Week 7
Plate Tectonics
Click on the link below to hear a mini-lecture audio segment.
Plate tectonics is arguably the most important scientific revolution of the 20th century. It is so compelling because it provides and explanation for practically all of the things we observe on the surface of the earth, and those processes that we observe originating below the surface of the Earth. the earth is covered with 14 major plates that move around the surface of the Earth and have done so for over 500 million years.
Plate Boundaries Fit
Click on the link below to hear a mini-lecture audio segment.
Plate tectonics was first proposed in the 1800's by Antonio Snyder-Pellegrini based on the fact that the continents of the Earth fit together like pieces of a jigsaw puzzle. Based on the fit, he proposed that the continents were once together, forming a single large landmass.
Other Plate Tectonics Evidence
Alfred Wegener in the 1920's first proposed the concept of "Continental Drift" based on evidence he found on the margins of continents.
If you look at the continental boundaries there is much overlap of fossil assemblages, structures, and rock types up to the time in geologic history when the plate split apart.
If you look at glacial deposits from the past, before the split, there is also overlap.
Now, see if you can solve for the fit of a hypothetical groups of plates that were once part of a megacontinent, but have since broken off into four distinct plates.
Plate Movement History
Mechanisms of Plate Movement
Plate Boundaries
Click on the link below to hear a mini-lecture audio segment.
Convergent Boundaries
Convergent boundaries are plate boundaries in which one plate (Oceanic) is pushed down under another (Continental) plate. Oceanic crust is destroyed at this type of boundary. There are three types of convergent boundaries.
Ocean-continent
Ocean-ocean arc
Continental collision
Things Found at Convergent Boundaries:
Metamorphism
Volcanism
Reverse Faulted Mountains
Sedimentary Wedges
Divergent Boundaries
Divergent boundaries occur when plate are rifted apart and begin to move apart, creating large expanses of oceanic crust. Crust is created in this type of boundaries.
Things Found at Divergent Boundaries:
Volcanism
Normal Faulted Basins
Sedimentary Deposits of Arkosic Sandstone
Oblique-Slip Boundaries
Oblique Slip boundaries are boundaries in which one plate slide past another with neither loss or creation of crust.
Things Found at Oblique-slip Boundaries:
Dynamic Metamorphism
Faulting
Minor Igneous Structures (occasionally)
Summary of Plate Boundaries
A number of processes occur at the various types of plate boundaries. These processes result in many of the features we observe on the surface of the earth near these boundaries. In the exercises below, you may test your knowledge of the processes.
Sea Floor Spreading
Click on the link below to hear a mini-lecture audio segment.
The ocean floors provide a great deal of evidence about the mechanisms of plate movement and also provide some of the most compelling evidence in support of plate tectonics. For reasons that are not completely clear, the magnetic field of the earth periodically reverses itself. When the field is normal, the magnetic pole is roughly coincident with geographic north. When the field is reversed, the magnetic pole is south. The magnetic field at the time is preserved in any igneous rock that crystallizes at the time, because the the magnetically susceptible minerals point in the direction of the magnetic pole at the time of crystallization. This results is stripes of normal and reverse polarized stripes of oceanic crust that represent the magnetic field at the time that the particular part of the crust was created. This striping is shown below.
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