Just something i’ve been wondering about: some sources say Taiwan appeared about 4 million years ago, other sources mention 2 million years. And Taiwan is said to have about 200 mountains that are over 3000m high (with the highest one reaching about 3950m), so if we calculate an average mountain growth rate of a 3000m high mountain we get the following results:
3000m in 2,000,000 years would amount to 1.5mm/year and 3000m in 4,000,000 years would amount to 0.75mm/year
Considering that erosion whittles away at the growing mountains, what is the actual average uplift rate? I have seen various papers (written by geologists and others) that mention values ranging from 0.5cm to 8cm per year, with values around 2cm being mentioned more often. With all that conflicting data floating around i am not confident i’ve gotten the real picture here.
It varies around the island a bit, higher in the south and lower in the north. Northern Taiwan is actually being pulled apart (forming the Yilan Plain). 8 cm/yr isn’t an uplift rate, it’s a convergence rate - that’s how fast the Philippine Sea Plate is moving towards the Eurasian Plate. Today, Taiwan is generally considered to be in equilibrium, where uplift is balanced by erosion and the mountains are therefore not getting any higher (It’s actually one of the classic examples of a ‘steady-state’ mountain belt). So the kind of calculations that you did aren’t valid.
Obviously it’s complicated, but if you want to pick a single representative number for the rate of rock uplift and erosion, 5 mm/yr is the one most commonly bandied about.
Darn, thought the heading said “upskirt”
AAAhhh now ‘Taiwan Up’ makes sense. Finally!
Don’t forget to include the retraction of the oceans … sea level now was not sea level so many years ago …
Yes. But that is very silly.
My grandmother gets shorter every year, but that doesn’t make me taller now, does it?
Mountains are measured from sea level … not from their base underwater as your granny is … so put you granny in a bucket filled with water, measure her from water level … later remeasure her from the same water level as the water has evaporated …
With ocean levels rising, Taiwan’s mountains will actually shrink …
[quote=“Belgian Pie”]Mountains are measured from sea level … not from their base underwater as your granny is … so put you granny in a bucket filled with water, measure her from water level … later remeasure her from the same water level as the water has evaporated …
With ocean levels rising, Taiwan’s mountains will actually shrink …[/quote]
There are different ways of measuring elevation. You do need to have a reference frame, and sea level is a convenient one, but you can also use a reference ‘geoid’ that does not necessarily correspond to local sea level. When people talk about the elevation of things in the past, they do it relative to an unchanging reference frame. Otherwise it would get crazy, you can’t go around changing your reference frame.
Anyway, most measures of uplift are not affected by sea level changes. For the few that are (ie. uplifted marine terraces), sea level is definitely taken into account. Like I said, the mountains are currently not getting higher. They’re being eroded down at the same rate that they’re being pushed up. No net change (on average, with some complications, of course). I’m not sure that anyone has really pinned down how long the mountains have been in this steady state, but probably for a while - uplift rate data covering the past 1-2+ million years generally agrees with data covering much shorter time periods.
Didn’t the 921 earthquake push them up a bit, like sort of substantially in some cases (in geological terms, obviously)?
HG
[quote=“Huang Guang Chen”]Didn’t the 921 earthquake push them up a bit, like sort of substantially in some cases (in geological terms, obviously)?
HG[/quote]
It did, but the earthquake also caused a LOT of landslides. If you look at the volume of uplift and the volume of material that ended up in landslides, they may be comparable (I know that people have showed this for the 2008 Sichuan earthquake, not sure that it’s been definitely done for Taiwan). In the years following the earthquake, the amount of sediment coming out of the rivers increased dramatically. A study looking at one river found that something like 30% of the volume added in the earthquake is already gone, just 10 years later (and a lot more might still be sitting in landslides, still in the river, etc). The return time of such as earthquake is though to be hundreds of years (200 years is the lowest I’ve seen), which leaves a lot of time for erosion to continue catching up. In the long run (balanced over 1000’s or 100,000’s of years and many earthquakes) it all balances out.
All this sounds interesting 
- where do you find this information?
Obviously - so what would your calculation look like? 
TIA! 
If I put granny in a bucket, she might kick it. 
Anyway, aside from that small detail, I can’t imagine MSL being changed anytime soon. The implications of doing this are very far reaching.
Absorbed over many years of paper reading, conferences, discussions, classes, etc. I kind of do this sort of thing for a living.
[quote=“yuli”]
Obviously - so what would your calculation look like?
TIA! [/quote]
Ah, you’re talking a Ph.D thesis or two (or three). There are a bunch of different ways to measure erosion and uplift over a range of timescales (from days to millions of years). Getting all of that data is the first step. Then you can compare different data sets and look at what the rate are and whether things change through time or remain steady.
As for getting at how fast topography developed and the mountains went from 0 to 3000+ back in the day, that is a difficult thing to measure, particularly in places like Taiwan, where the types of deposits you would need to even make a stab at it are just not preserved. Fortunately, though, Taiwan has another thing going for it - the mountain building is actually propagating from the north to the south. So the northern mountains are oldest, and the mountains down in Hengchun are just getting started and are still going up (and there are more to south still working on making it above water). So you could take the time for the onset of collision in the Hengchun Peninsula and the height of the mountains there and do this sort of calculation. I don’t remember what the timing of collision down there is though to be (and don’t feel like looking it up), but probably about a million years or less.