Headlands - Lesson Plan

Lesson Plan

  1. Before going on this trip, review any of the science concepts that students should have fresh in their minds: rock cycle, basic geology, stratigraphy, geologic time, seafloor spreading, and plate tectonics
  2. Go to the Point Bonita Lighthouse in the Marin Headlands.
  3. Investigate the pillow basalts:
    • Begin by asking students to draw or photograph the pillow basalts.
    • Model the formation of pillow basalt with ice water and Magic Shell. Fill a cup with ice water and then pour Magic Shell into the water – the cold water makes the outside of the chocolate syrup harden and the chocolate soon piles up in pillow shaped piles.
    • Discuss the model with the students, pointing out how the syrup represents magma welling up on the ocean floor.
    • Ask students where magma might well up from the ocean floor: volcanic islands and mid-ocean ridges.
  4. Travel to Rodeo Beach.
  5. Investigate the sandstones:
    • Ask students to draw or photograph the sandstones. Point out the borders between each large layered sandstone bed.
    • At the water’s edge, observe how sand particles act in the water. Draw connections to the soil separation test and the tiny particles remain suspended in water but the larger particles rapidly settle to the bottom.
    • Point out the sorting of the sediments in each bed.
    • Discuss how much sand would need to be dumped at one time to create each bed.
    • Ask students what might cause this much sediment to be dumped at one time: under-water landslides.
    • Point out how large landslides are known to occur at the edge of a continent, particularly near subduction zones.
  6. Lead students on a counter-clockwise walk around Rodeo Lagoon.
  7. Stop at the chert outcrop and investigate the chert:
    • Ask students to draw or photograph the chert. Pass out hand lenses and encourage students to look for fossils.
    • Review the law of original horizontality. Ask students how these layers must have been laid down originally. How did the bodies of these fossils originally form layers? (By millions of years of accumulation on the ocean floor.)
    • Ask students for initial hypotheses about how these layers could have become so folded.
    • Pass out cookies and model the subduction of an oceanic plate with cookies. Carefully remove the top cookie, leaving the filling on top of the lower cookie. Hold the top cookies still while gradually moving the lower cookie so that it subducts unter the top cookie, scraping off the filling as it goes.
    • Discuss the model, pointing out what the cookies and filling each represent. Observe the similarities between the now wrinkled filling and the folded layers of chert.
    • Collectively piece together the story of how this chert was originally laid down and how it became wrinkled.
  8. Return to the parking lot and go back to school (or play on the beach a little first).
  9. At school, review the stories of the 3 rocks that were investigated. Discuss how each type of rock must have formed, specifically focusing on where it must have formed.
  10. Spend at least half an hour piecing together the story of how all 3 types of rock, sandstone, chert and pillow basalt, came to all be found in one place. Once the story is filled out, diagram it on the board.
  11. Use the relief map of California to show the 3 major geologic zones in California – the Sierra Nevadas, the Central Valley, and the Coast Range mountains.
  12. Tell the story of how each of the 3 zones formed. Create a timeline to show students what was happening at different periods.