Section 2: Determining Ages of Rocks

One way to determine the age of a fossil is by finding the age of the rock it was found in. A rock’s relative age is compared with the ages of other rocks.  The absolute age of a rock is the number of years since the rock formed.  While it may be impossible to know the absolute age exactly, a geologist can sometimes determine it within a certain number of years.  Geologists use the law of superposition to determine the relative age of rocks.  For example, in horizontal sedimentary rock layers, the oldest layer of rock is at the bottom, and each higher layer is younger than the layer below.

Geologists look at other clues besides the position of rock layers to determine the relative age of rocks.  An extrusion is lava that hardens on the surface and is always younger than the rocks below it.  When magma cools and hardens into a mass of igneous rock beneath the surface, it’s called an intrusion.  An intrusion is always younger than the rock layers around or beneath it.  A fault is a break in the Earth’s crust. It is always younger than the rock it cuts through.  An unconformity is a gap in the geological record where some rock layers have been lost because of erosion.

Certain fossils, called index fossils, represent an organism that existed only briefly and was widely distributed, meaning it lived in many different areas.  To date rock layers, geologists use index fossils to match layers, and then they can give the same age to those layers of rocks at other locations.

Most elements are stable, but some exist in unstable forms, meaning they break down.  An element is when all the atoms of a particular type of matter are the same.  When an element breaks down or decays, it releases particles and energy in a process called radioactive decay.  These unstable elements are said to be radioactive.  The rate of decay of each radioactive element is called a half-life, which is the time it takes for half of the radioactive atoms to decay.  The decay will continue at a steady rate, slowly changing the original radioactive element into another, changing the composition over time.  This change causes the amount of the radioactive element to decrease while the amount of the new element increases.  Geologists also use radioactive dating to determine the absolute ages of rocks by first determining the amount of radioactive elements in a rock.


  1. What is the law of superposition?
  2. Why are index fossils important?
  3. What is radioactive decay?

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