Laboratory #7: Plate Tectonics

Materials Needed:1. Pencil

2. Colored Pencils

3. Metric/Standard Ruler

4. Calculator

5. Tracing Paper

Plate TectonicsThe Earth is composed of layers. At the center is a nickel and iron core, which is separated into a

solid inner core and a liquid outer core. Next is the mantle, which is contains the rigid

mesosphere, the plastic and ductile asthenosphere, and the rigid upper mantle. The crust and

lithosphere are a part of the upper mantle.

Plate tectonics has been an accepted theory since the 1960’s. According to this theory, the crust

of the Earth is composed of plates that move over the asthenosphere. There are two basic types

of plates: heavy, thin and dense oceanic plates, which are primarily composed of basalt; and

thick, lighter continental plates, which are comprised of silicate rocks.

There are three types of plate boundaries. Divergent plate boundaries (also called spreading

centers) occur where two plates are moving away from one another. Convergent plate

boundaries exist where two plates are moving towards one another. Convergent plate

boundaries are classified based upon the type of crustal plate involved: ocean-ocean, ocean-

continent, and continent-continent. Finally, transform plate boundaries are located where two

plates are sliding past one another. The largest plate is the Pacific plate, followed by the African

plate, Eurasian plate, Australian-Indian plate, Antarctic plate, North American plate, and South

American plate. Smaller plates include the Nazca plate, Philippine plate, Caribbean plate, Cocos

plate, and Juan de Fuca plate.

Most continental shorelines do not coincide with plate boundaries (one important exception is the

west coast of South America). Nor are most shorelines zones of earthquake activity. For this

reason, continental shorelines are referred to as "passive margins," places that may have been

plate boundaries in the past, but are no longer active. The east coast of North America is a

passive margin, as are the east, south and west coasts of Africa, northern and W estern Europe,

and others. The eastern boundary of the North American plate is in the middle of the Atlantic

Ocean, where there is a belt of active seismicity. Thus the North American plate is made up of

both continent and ocean. In fact, most plates consist of both continental and oceanic material.

There are several entirely oceanic plates (Nazca, Cocos), but no entirely continental plates (with

the possible exception of the Arabian peninsula; it depends on the criteria one uses to define

individual plates and how much ocean is required to be considered "ocean"). The fact that

continents are included as part of plates made of both continent and ocean suggests that the

continents do not move independently of the oceans as W egener envisioned, but rather that

continent and ocean move together as part of a single plate. Thus from a geodynamical

perspective, a "plate" appears to be a more fundamental unit than a continent or ocean.

Laboratory #7: Plate Tectonics Name: ___________________

Part I: Crustal Plates and Plate Boundaries

1. Figure 3 shows the major crustal plates and plate boundaries of Earth.

a. Color all of the divergent plate boundaries red, subduction zones blue, continent-

continent convergent boundaries green, and transform faults yellow .

b. Shade in areas with chains of volcanoes using a blue pencil.

c. Make a key for the colors on your map.

d. Next to each plate boundary, place a set of arrows to indicate the relative rate of

movement for major plates.

2. How many plates are there?

3. W hich plate is totally surrounded by a convergent plate boundary?

4. W hich plate(s) do not contain significant areas of continental landmasses?

5. In general, do continental shorelines coincide with plate boundaries?

6. Are there any plates that have both continents and oceans? If so, name them.

7. List the plates that are bounded in part by:

The Mid-Atlantic Ridge:

The East Pacific Rise:

8. What major island mass lies on the axis of the Mid-Atlantic Ridge?

Laboratory #7: Plate Tectonics Name: ___________________

Part II: Movement of the Volcanoes in the Hawaiian Ridge over theHawaiian Hot Spot.

The premise behind plate tectonics is that the crustal plates are moving with respect to one

another over geologic time. The rates of movement of crustal plates can be determined by using

data from the plate margins along the mid-ocean ridges, where the amount of movement can be

measured.

To measure the movement of two adjacent crustal plates along the margins of a divergent plate

boundary, two things must be known: 1) two points on adjacent diverging plates that were once at

the same geographic coordinates but have since moved away from each other over a known

distance; 2) the time required for the two points to move from their original coincident positions to

their present positions. Determining the age in actual years of the two points requires knowing the

age of the rocks in that location. As new crust forms on either side of a spreading center, this rock

becomes magnetized according to the polarity of the Earth at the time of the rocks formation.

Thus, if the absolute age of the magnetic anomaly is known, then the age of the rocks in that

location are also known.

The map of below shows part of the Hawaiian Ridge and the absolute dates of lava in bold black

numbers that millions before the present. Hawaii contains an active volcano, Mauna Loa, so the

lava from it is zero years old. The lava on Nihoa Island is 7 million years old. Thus, according to

the hot spot hypothesis, Nihoa Island was once an active volcano standing where Hawaii stands

today.

The Lab:

Figure the rate in centimeters per year using the distances from Hawaii to each of the three dated

lavas on the map located on the next page. Make your distance measurements from the center of

the zero on Hawaii to the center of each of the boldface numbers on the ridge.

Laboratory #7: Plate Tectonics Name: ___________________

1. From Hawaii (0 m.y.) to Nihoa Island (7 m.y.)

2. From Nihoa Island (7 m.y.) to just west of Necker Island (10 m.y.)

3. From just west of Necker Island (10 m.y.) to Midway Island (20 m.y.)

4. Do the rates of movement based on the three dates indicate that the movement has been

constant or variable?

Map of the Hawaiian Ridge in the Pacific Ocean.

Contour interval = 1000 meters Scale: 1 cm = 170 km.

Laboratory #7: Plate Tectonics Name: ___________________

Part III: Restoration of the South Atlantic Coastline 50 Million Years beforePresent

Given the evidence of spreading along the Mid-Atlantic Ridge, it can be deduced that the Africa

and South America plates are moving away from each other carrying the continents of Africa and

South America with them. By using the pattern of magnetic lineations shown on the map on the

next page, it is possible to reverse the spreading process and restore the positions of the African

and South American coastlines to a time when a particular set of magnetic lineations was being

formed on the Mid-Atlantic Ridge. For the purposes of this exercise we win use anomaly number

21, which, according to the magnetic lineation time scale page 51, was formed 49.6 or roughly 50

million years ago. Proceed as follows.

1. On the map on the South Atlantic Ocean, draw a red line over each of the magnetic

lineations of anomaly number 21 on the South American side of the Mid-Atlantic Ridge.

Connect the segments of the number 21 anomaly with a red line drawn along the fracture

zones against which they terminate. Start with the point where anomaly 21 touches the

Ascension F. Z. Follow Anomaly 21 with your red pencil southward until it reaches the

Bode Verde F. Z., then along the Bode Verde F. Z. westward to the northern end of the

next fracture zone. Continue until you have reached the southernmost fracture zone on

the map.

2. Attach a piece of tracing paper over the map with tape or paper clips, and repeat the

process described above for Anomaly 21 on the African side of the Mid-Atlantic Ridge.

Draw this line in red pencil on the tracing paper.

3. W ith the tracing paper still in place, trace the coastlines of Africa and South America on

the tracing paper with black pencil. Also, trace on the tracing paper the boundaries of map

and the 20° South latitude line in black pencil.

4. Detach the tracing paper and slide it toward South America until the red line on the tracing

paper matches the red line on the map. W hen the two lines are matched as closely as

possible, hold the tracing paper in place and trace the coastline of South America in red

pencil on the tracing paper. Trace also the 20° South line on the tracing paper in red

pencil.

5. The map you have constructed on the tracing paper shows the Mid-Atlantic Ridge as it

existed when magnetic anomaly 21 was being formed. Your tracing paper also shows the

relative positions of segments of the coastlines of Africa in black pencil and South

America in red pencil as they were approximately 50 million years ago. This

reconstruction is based on the assumption that the continents of Africa and South

America were fixed to their respective plates during the spreading process over the past

50 million years. The continents moved with respect to each other because the tectonic

plates to which they were attached moved as spreading continued along the Mid-Atlantic

Ridge. W hat is the evidence that the movement of the two plates was not strictly in an

east-west direction?

6. W as the earth's magnetic field normal or reversed at the time represented by your map on

the tracing paper?

Laboratory #7: Plate Tectonics Name: ___________________

Map of the South Atlantic Ocean showing part of the Mid-Atlantic Ridge in black bars, with east-west fracture

zones, and selected magnetic anomalies. The ages of the numbered anomalies or magnetic lineations can be

determined from the magnetic time scale in the next page. (From Magnetic Lineations of the World’s Ocean

Basins, Copyright 1985 the American Association of Petroleum Geologists.

Marine magnetic anomalies. (A) The

black line shows positive anomalies

as recorded by a magnetometer

towed behind a ship. In the cross

section of the oceanic crust,

positive anomalies are drawn as

black bars and negative anomalies

are drawn as white bars. (B)

Perspective view of magnetic

anomalies shows that they are

parallel to the rift valley and

symmetric about the ridge crest.

Magnetic Time Scale. Magnetic anomaly identification numbers are given on the top and

absolute time scale in millions of years (Ma) is given on the bottom. Normal magnetic

anomalies are shown in black, and reversed polarity is shown in white. The absolute age of

the anomalies and their polarities can be read directly from the chart. For example, anomaly

4 is 7 million years old, and it is a positive anomaly.