A well demonstrated answer would include the following: . The graph is plotted as age(on the vertical axis) against distance (on the horizontal axis). . The graph has been given at title, the axes are labelled and units have been included where appropriate. . The scale used on each axis has been chosen in an appropriate way. By ‘appropriate’ here we mean that the scales make the best use of the space available on the graph paper and allow all of the points to be plotted clearly. . The points on the graph have been correctly plotted and an appropriate best-ﬁt straight line drawn, with roughly half the points above the line and half below.
Advise on question b(ii and b(iii
Choose two well-separated points on the best-ﬁt straight line, measure the rise and run between them in the units shown on the graph, and then ﬁnd the gradient using the relation gradient =rise/run.) In this case, even when you have found the gradient, you will still need to do a little more work to determine the average speed of the plate (in kilometres per million years) since the gradient is not itself a speed.
As in any question that asks for a calculation you should clearly identify any equations used, show all of your working and clearly state all units used. Units are particularly important in this case, since they will help you to ﬁnd the relationship between the gradient of your best-ﬁt line and the speed required as a ﬁnal answer.
The ﬁnal step, in part(b)(iii), is just a unit conversion to express your answer in millimetres per year.