GEOL 101 1nd Edition Lecture 31 Outline of Last Lecture I Continental Drif II Seafloor Spreading III Plate Tectonics Outline of Current Lecture I Plate Boundaries II Convergent III Divergent IV Transform Current Lecture Plate Boundaries Plate boundaries found at the edges of a plate where it meets another plate The movement of the plates results in three different kinds of boundaries o Convergent two plates moving toward each other o Divergent two plates moving away from each other o Transform two plates sliding past each other These plate boundaries result in several geologic processes Earthquakes present in all three types Volcanism present in convergent and divergent boundaries Convergent boundaries tend to form mountain ranges increasing the thickness of the crust Divergent boundaries tend to create rifs which causes the crust to be thinner Review The lithosphere crust uppermost mantle is broken up into pieces called plates The plates are in constant motion due to convection in the mantle The edges of the plates are called plate boundaries Divergent 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 Most divergent plate boundaries occur at the ocean ridges seafloor spreading centers Mid Ocean Ridges Two plates are pulling apart creating a rif valley The lithosphere is very thin which allows for hot mantle rock to rise and partially melt generating volcanism The volcanoes produce basaltic lava o Basalt is the most abundant rock on the face of the Earth If the lithosphere is thinning why are the spreading centers elevated o The newly formed crust is much hotter than the older crust and is therefore more buoyant causing the ridges to sit higher o As the rocks are pushed further away from the spreading center they cool and become more dense sinking lower Features of the ocean ridges Layers of the oceanic crust o Deep sea sediment o Basaltic pillow lavas As lava comes into contact with sea water it is cooled instantaneously forming pillow like shapes o Sheeted dike complex Recall dikes are vertical tabular volcanic intrusions o Gabro coarse grained equivalent of basalt o Peridotite upper mantle rocks Together this sulfite of rocks makes up an ophiolite complex Pillow basalts Normal faults o Due to extensional forces at spreading centers normal faults are common Black smokers o Fractures from the faults and from the sheeted dikes allow sea water to percolate deep into the crust o The water gets heated from the magma chamber and circulates back up towards the surface o As it is circulating it dissolves metallic ions from the basaltic crust o When the water reaches the surface and mines with the cold sea water the ions come out of solution and from particle clouds or black smoker o These hot ion rich waters are home to hundreds of species of life that thrive in these extreme environments Vent communities Divergent boundaries can also develop within a continent splitting the land into smaller segments The breakup of Pangaea Today Eastern Africa is undergoing continental rifing as a result of extensional forces The East African system represents the initial stage in the breakup of a continent Like at seafloor spreading centers the lithosphere is thinning as it is being pulled apart Hot mantle material rises up and causes partial melting Volcanism occurs at the surface Since we re dealing with continental crust magma is much more silica rich as opposed to basalt The East African Rif has produced volcanoes such as Kilimanjaro and Mt Kenya o The extension also causes normal faults Progression of continental rif to a new ocean basin and seafloor spreading center Tensional forces stretch the crust producing volcanism and a continental rif A and B o Kilimanjaro and Est Africa rif valley Continued spreading generates a narrow seaway C o Modern Ex Of Red Sea Eventually a new ocean basin is formed with a seafloor spreading center generating new ocean crust D o The Atlantic ocean afer the breakup of Pangaea Convergent At convergent boundaries the plates are moving towards each other There are three different scenarios that can take place depending on the type of plates involved o Ocean and Continental plate o Ocean and Ocean plate o Continental and Continental plate The crust type and density differences in the plates determines the geologic features that will form Oceanic Continental Convergence The oceanic plate is much more dense and will sink down into the mantle while the continental crust floats above creating a subduction zone Water that is trapped in pore spaces and within mineral bonds of the subducting plate gets squeezed out into the overriding continental plate This water triggers the melting in the continental crust which triggers volcanism o Continental volcanic arc Continental volcanic arcs typically produce stratovolcanoes and an overall intermediate composition andesite o Recall that stratovolcanoes produce large violent eruptions Pinatubo Vesuvius St Helens Almost the entire Pacific ocean is lined with subduction zones o The Pacific Ring of Fire o Andes Sierra Madres Cascades Aleutians Japan Indonesia New Zealand The surface expression produced by the subducting plate is an Oceanic Trench These trenches can be thousands of miles long Peru Chile trench and up to 12 km deep Mariana trench Oceanic oceanic convergence The process is essentially identical to oceanic continental subduction The relatively denser oceanic plate sinks below the more buoyant oceanic plate Water is driven off the subducting plate into the upper oceanic crust initiating volcanism o In this case we get an oceanic island arc Examples of volcanic island arcs include o Aleutians Japan New Zealand and most of the South Pacific islands In all cases the subducting oceanic plates are older and denser making them sink more readily The volcanoes formed along island arcs are also stratovolcanoes and produce violent eruptions o e g Tonga eruption Most tectonic plates have both oceanic and continental crust attached o When the oceanic crust runs out the continental masses will collide in a contintent continent collision o When two continental masses collide there is no major density contrast so neither plate will want to subduct Instead the two landmasses will grow vertically generating mountain ranges Such was the case when India collided with Eurasia forming the Himalayas also formed the Appalachians Continent