CPT Q. 070: Is existence of oceanic lithosphere in the pre-Flood world merely an assumption?

Q. 70. In your response to question 49, you asked an excellent, provocative question: “Let me conclude with a question. Since the pre-Flood ocean lithosphere all seems to missing from the earth’s surface today, where else might it be except at the bottom of the mantle?” There are at least two flood models which assume a pre-Flood earth that has no ocean lithosphere. For example, in the hydroplate model, the ocean floor became exposed for the first time during the Flood. Would you not agree that the existence of a pre-Flood ocean lithosphere is an assumption?

Response: I would describe its existence as a reasonable inference. The crucial factor that distinguishes the continental regions from the deep ocean basins today is the presence or absence of a 30-40 km thick layer of buoyant granitic crust. Those portions of the earth’s surface without this granitic layer are everywhere oceanic, generally covered with 3000-4000 m of seawater. The difference in height of the respective rock columns is a direct consequence of isostasy. Based on clues from today’s continental granitic basement rocks such as the presence of radiohalos and measured radioisotope levels, the case seems strong that the continental crust was formed before God created life on earth, which places its formation during the very first days of Creation Week. It further appears that not only the continental crust but also the continental lithosphere, consisting of a keel of about 200 km of strong, anhydrous mantle rock overlain with the 30-40 km thick layer of granitic crust, also must have been formed by God early during Creation Week. This formation would have occurred before the culmination of the events which unfolded in response to God’s declaration on Day 3, “Let the waters below the heavens be gathered into one place, and let the dry land appear.”

Although the original continental lithosphere appears to have since been fractured into several pieces and kilometers of its original thickness appears to have been beveled away in places, it seems that most of this continental lithosphere did manage to survive the Flood. This survival seems to be largely because of the extreme buoyancy of continental crust. This buoyancy tends not only to prevent the crust itself from subducting but, because of the buoyancy it confers to the underlying layer of mantle rock to which it is attached, it also prevents the continental lithosphere from subducting. Given this strong tendency for the continental lithosphere to survive, one can surmise that the fraction of the earth’s surface covered by continental lithosphere before the Flood was likely similar to that of today.

What then would the remainder of the pre-Flood surface have been like? Just as today, regions of the earth surface without this special layer of buoyant rock before the Flood almost certainly would be topographically 3-4 km lower than the continental areas. Genesis 1:9, 20, and 21 speak of ‘waters’ on the earth’s surface, so these regions would then surely correspond to ocean basins, and the cool rock layer beneath them would correspond to ocean lithosphere.

To reiterate a bit, continental lithosphere, especially that associated with continental shields and platforms, seems to have survived the Flood cataclysm more or less intact. The reason for this is the striking buoyancy of continental crustal rock relative to mantle rock, with a density difference of about [[3300 - 2800 = 500 kg/m^3]], or about 15%. This layer of low density continental crust, generally 30-40 km thick, is sufficient to make the entire lithospheric layer, which includes the layer of crust plus a significant layer of underlying mantle rock, buoyant and difficult, if not impossible, to subduct. Since this significant amount of continental lithosphere appears to have survived the Flood cataclysm, it seems likely that, not only did most of the pre-Flood continental crust survive the Flood because of its remarkable buoyancy, but also did most of the pre-Flood continental lithosphere. (Exceptions occur in tectonic belts such as in parts of the southwestern U.S., where it appears, probably because of shallow subduction, the mantle portion of the continental lithosphere has ‘delaminated’ and fallen away.)

In short, the implication is that the amount of pre-Flood continental lithosphere likely was similar to the amount of continental lithosphere that exists today. The issue, then, is what did the remainder of the surface of the pre-Flood earth look like? Almost by definition it had to be deep ocean basin. Why is this? Basically, it is because of isostasy. Consider two equal diameter columns of rock rising up a common depth in the weak asthenosphere, one through continental lithosphere and the other through what would correspond to oceanic lithosphere. Isostasy requires both columns to weigh the same. Because the oceanic column consists mostly of high density mantle rock, while the continental contains a substantial thickness of low-density crustal rock, the total height of the oceanic column must be substantially less in order for the total weights to be equal. This accounts for why the ocean bottom is several km lower in elevation today than the mean height of the continents. In other words, if the earth’s surface is in isostatic equilibrium, portions that do not have the nominal thickness of continental crust will tend to be several km lower in elevation and be covered with several km of water. There is no good reason to believe that this same state of affairs would not have existed on the pre-Flood earth.

Another indicator of the existence of ocean lithosphere in the pre-Flood world is the presence of ophiolites throughout the portion of the geological record associated with the Flood. Ophiolites are interpreted to represent pieces of ocean lithosphere that have been tectonically emplaced and preserved in continental environments. Typically, the vertical sequence, from top to bottom, in an ophiolite consists of a layer of pillow lavas, then a laterally extensive sheeted dike complex, transitioning into gabbro, layered gabbro, and cumulate ultramafic rocks. Sometimes there is a layer of chert on top. Ophiolites from the portion of the rock record associated with the earlier part of the Flood almost certainly correspond to actual pieces of pre-Flood ocean lithosphere. In other words, there are places in the world today where one can actually visit and inspect pieces of pre-Flood ocean lithosphere.