https://en.wikipedia.org/wiki/Giant-impact_hypothesis Following the suggestion, that the Moon was formed 4.5 bya by a large impact of a heavenly body labeled Theia and one labeled Young Earth, I wondered if maybe that's the reason we have continents? Just like there are visible differences between the Earth's ocean floors and its continents, there are similar differences on the surface of the Moon, dark and light patches, the dark patches usually being called "maria", from 'mare' for ocean in Latin, e.g. Mare Tranquillitatis, where Apollo 11 landed. Please Register or Log in to view the hidden image! Please Register or Log in to view the hidden image! Also, the backside of the moon has almost no dark spots. Please Register or Log in to view the hidden image! So, if the Moon was formed after an ancient impact between Young Earth and this Theia, would the dark patches on the Moon and the Earth's ocean floors be the original composition of Young Earth, and the light patches on the Moon and the Earth's continents be the original composition of Theia? (Or maybe vice versa, I don't know.) The side of the Moon with the dark patches always angle that side towards the Earth, so after an ancient impact, material from both bodies mixed together in the new Earth to form continents and ocean floors (with are then subject to plate tectonics), and in the Moon to form the light and dark patches. Anyway, just something to get out of my head after watching a couple of astronomy documentaries. A simple thing could answer this: Do other bodies in the Solar System have similar continent-like structures? Which may or may not be the result of similar ancient impacts and mix of planetary rocky materials, distinct from each other? Using the Lunar samples from the Apollo missions, is there a similar chemical structure between samples from the dark patches and samples from ocean floors, and conversely between samples from light patches and continents? These two types being distinct from each other.
This is the quick answer: The continents on the Earth are a consequence of the end product of fractional crystallisation of mantle material producing a comparatively light rock of broadly granitic composition, while the ocean basins contain "new" basaltic magma derived by partial melting of mantle. The moon was formed from completely homogenised material following the impact event. An anorthosite crust formed and this constitutes the bulk of the lunar Highlands. The maria are a result of basaltic vulcanism at the end of the Late Heavy Bombardment. Your suggestions are inventive and thoughtful and imaginative, but largely wrong. Time permitting I'll suggest some material to read that will give you a more detailed picture that expands on this grossly simplified summary.
Very good reply, thank you. I wouldn't be one to rule out Earth being geologically active, while the Moon seems to be geologically dead. But I've also been wondering why all continents would've been once all one super continent, the Pangea. By consensus, it's only some 175 mya, that Pangea started to break up into the seven continents we have today. Why would the other side of Earth be "blank" for so long? If the continents for a very long time were one single structure, I could fathom that this was due to Theia slamming into one side of Young Earth, leaving behind massive material to form Pangea, which then much, much later started to break apart due to tectonics. (I don't know if expanding Earth hypothesis might fit in to that as well, that's a different angle.) Please Register or Log in to view the hidden image! And also, if the Lunar maria are the result of vulcanism at the end of the Late Heavy Bombardment 4bya (thank you), why is it only on one side of the Moon? Again, it might be on the one side, the one always facing Earth, 'cause that's where Theia impacted with Young Earth and either lost a lot of crust material (so we'd be looking 'into' the Moon in the Maria), or maybe gained some from Young Earth in the Maria. But anyway, I'm not married to the concept, it's just a thought and a lot of maybes. It could be answered by comparing the chemical structures of said regions, which should be feasible. Or maybe someone has already done that?
Also, when you say "the moon was formed from completely homogenised material following the impact event," does that mean, that the light and dark patches are chemically the same?
You ask some very good questions. To properly answer them would take many pages, or ideally several books. Keep in mind, therefore, that my answers are by way of summaries and simplifications. There is ongoing exploration as to when plate tectonics began on Earth and to the extent that continents have grown over time. However, there appears to have been a series of Wilson Cycles. A Wilson cycle involves the periodic assembly of continents, by collision, and there subsequent breakup, by rifting. So, Pangea was only the latest in a series of such supercontinents. It is projected that in a couple of hundred million years the continents will reassemble again. The complexity of continental geology is due, in large measure, to the juxtaposition of continental fragments (terranes) that were once widely separated. The Expanding Earth hypothesis has no meaningful supporters today. In terms of serious investigators there are fewer than a handful who would argue for it. I recommend disregarding it. It merited consideration in the early 1950s, but no longer. This has also been the subject of much debate. This lunar dichotomy may simply be the result of chance. We see a similar dichotomy of lowland/highland on Mars that may be a consequence of single large impact in the Northern Hemisphere. The lunar situation could be due to a handful of large impacts concentrated on the nearside. I don't think you are quite appreciating the devastation of the impact. The moon formed from fragments solidifying from the vaporised and molten ejecta from the collision. These were spread out in a ring around the Earth, but came together to form the moon in a geologically short time. No. Once the moon formed, differentiation occurred. A small iron core formed. Above this a magma ocean differentiated into a plagioclase rich crust (anorthosite) and a mafic (Fe, Mg) rich mantle. Later partial melting of the mantle provided the basaltic lavas that flooded the maria. The maria began life as impact basins from the Late Heavy Bombardment.
A follow up: What's the collective mass of the Earth's continents? And if the continents would somehow be sheared off material of a young Moon, and the dark patches on the Moon represents where material was sheared off in an impact, thus showing us the Moon partly stripped of its original mantle, would the mass of Earth's continents match those dark patches? Or would that ammount of mass be way too thick, if theoretically placed on the Moon? Please Register or Log in to view the hidden image! Moon's rift valleys
