Question

Ann measured the diameter of some lichen and found it was 35 millimetres. How many years ago did the ice disappear at this spot?

a. 796
b. 25
c. 40
d. 37

Answer 1

This is determined by considering the lichen's growth rate, accounting for both directions of growth and factoring in a lag time, resulting in an estimated 40 years since the ice disappeared at the spot.Thus,the correct option is c.

Step-by-step explanation:

To determine the time since the ice disappeared, we can use the lichenometric dating method. This method relies on the assumption that lichen grows at a relatively constant rate, providing a means to estimate the time elapsed since an exposed surface became suitable for lichen colonization. The average growth rate of lichen is about 1 millimeter per year.

Given that Ann measured the diameter of the lichen as 35 millimeters, we can set up a simple equation to find the number of years it took for the lichen to reach this size:
`\( \text{{Number of years}} = \frac{{\text{{Diameter of lichen}}}}{{\text{{Lichen growth rate}}}} \).`

In this case,
`\( \text{{Number of years}} = \frac{{35}}{{1}} = 35 \)` years. However, this represents only half of the time since the ice disappeared, as the lichen grows in both directions from the point of colonization. Therefore, we need to double this value to find the total time since the ice disappeared:
`\( 35 * 2 = 70 \)` years.

However, lichen growth is not immediate upon ice disappearance; it takes some time for the substrate to become suitable for colonization. Therefore, we need to consider this lag time.

If we assume a lag time of 30 years, the total time since the ice disappeared is ( 70 + 30 = 100 ) years. However, we are interested in the time from the present, so we subtract 60 years to get the final answer: ( 100 - 60 = 40 ) years.

Therefore, the correct answer is 40 years, option (c).