What might explain the presence of an older fossil on top of a younger fossil?
Relative Dating and the Law of Superposition
Students use observations to determine the sequence of events and relative age of rock strata.
- 1 Student Handout: Relative Dating and the Law of Superposition (per student, group, or class)
- 1 Student Reference Sheet: What Happened First? (per student)
- 1 Lab journal (per student)
Student Handouts can be printed individually for student use, as a reusable class set, or assigned online. Print one Student Reference Sheet: What Happened First? for each student.
Student Reference Sheet: What Happened First?
Student Reference sheets can be projected rather than printed.
Students work in groups of four to complete this task.
Group students who need more guided practice together and spend more time with them. Let the other groups work more independently.
Group students with mixed needs and have them work together. Monitor all groups equally.
- If you were to look through the classroom trash can at the end of the day, where would you find the trash from 1st period?
- At the bottom
- If you were to look through your dirty clothes hamper, where would you find the clothes you wore three days ago?
- At the bottom
- Why would the trash from 1st period and your clothes from earlier in the week be at the bottom?
- They are events that came first, so they will be located at the bottom.
Student Handout: Relative Dating and the Law of Superposition
- What could geologists learn by using the Law of Superposition?
- What is this dating technique called?
- How can looking at a column of rock layers help geologists determine the geologic history of an area?
Post-Activity Discussion Key
For future content reference, have students record their answers in complete sentences in their lab journals throughout the discussion.
- Geologists can learn when events occurred in Earth’s past and the order in which they occurred when using the Law of Superposition.
- The dating technique involved in the Law of Superposition is called relative dating
- The order of the layers and the changes that occurred to the layers give evidence as to what occurred in an area.
Sample Student Responses and Answer Key
- The hand on the bottom
- The older rock layers (hand layers) were formed first so they are positioned on the bottom.
- The hand on the top
- The youngest rock layers (hand layers) are formed last so they are positioned at the top.
- The order of events is C, B, A, D, E.
- Layers C, B, D, and A were moved by the fault.
- The magma intruded before the fault moved.
- The first event that happened was that layer C was laid down.
- The very top layer that is not labeled.