The not-so-dark side of the Moon
Published: 23 August 2011 (GMT+10)
Evidence of siliciclastic volcanism on the far side of the moon supports a young age for the Moon.
Humans have walked on the Moon, collecting samples which were thoroughly analyzed, and some became travelling exhibits. I still recall the emotion of watching a thin section of a moonrock under my microscope while still a university student [too!] many years ago. And what I saw so nicely matched the model I have already assimilated from my Profs: the Moon is probably a chunk of the primitive Earth ripped off during the late accretion stage of the solar system because of the impact with a massive body (possibly a proto planet). And because it was much smaller than the earth, the Moon cooled faster, its surface the frozen image of the molten inferno it was in the beginning. Once the surface solidified, it started preserving the scars of numerous impacts, which were filled with flat-laying basalt flows—maria—a landmark of our moon (or should we call them a ‘moonmark’?). Furthermore, because the cooling took place before a full gravitational separation of all chemical elements (with the accompanying gravitation friction heat) took place, there was not enough time for plate tectonics to be initialized. Plate tectonics on Earth are believed to be a recycling mechanism of rocks which keeps the mantle active and hot-enough to drive it.
It soon became dogma that moonrocks are essentially mafic i.e. rich in magnesium and iron. It therefore came as a surprise when Apollo 14 brought back some fragments of genuine granite. As these granites have been radiometrically dated at nearly 4 billion years, it is now believed to be evidence of water having been present in the infancy of the Moon, before its ‘thermal death’.
NASA has recently brought new evidence that is putting all that neatly assembled interpretation of field data into question. On the dark (or ‘far’ depending on one’s taste) side of our moon, evidence of silicic volcanism was revealed by the successive data provided by the Lunar Prospector in 1998 and Lunar Reconnaissance Orbiter (2011). In order to understand why this is so important, a little petrographic ‘excursion’ is needed.
Geologists classify igneous rocks (rocks formed from the cooling of melted mixtures) first by their assumed position of cooling. They would thus separate plutonic i.e. intrusive rocks (believed to have cooled and solidified very slowly inside the Earth’s crust) and volcanic i.e. extrusive rocks (which have reached the surface and cooled and solidified quickly). Petrologists (geologists specialized in the study of rocks as chemical and physical entities) have classified igneous rocks into a sequence, based on their SiO2 (silica) contents, from ultramafic (less than 45% silica) to felsic (more than 69% silica). And they all have intrusive and extrusive equivalents. So, for example, a gabbro is a mafic plutonic rock with basalt as its volcanic correspondent i.e. basalt is essentially a gabbro that made it to the surface and solidified there. Granite is a felsic plutonic rock with rhyolite as its volcanic correspondent. To the geologist, the existence of rhyolite on the surface is almost always evidence of a deep granite magma chamber.
Back to the Moon now: NASA reported the discovery clear volcanic structures, cones and calderas with steep slopes and depressions at the top, all consistent with viscous (silicic) volcanism on Earth. It is interesting that such silicic (felsic) volcanic structures have been identified on the Moon before, both on the nearside and the farside1 but they were considered minor exceptions that did not raise serious problems for the standard model. This new discovery came as a serious surprise and like always, when well-established and popular theories and models are questioned by facts, the scientists embarked on the ‘universal inference train’ simply inferring whatever wild explanation their minds can fetch that will keep the established model afloat. And inferences and speculations abound in the latest reports.1 Unlike the previously discovered silicic volcanoes—which were found within mare volcanic areas i.e. impact related basaltic lava flows—the recent discovery is not only outside mare volcanism, but it appears as an isolated and very well defined feature. The fact they are not affected by the usual impacts—on the farside of the moon which is the most hit!—is evidence for a younger age. And without plate tectonics, such felsic volcanism has to be the result of deep melt in the mantle since there should be no active granite magma chambers in the crust, if the model is correct. A deep melt in the mantle would have to ascend through about 50 km of lunar crust (according to the seismic data) and that is not something geologists would expect. So the speculations go on with words like ‘could’ and ‘might have been’ as panacea. The scientists seem to be undeterred by the convoluted explanations required to explain the chemical and physical processes involved (extreme chemical fractionation i.e. a clean separation of mafic and silicic components in a chamber inside a static crust) in their scenario. They nonchalantly conclude that ‘compositionally evolved volcanism did occur far removed from mare basaltic volcanism’, while openly admitting that to be ‘an enigma’. It is perfectly scientific to use words like ‘maybe’ when attempting explanations of facts, but why would that be restricted to naturalistic, evolutionary explanations? Why are so many creationist explanations ridiculed when using the same kind of wording? The question is manifestly rhetorical!
We have been following the Lunar Reconnaissance Orbiter’s mission with interest and pointed out that the discovery (2010) of small scarps confirms a young age of the Moon. The secular scientists explained the scarps as the result of the shrinking of the lunar crust during cooling and by use of the ‘universal lunar clock’ i.e. the impact craters (intersected by the scarps which therefore must be younger) suggested an age of 100 million years. Well, the discovery of recent silicic volcanism suggests a better explanation of the origin of the scarps: the Moon’s mantle is still active and that can cause tectonics (faulting) on the surface (the very opposite of shrinking, faulting).
To the creation scientists a far simpler and straightforward answer is not only available, but perfectly congruent with known igneous activity as well as a Christian worldview: the Moon, like the Earth, is young enough to have an active mantle to cause advanced chemical fractionation of magma and heat to ensure its ascent to the surface. Unlike with the Earth, the Moon’s mantle could not trigger plate tectonics, most probably because the Earth’s strong gravity has prevented it. The marked chemical difference between the rocks on the near and far sides of the Moon is in accord with a gravitational pull of the heavier elements inside the mantle and core towards the earth, making the Moon slightly unbalanced in its rotation. This may be a valid explanation for lunar libration. An asymmetry of the core and mantle may very well prevent plate tectonics.
- Jolliff, B.L. et al., Non-mare silicic volcanism on the lunar farside at Compton-Belkovich, Nature Geoscience 4:566–571, August 2011. Return to text.
This article confuses the “far side” of the moon (the side which always faces away from Earth) with the “dark side” of the moon (the side facing away from the sun), even though they are not the same, as the sun lights up the far side of the moon as it orbits around the earth. I knew what you meant, but to perpetuate confusion is not good, especially given that others reading the article might not know this beforehand.
Emil Silvestru responds:
I’m sorry for the confusion! I was hinting—in tying to lighten the style—to the Pink Floyd album “The Dark Side of the Moon”. In astronomy it only coincides with the far side at full moon. But of what I know the two are popularly used as synonyms.