Geology 101 Lab: Plate Tectonics (Using Excel)

Answer sheet (click here).

What you'll learn:

What you'll turn in, with your name on top of each page:

  1. Answers to all written questions, written on the answer sheet.
  2. Excel spreadsheet for part 2 with all your data in it (a 1-page printout).
  3. Completely labeled XY (Scatter) graphs for parts 2 and 3, with white (blank) backgrounds, produced in Microsoft Excel.
  4. The maps you used and drew on for Part 1.
  5. The scale on the edge of a piece of paper you used for measuring distances on the map for Part 2.

Part 1: Four Magnetic Profiles

Look at the photocopy of Figure 17.8 (from Jones and Jones, Laboratory Manual for Physical Geology, McGraw Hill Higher Education, 2003). It shows magnetic measurements in four profiles conducted in the mid-1960s across along the Pacific-Antarctic Ridge. The data were gathered using an instrument called a magnetometer that was towed behind the ship to measure the magnetism of the seafloor. The ship made four passes across the Pacific-Antarctic Ridge, at right angles to the ridge. These profiles are labeled A, B, C, and D in Figure 17.8.

The dashed line marks the crest, or axis, of the ridge.

The positive parts (peaks) of the magnetic profiles are where the rocks are magnetized pointing to magnetic north in the same direction as magnetic north is today.

The negative parts (valleys) of the magnetic profiles are where the rocks are magnetized pointing to magnetic north in the opposite direction as magnetic north is today.

The theory of seafloor spreading, part of plate tectonics theory, predicts that the profiles should be mirror images (symmetric, equally spaced) on either side of the ridge. In addition, each magnetic anomaly should form a line or stripe parallel to the ridge. You will investigate these predictions. You will also use the spacing and age of the magnetic anomalies to calculate rates of seafloor spreading.

First, see if you can find the same pattern in each of the four profiles. Finish labeling anomalies 1, 2, 3, 4 and 1', 2', 3', 4' in each profile. Then USE A RULER to draw straight lines connecting each anomaly.

The ages of the magnetic anomalies have been measured. To within 0.2 Ma (millions of years ago), the magnetic peaks labeled 4 and 4' are 7 million years old.

Now calculate at what rate, on average, the plates have been spreading away from the ridge at profile D since 7 Ma. Show your work on the answer sheet.

In other words, calculate the rate of motion of peak 4 since it left the ridge, and peak 4' since it left the ridge.

Recall that a rate of motion, commonly called "speed," is a distance divided by the amount of time it took to cover that distance. Divide the distance from peak 4 (measured in km) by the age (in Ma) to get the average rate of plate motion away from the ridge.

In the case of magnetic anomalies at peaks 4 and 4', age is 7.0 Ma.

The units of plate motion will be kilometers per million years (km/Ma).

Similarly, divide the distance from peak 4' to the ridge (km) by the age in (Ma) to get the average rate of plate motion away from the ridge on that side.

To convert kilometers per million years (km/Ma) to centimeters per year (cm/yr), divide the km/Ma number by 10, or multiply by 0.1 (same result either way).

Now complete the following instructions and write down answers to the following questions.

(WORD OF ADVICE: NEVER SUBMIT QUANTITIES OF UNITS WITHOUT WRITING DOWN THE UNITS. NEVER.)

  1. Magnetic anomalies can be used to track the spreading of oceanic plates on either side of a mid-ocean spreading ridge ( a divergent plate boundary on the ocean floor).
    1. According to plate tectonic theory, would the plates on either side of a mid-ocean ridge, at a particular point on the ridge, be spreading at close to the same rate?
    2. According to plate tectonic theory, would the spreading rate be the same on one end of the ridge as at the other end of the ridge? Explain your answer.

  2. Along profile A, what is the rate of plate motion on the peak 4 side in terms of both kilometers per million years and centimeters per year?

  3. Along profile A, what is the rate of plate motion on the peak 4’ side in terms of both kilometers per million years and centimeters per year?

  4. Along profile D, what is the rate of plate motion on the peak 4 side in terms of both kilometers per million years and centimeters per year?

  5. Along profile D, what is the rate of plate motion on the peak 4’ side in terms of both kilometers per million years and centimeters per year?

  6. Are the spreading rates roughly similar on both sides of the ridge?

  7. Are the spreading rates the same on profile A as they are at profile D?

  8. If the spreading rates are different at A and D, what may be the reason?

  9. Are the magnetic anomaly profiles perfectly complete, with no gaps?

  10. Do you think that a meaningful analysis of such data can be conducted even if there are any gaps in the data? Why or why not?

  11. Are the lines connecting the profiles at least approximately parallel to each other?

  12. Are the lines connecting the profiles at least approximately parallel to the ridge crest?

  13. Are the magnetic anomalies consistent with the theory of seafloor spreading?

Part 2: Spreading from the Juan de Fuca Ridge

Look at the colored map of magnetic anomalies on the ocean floor, off the coast of the Pacific Northwest. The thick dashed line is the crest of the Juan de Fuca Ridge. North is to the top, west is to the left, east is to the right.

If the instructor provides you with a color map copy, do not print one yourself. Click here for a bigger copy of the colored magnetic anamolies map, better for printing.

For a map scale, one cm (centimeter) equals 50 km.

BACKGROUND: These data were gathered by a magnetometer towed behind a ship. Enough data were gathered to allow the magnetic anomalies to be mapped out as color contours. Correlating the seafloor magnetic patterns with the Earth's magnetic polarity reversal timeline, the ages of the anomalies on the seafloor can be narrowed down within a precision of 0.1 Ma (millions of years ago) on this map. (On other, more precise maps, precision to within better than 0.01 Ma can be achieved).

The colored areas are where the ocean floor has normal magnetism, with north in about the same direction as it is now.

The white stripes between the colored stripes are areas where the ocean floor has reversed magnetism, with north and south switched around.

Oceanic crust gets its magnetism from the Earth's magnetic field. As new oceanic crust forms from solidifying lava, magnetic minerals suspended in the liquid mixture line up like compasses. Then, once the new crust has become solid rock, the magnetism is frozen in permanently. It acts as a record of which way magnetic north was on Earth at the time the oceanic crust formed.

The magnetic anomaly map pattern is not perfectly symmetric, due to faults in the Earth's crust. The faults show up on the map as straight, solid back lines offsetting the colored magnetic strips.

The data collection is less than perfect. In real research, data collection is seldom perfect; the goal is for it to be good enough to be statistically meaningful. Despite these complicating factors, one can nonetheless discern mirror image (symmetric, equally spaced) magnetic anomaly patterns on either side of the ridge, once the offsets along faults are removed or worked around.

INSTRUCTIONS: Your first job is to measure distances from the center of the Juan de Fuca Ridge to the magnetic reversals that appear on the map as the edges of the colored zones.

MEASURING DISTANCES:
Use a ruler with cm on it. Measure the distances in km, using the scale of 1 cm = 50 km.. Once you measure a distance in cm on the map, multiply it by 50 to get km.

Measure the distances out from the center of the ridge, at right angles from the ridge. Measure anomalies on both sides of the ridge, east and west.

Avoid crossing faults with your measuring lines.

Each magnetic reversal appears on the map as boundaries between a colored area and a white area.

Measure the distance from the ridge to the boundaries along the edge of each color zone (see table below). Again, avoid crossing faults with your measuring lines.

MEASURING AGES:
The second set of measurements that you must make is to measure the age of each magnetic reversal.

The ages can be found in the magnetic reversal timeline, which is the colored column in the bottom left part of the map. The magnetic reversal timeline is labeled "Age of oceanic crust".

The "Age of oceanic crust" magnetic reversal timeline is marked and labelled at two million year intervals (every 2.0 Ma). Read the ages of the magnetic reversals (such as the age at the end of the red color zones) to the nearest 0.1 Ma, and write them down in the table below.

Write down all of your measurements of the distance to the edge of each color zone (from the map), and its age (from the "Age of oceanic crust" timeline), in the table below.

Magnetic anomaly color boundary (corresponds to a magnetic reversal, except for the current center of the ridge)
Age (Ma)
Distance west of center of ridge crest (km)
Distance east of center of ridge crest (km)
red begins (center of ridge)
0.0
0
0
red ends
      
orange begins
      
orange ends
      
yellow begins
     
yellow ends
     
light green begins
     
light green ends
     
dark green begins
      
dark green ends
      

Now put your data into an Excel spreadsheet for further analysis.

In an Excel formula the asterisk, *, stands for multiplied by, and the forward slash, /, stands for divided by.

All mathematical formulas (all calculations) in Excel must begin with the equals sign, =.

Now calculate the spreading rates of the plates on either side of the Juan de Fuca Ridge.

Recall that a rate of motion, or speed, is a distance divided by the amount of time it took to cover that distance.

Calculate the rates of plate motion in terms of both kilometers per million years and centimeters per year.

In Excel formulas, the slash symbol / means divided by.

Now convert kilometers per million years (km/Ma) into centimeters per year (cm/yr).

There are 100,000 cm in one km and there are 1,000,000 years in one Ma. Unit conversion analysis shows that, to make it quick and simple, you just need to multiply km/Ma by 0.1 (one tenth) to convert to cm/yr.

Now find the average rates of spreading.

That completes your numerical analysis.

Now make a graph that depicts and compares how the plates have apparently been spreading, on the west and east sides of the ridge.

When you are done creating your graph (chart), print it WITH A WHITE BACKGROUND. DO NOT PRINT GRAPHS WITH COLORED OR SHADED BACKGROUNDS. Also, be sure your name is on the graph. To clear a shaded or colored background area on the map, right-click on the area and then choose Clear from the bottom of the menu, or else choose Format Plot Area and choose the white box for the area color.

(WORD OF ADVICE: A GRAPH WITHOUT LABELED AXES SHOULD NEVER BE SUBMITTED.)

Now answer the following questions, with reference to your chart (graph).

  1. On the map, what is represented by the dark, solid straight lines that offset the colored magnetic anomaly patterns?

  2. The theory of seafloor spreading, which is part of plate tectonics theory, suggests that plates tend to spread away from divergent plate boundaries at close to equal rates on either side of the ridge. If that is true for the Juan de Fuca Ridge, then the two lines on your graph should be more or less on top of each other. Are they?

  3. If they are not, do you think it is because the plates have not been spreading symmetrically, or do you think it might be largely due to uncertainties in your measurements due to complicating factors such as the faults that offset the magnetic anomalies?

  4. (The Pacific Plate is the plate on the west, or left, side of the ridge.) To the nearest cm/yr, at what rate has the Pacific Plate been spreading away from the Juan de Fuca Ridge over the last 10 Ma or so? (out to the oldest-age, farthest-out anomaly you measured on that side)


  5. (The Juan de Fuca Plate is the plate on the east, or right, side of the ridge.) To the nearest cm/yr, at what rate has the Juan de Fuca Plate been spreading away from the Juan de Fuca Ridge over the last 10 Ma? (out to the oldest-age, farthest-out anomaly you measured on that side)

Part 3: The Hawai'ian Hot Spot and Plate Motion

(The following text, pictures, and data table are copied from a Web page by Hull and Langkamp, supported by the National Science Foundation under Grant No. 9980740, on the Web site http://www.seattlecentral.org/qelp/index.html, on the Web page http://www.seattlecentral.org/qelp/sets/073/073.html.)

About the Hawai'i-Emperor Chain:

The Hawai'i-Emperor chain of seamounts (volcanoes resting on the ocean floor) stretches from its active end at the Big Island of Hawai'i west and north across the Pacific Ocean floor to the Aleutian trench near the Kamchatka Peninsula (first figure). There are about 110 individual volcanoes in the Hawai'i-Emperor chain (see the data table), which is about 6000 km (3800 miles) long altogether. The Hawai'i-Emperor chain is divided into two segments, the WNW-trending Hawai'ian chain and the N-trending Emperor chain. The two chains meet at a prominent bend, around the underwater seamounts Daikakuji and Yuryaku.


The active end (youngest end) of the Hawai'i-Emperor chain is at the Big Island of Hawai'i and the offshore, still underwater volcano Loihi. Kilauea volcano on the Big Island is active today, and other centers on the Big Island and on Maui have erupted recently. As one progresses towards the west-northwest, the volcanoes of the Hawai'ian Islands get progressively older (see data table). Once active volcano building through eruptions of lava ceases, the erosional forces of tropical weathering, landslides, river erosion, and wave action overcome the island, and erodes it down to sea level (second figure). The extinct volcano evolves to a flat-topped mesa ringed by coral reefs, and then to an atoll with nothing but the circular reef showing. Finally the volcano sinks beneath the waves, and becomes an underwater seamount.


The Hawai'i-Emperor chain is a classic example of a hot spot track. The standard explanation begins with a hot spot whose source of magma is rooted deep in the Earth's mantle. The hot spot magma source is thought to be fixed in the deeper mantle, with a slab of ocean crust and uppermost mantle (called a plate) moving laterally above the hot spot. As the Pacific Plate moves over the Hawai'ian hot spot, magma punches up through the Pacific Plate, creating an active volcano. Plate motion carries the active volcano away from the magma source, the volcano goes extinct, and a new volcano grows over the hot spot. As the extinct volcano is carried farther and farther from the hot spot source, the volcano sinks beneath the waves mostly due to aging and cooling of the ocean crust underneath the extinct volcano; this cooling causes subsidence of the ocean floor.

Hot spot tracks are very important geologic features for determining both the direction and speed of the plate upon which the seamounts rest The direction of plate motion is given by the orientation of the chain of seamounts and volcanoes. Using your "hands of science", you can quickly determine that the plate "moves towards the oldest volcano." As can be seen in the first figure, the Pacific Plate moved almost due north during "Emperor time" (from 75 to 42 million years ago), and then changed direction about 42 million years ago (the age of the volcanoes at the bend in the chain), to move west-northwest during "Hawai'i time" (from 42 Ma to the present). Note that the azimuths cited here assume no rotation of the Pacific Plate during the last 75 Ma.

Hot spot tracks also give the speed of plate motion, if the length of the chains of volcanoes and seamounts, and the ages of the volcanoes and seamounts are known (data table). Plates typically move about 1-10 cm/year, which is equivalent to 10-100 km/Ma (kilometers per million years). These speeds are about the rates at which fingernails grow, and may seem rather slow on the human time scale, but are very fast on the geological time scale. The Earth is 4.55 billion years old; one million years is a brief moment in Earth time. The speed of the North American plate (for example) is fairly typical for plates, about 6 cm/year, whereas the Marianas plate is one of the fastest (today), moving about 13 cm/year.

Distances from the active Kilauea volcanic center (measured parallel to the Hawai'i-Emperor chain) and ages of each volcano and seamount are given in the data table (Clague and Dalrymple 1989). These data have been compiled from a wide variety of sources and researchers, which can introduce uncertainties. For example, different geochronologic laboratories determined the ages of the volcanic rocks from these seamounts, and different labs often use different machines, different standards, and different analytical techniques. Even with the highest quality of work, the ages have uncertainties which vary from sample to sample. Furthermore, volcanoes do not have a single age; a typical Hawai'ian volcano builds up over half a million years or more. Who is to say that the volcanic rocks dredged up from the underwater seamount Jingu (for example) are representative of Jingu's eruptive history? It is very difficult to sample Jingu's older rocks; they are covered by the young lavas. The data in the table are not without problems, and students should not simply accept the data at face value.

 
Distance versus Age for the Hawai'i-Emperor Chain
data from Clague and Dalrymple (1989)
best known volcano age in millions of years (Ma)
distance from Kilauea measured along the chain
for discussion of uncertainties, see original sources
volcano volcano age distance age ± distance ±
number name (Ma) (km) (Ma) (km)
1 Kilauea 0.20 0 0.20 1.5
3 Mauna Kea 0.38 54 0.05 1.8
5 Kohala 0.43 100 0.02 2.0
6 East Maui 0.75 182 0.04 2.5
7 Kahoolawe 1.03 185 0.18 2.5
8 West Maui 1.32 221 0.04 2.7
9 Lanai 1.28 226 0.04 2.7
10 East Molokai 1.76 256 0.07 2.9
11 West Molokai 1.90 280 0.06 3.0
12 Koolau 2.60 339 0.10 3.3
13 Waianae 3.70 374 0.10 3.5
14 Kauai 5.10 519 0.20 4.2
15 Niihau 4.89 565 0.11 4.5
17 Nihoa 7.20 780 0.30 5.6
20 unnamed 1 9.60 913 0.80 6.3
23 Necker 10.30 1058 0.40 7.1
26 La Perouse 12.00 1209 0.40 7.9
27 Brooks Bank 13.00 1256 0.60 8.2
30 Gardner 12.30 1435 1.00 9.1
36 Laysan 19.90 1818 0.30 11.1
37 Northampton 26.60 1841 2.70 11.3
50 Pearl & Hermes 20.60 2291 0.50 13.6
52 Midway 27.70 2432 0.60 14.4
57 unnamed 2 28.00 2600 0.40 15.3
63 unnamed 3 27.40 2825 0.50 16.5
65 Colahan 38.60 3128 0.30 18.1
65a Abbott 38.70 3280 0.90 18.9
67 Daikakuji 42.40 3493 2.30 20.0
69 Yuryaku 43.40 3520 1.60 20.1
72 Kimmei 39.90 3668 1.20 20.9
74 Koko 48.10 3758 0.80 21.4
81 Ojin 55.20 4102 0.70 23.2
83 Jingu 55.40 4175 0.90 23.6
86 Nintoku 56.20 4452 0.60 25.1
90 Suiko 1 59.60 4794 0.60 26.9
91 Suiko 2 64.70 4860 1.10 27.2
 

(The above text, pictures, and data table about Hawai'i-Emperor are copied from a Web page by Hull and Langkamp, http://www.seattlecentral.org/qelp/sets/073/073.html, supported by the National Science Foundation under Grant No. 9980740, on the Web site http://www.seattlecentral.org/qelp/index.html.)

Analyze the Hawai'i-Emperor data using an Excel worksheet. Get the Hawai'i-Emperor data (above) into an Excel spreadsheet on your computer by one of three possible ways:

1. Just highlight the data above, copy, and paste it into an Excel worksheet you have opened on your computer.

2. Or, download the data table from Hawai'i EMPEROR DATA TABLE/ EXCEL FILE.

3. A third possibility is that the instructor may have a copy of the data file on a Wenatchee Valley College network or storage disk. Open it and save it in your own Excel file on your computer.

Once you have the Excel spreadsheet open and saved on your computer with the above data in it, you can make an XY (Scatter) graph (chart).

Make sure your chart of the Hawai'i-Emperor hot spot data is just dots, with no lines connecting the dots.

Now fit a best straight line to the data on your Chart. In Excel, go to Chart/Add Trendline/Trend-Regression Type/Linear. Excel will draw the line on your chart for you.

Double-click on the trendline and and select Format Trendline/Options/. In the options, turn on "Set intercept =0.0", "Display equation on chart", and "Display R squared value on chart".

The equation of a line is y=mx + b, where m is the slope of the line. The R squared value is how closely the data follow a linear trend, with R squared values close to 1.00 indicating a good linear trend.

When you are done creating your graph (chart), print it WITH A WHITE BACKGROUND. DO NOT PRINT GRAPHS WITH GRAY OR COLORED BACKGROUNDS. ALSO, BE SURE YOUR NAME IS ON THE GRAPH.

Print your graph, with your name on it, once you are sure you have a white background, a chart title, and labeled X and Y axes.

(REMEMBER: A GRAPH WITHOUT LABELED AXES SHOULD NEVER BE SUBMITTED.)

Answer the following questions.

  1. Look at the map of the Hawai'ian Island-Emperor Seamount chain again. It has a bend where it changes from the Hawai'ian Islands to the Emperor Seamount chain. Why? Give an explanation of why the the Emperor seamount chain is oriented more along a north-south line(as seen on the map) whereas the Hawai'ian island chain is oriented more along a northwest-southeast direction.


  2. Not all the data points on your graph fall onto a single straight line. Does this prove that the Pacific Plate is not really moving across the hot spot?


  3. Give an explanation of why some of the data points on your graph are scattered above and below each other, rather than spread out on a single line.


  4. To what does the slope of the line correspond? (Slope is rise over run, or Y value divided by X value. Look at the Y units and divide them by the X units. What do you call the result?)


  5. At what rate, on average over the whole Hawai'ian-Emperor chain, has the Pacific Plate been moving across the hot spot?
    (Hint: Use the slope of the line.)

  6. Suggest an independent method, not using hot spot volcanoes, for measuring the velocity of motion of the Pacific Plate, which could be used to compare with the velocity of Pacific Plate motion determined here by the Hawai'i-Emperor volcano age-distance data and map.

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