Download Earth Revealed_lab07..

Laboratory #7: Plate Tectonics
Materials Needed:
1.
2.
3.
4.
5.
Pencil
Colored Pencils
Metric/Standard Ruler
Calculator
Tracing Paper
Plate Tectonics
The 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, oceancontinent, 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plate boundaries in the past, but are no longer active.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 Western 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 Wegener 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, continentcontinent 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?
14
1) Eurasian Plate, 2) Australian Plate, 3) Philippine Sea Plate, 4) Pacific Plate, 5) Cocos
Plate, 6) N.A. Plate, 7) Caribbean Plate, 8) Nazca Plate, 9) South American Plate, 10)
African Plate, 11) Arabian Plate, 12) Iran Plate, and 13) the Antarctic Plate.
3. Which plate is totally surrounded by a convergent plate boundary?
The Philippine Sea Plate
4. Which plate(s) do not contain significant areas of continental landmasses?
The Pacific Plate, the Philippine Sea Plate, the Nazca Plate, and the Cocos Plate.
5. In general, do continental shorelines coincide with plate boundaries?
No.
6. Are there any plates that have both continents and oceans? If so, name them.
The Australian Plate, the North American Plate, the South American Plate, the African
Plate, the Eurasian Plate, and the Caribbean Plate.
7. List the plates that are bounded in part by:
The Mid-Atlantic Ridge: the North American Plate, the South American Plate, the
African Plate, the Eurasian Plate, and the Antarctic Plate.
The East Pacific Rise: The Pacific Plate, the North American Plate, the Nazca Plate,
the Cocos Plate, and the Antarctic Plate.
8.
What major island mass lies on the axis of the Mid-Atlantic Ridge?
Iceland!
Laboratory #7: Plate Tectonics
Name: ___________________
Part II: Movement of the Volcanoes in the Hawaiian Ridge over the
Hawaiian 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.)
5 cm x (170 km/1 cm) = 850 km
850 km x (1000 m/1 km) x (100 cm/1 m) = 85,000,000 cm
85,000,000 cm / 7 m.y. = 12,142,857.14 cm/m. y.
12,142,857.14 cm/m. y. / 1,000,000 yrs = 12.14 cm/yr
2. From Nihoa Island (7 m.y.) to just west of Necker Island (10 m.y.)
2.3 cm x (170 km/1 cm) = 391 km
391 km x (1000 m/1 km) x (100 cm/1 m) = 39,100,000 cm
39,100,000 cm / 3 m.y. = 13,033,333.33 cm/m. y.
13,033,333.33 cm/m. y./ 1,000,000 yrs = 13.03 cm/yr
3. From just west of Necker Island (10 m.y.) to Midway Island (20 m.y.)
7.1 cm x (170 km/1 cm) = 1207 km
1207 km x (1000 m/1 km) x (100 cm/1 m) = 120,700,000 cm
120,700,000 cm/ 10 m.y. = 12,070,000 cm/m. y.
12,070,000 cm/m. y./ 1,000,000 yrs = 12.07 cm/yr
4. Do the rates of movement based on the three dates indicate that the movement has been
constant or variable?
Slightly variable, but not by much!
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 before
Present
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 MidAtlantic Ridge. Draw this line in red pencil on the tracing paper.
3. With 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. When 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. What is
the evidence that the movement of the two plates was not strictly in an east-west
direction?
6. Was 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.