Explain what is required for a planet’s magnetic field to be generated by a dynamo. Describe the evidence that plates move, based on observations of magnetic patterns. Use paleomagnetic data to reconstruct past plate motion. Compare the Earth’s magnetic field with those of other planets and explain what the observations of those planets reveal about internal composition and structure. To Instructor: Students should be able to… The hotspot activity is directed toward achieving the 2 nd and 3 rd of these goals. Prior information should include generation of Earth’s magnetic field, magnetic reversals, sea floor spreading, and the development of remanent magnetism, both in igneous and sedimentary rocks/sediments. See “notes” pages under subsequent slides for information

Earth’s Magnetic Field - Inclination + - Angle between Earth’s surface and magnetic field lines.

Earth’s Magnetic Field – Inclination as a function of latitude At Vancouver: Latitude = 49 degrees North Inclination = ~67 degrees

Magnetic inclination: evidence for WHERE rocks formed HOT SPOT VOLCANOES: Chains of volcanoes (and single volcanoes) on the sea floor

The Mantle Plume Hypothesis 1. Hot spots form where narrow plumes rise by convection from the core-mantle boundary 2. The plume locations are stationary in the mantle IF this is true, what would the evidence look like?

Hotspot tracks (animation) (animation) Lithospheric plate motion Mantle plume Resulting chain of volcanoes “Fixed hotspot-moving plate” model

For instructor: Have class do activity 1 before continuing Next slide, discuss answers to activity 1

What does the “fixed hotspot – moving plate” model imply about the direction of motion of the Pacific Plate in the past? 0-45 Ma: NW-ward >45 Ma N-ward Did this happen?

How are hot spots? (and how could we tell?) TESTING THE FIXED HOTSPOT MODEL WITH INCLINATION

How are hot spots? What do you think happened? SeamountAge (Ma)Inclination (º)Paleolatitude (º)Corals? Detroit78 Ma60No Suiko61 Ma45No Nintoku56 Ma44No Koko49 Ma38Yes Hawaii0 Ma34Yes Data from Tarduno et al., Science, 2003

SeamountAge (Ma)Inclination (º)Paleolatitude (º)Corals? Detroit78 Ma60No Suiko61 Ma45No Nintoku56 Ma44No Koko49 Ma38Yes Hawaii0 Ma34Yes

 Moving plate + moving hotspot! Hotspot moving south Hotspot mostly stationary Corals No Corals

Demo for after this activity (make the Hawaii-Emperor seamount chain): Materials: blank overhead and overhead pens You be the hotspot (pen). Get a volunteer to be the plate (overhead). Ask the volunteer to move the “plate” across the overhead projector to the NW (like the Pacific Plate) 1.You be a “fixed” hotspot. Result: linear chain 2.You be a hotspot that moves south for a while, then becomes stationary. Result: something very like the Hawaiian-Emperor Seamount Chain, with a sharp elbow