Plate Tectonics

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Terms §

• Crust, mantle, core
• Lithosphere
• Asthenosphere
• Topography and bathymetry
• Magnetic reversals
• 3 types of plate boundaries
  • Convergent
  • Divergent
  • Transform
• Convection

Things to think about §

• Relationship of topography, earthquakes, and volcanoes to plate margins
• The distinctive features associated with the 3 different types of plate margins
• What do magnetic reversals tell us about past plate motions

Basic Earth Structure §

• We know what the Earth’s interior looks like through seismology (study of earthquake energy release and propagation)
  • We also can tell things based on igneous rocks that are brought to the surface
  • And Structural features at the surface
• 
  • The crust is actually very small. Upper/Lower crust on this picture is the Mantle.
  • The outer core is liquid, and the inner core is solid
• Radius of the Earth is approximately 6371 kilometers (at widest)
• Mechanical vs Compositional Classifications
  • Compositional = what it is made of
  • Mechanical = how it behaves
  • We have the Continental Crust, and the Oceanic Crust
• The crust and top roughly 100 km of mantle is the Lithosphere. The lithosphere is mechanically strong and rigid. This is where the plates are.
• The next 100 kilomters (100-200 km depth) is the Asthenosphere. It is mechanically weak and partially molten. The movement in the Asthenosphere is what allows the Lithosphere to move and thus the plates.
  • The rock composition is roughly the same, but the fact that it is partially melted is what makes it important and different
• The Asthenosphere and Lithosphere are both part of the mantle. The Lithosphere also contains the crust

Tectonics §

• Topography = elevations above sea level (usually green to brown on maps)
• Bathymetry = depth below sea level to ocean floor (blues, sometimes black too)
• These combined help show plate tectonics
• There are several large plates that make up the Earth
• There are 3 different types of boundaries that are found where the plates meet, and most plates have all types somewhere

Features of Earths Surface §

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  • Divergent Boundary
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  • Convergent Plate Boundary
  • Subduction Zones
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  • Volcanoes
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Plate Boundaries §

• 3 types of plate margins
• Divergent Boundaries
  • Plates move apart
  • 
  • At the very top of mid ocean ridges, molten material is coming up to the surface and solidifying further increasing the size of the plate
  • 
  • The rocks are magnetized and when solidified you can see the effects of the magnetic field on the plates, showing its spread.
    • This leads to magnetic stripes on the seafloor, they are symmetric across the ridge
  • Aside:
    • Earth’s magnetic field
    • Inner core transfers heat to liquid outer core
    • Outer core is convecting
    • Movement of molten iron affected by Earth’s rotation, produces electrical currents and a magnetic field
    • Magnetic field currently flows south to north (normal polarity)
    • The magnetic field can change directions! North and South can switch
      • We can see the change in magnetic field in the rocks
  • How fast are plates spreading? Usually millimeters per year. Sometimes centimeters per year
• Convergent Boundaries
  • Plates move toward each other
• Transform Boundaries
  • Plates move horizontally past one another

Convection §

• Movement of heat with mass in the Earth (or anywhere)
• convection-water-top-bottom-temperature-molecules-process.webp
• This happens in the mantle, more specifically the Asthenosphere. It’s the motion of convection in the asthenosphere that pulls plates together or moves them apart
• 
• Convection works via changes in density of the rock

1. Rock material at the bottom gets hotter than surroundings
2. It expands
3. Same material in more volume = less dense
4. Less dense than surroundings = floats
5. Gets to top, cools off, contracts, gets more dense
6. Sinks
7. Starts again