The University of California Museum of Paleontology (UCMP) created a fabulous introduction to the geologic time scale on the web called “Understanding Geologic Time”. Students are led through a series of interactive web pages covering a wide range of earth history concepts: relative vs. absolute time, the law of superposition, radiometric dating, the geologic timescale, and the origins and evolution of life on Earth. While the teacher section includes assessment materials including a “Scavenger Hunt” activity, I have included an alternative worksheet for students to follow as they navigate through the website and some extra credit questions in UCMP’s online “Geology Wing” for students that finish early. Links to activities that teach radiometric dating are included in the Going Further section.
Note: The simplicity of this lesson makes it appropriate for a substitute teacher to lead. On the other hand, the concepts covered are central to the curriculum and it is recommended that this lesson follows the Personal Timeline activity and precedes the Geologic Timeline activity.
Can read information from a Geologic Time Scale
Can explain the difference between relative and absolute time and how each is inferred from geologic evidence
Develop a sense of the vastness of geologic time compared to everyday experience or even the existence of modern humans
Law of superposition
Geologic time scale
Individual, although pairs of student sharing a computer also works well
Classroom or computer lab
This lesson gradually leads students to a basic understanding of how the geologic timescale is organized and how it was created from evidence in rocks. See background information about Nicolas Steno (from the Layers Upon Layers lesson) and William Smith (from the My Time lesson) for preliminary background on arranging earth history by relative time. Below is a brief overview of determining the absolute age of rocks by radiometric dating. For a deeper understanding of the geologic time scale itself, see the Geologic Timelines lesson.
A great advance in the field of geology came in the form of the mass spectrometer, a device that measure the radioactive decay of elements. Simply put, each element in the periodic table contains the same number of protons but vary in the number of neutrons and thus can vary in their atomic weight. Atoms with the same number of protons but different numbers of neutrons are called isotopes. Over time, an unstable parent isotope will spontaneously eject parts of its nucleus and transform into a far more stable daughter isotope.
Geologists can use this to precisely determine the age of a rock layer, a method called radiometric dating. When a layer of igneous rock is laid down from a volcanic eruption, all the atoms begin as the parent isotope. Each parent isotope has a specific rate of decay that can be precisely timed. Some parent isotopes, like uranium-238 take an extremely long time to decay to the daughter isotope lead-206 (4.5 billion years for half of the uranium-238 in a given rock to decay to lead-206). Others have a very short decay rate or half-life (5,700 years for carbon-14 to decay to carbon-12). The ratio of parent to daughter element tells scientists precisely how old a given rock is.
No experience is required although familiarity with determining the relative age of rock layers through the law of superposition (see Layers Upon Layers lesson) and with the general organization of the geologic time scale (see My Time lesson) is helpful.
UCMP has a fabulous collection of web resources related to geologic time and paleontology called “Explorations Through Time”. Their description describes how these modules “explore the history of life on Earth, while focusing on the processes of science. Each module contains suggested lesson plans and an extensive teacher’s guide.” All are extremely well written and well worth a teacher’s time. This lesson is based on the “Understanding Geologic Time” lesson from this series.
For more information about radiometric dating, see:
Investigation and Experimentation
Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other three strands, students should develop their own questions and perform investigations. Students will:
g Interpret events by sequence and time from natural phenomena (e.g., the relative ages of rocks and intrusions).
Earth and Life History (Earth Sciences)
Evidence from rocks allows us to understand the evolution of life on Earth. As a basis for understanding this concept:
b Students know the history of life on Earth has been disrupted by major catastrophic events, such as major volcanic eruptions or the impact of an asteroid.
c Students know the rock cycle includes the formation of new sediment and rocks. Rocks are often found in layers with the oldest generally on the bottom.
d Students know evidence from geologic layers and radioactive dating indicate the Earth is approximately 4.6 billion years old, and that life has existed for more than 3 billion years.
e Students know fossils provide evidence of how life and environmental conditions have changed.
g Students know how to explain significant developments and extinctions of plant and animal life on the geologic time scale.