Activity 4: Determining the Rate of Sea-Floor Spreading | |

Spreading rates of midocean ridges vary
considerably. The
Mid-Atlantic Ridge spreads at about 2.5 cm/yr, whereas the East Pacific Rise has spreading rates between 6 and 8
cm/yr. By
plotting the age of
oceanic crust at several points versus the distance of these points from the
ridge, we can
calculate the spreading rate of the ridge. On a plot of distance
versus age, if
all points fall
along a straight line, the spreading rate has remained constant over the time
period plotted
(Fig. 1). If, however, the data define several straight lines, the spreading rate has not
been constant. The
time of the change in plate velocity corresponds to the age at which the slope of the line
changed (Fig. 1). | |

Fig. 1: Determining sea-floor spreading rate. | Fig. 2: Relation between slope and spreading rate. |

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The slope (steepness) of the line indicates the spreading rate (Fig. 2). The steeper the line,
the faster the
ridge is spreading. Because ridges with steep slopes are spreading rapidly,
they will produce
thick magnetic anomalies.
In contrast, slower
spreading defines
lower slopes and produces thinner magnetic anomalies for the same magnetic reversal
interval. If there is a break in the slope of the
line plotted, a
spreading rate must
be calculated for each
of the two different spreading intervals. The younger spreading
rate can be
calculated from the equation above by using the | |

Fig. 3: Calculation of spreading
rate from distance-age plot. | |

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