Themes of the Day:

• Structure of the Earth (continued)
• Plate Tectonics: Driving Force, Plate Boundaries
• Rock Types

Structure of the Earth

• Core - composition: primarily Iron (Fe) - inner core is solid, outer core is liquid
• Mantle - silicate composition, primarily the mineral olivine - solid, but near melting point in places
• Crust - silicate composition, primarily feldspars - thin scum riding atop the mantle, solid
• Oceanic vs. Continental Crust
• Lithosphere vs. Asthenosphere

Plate Tectonics: Driving Forces, Plate Boundaries

• Heat loss from the interior of the Earth drives convection of the mantle, which in turn drives Plate Tectonics A B
• Basic Principles of Plate Tectonics
  • Lithosphere forms a number of rigid plates that undergo little internal deformation
  • Lithospheric plates move relative to each other atop the ductile asthenosphere in response to convection in the Earth's mantle - Figure 1.12
  • Plate motion rates are on the order of centimeters per year (cm/yr) - about as fast as fingernails grow
  • The vast majority of geologic activity (earthquakes, volcanoes, etc.) occurs at plate boundaries
• Earth's Plates - Figure 1.11 left right - in Google Earth
  • Plate Tectonics is a unifying paradigm that explains many topographic/bathymetric features of Earth's surface - Figure 1.13
• Types of Plate Boundaries
  • Divergent - Mid-ocean spreading ridges - new crust created
  • Convergent - Subduction zones or Continent-continent collisions - deep-sea trenches, volcanic arcs, and major earthquakes at subduction zones - Subduction zone examples: Andes Mts on W edge of South America, Japan, Aleutian Islands - Continent-Continent collision examples: Himalayas, Alps
  • Transform - plates slide past each other - Strike-slip faults - San Andreas Fault
• For more on Plate Tectonics, see This Dynamic Earth: the Story of Plate Tectonics, from the U.S. Geological Survey.
• Note: Surficial geologic processes are the result of a combination of Plate Tectonics - driven by heat loss from the interior of the Earth - and atmospheric phenomena (weather) - driven by heat input from the Sun.

• Rocks are generally made up of one or more minerals.
• Igneous Rocks
  • Form by cooling and crystallization of magmas
  • Usually tight, interlocking crystals, generally silicate minerals
  • Crystal size (grain size) varies based on cooling rate - fast cooling = fine-grained, slow cooling = coarse-grained
  • Composition (chemical, mineralogical) varies from mafic to felsic
• Sedimentary Rocks
  • Form at or near the surface of Earth by reconstitution (lithification) of fragments or chemical constituents that result from the weathering process
  • Usually formed in stratified layers that obey Steno's Laws (Original Horizontality, Lateral Continuity, Superposition)
  • Clastic, Chemical, and Organic varieties
  • Often record important information about depositional environments
• Metamorphic Rocks
  • Rocks that have undergone changes (textural, mineralogical) in the solid state (no melting involved)
  • Often preserve information about their protolith (pre-metamorphic rock type) and the pressure and temperature conditions under which they equilibrated