Notes on chapters 11 and 12

The topic before us is so broad in scope that we can do no more than touch on the high points. Chapters 11 and 12 actually provide a good introduction to earth dynamics and the role of plate tectonics, but we will not have time to discuss all of these topics in class. These notes provide an overview of the main issues you should be familiar with. There are also some excellent web sites that provide a pretty good overview of plate tectonics.  The U.S. Geological Survey publication, This Dynamic Earth, is an excellent source. Another web page on plate tectonics from John Louie at the University of Nevada, Reno, is also worth reading.

Here are the topics covered in our discussion of these chapters. This list follows along more or less in sequence with the discussion in class and is not necessarily presented in the same order as the topics in chapters 11 and 12.

Chapter 11

• What is the earth made of? elements as building blocks of minerals; basic characteristics of minerals; rocks as assemblages of minerals; the importance of the silica tetrahedron as the basic building block of most rock-forming minerals in the earth's crust and mantle
• The eight most important elements in the earth's crust: oxygen, silicon, aluminum, iron, calcium, sodium, potassium, magnesium
• Basics of the earth's internal structure: crust, mantle, outer core, inner core
• difference between continental and oceanic crust
• the Mohorovicic discontinuity and the crust/mantle boundary
• difference between the lithosphere (uppermost mantle + crust acting together as a series of rigid plates) and the asthenosphere
• the core-mantle boundary and the difference between the molten outer core and the solid inner core
• composition of the layers: both outer and inner core primarily iron with some nickel; mantle composed of silicate minerals but with a large percentage of the heavier elements (iron and magnesium) and darker, denser minerals; crust also primariliy silicates, but less dense, less iron and magnesium, more aluminum, silicon, sodium, potassium
• oceanic crust is thinner (3-5 km) and denser (3.0 g/cm³) than continental crust (25-35, sometimes up to 70 km thick, 2.7 g/cm³)
• The age of the earth and the concept of "deep" geologic time; Hutton and uniformitarianism vs. catastrophism
  
• The rock and fossil record of earth history; mapping of rock layers and use of the fossil record to reconstruct earth history; use of relative dating methods to construct the geologic time scale; use of radiometric dating to put correct dates on the geologic time scale
• Major time periods in earth history - Precambrian vs. Phanerozoic (which includes Paleozoic, Mesozoic and Cenozoic)
• Importance of major extinctions as dividing lines in earth history
• How important are catastrophic events compared to the slow, steady pace of geologic processes? the concept of punctuated equilibrium
• Major terrain types on the ocean floors and the continents
  
• The rock cycle and the three main varieties of rock:
• igneous
• sedimentary
• metamorphic
• know the distinctive properties of each
  
• recycling of earth materials and transformation of one rock type to another: the rock cycle
  
• Introduction to plate tectonics:
• the three types of plate boundaries: divergent, transform, convergent
• relation of these plate boundaries to types of landscape features, e.g. mid-ocean ridges, trenches, volcanic arcs, etc.
  
• global pattern of earthquakes and volcanic activity in relation to plate boundaries
• dominant processes at each type of boundary
• continental rifting, sea-floor spreading, and upwelling of new magma from the mantle
• age distribution of rocks on opposite sides of mid-ocean ridges
• magnetic "stripes" on the ocean floor and their significance
  
• transform faults as sites of lateral offset forming fractures in the mid-ocean ridge
• subduction zones, trenches, and the recycling of ocean floor
• volcanic arcs associated with partial melting of descending slabs
• ocean-ocean boundaries and volcanic island arcs (Aleutians, others bordering the Pacific Ring of Fire)
• ocean-continent boundaries and volcanic mountain chains on land (Andes, Cascades)
• continent-continent boundaries: continental collisions and the building of the largest mountain ranges
• what about volcanism that isn't related to plate boundaries? hot spots and the Hawaiian Islands and what they tell us about plate motion
  
• relative motion of the plates
• what drives plate tectonics? convection in the earth's mantle
  
• Pangaea and the evolution of the continents and oceans over the last several hundred million years or so

Chapter 12

• Correlation between high relief and tectonic activity;contrast between western and eastern U.S.
  
• the terrane concept and the assembly of western North America
• relation between types of stress, types of plate boundaries, and types of geologic structures:
• compressive stress, convergent boundaries and subduction zones, folding and reverse faulting
• shear stress, transform boundaries and lateral offset along strike-slip faults (e.g.San Andreas)
• tension, divergent boundaries, normal faults and fault-block mountains, horst and graben
• orogenesis (mountain-building)
• Brief discussion of earthquakes
• focus and epicenter
• magnitude and intensity scales
  
• Brief discussion of volcanic features in relation to patterns of plate tectonics
  
• locations and types of volcanic activity:
• at hot spots (e.g. not a plate boundary; over a plume of hot magma rising from the core-mantle boundary)
• effusive eruptions, low viscosity (highly fluid) magma  with low gas content that makes lava fountains and lava flows
• high temperature, high iron and magnesium content, "mafic" composition - rock type is basalt
  
• shield volcanoes: broad, gently sloping
  
• example on ocean floor - Hawaiian Islands
• plateau basalts or flood basalts (e.g. Columbia Plateau in Pacific Northwest; Iceland)
  
• along mid-ocean ridges (divergent plate boundaries associated with sea-floor spreading)
• also basaltic magma, high temperature and low viscosity; mostly erupts underwater, creates new ocean floor rather than individual volcanoes
  
• fissure eruptions
  
• near subduction zones (e.g. convergent plate boundaries)
• result from partial melting of crust above subducting slab of oceanic lithosphere
• composition is intermediate to felsic, lower content of iron and magnesium and more silicon and aluminum
• magmas more viscous, not as hot, higher content of compressed gases
  
• explosive eruptions, dominated by ash deposits and pyroclastic materials; much more dangerous
  
• composite or stratovolcanoes (e.g. Mt. St. Helens, Mt. Pinatubo)