Subduction Zone Activity

 

 

INTRODUCTION

 

            The theory of plate tectonics is relatively new in the field of Earth Science.  Only within the last 30 years have scientists fully accepted the idea that the top layer of the Earth is made up of large, moving plates.  The oceans and continents embedded within the plates move with them.  As they move, the plates interact with each other at the plate boundaries.  An important type of interaction is the collision of two tectonic plates in what is termed a Convergent Boundary.

 

            When two plates collide mountain ranges, volcanic mountain ranges or volcanic islands may be formed.  Four fifths of historically active volcanoes are found at convergent boundaries.  The Ring of Fire is the name given to the volcanic islands around the rim of the Pacific Ocean.  These islands occur along the convergent boundary of the Pacific plate.  The Himalayas are an example of a mountain range formed by continent-to-continent convergence.  These mountains were formed and are still rising as a result of the collision of India with Eurasia. 

 

 

 

Figure 1 The convergence of the India with Eurasia and the resulting deformation of the crust into the dramatic Himalayan Mountain Range.  (USGS)

 

 

 

 

 

 

 

A more common convergent boundary occurs when two ocean plates collide or when an ocean plate is subducted beneath a continental plate

 

            An ocean plate is generally thinner and denser than a continental plate and as a result it usually slides underneath or is subducted.  A deep ocean trench with a line of volcanic mountains running parallel is characteristic of ocean–to–continent convergence.  The Andes are a volcanic chain of mountains that were formed by the subduction of the Nazca plate beneath the South American plate.  As the plate descends it heats up. At a depth of about 100 km below sea level the increased temperature and presence of fluids brought down with the plate causes partial melting of the subducted crust and surrounding mantle material.  The melt is lighter than the mantle and ascends buoyantly as magma to form the volcanoes.  The downgoing slab continues it’s descent and is eventually absorbed into the Mantle – a recycling process that keeps the Earth the same size!  Subduction zones reclaim ocean crust while new crust is generated at mid-ocean ridges.

 

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.

 

When two ocean plates converge one slides beneath the other and forms a deep ocean trench and Island Arcs.  These volcanic islands are formed in the same manner as above.  The arc of islands usually curves concavely in the same direction as the movement of the subducting plate. 

 

 

 

 

 

 

 

 

 

 

 

 

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Figure 3 The Ring of Fire around the Pacific Ocean.  The green lines show the location of Deep Ocean Trenches.  Note the concave arc of the Aleutian Islands (top, center) is to the North - the same direction as the movement of the subducting Pacific plate.  (USGS)

 

 

            The interaction between converging tectonic plates results in the build up of stress, which is released in the energy of an earthquake.  Over three fourths of the world’s earthquakes occur at convergent boundaries.  Not only are earthquakes more frequent in subduction zones but are also of the largest magnitudes.  The largest recorded earthquake occurred on May 22, 1960 in Chile along the Andean subduction zone, with a Richter magnitude of about 8.5.  The epicenter, defined as the location on the Earth’s surface, of this earthquake was near the Taitao Peninsula while its focus, the actual point source of energy release, was several kilometers deep within the Earth.

 

Most earthquakes occur at shallow depths, typically no more than 70 km below the earth’s surface.  Intermediate focus earthquakes occur from 70 to 300 km and those below this depth are called deep focus earthquakes.  When seismologists first began studying earthquake data they noticed that many deep see trenches also had a line of earthquake foci that dipped into the earth at about 40 – 60 degrees from the horizontal.  This characteristic dip is called the Wadati–Benioff zone after the two seismologist who first discovered it.  Today, we know that the line of earthquakes follow the down going slab.  It is the sticking and slipping of this subducting plate, which builds up strain in the surrounding rock.  This strain is available for release in large earthquakes.

 

Figure 4 Earthquake activity around the Pacific Ocean 7/25/00 - 7/25/01, showing magnitude and depth.  Purple (0-33 km), Blue (33-70 km), Green (70-150 km), Yellow (150-300 km), Orange (300-500 km), Red (500-800 km).  (IRIS)

 

 

In this exercise you are a member of a team of Science Officers on Planet X, a fictitious Earth Colony.  You will use seismic data to plot a Benioff Zone to examine converging plates in a subduction zone.  Next, you will plot depth of foci for a small slice of the Earth.

                       

 

 

 

 

PROCEDURE

 

1.      Plot foci from Table 1. onto Graph 1. of only those quakes within the range of 60 degrees north to 75 degrees north (latitude) and 60 degrees east to 90 degrees east (longitude).

2.      What can you assume about the plate interaction in this location?

3.      Draw a diagram of plate interaction showing the angle of the down-going slab.

4.      Plot foci from Table 2. onto Graph 2.

5.      Using World Map and Graph 2. draw a diagram of the plate interaction showing angle of the down-going slab and name the plates involved.

 

 

 

DATA

 

TABLE 1

 

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 5 Table of seismic data for Planet X.

 

 

TABLE 2

 

Event Date

(mth/day/yr)

Longitude

(Degrees West)

Latitude

(Degrees South)

Magnitude

(Richter)

Depth

(Km)

1/23/97

65.7

22

7.1

276

10/14/99

68.8

21.8

5.5

101

5/12/00

66.5

23.6

7.2

225

11/29/00

70.9

24.9

6.3

58

5/24/01

67.1

20.1

5.2

205

6/23/01

73.3

16.1

7.9

33

6/25/01

70.5

17.2

5.5

18

Figure 6 Table of actual seismic data for the Andes subduction zone.  (IRIS)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

GRAPH 1.

 

Figure 7 Graph showing the depth of foci of earthquakes on Planet X in the latitude range 60 - 75 degrees north and longitude range 60 - 90 degrees East.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

GRAPH 2.

 

Figure 8 Graph showing the location of Foci of actual earthquakes in the latitude range 15 - 25 degrees south and longitude 60 - 90 degrees west.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MAPS

 

 

PLANET X MAP

 

Figure 9 Map of Planet X. showing earthquake epicenter location in latitude range 30 – 90 degrees north and longitude 60 – 150 degrees east.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MAP OF SOUTH AMERICA

 

 

 

Figure 10 Map of World showing earthquake epicenter locations in the latitude range 0 - 90 degrees south and longitude range 0 - 180 degrees west.  (IRIS)