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EATS1010 LAB #4 ANSWERS!!!!!!!YORK UNIVERSITY


Department
Earth, Space Science and Engineering
Course Code
ESSE 1010
Professor
Gary Jarvis

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!Name:_____ANSWERS__________________
Lab. Session:_______
Student #______58 MARKS IN TOTAL_______
YORK UNIVERSITY
DEPARTMENT OF EARTH AND SPACE SCIENCE & ENGINEERING
Fall 2010
EATS 1010 3.0 Laboratory Exercise #4
PLATE TECTONICS
This lab is divided into three parts, each dealing with a different aspect of plate tectonics.
Read the lab over beforehand and bring a pencil, metric ruler and either your textbook or
your class notes to the laboratory. Allow yourself about one hour of time for each exercise.
Instructions are given for each part. Follow the procedures as outlined and fill in your
answers to all the questions on the work sheets provided.
Hand in your completed lab before the end of the session.
Be sure your name is on the front page and also on each of your answer sheets (in
case some of the pages come apart).
PART 1. ISLAND ARC/OCEAN TRENCH EARTHQUAKES [12 MARKS]
Introduction
In the western and northern Pacific Ocean there are many arc-shaped groups of islands. The
Aleutians, the Marianas, Japan and Indonesia are all examples. There are also island arcs in the
Caribbean and South Atlantic. Island arcs have two important characteristics: (1) they all have
deep ocean trenches nearby and (2) they are areas of volcanism and intense earthquake activity.
The location in the Earth where an earthquake occurs is called its focus. Geophysicists have
discovered that most of the world's intermediate and deep-focus earthquakes (below 100 km)
occur near island arc/trench systems.
For many years there was great curiosity about the unusual character of island arcs. There was
no explanation for them prior to the Theory of Plate Tectonics. It proposed that if oceanic plates
are created at a mid-ocean ridge, there must be some place where the far ends of the plates dive
back down into the mantle. This occurs in the vicinity of island arcs producing the trench as well
as volcanic and earthquake activity. Earthquake studies in such regions provide evidence of
subduction of the diving plate.

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Objective
In this exercise you will:
1) plot and interpret an earthquake focus profile, and
2) explain the seismic characteristics of a subduction zone.
Procedure
1. The table below lists the locations of earthquakes that have occurred in the Marianas
Trench area. Plot the location of each on the Earthquake Focus Profile provided as page
4.
2. Study your earthquake focus profile and answer the questions on page 3 in the spaces
provided.
Earthquakes of the Marianas Trench Area
Horizontal Horizontal Horizontal
Depth (km) Distance from Depth (km) Distance from Depth (km) D i s t a n c e
from
Trench (km) Trench (km) Trench (km)
0 16
0 50
0 72
0 96
32 168
66 132
83 168
83 184
116 232
116 248
150 200
150 260
200 216
200 248
216 264
232 254
250 264
300 360
350 364
350 400
383 408
416 432
448 448
472 416
472 432
500 450
500 458
516 432
516 456
550 500
580 450
580 464
600 480
600 508
600 524
624 508
624 524
650 532
667 548

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Page 3
Questions
2.What is the relation between the depth of earthquakes and their distance from the Tonga
Trench?
EARTHQUAKE DEPTHS INCREASE AS DISTANCE FROM THE TRENCH
INCREASES. [1 MARK]
3. Draw a line through the points on your profile. Describe the appearance of the line:
LINE DESCENDS UNIFORMLY FROM THE TONGA TRENCH TO A DEPTH OF
ALMOST 700 KM [ 1 MARK]
4.Give an explanation for the distribution of the earthquake foci. Does the shape of the
earthquake focus pattern support the theory of the plate tectonic concepts that you learned
in class. Explain.
ACCORDING TO PLATE TECTONICS, COLD RIGID PLATES ARE SUBDUCTED
INTO THE MANTLE AT SUBDUCTION ZONES, MARKED BY AN OCEAN
TRENCH AT THE UPPER SURFACE.
[1 MARK]
THE PLATES ARE BRITTLE AND FRACTURE, PRODUCING THE PATTERN OF
EARTHQUAKES IN THE PLOT. [ 1 MARK]
THE PATTERN OF EARTHQUAKES IN THE PLOT SUPPORTS THIS IDEA.
[1 MARK]
5.What surface feature(s) result(s) from the melting of the sinking plate and/or mantle
immediately above the sinking plate?
VOLCANOES (or ISLAND ARCS) (or VOLCANIC ISLANDS) [1 MARK]
6.As the lithosphere plate dives into the mantle, it begins to warm and soften. At what depth is
the plate no longer brittle enough to support earthquakes? __~700 km (> 667 km is OK)
[1 MARK]
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