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Plate Boundaries 3 types of plate boundaries: • Divergent (divide) • Convergent (collide) • Transform (slide past) Remember the plates are in motion…. • When plates move , they can either: • A) converge (collide) B) pull apart (divide) C) slide along past one another Either way, changes will occur along the “boundaries” of the plates. #1) Divergent Boundaries • Are boundaries between two plates that are moving apart (dividing) or “rifting” ß à • RIFTING causes SEAFLOOR SPREADING Divergent Boundaries.. • Most occur at the mid-ocean ridges • Divergent boundaries that occur on land are called rift valleys • (ex: Great Rift valley in Africa) What is a rift valley? How are rift valleys formed? • A rift valley is a deep valley that forms along a divergent boundary on land • Rift valleys form as two slabs of Earth’s crust slide apart What could eventually happen at the Great Rift Valley in Africa? • The rift may someday split the eastern part of Africa away from the rest of the continent Volcanic fissures • Divergent boundaries also cause volcanic fissures!!! • This is a vent through which lava erupts, but there is usually no explosive activity. #2) Convergent Boundaries • Boundaries between two plates that are colliding à ß • There are 3 types… Convergent Boundaries • A plate boundary where two plates move toward each other • Collisions can occur between: – oceanic crust and oceanic crust – oceanic crust and continental crust – continental crust and continental crust When two plates collide, the ______ of the plates determines which one comes out on the top. • density Type 1 • Ocean plate colliding with a less dense continental plate • And meet at a Subduction Zone: (where the more dense plate slides under the less dense plate) • VOLCANOES occur at subduction zones How do these volcanoes form? • High temperature from the magma causes the rocks to melt along the subduction slab as it goes under the other plate. • Magma seeps up along the plate boundary forming volcanoes. • Ex: St. Andes mtns along the Nazca plate and S. American plate boundary: Type 2 • When one oceanic plate collides with another oceanic plate, the more dense plate slides under the less dense plate creating a subduction zone called a TRENCH Type 3 • A continental plate colliding with another continental plate • Have Collision Zones: – a place where mountains will form Continental/Continental cont’d • Because the plates are less dense than the material in the asthensosphere, the two plates collide and crumble up forming mountain ranges. • There is little or no subduction here, so volcanoes do not form. Result of the Eurasian and the IndianAustralian Plates Colliding The Himalayas Where are the Himalayas? Quick Check #1: What occurs when plates carrying oceanic crust collide? • The more dense plate dives back into the mantle (subduction) • The collision between oceanic plates causes deep-ocean trenches #2 What occurs when plates carrying oceanic and continental crust collide? • The more dense oceanic crust sinks and plunges (subducts) beneath the less dense continental crust • The collision between oceanic plates and continental plates can cause a volcanic eruption #3 What occurs when plates carrying continental crust collide? • Plates crash head-on causing mighty mountain ranges. #3) Transform Fault Boundaries • Boundary between two plates that are sliding past each other • Crust is neither created nor destroyed What is a Fault? • Is a break in the Earth’s crust where slabs of rock slip past each other • Ex: San Andreas Fault where many Earthquakes occur: What Earth-changing event occurs on a transform boundary? • Earthquakes frequently occur at transform boundaries Video Review of plate tectonics and boundaries….. • Watch lesson one of the video review from the link below. Be prepared to answer questions after the video!!! • Video clip : Lesson 1 "Plate Tectonics" Review Questions... • What are the three types of boundaries? • What direction do the plates go for each type? • Which boundary has a subduction zone…what occurs at a subduction zone? So What Causes Plate Tectonics? (The continent’s slow dance) Convection Currents • Hot magma in the Earth moves toward the surface, cools, then sinks again. • Creates convection currents beneath the plates that cause the plates to move. What causes plates to move? • Because there is an unequal distribution of heat, hot plastic-like rock is forced to the top of the surface where it will cool, thicken (denser), and sink. • It’s the transfer of heat inside the Earth that provides the energy to move the plates and causes many of our surface features to form. Plates move at a rate of between ____ and ____ centimeters per year. • one • ten The North American and Eurasian plates float away from each other at a rate of fingernail growth. This is a rate of ____ per year. • 2.5 centimeters Figure 29: Posing Questions What questions would you need to answer in order to predict where the continents will be in 50 million years? • You would need answers to how fast and in what direction each plate is moving Review Questions... • What causes plates to move? • How is a convection current formed? How do we know the plates are really moving? • 1) We can feel it in the form of seismic waves from earthquakes!! • 2) We can see it as volcanoes erupt! • 3) We can see it from how our land changes from the volcanoes and earthquakes!!! Bellwork 10/31/13 • Retake this pre-‐test and see how much you have learned!! If you change an answer, rewrite it BESIDE your original answer. Practice Unscramble the following words: - gneivdret - boundary where two plates pull apart - areinmls - a non-living, inorganic solid - sarfmrotn - where plates slide past each other; earthquakes occur - utalf - a break in the Earth’s crust