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.

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.
The distance-age plot also permits calculation of the spreading rate (Fig. 3). For any straight-line segment plotted, the spreading rate can be calculated from the following relation:

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 x,t coordinates of the kink in the plot. The rate for the older spreading interval can be determined by subtracting the values of x and t at the inflection point from any other point along the older plot segment. Substitution of these values into the equation above will yield the older spreading rate.

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

 To complete Activity 4, select one of these files: