Name:_____ ANSWERS __________________
Student #______58 MARKS IN TOTAL_______
DEPARTMENT OF EARTHAND SPACE SCIENCE & ENGINEERING
EATS 1010 3.0 Laboratory Exercise #4
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]
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. Objective
In this exercise you will:
1) plot and interpret an earthquake focus profile, and
2) explain the seismic characteristics of a subduction zone.
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
2. Study your earthquake focus profile and answer the questions on page 3 in the spaces
Earthquakes of the Marianas TrenchArea
Horizontal Horizontal Horizontal
Depth (km) Distance from Depth (km) Distance from Depth (km) D i s t a n c e
Trench (km) Trench (km) Trench (km)
0 16 216 264 516 456
0 50 232 254 550 500
0 72 250 264 580 450
0 96 300 360 580 464
32 168 350 364 600 480
66 132 350 400 600 508
83 168 383 408 600 524
83 184 416 432 624 508
116 232 448 448 624 524
116 248 472 416 650 532
150 200 472 432 667 548
150 260 500 450
200 216 500 458
200 248 516 432 Page3
2.What is the relation between the depth of earthquakes and their distance from the Tonga
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 TONGATRENCH TO ADEPTH 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.
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.
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] Page4 Page5
PART 2: HOT SPOTS AND PLUMES [26 MARKS]
Hot spots are zones where unusually high heat flow has been recorded at the Earth's surface.
Several dozen are known. These are often found well within the tectonic plates, as for example,
the Hawaiian Islands. They are isolated and not associated with spreading centres or subduction
The cause of hot spots is believed to be plumes of abnormally hot rock welling up from deep
within the mantle. Although the origin of these mantle plumes is uncertain, these sites often
experience spectacular volcanic activity. The volcanoes of Hawaii are a famous example. Here,
eruptions of Mauna Loa have occurred recently. Northwest of Hawaii a north-trending row of
volcanic peaks is found on the ocean floor far below sea level. They are called the Emperor
Seamounts. Though the Hawaiian Islands and the Emperor Seamounts meet at an angle, they
appear to be part of a single, continuous chain of volcanoes.
It is interesting to note that of the entire chain only the island of Hawaii and Loihi (which is
located to the southeast of Hawaii) are volcanically active. These islands are located farthest to
the southeast and have the highest heat flow. The unusual arrangement of these volcanic islands
and seamounts could be explained by the movement of the Pacific Ocean floor over a hot spot.
If so, this evidence would provide a verification for the theory of plate tectonics and sea floor
In this exercise you will:
7.Explain how a mantle plume may produce a chain of volcanoes
8.Understand how the direction of movement of a tectonic plate can be determined
9.Describe how a change in plate-movement direction can be detected
10.Compute ocean floor spreading rates, and
11.Explain how hot spots help to verify plate tectonics theory.
Use the map on page 6 and a metric ruler to determine the distance from the island of Hawaii to
each island or seamount listed below. Make all measurements along the trend of the island
chain. Fill in the distances on the chart and answer the questions on page 7. Page6 Page7
Distance (km) Date of Last Eruption
(Millions of Years ago)
Hawaii 0 Recent
West Maui 150 1.3
West Molokai 212 1.8
Waianae (Oahu) 294 3.5
Kauai 396 5.6
Nihoa 573 ±10% is OK 7.5
Necker 792 10.0
Midway 1775 17.7
Kanmu 2546 38.0
Koko 2758 46.0