PLATE TECTONICS ACTIVITY

 

INTRODUCTION

 

Are the Himalayas getting higher?

Is Australia becoming more tropical?

Will Los Angeles one day be a suburb of San Francisco?

Is the Atlantic Ocean growing and the Pacific Ocean shrinking?

Are we standing on large plates of the Earth’s crust that are moving at the rate a fingernail grows?

 

The answer to each of the above questions is yes!  And, it is all due to Plate Tectonics.

 

            The basic idea of Plate Tectonics Theory is that the Earth’s oceans and continents are embedded in several large, flat plates that are moving slowly (about 5cm/year).  These plates are an average of 100km thick and are composed of the Crust and the top 60km – 75km of the Mantle, together called the Lithosphere.  The lithospheric plates rest on top of the solid, but plastic, lower Mantle or Asthenosphere.  Scientists believe that convection currents in the Asthenosphere, due to the Earth’s escaping heat, provide the driving force of Plate movement. 

 

            There are three types of Plate movement – Convergence, Divergence, and Transform.  Convergence occurs when two plates move towards each other.  Divergence is the spreading apart of plates and Transform faults occur when two plates slide past each other.

Figure 1 Schematic view of plate boundary interactions and the resulting landforms.  "This Dynamic Planet.”  (USGS)

When Continental plates converge, the crust buckles and dramatic mountain ranges, such as the Himalayas, are formed. When Ocean plate converges with thicker, but lighter Continental plate, it slides underneath in a subduction zone.  A deep ocean trench with a chain of volcanic mountain ranges running parallel characterizes this zone.  An example of this is the subduction of the Nazca Plate beneath the South American plate and the resulting Andes.

 

 

Figure 2 Subduction zone of continent-to-ocean convergence.  As the crust descends into the mantle it heats up.  At around 100 km below sea level, hot fluids are sweated out which melt the surrounding material.  This light material ascends buoyantly and forms a line of volcanoes.

 

 

 A subduction zone also occurs when Ocean plates converge, resulting in the formation of curved chains of volcanic islands or Island Arcs.  The arc of islands is curved concavely in the direction of the movement of the down-going plate.  The Ring of Fire is the name given to the volcanic islands around the Pacific that are the result of the subduction of the Pacific Plate.  The Aleutian Islands located below Alaska, are curved concavely to the north, (see figure 3 below.)  The Pacific plate is subducting beneath the North American plate in a northwesterly direction in this region.

 

           

Figure 3 Ring of Fire around the subducting Pacific plate.  (USGS)

            Divergence occurs in the Mid-Ocean Ridges where new crust is formed to take the place of the subducted crust keeping the Earth the same size.  A mid-ocean ridge that encircles the earth runs through the center of the Atlantic Ocean.

 

 

            A Transform boundary occurs when two plates slide past each other without any upward deformation of the crust but may result in localized cracks or “faults”.  The San Andreas Fault in California is a well-known example of this type of “Strike-Slip” fault.  Los Angeles on the Pacific plate side is moving northwest, while San Francisco on the North American plate side, is moving southeast.

 

Figure 4 The San Andreas Fault.  (USGS)

 

 

            As the plates move they grind against each other at Plate boundaries.  Here, stress in the crust builds up until it is released in the energy of an earthquake.  The location of these earthquakes is one of the clues used by Scientists to help locate Plate boundaries. The location of volcanoes, Island Arcs, Mid-Ocean ridges and Hot Spots (stationary plumes of convecting Mantle) are also used to determine the Plates and their movement.

 

 

 

 

            In this exercise you are a member of a team of Science Officers on Planet X, a fictitious Earth colony.  Given an incomplete map of earthquake activity and recent seismic data, you must determine the location and movement of the tectonic plates of this planet.  You will create a file of “clues” to help solve the Plate Tectonics puzzle.

 

 

 

 

 

PROCEDURE

 

 

  1. Examine Figure1. Global Seismicity and compare with Figure 2. Major Tectonic Plates.  Write down three observations:

 

 

 

 

 

 

 

 

 

 

 

  1. Which one of the following patterns of seismicity would you choose to show a relationship between seismicity and tectonic plate location?

 

 

Figure 5 Clustered, line, or random patterns of seismicity.

 

 

  1. Map epicenter location of events 1 – 8 onto Map 1. Incomplete Seismicity of Planet X.  (Check key for correct color code.)
  2. Draw tectonic plate boundaries of Planet X. onto Map 1. and name plates.
  3. Complete Modeling Clay Activity, Ping Pong ball Activity, and Hot Spot Activity to obtain a “clues” folder.
  4. Use Maps and Clues to determine plate movement.

 

 

 

 

 

 

 

DATA

 

Event #

Longitude (Degrees East)

Latitude        (Degrees North)

Magnitude (Mw)

Depth        (Km)

1

60

80

5.1

32

2

90

60

6.9

25

3

45

80

7.1

20

4

45

45

6.3

27

5

68

60

5.2

250

6

75

70

5.1

200

7

82

70

3.2

60

8

70

75

5.5

275

Figure 6 Table of seismic data for Planet X.

 

 

CLUES

 

1.      Location of Earthquakes.

2.      Location of volcanoes

3.      Hot Spots characteristics. (See above Activity.)

4.      Subduction zone characteristics.  (See above Modeling Clay Activity.)

5.      Island Arcs.  (See above Ping Pong ball Activity.)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

WORLD MAPS

Figure 7-8 World map showing location of tectonic plates and location of earthquake (USGS). 

 

 

 


PLANET X. MAPS.

 

Figure 9 Incomplete Planet X. map showing location of quake epicenters.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PLANET X. MAPS CONTINUED

 

Figure 10 Active Volcanoes of Planet X.