Introduction to Physical Geography

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GEO 200 ICA 2: Plate Tectonics

The model of plate tectonics is the starting point for understanding the distribution and formation of many collections of landforms around the world. The lithospheric plates are 65 to 100 kilometers (40 to 60 miles) thick and consist of the crust and upper mantle. The plates move over the layer of the mantle known as the asthenosphere at speeds averaging from 2.4 to 10 centimeters (1 to 4 inches) per year.

Major Lithospheric Plates.

The three different kinds of plate boundaries are associated with different kinds of topographic features and tectonic activity.

Divergent Boundaries: At divergent boundaries (also called “spreading centers”) plates are moving apart. The most common kind of spreading center is the midocean ridge where new basaltic ocean floor is created. Spreading may also take place within a continent. In this case, blocks of crust may drop down as the land is pulled apart, producing a continental rift valley (such as the Great East African Rift Valley).

Convergent Boundaries: At convergent boundaries, where plates collide, three circumstances are possible:

1. If the edge of an oceanic plate collides with the edge of a continental plate a subduction zone is formed. The denser oceanic plate is subducted below the continent, producing an oceanic trench. As the oceanic lithosphere descends, water and other volatile materials are driven out of the ocean rocks, leading to a partial melting of the mantle. The magma that is generated rises, producing intrusions of plutonic rock such as granite and a chain of andesitic volcanoes, such as the Andes in South America or the Cascades in North America.

2. If the edge of an oceanic plate collides with the edge of another oceanic plate, subduction also takes place. An oceanic trench forms, along with a chain of andesitic volcanic islands known as an island arc, such as the Aleutian Islands in Alaska and the Mariana Islands of the western Pacific Ocean.

3. If the edge of a continent collides with the edge of another continent, the relatively buoyant continental material is not subducted. Instead, a mountain range is uplifted. The Himalayas are a dramatic example of this kind of plate boundary interaction.

Transform Boundaries: Plates slide past each other at transform boundaries, such as along the San Andreas fault system in California.

Evidence supporting the theory of plate tectonics comes from global patterns of landforms and tectonic activity. In addition to the matching shape of the continental margins on both sides of the Atlantic Ocean (which spread apart from the Mid-Atlantic Ridge), the age of the ocean floor provides evidence of movement. The ocean floors are youngest at midocean ridges, where new lithosphere is being formed, and become progressively older away from a ridge in both directions. This was verified through ocean core samples, as well as paleomagnetic evidence (changes in Earth’s magnetic field that have been recorded in the volcanic rocks of the ocean floor).

Plate boundaries are often the sites of significant volcanic activity. At spreading centers, magma is moving up to the surface, creating new lithosphere as the plates spread apart. Magma generated in subduction zones can produce a chain of continental volcanoes or a volcanic island arc.

The distribution of earthquakes also provides clues to plate activity. Most earthquakes around the world occur in association with plate boundaries. Shallow-focus earthquakes, within about 70 kilometers (45 miles) of the surface, occur at all plate boundaries. However, in subduction zones, bands of progressively deeper earthquakes are observed, produced when an oceanic plate is thrust deep down into the asthenosphere.

Hot Spots

One of the important modifications of basic plate tectonic theory is the concept of the hot spot. These are locations where a fairly narrow plume of magma is rising from the asthenosphere to the surface, producing volcanoes. Many hot spots apparently develop from mantle plumes that originate deep within the mantle.

Hot spots may occur well away from plate boundaries, often in the middle of a plate. It is not yet completely understood why these hot spots occur where they do, but the existence of hot spots has been helpful in verifying plate motion.

Evidently, hot spots can remain active in the same location for millions of years. While the hot spot remains in the same place, the plate above continues to move above it. Currently active volcanoes are found directly over the hot spot, while the moving plate carries older volcanoes off the plume, at which time they become inactive. Ongoing plate motion carries these old volcanoes farther and farther away from the hot spot, resulting in a chain of extinct volcanoes.

The Hawaiian Islands are the best-known example of an island chain produced by a hot spot. The only currently active volcanoes are found on the island of Hawaii in the southeast part of the island chain. It is believed that this island is currently over the hot spot.

The map below shows the ages of volcanic rocks in the Hawaiian chain. Notice that the ages (in millions of years) of the volcanic rocks becomes progressively older as we follow the islands to the northwest. The pattern of islands in the Hawaiian chain shows the general direction of movement of the Pacific Plate, and from the ages of the rocks, we can infer the rate of plate movement.

Island Ages of the Hawaiian Archipelago.

Activity

Study the tectonic map of a hypothetical ocean basin (page 6 of this packet). The map shows the location of volcanoes, earthquakes, and the age of ocean floor rocks. From this map, you will determine the probable location of the plate boundaries and the locations of major topographic features in the region.

The edges of two continents are shown (in the upper right corner and lower left corner). Six islands are also shown. The symbols used on the tectonic map are described below.

Earthquake Epicenter Location and Depth:

The locations of earthquake epicenters are shown with letters. The depth of the earthquake (the distance of the earthquake hypocenter or “focus” below the surface) is indicated with an “S” (shallow focus), “I” (intermediate focus), or “D” (deep focus):

S = Shallow Earthquakes 0-70 kilometers (0-45 miles) deep

I = Intermediate Earthquakes 70-200 kilometers (45-125 miles) deep

D = Deep Earthquakes 200-500 kilometers (125-310 miles) deep

Active Volcano:

Continent or Island:

Activity Procedure:

The first step of the activity is to draw in the approximate plate boundaries as indicated by the tectonic activity on the map.

Clues include:

a) The pattern of earthquakes. For example, subduction produces a pattern of deeper and deeper earthquakes as one plate plunges below the other.

b) The age pattern of volcanic ocean floor rocks suggests the location where new ocean floor is being created at a midocean ridge.

c) Volcanic activity may be associated with subduction, spreading centers, or hot spots.

Use the following symbols to indicate the extent of all plate boundaries. Both the map symbols, and a side view of the circumstance they represent, are shown below. Arrows indicate direction of plate movement.

Convergent Boundary (Subduction) Divergent Boundary

Map symbol to be used: Map symbol to be used:

Side view: Side view:

Note: No transform boundaries are found on the map. Assume that only one of the volcanoes on the map is associated with a hot spot.

Draw the plate boundaries using the appropriate symbols on the tectonic map, and then answer the questions on the following page. When asked to cite evidence to support your answers, only cite evidence that you can see on this map.

1. How many different plates (not boundaries) are clearly shown on the map?

2. Indicate on the map the most likely location of a midocean ridge (such as the Mid-Atlantic Ridge). Write the label “Midocean Ridge” on the map.

a. What type of plate boundary is this?

b. What evidence shown on the map indicates that this type of boundary is present?

3. Indicate the most likely location on the map of a major volcanic mountain range similar to the Andes in South America with the label “Volcanic Mountain Range”.

a. What type of plate boundary is this?

b. What evidence shown on the map indicates that this type of boundary is present?

4. Label all plate boundaries where oceanic trenches should be found with the label “Trench”.

5. Assume only one of the volcanoes on the map is a hot spot. Label this volcano “Hot Spot”.

a. Draw an arrow extending from this volcano, indicating the direction of plate movement (and the direction in which you would expect to find progressively older, extinct volcanoes).

6. In the space below, draw an approximate continuous cross section (“side view”) of the ocean basin from Point A to Point B (from lower left to upper right). Use the “side view” drawings for reference, and use arrows to indicate the relative direction of plate motion. If subduction is taking place, clearly show which plate is subducting beneath the other.

Use pages 22-23 (or pages 6-7 in the 23rd edition) in Goode’s World Atlas to answer the following questions.

7. The Sumatra-Andaman earthquake on 12/26/2004 was centered near 3° N latitude, 96° E longitude.

a. What type of plate boundary is present (for example, is this a convergent or a divergent boundary)?

b. Name the plates involved Check the Atlas or the figure “Major Lithospheric Plates” on the first page of this packet to find the plate names (for example, the North American plate and the Juan de Fuca plate).

c. Since this earthquake occurred under water, what additional natural disaster accompanied this earthquake?

8. Locate Tonga near 21° S latitude, 175° W longitude.

a. The Tonga islands are located along the Tonga Trench. What type of plate boundary is present?

b. Would you expect to find volcanoes in Tonga? Why or why not?

9. The Cascadia earthquake of 1700 occurred at 47° N latitude, 122° W longitude, and generated a tsunami that traveled to Japan.

a. What type of plate boundary produced the Cascadia earthquake?

b. When oceanic crust meets continental crust, which type of crust is subducted? Explain your answer.

c. Name two active volcanic mountains located in this area.

10. Locate the East African Rift Valley and the Red Sea. Name the plates involved and describe the tectonic activity (which is happening here: subduction or rifting?).

11. For each of the following locations, identify the plate that is being subducted.

a. Santiago, Chile

b. Attu Island, Alaska

c. Taipei, Taiwan

For the remaining questions, use the maps and Index in Goode’s World Atlas to identify the features, and draw them on your copy of the blank world map (last page of this packet). Be precise in your drawings.

12. Locate and draw the boundary between the Nazca and South American plates.

a. What type of boundary is this (convergent or divergent)? What tectonic activity occurs here (subduction or rifting or sea-floor spreading)?

b. List two tectonic landforms that occur here.

13. Locate and draw the boundary that runs south from Iceland to near Antarctica.

a. What type of boundary is this? What tectonic activity occurs here?

14. Locate and draw the boundary between the Philippine and Pacific Plates.

a. What type of boundary is this? What tectonic activity occurs here?

b. List two tectonic landforms that occur here.

15. Locate the Hawaiian islands on the map.

a. Draw an arrow that shows the direction of movement of the Pacific Plate. The arrow should point in the same direction as the oldest islands.

b. What is the term that describes the volcanic activity that occurs far away from a plate boundary?

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