Yes I have been remiss in writing this post. This is particularly ironic since as James was late on his last post I was beginning to think about what I should put in my “Because James is a Slacker” post. James posted before I had a chance to follow through with that idea. In any case, I will continue with the discussion of the picture that science gives us of the world we live in and how it came about. I previously posted on Astronomy, my real area of expertise, but now I will delve into another subject near to my heart – earth science. For me, the convincing aspect of scientific cosmology is that it has produced an overarching, coherent theory that explains essentially all of the data. There are still missing puzzle pieces, but by in large when we look out at the cosmos we see a concordant story of its history – Big Bang Cosmology.
Over the past century, geology has also experienced a revolution and much of the overarching framework of the history of earth has come into focus. The theory that produced this revolution was plate tectonics. The motion of earth’s plates and the eroding power of water are the primary forces that have shaped earth’s surface over its history. In addition, earth scientists now have dozens of dating techniques available for the purpose of studying the timing of past geological events. These widely varying techniques provide a means of double-checking and further solidifying hypotheses of past events. In order to help you appreciate this picture I will take one particular example dealing with both plate tectonics and dating – as well as some basic physics. What I hope to show you is not a comprehensive picture of why geologists think the earth is old, but rather a specific representative example to demonstrate the kinds of coherent stories that geology is now producing.
Oceanic plates begin their life at a mid-ocean ridge, where molten rock is constantly spewed forth to cool and produce more and more solid rock. The plate then gradually marches its way across the bottom of the ocean like a conveyor belt to be pushed back into the depths of the earth and melted once again in a subduction zone. Because this process is constantly recycling the oceanic plates, they are all relatively young (up to about 200 million years) compared to continental plates. Thus the youngest oceanic crust is near mid-ocean ridges, and the oldest is near subduction zones. However, another curious correlation can be observed. If you look at the average depth of the ocean along with the age of the crust at that location you find a strong correlation between the two. Specifically, the depth of the ocean is roughly proportional to the square root of the age of the rock beneath it. So, the ocean is relatively shallow near the mid-ocean ridge and drops off gradually as you get further and further away. This is demonstrated in the figure below.
The obvious question is why does ocean depth correlate with age of the rock at the bottom, and why is it specifically with the square root of age. The earth can be divided up into two zones: the lithosphere, which is the part of the earth near the surface where rock behaves like a brittle substance, and the asthenosphere, which begins at the depth at which rock starts to flow like a fluid and can deform plastically (like silly putty) over geological time scales. We’ll leave it to James to give us the etymology of those words. Near the mid-ocean ridges, where the plates are being formed and are very young, the lithosphere is very thin because it is so hot. Rock also is a very good insulator, and cools very slowly through conduction. However, as you get further and further away from the mid-ocean ridges the rock has had a lot of time to cool, and because of this the lithosphere is much thicker. Another thing one must realize is that rock is more dense when it’s cold. So, the dense, cooled-off lithosphere is floating on top of the hot plastic asthenosphere. Because the lithosphere is more dense it actually wants to sink, but it can’t because the strength of the rock holds it in one piece. It’s kind of like when you have some hot grease and as it starts to cool you get a thin skin of solid fat on the surface. That fat is denser but doesn’t sink because it’s all stuck together. However, since the cooler parts of the lithosphere are thicker they want to sink into the fluid below even more than the thin hot parts. So, as the rock cools it thickens and sinks further down into the fluid below until it obtains a state referred to as hydrostatic equillibrium, when all of the pressures in the fluid balance out.
So this nice little story explains why the deeper parts of the ocean are also the older parts. However, we can even show with a simple calculation that this relationship should go with the square root of age. As I alluded above, the heat loss from the rock is mostly through conduction. If one sets up the proper equations then one finds that the temperature follows the equation below, where kappa is the thermal diffusivity of the rock (how quickly it conducts heat):
The math buffs will immediately recognize this as a diffusion equation, and in its solution the temperature will vary with the square root of time. Thus, since we know the thermal diffusivity of rock, one can do a back of the envelope calculation to confirm the sort of relationship we see above. Hence our story has completed a circle. Basic physics provides a mechanism to explain what we observe, and since we know the conductivity of rock this can be used independently to verify that the observed ages of the rock are in the correct ballpark. All of it hangs together. This is the hallmark of a solid scientific theory.
The young earth creationist would not throw out the theory of plate tectonics. Rather, by my understanding, they typically hypothesize that a huge amount of plate motion and disturbance occurred during Noah’s flood. However, were this the case, it destroys our nice story above, for one would expect that the plate all formed at about the same time and would be cooling and sinking roughly uniformly. If the young earth creationist wishes to throw out this idea of rapid plate movement then there is still a problem, because the sort of sinking discussed above takes millions of years to occur (because rock is a very good insulator). One can at least say that the burden is on the young earth creationist to provide an equally convincing mechanism to explain this striking observational result.
Hope I haven’t bored you all to death. This one turned out to be quite scientific.
-Matt, the elder brother