Original Image from this ‘teaching unit’. As a certified teacher I’m permitted to use this image under the blanket authorization of their front page banner.
I find this graph interesting. Yes, way too simplistic. It would be nice to know, for example, exactly which years of the Quaternary matched those formations. (We’re presently in the Holocene part of the Quaternary, so it kind of matters… the older part is about 2 million years ago).
But I note that it says “Late Quaternary” for the submerged reefs. One can presume that means from during the last ice age, prior to the Holocene, when sea level was at about that level. That blue patch labeled just “Quaternary” is a ‘bedrock’ so it’s been around long enough to be changed from a ‘reef’ into a ‘rock’. It’s also not labeled with “late”. So less than 2 million or so, but older than 20,000 or so years.
Why mention that? Because it’s limestone. Limestone made from reefs that grew under the ocean.
So we can pretty well say that large parts of Florida has been under water for very long periods of time. Long enough for reefs to grow, grow old, die, and become limestone.
First off, the notion that reefs dying is going to be a catastrophic thing is clearly wrong. Reefs die all the time and have for millions of years. They grow, grow old, have the water get too deep (like those “late quaternary” reefs) or too shallow (like the ones that are now “bedrock”) and die. The organisms move on to new homes in the new places that are now hospitable to them (or rather their progeny move on) as seen in the reef building in the Keys today. There is nothing at all unusual, unexpected, or bad about the notion that a reef might die. The only bad is if it were to die before it’s time or at our hand.
Second, that limestone bedrock. Water has been higher, or the land lower, or both. Florida is not at a great tectonic boundary. It’s a pretty stable place. This is the point where it would be nice to have the exact ages of the rocks. “Pretty stable” can be a rocket ride in million year time scales. But the generally accepted wisdom is that Florida doesn’t rise much and the limestone comes from prior times when the ocean was higher.
But that Blue Bit has me wondering.
Why? Because we’ve had ice ages on a 100,000 year or so schedule for quite a while. Basically, for the length of the quaternary. (Again, it would be a good thing to have exact dates, at this point its just a gun, and smoke near by…) So if that is quaternary rock, it had to come from some PRIOR interglacial when waters were higher. A natural flooding of Florida. (The interglacial just prior to this one was warmer, so it’s a candidate).
These folks propose isostatic rebound from the limestone getting moth eaten as rainfall erodes it:
Rain is ‘fresh water’ and does dissolve limestone. Florida does get ‘sink holes’ in the surface from the dissolving processes. (When I was working in Florida for 6 months, they ate 2 lanes of freeway, one fire station driveway, and a very large house, with lawn. Those were the ones near Orlando that made the news…)
But that still leaves a question for me:
Sure, it’s dissolving NOW, since it’s above ocean level. But it had to form, and ALL of it, below water level. It’s presently above water level (by about 0 to 100 feet as you move away from the coast). So if it was not there before it formed, would not the land under it have already been ‘up’? Then limestone forms pushing it down. For that ‘isostatic rebound’ to happen, it had to be pushed down from where it was when it was unburdened AS the burden formed UNDER water.
So this looks to me like the basic process had to start with water higher than today. High enough to get the rock started, and the base sinking, to build the layer that now is above the water.
I’m thinking this isostatic rebound explanation needs some work.
Peter Adams at the University of Florida at Gainesville. Adams is the lead author of a paper on the matter in the June issue of the journal Geology.
Since tectonic collision can’t be called on to explain the matter, Adams decided to put the cavern-ridden rebound idea — first proposed in 1984 by his co-author Neil Updyke — to the test with a model.
The numerical model combined estimated sea level fluctuations over more than a million years, the length of time the land has been out of the water, rainfall, cave-formation rates and isostatic uplift to see if the idea has any legs.
“I’m just seeing if mechanically those ridges fit with the ages,” said Adams.
The model suggests the ages of north Florida’s Trail Ridge, Penholoway Terrace and Talbot terrace are 1.44 million, 408,000, and 120,000 years old respectively. That basically matches the fossil evidence in the rocks, Adams said.
OK, so we’ve got a model being used to “test” things again… but we do get some nice numbers from the end. The youngest rocks are 120,000 years old. Gee, right about the time of the last interglacial…
I’ve used the following chart before in this posting:
last interglacial was just about when the rocks formed…. and the ice was quite low (the graph is “upside down” with a high line being low ice) then. Even lower than now, per the graph. So the oceans ought to have been higher.
So ALL it takes to cover Florida in water is a regular old Interglacial. The fact that it’s NOT covered during this one is, to some extent, an accident of nature (as our peak didn’t get as high as the last one) and blaming it on people is a pretty dim thing to do.
I note in passing that the 408 kilo year stone formation lands right on top of the EARLIEST interglacial on that chart as ice volume just crosses the dotted line …