Montana Earth Science Picture of the Week

Like rock, papers, scissors . . . sort of

This roadcut a few miles south of Cascade along I-15 illustrates one of the basic principles used by geologists to determine the relative ages of rocks. The horizontal layers are sandstones made up of sediment laid down near the coast of a shallow sea that was present here millions of years ago. The darker rock that cuts vertically through the layers of sandstone is type of igneous rock formed as magma solidified beneath the surface. These types of igneous formations, called “dikes,” are common in the Cascade area.

Relative age. . .
Since the igneous rock “cuts across” the sandstone layers, geologists conclude that the sandstone was there first. In other words, “relative” to (compared to) the igneous rock, the sandstone is older. This is called the “principle of cross-cutting relationships” . . . If a crack, a fault, a vein, a dike, etc. cuts across another rock, then that which “cuts across” happened after the older rock was formed. For example a crack through part of a sidewalk could have only happened after the sidewalk was built.

Absolute age . . .
Determining relative ages of rocks is important, but where igneous rocks are found radiometric dating techniques can provide a more accurate age. The Ar 40/Ar 39 and K/Ar (potassium/argon) methods can be used for dating the type of igneous rock shown in the photo. Geologists have determined the igneous rocks in the Cascade area solidified about 75 million years ago. Based on this “absolute age” of the igneous rock, its fair to say that the sandstone shown in the photo is over 75 million years old.

terms: inert, radioactive decay


*More about radiometric dating (more photos too)
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By Rod Benson
Earth Science Teacher at Helena High School

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