Today’s plan: • Plate Tectonics – • Review magnetism and sea-floor spreading • Modern evidence for plate tectonics • Plate boundaries, drivers of plate motion • Discussion of Giere reading Announcements: • Lab starts this week – meet in Fairchild 414 • Quiz 1 on Friday • Office hours – see Canvas page (Instructor and TA Contact Information) Earth’s Magnetic Field Earth’s magnetic field produced by flow in the outer core It is a dipole - it has two poles (north and south poles) similar to a bar magnet If there are two bar magnets – like poles repel, opposites attract A dipole can be shown as an arrow that points from the south to the north pole Currently the south pole of Earth’s dipole is at the north geographic pole By convention, we still call this the “north magnetic pole” Hess/Dietz Sea-floor Spreading Hypothesis An “Essay in Geopoetry” Convergent plate boundary Divergent plate boundary Old, cold, dense crust sinks back to the mantle Modern technologies that supported sea-floor spreading • Magnetic profiles • Ocean sediment and rock drilling US Coast and Geodetic Survey Ship Pioneer. First ship to drag magnetometer to measure sea-floor magnetism (1946- Ocean Drilling Program ship 1956) Plate Tectonics – Modern Evidence Magnetic profiles show “Marine Magnetic Anomalies” Earth’s Current Magnetic Reversal – Magnetic Field – “Reversed Polarity” “Normal Polarity” Magnetic Reversals Record of Earth’s magnetic field reversals (dark gray shows “normal” polarity, light gray shows “reversed” polarity) Plate Tectonics – Modern Evidence Magnetic profiles show “Marine Magnetic Anomalies” Plate Tectonics – Modern Evidence Magnetic profiles show “Marine Magnetic Anomalies” Plate Tectonics – Modern Evidence Mid-ocean Marine Magnetic Anomalies ridge Negative anomaly Positive anomaly Earth’s field Brunhes 0-700,000 yrs Key advances: Matuyama (0.7-2.5 Ma) Gauss 1) Symmetry on either side of the mid-ocean ridge (2.5-3.4 Ma) Gilbert 2) The width of the sea-floor anomalies is proportional (3.4-4 Ma) to the duration of the known ages of the reversals Plate Tectonics – Modern Evidence Ocean drilling has shown that: 1. Sediment on the sea floor thickens away from mid- ocean ridges 2. Oceanic crust is different in composition from continental crust 3. Heat flow varies across the ocean floor Plate Tectonics – Modern Evidence Age of the Sea Floor Plates and Plate Boundaries •Earth’s lithosphere is broken into plates made of continental and oceanic lithosphere •~15-20 plates (and many micro-plates) •Contacts between plates = plate boundaries Plate Boundaries - defined by the relative motion of plates Divergent Convergent Transform Drivers of Plate Motion Convection in the mantle Ridge push Slab pull.