Seamount chain

Hi guys!

I got this question to solve and it makes me pretty confused. ( I wonder if any of you could help. Thanks a lot in advance.

Question:

The Sala y Gomez Seamount chain is a row of submarine volcanoes (seamounts) in the Pacific Ocean, on the Nazca Plate. The seamounts form a roughly E-W trending line, which changes to an ENE-WSW trending line closer to the South American continent, in which the submarine volcanoes become progressively younger to the west. Table 1(below) list a subset of seamounts from the Sala y Gomez Seamount chain, showing the age of each seamount and the distance from the youngest ( Umu) to each successive seamount moving towards South America. The change in direction of the chain occurs between seamounts 8 and 9 ( at 21 Ma).

(i) Plot the data in Table 1 by hand on a graph of the age (Ma) versus distance (km) for the Sala y Gomez Seamount chain, and draw two best fit lines, one through seamounts 1-8 and one through seamounts 8-14.

I ploted the data on the graph, and have done the following calculationsB]

(ii) Use the best-fit lines to calculate teh average speed of the Nazca plate before and after 21 Ma.

Before 21 Ma

Gradient of the line through seamounts 8-14:

Gradient = y2 – y1 / x2- x1
= 32-25 / 2800- 2320
= 7/ 480
= 0.014583333

Average speed = 1/ 0.014583333
= 68.57 km Ma-1

After 21 Ma

Gradient of the line through seamounts 1-8:

Gradient = y2- y1/ x2- x1
= 16.5- 8/ 1400- 300
= 8.5/ 1100
= 7.72 x 10-3

Average speed = 1/ 7.72 x 10-3
= 129.53 km Ma-1


(iii)

Using your graph to help you, briefly describe how the average speeds calculated in (ii) have changed and explain the significance of change in orientation of the Sala y Gomez Seamount chain at 21 Ma.