Q8

[jennatreftlin]

I'm a bit confused as to why D is the correct answer. Is it because it relates to mountains which are related to the topic addressed in the passage (and are mentioned)?

[giladedelman]

Good question. (D) didn't jump out at me at first, either; it was simply the last answer standing once I eliminated the rest.

Here's the thing: we're told that lichenometry works by measuring the extent of lichen growth on rocks that became exposed during earthquakes. The more lichen, the longer ago the rocks were exposed. That's the key for understanding (D): lichenometry might be applicable here because we could measure the lichen growth on rocks that were exposed as the glacier receded. The more lichen, the longer ago the glacier ceased to cover the rocks.

But again, the best way to get here is by eliminating the other answers:

(A) is out because we don't have any connection between lichenometry and flooding, and because it's about dating things, not determining the frequency of an event.

(B) is out because there's no indication that lichenometry is applicable to dating fossils; in fact, it doesn't sound as though lichens grow on skeletons.

(C) is out because, again, underwater?

(E) is way out there -- what do rainfall rates have to do with this?

Hope that clears this up for you!

[irene122]

Hi giladedelman,I am confused by C and D--we don't know whether there's rock exposed to lichen in D and what role does "underwater" play in the question?

Could you explain more in detail? Thank you very much!

[d.andrew.chen]

Struggling on this one. The issue I had with (D) was that the passages clearly states that lichen grow very slowly "slow but constant rate for as long as 1000 years" and that one particular NA species grew at 9.5 mm per century. Okay, so the 9.5 mm species is only ONE species, but they did state that they all grow slowly (and I assumed that that was at some rate not too ridiculously far off of 9.5 mm per century...for instance, it wouldn't be 20 meters per century...as they all had to be "slow").

With that in mind, how the heck are you going to identify the rate of anything in kilometers per century? It's only growing to grow for 1000 years and you only are going to get about 100 mm of growth (if we're looking at that species)...how are you going to measure rate at km per century? I just saw a huge scale issue with this problem.

It's definitely the best answer, but one that I really don't think is warranted. A and D are probably next close and both of them presented severe problems, too.

[heythatslife]

I struggled with this one too - I narrowed the candidates down to A and D then ultimately went with A. I think I was thrown off by the apparent discrepancy in sheer scale between the rate of lichen growth and the rate of glacier recession. Also, I live in a part of the world where floods happen quite often and I had this mental image of gushing torrents of water sweeping trees and rocks away!

But going over the answer choices again, I can see why the correct response is D. First, the scale of lichen growth itself has no direct relation to the scale of glacial movements. When a portion of glacier melts in a given year, it would release rocks in that area and lichen growth on those rocks would begin or be reset. The timing is all that matters, and the absolute size of the lichens (as opposed to their relative sizes) does not figure into this. Second, the passage mentions the word "rockfalls" twice (lines 23 & 26) regarding the process of lichen formation. This definitely tips the scale in favor of D. Hope this helps!

[htumonroe]

I eliminated A-C based on the age. The passage says that its best if used in the past 500 years so ancient beaches, 1000 year old floods, and fossilized skeletons are out. That left me with D and E. I actually took E because glaciers once again IMO are ancient and Im not sure if you could calculate out the number of KILOMETERS per century it has been receding with something that is best used in 500 years.

E like D are both subject to water but it states that it fit in the 500 year time frame that lichenometry works BEST. The passage further states that outside influences can be taken into account when making their calculations.

[hj8765]

Can someone please help me with this question?
I agree with htumonroe...
I eliminated A-C because they go far beyond 500 years...



But for D...E... I'm so lost....
Identifying the rate? vs... Local trend..?
They both sound unlikely to me...


Please help..

[sh854]

Having trouble understanding why we would know the rate of a glacier receding up a mountain valley through studying lichen growth.

[rinagoldfield]

Thanks for your posts. This is a hard question!

Lichenometry works by “measuring the size of lichens growing on the rocks exposed by rockfalls” (lines 23-24). This is relevant because “lichen will colonize newly exposed rock surfaces” (lines 25-26). None of the answer choices discuss rockfalls, so we’re looking for something else that indicates rocks were newly exposed.

(D) talks about glacier recession, which we can infer exposes rocks. As the glacier recedes, new rock surfaces will emerge.

(E) does not talk about rock exposure. Rainfall generally does not reveal new rock surfaces.

(A) talks about the number of times a river flooded… this would only work if each flooding revealed new rock or something, which we just cannot assume.

(B) is really irrelevant. Lichonometry is not connected to fossil dating.

(C) is the opposite! It talks about rocks that have been concealed (they’re underwater now) rather than revealed!

[greenapples]

Thanks for the detailed explanation above.

I'm afraid I still don't FULLY get why D is right, but since it's most strongly supported, I figured I should go by the process of elimination.

Both on my first try and during my blind review, I successfully eliminated choices A, B, and E. I guess I held onto C because it had a familiar range--30 km, which I guess LSAC put it in there to trap people like me.

Once I knew to just focus on the process of lichenometry-- they grow on rocks exposed by rockfalls, I know now to eliminate C. Thanks!

[MeganL677]

I was also stuck btw D and E.

To address answer choice D's scale problem, I think both glacier and earthquake are a very large scale event compared with the growth rate of lichens, so the scale shouldn't be the problem here. I chose it wrong because I didn't understand how the lichen method work.

The pattern is:
Firstly, lichens quickly colonize on newly exposed rocks. That is, to set foot on a "point" of the newly exposed stone within a relevant short time.
Secondly, they expand from the original "point" very slowly on these newly exposed rocks. After a few centuries(e.g 500 years), when those newly exposed rocks become old rocks, they only expand: 5*9.5mm/century=47.5mm in total. That is still a very small corner on a rock, which implies it's nearly impossible for the lichens to connect together from rocks to rocks in several hundred years....
However, if we deny that fact, if some lichens could grow very rapidly on the rocks, too soon they would have expanded to connect together to see when the rock fell...
Along the earthquake line, there are a lot of such rocks with spotted lichens, this is how we can detect the location of the earthquake.

Let's roughly set a formula:
largest lichen diameter/ lichen's constant growth speed= how many years the lichen has settled on the rock (ignore the quickly colonize time)= for how many years the rocks were exposed.
Thus we could know when did the earthquake or glacier receding (whatever event that would expose some new rocks) occurred.
This formula could also explain why the conditions influencing lichen's growth rate can be disturbing- they can't simply divide it when it's not constant and must factor in those conditions in the formula, which would make the formula much more complex.

In a nutshell, it is exactly because of the slow growth rate of lichens, rather than the rapid growth rate, can we determine when and where did those geological events happen.

I don't know if the above understanding is accurate, please note me if I was wrong!