see around us today operated in the past as well.

For example, if erosion and deposition occur slowly now, they probably have always occurred slowly. Hutton discovered places where sedimentary rock beds lie on an eroded surface. Such a formation is called an unconformity , or a gap in rock layers, where some rocks were eroded away. Hutton reconstructed the sequence of events that led to this formation. For example, consider the famous unconformity at Siccar Point, on the coast of Scotland Figure Filters 0.

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