Transform (conservative) – plates slide past each other Convergent (destructive) – plates move towards each other, one plate is subducted, crust is destroyed Plate margins – volcanic activity, earthquakes, mountains (orogeny) *begin looking at the pictures in the book and being able to recognize what is happening in the pictures* 1. Divergent Boundaries oceanic plate separation (Mid-Atlantic Ridge) new material is seeping through the opening, causing rifting, volcanoes, and earthquakes Ocean floor topography; mostly occur in ocean basins. Marked by mid-ocean ridges (Mid-Atlantic Ridge, East Pacific Rise, etc.) Ridges and Convection – text Passive upwelling: ridges mark sites of the mantle that wells up passively in response to plates moving apart. Example: two wooden blocks with water underneath, when the blocks are moved the water will seep in through the middle Plate forces:
Chapter 2 – Plate Tectonics 22:12 Immediate forces driving plate movement – ridge push, downhill sliding, slab pull Magma formed in the mantle below ridges forms intrusive bodies and some is erupted along the ridge. Rates and History of plate motion: arose from studying the Mid-Atlantic Ridge and magnetic anomalies. Measures the strength of magnetic signals coming from rocks Earth’s magnetic field originates in the liquid outer core. Lines of force exit and enter earth at magnetic poles (offset from geographic poles) Present day-normal polarity. Lines of force exit earth at magnetic south, reenter at magnetic north. Reversed polarity. Lines of force exit earth at magnetic north, reenter at magnetic south. When it is reversed, the magnetic pull is essentially the same but the magnetic field lines of enter/exit are reversed. Paleomagnetism and reversals: rock records magnetic field at time of crystallization Magnetic anomalies – Rekjanes Ridge Surveys of magnetic field over ridges revealed field not uniform. Anomalies form “strips” parallel to the ridge Absolute ages of reversals: polarity reversals also recorded by lavas erupted on land. Samples can be recovered and dated giving age of reversals. Drilling and age of oceanic crust: increase in age of crust with distance from ridges confirmed by drilling – samples recovered and dated
Chapter 2 – Plate Tectonics 22:12 QUESTION: This is a magnetic profile across an oceanic ridge system. The profile is a plot of ____ vs. _____. Declination vs. distance from ridge Distance from ridge vs. magnetic intensity Magnetic inclination vs. magnetic polarity Life along the ridges: new life forms have been found along the ridges. These include tube worms with a sulfur based metabolism that is unique on earth. At these depths, life
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