New York State High School Regents Exam Prep
Earth Science

Words to Know to Interpret the Relative Ages of a Geologic Profile

Law of Superposition states that the rock layer on the bottom is the oldest and the layers get younger as you move up the profile. This is true for any undisturbed rock exposure.

Igneous Intrusions are formed when magma is injected into older rock layers in the crust. Therefore they are younger than the rock they are found in. Look for contact metamorphic rock in the layer above and below the intrusion.

Igneous Extrusions are rocks that formed from lava on the surface of the earth. They are younger than the rock layers below. Look for contact metamorphic rock on the bottom only.

Folds are bends in the rock layers. They occur after the rock layers formed.

Faults are cracks in the rock layers where some movement has taken place. Faults produce offset layers.

Unconformity, a buried erosion surface, is formed when an area of the crust was uplifted above sea level and then eroded. Some time after that the area subsided below sea level and new sediments were deposited on top of the eroded surface


Test Your Knowledge

Use the geologic profile above to answer the following questions.
You may want to print this page for future reference.
1. What does the letter X represent?


2. Of the rock layers A - J, which is the oldest? ...... youngest?


3. What type of metamorphic rock would be found where layer E and layer H come in contact?


4. Of the rock layers A - G, which is the youngest?


5. What is Y? How can you tell?


6. Is the igneous rock formation H younger or older than Y?



New York State High School Regents Exam Prep Center: Earth Science