GEOG103: Physical Geography

Lecture 14-15: THursday-Tuesday, Novemnber 3 & 8, 2011

Topics
1. The Concept of Geological Time Scale
2. Earth's Structure and Internal Energy
  3. The Geologic Cycle
4. Plate Tectonics

The Concept of Geological Time Scale
• Human Beings strated to live on Earth about 1.8 million years ago
• Human Beings did not exist for 99.9% of the Earth history
• Birds did not exist for ~96% of the Earth history
• Mammals did not exist for ~96% of the Earth history
• Land plants did not exist for ~91% of the Earth history
• Animals did not exist for ~85% of the Earth history
• 87% of the Earth history is called Precambrian because no animals lived on Earth surface during this long period of time

• Find the times when Dinosaurs lived from the geological time table (Fig. 11.1, p. 299)

• Uniformitarianism: The basic principle for geological research
      The same physical processes active in the environment today have been operating throughout the geological time OR
      The present is the key to the past

Earth's Structure and Internal Energy
• Earth's Interior layers: Read Fig. 11.2, p. 301, understanding the following:
      Crust:
         Oceanic Crust
         Continental Crusts
      Mantle
         Upper Mantle
         Lower Mantle
      Core
         Outer Core
         Inner Core
      Lithosphere and Asthenosphere

• How do we know the outer core is liquid?
      Primary (P) seismic waves: Compressional, can penetrate liquid
      Secondary (S) seismic wave: transverse or shear force, cannot penetrate liquid
      By recording the time differences between P and S waves across the Earth suggests a liquid outer core

• Oceanic crusts: 3.0 g/cm³ (Basaltic); Continental crusts: 2.7 g/cm³ (Grantic);
• Isostatic Adjustment:
       Asthenosphere is plastic
       Mountains have roots
       Erosion of mountains or melting of continental ice sheets causes the loss of weight
       Isostatic uplift of the mountain areas

The Geologic Cycle
• Minerals and Rocks:
       O and Si are the most abundant elements in the Earth's crust
       Silicates (minerals made of SiO₄^4- and cations such as Al³⁺, Ca²⁺, K⁺, etc.) are the most abundant minerals in the Earth's crust
       Three types of rocks: Sedimentary rocks, igneous rocks, and metamorphic rocks

• Rock cycle:
       Igenous/sedimentary/metamorphoric rocks can be weathered, transported to the sedimentary basins (e.g., oceans and lakes, etc) to deposit;
       Burial of sediments form sedimentary rocks
       Igenous/sedimentary/metamorphoric rocks can be subducted downward along subduction zones, metamorphized, remelt and erupt back to the Earth surface.

Plate Tectonics
• Continental Drift Hypothesis:
       Alfred Wegener (1880-1930): Geographic Fit of the Continents, e.g., Africa and south America ==> these continents must have been together at some time and late split and drifted away
• Seafloor Spreading:
       The volcanic eruptions underneath the seafloor, when cools down to the Curie point, record the Earth’s Magnetic field
       The Earth's Magnetic field reverses from 20-30 thousand years to millions of years (Why does that happen is still a puzzle)
       By dating the volcanic rocks and comparing the magnetic strips, we know that the symmetric strips along the modern ocean ridges
       Conclusion: the seafloor is spreading along the ocean ridges

• Plate Tectonics:
       The Lithosphere consists of seven major and a number of smaller pieces of plates;
       There are three types of plate boundaries (divergent, convergent, and transform faults)
       Deformation (e.g. mountain-building) occurs at plate boundary; relative little deformation in plate interior

• Plate Boundaries:
       Divergent plate boundaries: are seafloor spreading centers where upwelling material from mantle forms new seafloor. Example: Atlantic Middle Ocean Ridge, Eastern African Rift
       Convergent plate boundaries: are collision zone where areas of continental and oceanic lithosphere collide. Example: Himalaya Mountains, Eastern Pacific--Western US
       Transform plate boundaries: Plates slide laterally past one another at right angles to a seafloor spreading center, neither diverging nor converging, usually without vigorous volcanic eruptions. Example: San Andreas Fault in western US

• Earthquakes and Volcanoes are mostly common at the platform boundaries
• "Ring of fire": volcanoes surrounding the pacific plate boundaries