The K-Pg Extinction Event

Activities to try

1. 1

   Assemble the case file: what killed the dinosaurs?

   Why it works: A scientific conclusion is built from converging independent evidence: chemistry (iridium), geology (shocked quartz, tektites, crater), and matching ages all point the same way, and drilling into the crater closed the link between the local crater and the global layer.

   1. Open a 'case file' and visit several boundary sites on a world map (e.g. Italy, Denmark, New Zealand, and a land site such as Hell Creek in Montana, USA); at each, examine the rock column and find the thin clay band at the boundary. At land sites like Hell Creek, non-avian dinosaur fossils are found below the band and none above it; at old sea-floor sites like Italy and Denmark, many kinds of tiny sea fossils found below the band disappear above it.
   2. Run an iridium measurement at each site and record where the reading spikes — notice the spike lands at the clay layer at each site you visit, and that similar spikes have been found at the boundary at many sites around the world.
   3. State in your own words why iridium at this layer points to space: iridium is rare in Earth's crust but common in asteroids, so a worldwide layer of it suggests material that fell from the sky everywhere at once.
   4. Examine two more independent clues from a large impact: shocked quartz, which forms under the intense shock pressure of an impact, and tektites, which are droplets of rock melted and flung out by the impact's heat and energy — both are products of one large impact rather than both being pressure-formed.
   5. Reveal the buried ~180 km Chicxulub crater in the Yucatan on a geophysics view, and check that its age matches the clay layer.
2. 2

   Run the aftermath: trace the causal chain

   Why it works: Mass extinction was caused by ecosystem collapse — blocked sunlight shutting down the food supply across the whole planet — rather than by the impact directly killing animals far from the crater; and survival of birds means dinosaurs are not entirely extinct.

   1. Starting from a thriving Cretaceous scene, release the dust and soot from the impact and reduce incoming sunlight (for example, by lowering a sunlight slider) to model an 'impact winter'.
   2. Observe and describe, in order, what happens as sunlight drops: plants dim and die, then plant-eaters (herbivores) starve, then meat-eaters (carnivores) that fed on them starve.
   3. Explain in your own words why plant-eaters are affected before meat-eaters, using the idea that energy enters a food chain through plants/photosynthesis.
   4. Write the full causal chain as a sequence: impact -> dust/soot/aerosols -> sunlight blocked -> photosynthesis crashes -> plants die -> herbivores die -> carnivores die -> ecosystem collapse.
   5. State clearly that worldwide extinction came from these after-effects, not from the blast crushing or burning dinosaurs everywhere, and note that the impact itself mainly devastated the local area.
3. 3

   Weigh the rival hypothesis

   Why it works: Scientific knowledge is based on empirical evidence and is provisional: competing hypotheses are judged by how well they explain the evidence, and a genuine rival idea can still have supporters while the main case strengthens.

   1. Compare two explanations for the extinction: the asteroid impact and Deccan Traps volcanism, considering what each can explain and whether both might have played a part.
   2. List which pieces of evidence (worldwide iridium layer, shocked quartz, matching-age crater, iridium inside the crater) fit the impact explanation.
   3. Discuss or write why the impact case grew stronger over decades as new evidence arrived, and why a minority of scientists still support the volcanism idea.
   4. Conclude with a statement about how scientists decide between rival ideas — by how much evidence each explains, not by authority — and that scientific conclusions can be revised as evidence accumulates.