# GEOL 1305 University of Houst

Calculate the Rate of Seafloor Spreading from the Age of the Seafloor

TECTONIC PLATES ON EARTH move relative to one another, diverging along mid-ocean ridges, converging along subduction zones, and sliding past one another on transform faults. Relative movement of the plates causes most earthquakes and volcanic eruptions and explains the present distribution of continents. A key aspect of plate tectonics involves seafloor spreading, where two plates move apart relative to one another, generating new oceanic crust in the process.

Figure 1 shows the age of seafloor as determined by the measuring of the magnetism and assigning ages by comparing the magnetic patterns to the geomagnetic timescale. Ages

are also determined by collecting and dating samples of rocks and sediment from the seafloor. The youngest seafloor is colored orange, intermediate ages are yellow and green, and the oldest is blue and purple. No seafloor is older than 200 m.y. old because older seafloor has been subducted into the mantle at some point in the past.

Mid-ocean ridges are represented by the dark lines within the most recently formed seafloor (shown in orange). Note how the color patterns representing the age of seafloor are symmetrical with respect to most mid-ocean ridges, such as those in the Atlantic Ocean. The age patterns are locally truncated in the Pacific Ocean where subduction has consumed the eastern part of so e plates. Examine the patterns in each ocean.

Analysis and communication

1. Can you tell which ridge segments are spreading more rapidly than others? (Write a complete answer by using subjects, verbs and appropriate explanation)

We can determine the rate of seafloor spreading if we know the width of oceanic crust that has been generated in a specific length of time. Width divided by time is the rate of spreading:

Spreading Rate = Width of Oceanic Crust / Time During Which That Crust Formed

For example, if a width of a belt of oceanic crust is 80 km, and it took 10 m.y. to form that crust, the equation is:

Spreading Rate = 80km wide/10m.y. duration = 80 km/m.y.

For seafloor spreading, we generally refer to rates in centimeters per year, so we need to convert our units from kilometers (km) to centimeters (cm) and from million years (m.y) to years (y). To convert km to cm:

1 km = 1km · 1,000 m/1km · 100cm/m = 100,000cm

so we multiply km by 100,000 (1 x 105) to get centimeters. To convert millions of years into years, we multiply our value for m.y. by a million (1,000,000 or 1 x 106). The spreading rate equation becomes:

Spreading Rate (cm/yr) = [width (km) · 100,000 cm/km]/[Duration (m.y.) · 1,000,000 (yr/m.y.)]

When you cancel out the zeros (100,000 over 1,000,000) there is one zero left over in the denominator, so to get from km/m.y. to cm/yr, we simply have to divide by 10. Thus, the total process is:

1 – measure the width in kilometers;

2 – note how long it took that oceanic crust to be formed (the duration);

3 – divide the width by the duration;

4 – divide by 10 to get cm/yr.

For our example the result is 8 cm/yr, a typical rate.

Spreading Rate = 80km wide/10m.y. duration = (80 km/m.y.)/10 = 8.0 cm/yr

Representation and manipulation

On the map in Figure 1 there are thick lines across mid-ocean ridges in the Atlantic Ocean (A-D) and Pacific Ocean (E and F).

Table 1 lists widths represented by each line and the duration over which the oceanic crust within that width was formed. Calculate the spreading rate in cm/yr for each line, show your calculations and write your answers in the table.

Table 1. Spreading rates of the oceanic floor

Line Width

(km) Duration

A 1050 55

B 5850 180

C 4530 120

D 6130 135

E 3350 20

F 5420 100

2. My calculations:

Analysis, assumptions and communication

3. Which part of the Atlantic Ocean opened first and is that segment’s spreading rate faster slower or the same as those for the other three lines across the Atlantic? When writing your answer use appropriate grammar and use the data in the table to support your statements.

4. How do the two spreading rates for lines C and D compare? Can you explain the difference in rates and why the age widths (and ocean) widen to the south? When writing your answer use appropriate grammar and use the data in the table to support your statements.

5. How do the spreading rates in the Atlantic Ocean compare with those in the Pacific Ocean? When writing your answer use appropriate grammar and use the data in the table to support your statements.

6. The correctness of this exercise relies on a series of assumptions. Order them by the most (1) to the least (5) important:

[_] The endpoint of every line in Figure 1 at some time in the past were in the same location

[_] The spreading rate that we compute is an average between burst of higher activity and dormant phases

[_] The same geologic processes as today occurred millions of years ago

[_] Dating method of the ocean seafloor are reliable

[_] The location of plate boundaries is accurate

Interpretation, analysis and communication

– Look carefully at the location and distribution of colors along the line F

– Look carefully at the location and distribution of colors along the line C

One line is in the northern part of the Pacific Ocean and the other line is in the Atlantic.

7. Use two concept sketches to show the geometry of plate boundaries along line F and line C. Can you identify the spreading centers, the location of a subduction zone and passive margins? Remember to include labels and short sentences explaining the key geologic processes.

[Take pictures of your sketches and insert them on this document. Add pages as needed, make sure all the labels and sentences are clearly readable. If you prefer you can draw on a printed version of this file.]

Concept sketch 1 – Plate boundaries along line F

Concept sketch 2 – Plate boundaries along line C

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