geotectonics

Page 1: Types of Crust

• Oceanic Crust

  • Composition: mostly olivine, basalt, and silica

  • Formation: created from magma filling gaps between diverging plate tectonics; consumed in subduction zones

  • Characteristics: 5.7 km average thickness, denser and thinner than continental crust, younger than continental crust

  • Solidifies into igneous rock at mid-ocean ridges at ~1488 °C

• Continental Crust

  • Composition: mostly andesite (diorite)

  • Characteristics: older and thicker (100 - 300 km), weaker than oceanic crust; primarily made of quartz, less dense

  • Continuous remaking through tectonic plate movement

  • Historical context: formed from debris collisions, ancient Earth was molten

• Plate Tectonics

  • Processes: convection currents drive tectonic movements

  • Rules: Plate tectonic plates move on small circles relative to one another around an "Euler Pole"

  • Transform faults and spreading ridges interact with these movements

Page 2: Geophysical Evidence for Plate Tectonics

• Measurement Techniques

  • GPS and various other methods confirm tectonic movements

  • Isostacy explained via variations in plate density and thickness

  • Continental crust's lighter density keeps it elevated

• Heat Flow and Gravity Maps

  • Topography related to crustal density variations

  • Red ridges on heat maps indicate magma presence near plate boundaries

  • Larger mountains correlate with higher gravity due to density differences

• Cooling and Thickness Relationship

  • Time of cooling is proportional to crust thickness

  • Magma rises, causing crustal thickening and deformation

Page 3: Active vs. Passive Margins

• Active Margins

  • Features: mountain building, earthquakes, volcanic activity

  • Locations: where continents meet oceans

• Passive Margins

  • Features: transition from land to sea, lesser tectonic activity

  • Examples: coastal plains, continental shelves

  • Sedimentation processes lead to coastal erosion and deposition

• Rifting and Sediments

  • Rocks in continental rifts contain alkalite minerals (olivine, pyroxene, plagioclase)

  • Age-related depth increases above mid-ocean ridges due to convection processes

Page 4: Arc Collisions

• Types of Arc Collisions

  • Arc-Arc: two arcs migrate toward one another, merging into one

  • Arc-Continental: creates a break or flip leading to subduction

  • Denser oceanic crust descends at convergent plate boundaries

• Microcontinent Formation

  • Small pieces of ophiolite indicate former ocean basin remnants, reflecting tectonic history