1 MAR110 Lecture #4 Fundamentals of Plate Tectonics The Ocean Sea Floor is formed Along the Mid-Ocean Ridge Spreading Centers The Ocean Sea Floor is destroyed in the Subduction Zones Figure 4.2 Convection Drives Plate Tectonics A schematic diagram of the major plate tectonic features in the upper 450km of the Earth. Convection in the asthenosphere causes the lithospheric parts of the plates to pull apart at the ocean ridges, move across ocean basins, and subduct to great depth in the convergent zone where plates collide. The subducting lithosphere melts at depth and rises as magma to form a volcanic island arc. (ItO) 1
2 Figure 4.3 Mid-Ocean Ridge System: Volcanoes Rising magma in the rift zone, where oceanic crust is being formed, forces its way through the oceanic crust to form underwater volcanoes some reaching the surface; Bermuda is such an example. These volcanoes ride the moving oceanic crust away from the rift, thus cutting off its magma supply, rendering it extinct or inactive. (NH) Figure 4.4 Mid-Ocean Ridge System: Iceland (NG ) 2
3 Figure 4.5 Oceanic Crust Formation In the 1950s, geophysicists, making deep ocean magnetic measurements near Iceland, discovered bands of seafloor rocks on both sides of the mid-ocean ridge with symmetrically-varying magnetic strength anomalies (small departures from the norm). These magnetic anomaly bands, which are parallel to the mid-ocean ridge axis, represent compelling evidence that new oceanic crust is being formed at the midocean ridge.(?) Figure 4.6 Mantle Convection Interior heat escape causes convection cells, with hotter rising plumes and the cooler sinking plumes to form. The lithospheric plates at the surface move with the lateral flow that connects the plumes. The movement of the electrically-conducting mantle material in the convection cells creates the earth s magnetic field (?) 3
4 Figure 4.7 Earth Magnetism Scientists now believe that convection in the mantle generates Earth magnetism in which its north and south magnetic poles approximately aligned with the Earth s rotational poles. The geologic record reveals that the Earth s magnetic polarity has reversed every few million years in the past (sometimes as rapidly as within 1000 years) (?) 4
5 Figure 4.8 Sea Floor Magnetic Anomaly Formation This schematic illustrates the reason for the observed sea floor magnetism. (top) Iron particles within the magma (molten rock) align magnetically with the Earth s prevailing magnetic lines of force (normal polarity dark brown). When the hot magma hits the cold ocean waters of the mid-ocean ridge, it solidifies to form solid rock and in so doing freezes the magnetic orientation of its iron particles forever. Ocean crust formation proceeds for many years, pushing symmetrical bands of older rocks further away from the ridge (middle) Every million years or so the magnetic polarity of the whole Earth changes; yielding bands of rocks with a reversed polarity (R, light brown). (bottom) This idealized record of magnetism in these rocks shows the alternating bands of small departures in magnetism consistent with the actual measurements shown next. (ItO) 5
6 Figure 4.9 Dating Magnetic Reversals Schematic of a magnetic anomaly record showing the major events or reversals in the earth s magnetic field in the past five million years and the names given to certain eras. (LEiO) Figure 4.10 Oceanic Plate Movement Rates The distributions of magnetic anomalies relative to the axes of three midocean ridges (spreading centers), namely the (top) South Atlantic, (middle) North Pacific, and (bottom) Pacific/Antarctic, respectively. Several of the Earth magnetic reversal peaks of known ages (see the lines) can be used to calibrate the records and estimate the sea floor spreading rates; which differ from basin to basin (LEiO) 6
7 Figure 4.11 Mid-Atlantic Ridge Faulting and Earthquakes Most earthquakes near spreading centers occur in and around the transform faults that break up the single mid ocean ridge into many smaller divergent faults. (?) Figure 4.12 Plate Tectonics Schematic Magma rises to form new ocean crust at the oceanic spreading center along the mid-ocean ridge (MOR), which exhibits shallow earthquakes (dots). The oceanic crust, exhibiting magnetic stripes, moves away from the MOR across the ocean basin. Upon collision with a continent, the oceanic crust subducts under the edge of the continental plate. (LEiO) 7