GEOL 101 1nd Edition Lecture 31 Outline of Last Lecture I. Continental DrifII. Seafloor SpreadingIII. Plate TectonicsOutline of Current Lecture I. Plate BoundariesII. ConvergentIII. DivergentIV. TransformCurrent LecturePlate 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 othero Divergent – two plates moving away from each othero 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 boundariesDivergentThese 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 lavao 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 highero 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 sedimento Basaltic pillow lavas As lava comes into contact with sea water, it is cooled instantaneously, forming “pillow” like shapeso Sheeted dike complex Recall dikes are vertical tabular volcanic intrusionso Gabro (coarse-grained equivalent of basalt)o Peridotite (upper mantle rocks)- Together, this sulfite of rocks makes up an ophiolite complex- Pillow basalts- Normal faultso Due to extensional forces at spreading centers, normal faults are common- Black smokerso Fractures from the faults and from the sheeted dikes allow sea water to percolate deep into the crusto The water gets heated from the magma chamber, and circulates back up towardsthe surfaceo As it is circulating, it dissolves metallic ions from the basaltic crusto 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 smokero 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. Kenyao 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 PangaeaConvergent-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 involvedo Ocean and Continental plateo Ocean and Ocean plateo 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 volcanismo 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 zoneso The Pacific Ring of Fireo 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 volcanismo 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 eruptionso e.g. Tonga eruption- Most tectonic plates have both oceanic and continental crust attachedo When the oceanic crust runs out, the continental masses will collide in a contintent-continent collisiono When two continental masses collide, there is no major density