Geologic Time: The Ticking of Our Planet’s 4.6 Billion Year Clock

Geologic Time: The Ticking of Our Planet’s 4.6 Billion Year Clock


Phoebe Cohen
Education and Outreach Lead
MIT NASA Astrobiology Team
Cambridge, MA

Francis MacDonald
Assistant Professor
Harvard University Department of Earth
and Planetary Sciences
Harvard University
Cambridge, MA

*MIT BOSSOMS thanks the Harvard Museum of Natural History for allowing us to film this lesson at several of their exhibits.

Lesson Feedback


The Earth is 4.6 billion years old. That’s a hard number to conceptualize. What does 4.6 billion look like, and what happened during all those hundreds of millions of years between the formation of our planet and now? This BLOSSOMS lesson will help students conceptualize the enormity of geologic time and learn about important events in Earth’s history. Students will also learn how geologic time can help explain seemingly incomprehensible processes, like the formation of the Himalayan Mountains from a flat plain to their current height, and the evolution of a tiny group of reptiles into enormous dinosaurs. The lesson will take approximately 45 minutes. Students should have a basic understanding of biology and a familiarity with geology is helpful but not necessary. The supplies required include a measuring tape that is at least 5 meters long, a 5 meter long piece of string, ribbon, or rope, index cards or other stiff pieces of paper, and calculators. During the breaks, students will construct a geologic timeline of their own in the classroom and do simple calculations to determine how long amounts of time can lead to impressive changes in the height of the Himalayan Mountains and the size of a group of reptiles. 

[ Correction: There is an incorrect statement in this video lesson that the pyramids were built 2.500 years ago. They were built in 2.500. B.C. ]

Instructor Biography

Phoebe Cohen is the Education and Outreach Lead for MIT’s NASA Astrobiology Team. With a Ph.D. in paleontology and experience in informal science education, Phoebe develops educational content and facilitates the outreach initiatives of her Team. She is devoted to promoting a greater understanding of the natural world and to helping scientists better communicate the amazing things they do.

Francis MacDonald is an Assistant Professor of Geology in the Earth and Planetary Science Department at Harvard University. Francis' research focuses on the interactions of climate, life, and ocean geochemistry in deep time. This work begins with field studies of Neoproterozoic and Paleozoic strata in Arctic Alaska, northwest Canada, Mongolia and Namibia.

Additional Online Resources

Nature: Adaptive differentiation following experimental island colonization in Anolis lizards
This is a link to the original article on the lizard research done by Jonathan Losos and his colleagues, entitled Adaptive differentiation following experimental island colonization in Anolis lizards.

Starting Point: Teaching about Radiometric Dating
This site, presented by Starting Point: Teaching Entry Level Geoscience, provides resources for teaching about radiometric dating.

Time Scale Creator
This website, Time Scale Creator, enables you to explore and create charts of any portion of the geologic time scale from an extensive suite of global and regional events in Earth History.

Back-of-the-Envelope Calculations: Height of the Himalayas
This site, sponsored by teaching Quantitative Skills in the Geosciences, provides a back-of –the envelope calculations exercise on figuring the height of the Himalayas.

The National Science Digital Library
This site, The National Science Digital Library, provides general resources for teaching Earth Sciences and Evolution.

The Science Education Resource Center
This site is presented by The Science Education Resource Center (SERC), an office of Carleton College that works to improve education through projects that support educators. There are numerous resources for Earth Science.

Long Times Intervals
Harrison Brown discusses the significance of long time intervals and describes how radioactive dating arrives at an estimate for the age of the earth. For more info on PSSC, its history, and films, visit here.