Plate Tectonics


Introduction
Plate Tectonics theory is based upon the concept that portions of the Earth's outer surface move relative to each other. The details of this theory can be found at the following links and references:
Dynamic Earth (U.S. Geological Survey)
Online Textbook, Plate Tectonics
Textbook: McKnight's Physical Geography, Hess, 10th Ed. - Chapter 14
Textbook: Essential Geosystems, Christopherson, 6th ed. ,- Chapters 8 and 9


The following are basic concepts that should be understood before completing the plate tectonic activities for this class:
1. Plate Tectonics is the concept that portions of the Earth's outer surface move relative to each other.
2. The portion that moves is called the "lithosphere" and consists of all of the crust and rigid uppermost mantle (see figure below).
The lithosphere is rigid (brittle) and moves over a more plastic (and, in some cases, partially melted) lower layer called the "asthenosphere". The lithosphere layer is broken into several pieces. These pieces are the "plates" of plate tectonics.

3. An important aspect of the Earth's landforms (and thus physical geography) is how the crust is deformed by interactions between (at plate boundaries) and within plates.
4. The three types of plate boundaries are shown in the figure below. The relationships at plate boundaries can be complicated, but each boundary is associated with one of the principle types of stress (tension, compression, shear).
a. At divergent boundaries plates pull apart and new plate is created. This boundary is associated with tensional stress.
b. At convergent boundaries plates push together and often one plate is pushed under another (subduction). This boundary is associated with compressive stresses.
c. At transform boundaries one plate slides by another to accommodate horizontal motion between two plates. This boundary is associated with shear stress.


5. Plates consist of oceanic and/or continental crust. The North American plate consists of a continental portion (that includes the North American continent) and an oceanic portion (that consists of half of the north Atlantic oceanic crust). Some plates (Pacific, Juan de Fuca, Nazca, etc) are primarily made of oceanic crust. Oceanic crust is denser than continental crust and therefore sinks to a lower elevation. Additionally, oceanic crust will subduct beneath less dense continental crust or younger, less dense oceanic crust.













Each of the three plate boundary types results in specific landforms. These are shown in the plate boundary figure above.
Oceanic Divergent Plate Boundaries result in oceanic ridges, often with rift valleys at their crest. Continental Divergent Boundaries also result in ridges and rift valleys. Both are associated with volcanic activity.
Convergent boundaries between oceanic plates or oceanic and continental plates result in trenches (where one plate pushes below another), and mountains and volcanoes in the over-riding plate. Convergence of two continents results in elevated mountains without a trench, and often without volcanism.
Transform boundaries result in linear valleys and streams channels and other features that are offset by faulting.

The following link provides a Google Map showing plate boundary locations. This may help to visualize the landforms at these locations.
Link to Interactive Plate Boundary Map

Activity Questions
The Google Maps below shows the landforms associated with plate boundaries. Study these images and answer the questions below each map.

View Oceanic Plate Boundaries in a larger map
1. Click on each of the placemarks in the map above to read about plate boundaries A, B, and C. List the plate boundary type and associated landform for A, B, and C.


View Continental Rifting in a larger map
2. Click on each of the placemarks on the map above to read about the plate boundary shown. List the type of plate boundary shown and describe how the landforms present are consistent with this type of plate boundary. Also, state the portion of this boundary that is the oldest and state your reasoning for this answer.


View Oceanic-Continental Convergent Boundary in a larger map
3. Do you know the type of plate boundary shown in the map above? Click on the placemark to find out. Describe the landforms associated with this boundary. Try switching to "TER" view (terrain) to see the landforms of the continent in this region.


View Convergent Boundary in a larger map
4. How does the convergent boundary shown in the map above differ from the one shown in question #3? Describe the landforms associated with this type of plate boundary. Is there a trench? Is there volcanism? List the name of the famous mountain shown.


View Plate Boundary in a larger map
5. Click on the placemarks and investigate the landforms in this area to determine the plate boundary at this location. This type of plate boundary is usually within the oceanic lithosphere, but this continental boundary results in a famous fault running through much of the length of California. List the plate boundary type and the evidence for plate motion at this location.


View Local Plate Boundaries in a larger map
6. Use the zoom tool in the map above to view the landforms associated with the three plate boundaries shown by the blue (A), green (B), and yellow (C) placemarks. List the type of boundary at each of the placemarks (boundaries A, B, and C). Also, describe how the feature shown by the green arrow is related to this plate boundary setting. What is the feature shown by the green arrow? You may want to use the TER (terrain) view to investigate this feature.