GEOL 101 1nd Edition Lecture 32 Outline of Last Lecture I Plate Boundaries II Convergent III Divergent IV Transform Outline of Current Lecture I Evidence of Plate Tectonics II Force Driving Plate Tectonics Current Lecture Evidences that support plate tectonics Fit of the continents fossil correlation etc Other evidences include observations from Ocean drilling Hotspots Paleomagnetism Satellite data Ocean drilling Between 1968 to 1983 the National Science Foundation sponsored several drilling projects that extracted drill cores from the seafloor The goal of these projects was to establish ages of the rocks on the seafloor It was observed that the youngest rocks were at the sea floor spreading ridges and the rocks got older as you move away in either direction o The oldest rocks on the seafloor have been radiometrically dated at 180 million years o Contrast to crustal rocks some with radiometric dates of 4 billion years Hotspots These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Mapping of the Pacific seafloor revealed several linear chains of volcanic seamounts submarine volcanoes Over 100 seamounts stretch a distance of 6000 km to make the Hawaiian IslandEmperor Seamount chain o Hawaii is the only island or seamount in this chain that is currently volcanically active o The ages of the seamounts get progressively older the farther they are away from Hawaii The most widely accepted theory for the formation for the Hawaiian Islands and Emperor Seamounts is that there is a hotspot currently active beneath Hawaii For reasons that are still widely debated hot mantle material rises up in the form of a mantle plume o The mantle plume initiates volcanism The Pacific plate moves over top of the stationary hotspot eventually cutting off one volcano s magma supply and forming a new volcano Paleomagnetism As lava is cooling iron rich minerals will align themselves with the earth s magnetic field like tiny compass needles o When the lava completely cools the iron rich minerals are frozen in the rock preserving the paleo magnetic field It has been observed in various igneous rocks throughout the world that the earth s magnetic field has gone through polar reversals o The iron rich minerals are oppositely aligned so that south is north and north is south As oceanographers were mapping the ocean bottoms they noticed magnetic variations within the basalts on the seafloor o When mapped out these variations exhibited a definite pattern especially at the mid ocean ridges known as magnetic stripping There are equally thick normal and reverse polarization patterns on either side of spreading centers o There are also equal dates for these rocks on other side of the ridge Satellite and GPS data Using satellites and global positioning system GPS technology we are able to track the motion of the plates Receivers placed around the world can track both the direction and rates of plate movements over time o Plates move between about 2 8 cm per year Driving Force Slab pull as a subducting oceanic plate sinks down into the mantle it pulls the rest of the plate along o Providing main driving force for plate tectonic o Plates with more subducting lithosphere tend to be faster moving plates Ridge push as more seafloor is generated at the ridges material gets pushed away by gravity driven mechanisms Subducting suction mantle circulation tends to drive the two plates together creating a suction zone at the trench o Similar to pulling the plug in a bathtub Ultimately the main driving force for plate tectonics comes from convection in the mantle Lower mantle heated by core This heated material is more buoyant and so it rises When it gets closer to the surface it cools off and gets denser The cooler denser material sinks back down towards the core 660 model This model proposes that there is convection occurring in the lower mantle and separate convection in the upper mantle The boundary between these convection cells is the 660 discontinuity Geophysics indicates that subducting slabs end up at the 660 many have nicknamed this boundary the subducted plate graveyard Whole mantle convection model In this model the subducting slabs sink past the 660 discontinuity into the lower mantle o Supported by some geochemical data Mantle plumes originate near the core mantle boundary o Not supported by new geophysical data Deep layer model This model features a deep mantle layer that is chemically different from the upper layer The two layers move like the liquids in a lava lamp allowing deep mantle material to rise up as hotspot plumes There is little geophysical evidence to support this model