Creation: The Story from Science (Part II – Geology)

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.

Taken from a draft verson of Anderson and Anderson's upcoming book on Geomorphology.

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


4 Responses to Creation: The Story from Science (Part II – Geology)

  1. Ann says:

    Until you got into the math, I was actually understanding this pretty well because I teach about plate tectonics to my 6th graders. It is basically a process by which the earth’s crust is constantly being recycled. In addition to new crust being formed at the mid-ocean ridge, you have subduction zones near the edges of the Pacific Ocean where the oceanic crust is being thrust under the continental crust and remelting. This is why we have the Pacific “Ring of Fire” where so many earthquakes and volcanoes are found.

  2. cheno says:

    Wow! Awesome post. I kind of wish the crazy guy would come back so I could make fun of him…. Oh well, you’ll have to just be content with simple praises.

    Could I make a request for you to explore the thermodynamics of the big bang? It’s one of the classes I’m taking currently, and my prof makes a big deal about how the second law isn’t actually proven. I guess it makes sense if you look at it as an inequality, as we do in class. (What I mean is Production of Entropy is greater-than-or-equal-to zero) but even if you ignore that and merely admit that nothing has been shown to possibly reduce total entropy, should we just ignore this when considering the Big Bang? I mean, with a Christian world view, it’s easy fore me to believe that God supplied that initial energy, but surely this is not the classical Theory of the Big Bang. Do I have an inadequate understanding of the Big Bang? How does the theory deal with the first and second laws of thermodynamics? More importantly, how could we recreate and harness “big” bangs in order to satiate our energy needs?

  3. Matt says:

    Cheno, there are many ways that I could go with a post on thermodynamics and the big bang (which I would be happy to do), so it might help to clarify things a bit more. Are you specifically referring to the general conception (sometimes put forward by creationists) that the big bang theory violates the first and second laws of thermodynamics? This seems like what you’re referring to. If so I can certainly expound on that a bit.

  4. cheno says:

    sure, either way. as i kind of said, i’m ok with believing in a “big bang” as God’s method for creating the universe, but i suppose a more generalized post on the Classical Big Bang Theory versus the first and second law(s) of Thermodynamics would certainly suffice.

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