Your mafic minerals are very magnesium iron rich minerals, which you can
kind of get from the name, mafic, the ma from Magnesium, and fic from ferron Iron.
So, you get your Plagioclase feldspar, you get your mafic minerals,
you put it together, and you get you an Anorthosite rock.
And many of the rocks brought back by the Apollo astronauts
where these Anorthosites.
And so, this photo here, was a rock brought back by Apollo 15 and
it's now called the Genesis Rock.
When scientists dated this rock using samarium-neodymium dating,
they found that the rock was 4.46 billion years old.
At the time, that was the oldest rock that we had in our moon rock collection.
And so everyone is really excited that we found this very, very old rock
that could provide a very important clue to the formation of the moon.
And even further, scientists realized that this rock is 98% plagioclase
which is a crazy amount of plagioclase.
On Earth we rarely find rocks this pure in plagioclase, so
whatever process formed this rock, should be an important indicator for
what happened to the moon as a whole.
Now an important clue about plagioclase, is that it's very light, and so
it have the tendency to float in say, a liquid magma.
And so, how would you get this global layer of anorthosite
covering the whole moon, if we know it's also very light?
Well, maybe we had a magma ocean.
And how would you cause a magma ocean?
You could have a giant impact.
And so today, the leading theory for moon formation is the giant impact hypothesis.
According to this hypothesis, you have a Mars sized impactor hitting
a proto-Earth, spewing off pieces of the impactor,
and parts of the Earth's mantle into what's called a proto-lunar disk.
Out of this proto-lunar disk, moonlets formed and
those moonlets coalesced into our moon.
Studies suggest that this process would have taken as little as a month.
Now we should go back and double check that this theory for moon formation,
does in fact, adhere to all the observables.
Well first, we had the conservation of angular momentum.
This observable can be satisfied by, allowing for
the correct math and velocity of the impacting body.
Second, we have the small core of the Moon.
This makes sense, because in the giant impact formulation,
you have your impactor and bits of the Earth's mantle making up the moon, and
you could imagine that there would be little Iron available
in that set of starting materials to make a very large core.
Our third observable was the Oxygen isotope ratio.
This observable can be adhered to, by allowing for the proper
ratio of impactor material and earth mantle material and the proto-lunar disk.
It's a bit more complicated than that, and I'll come back to that later.
And finally, our last observable was the anorthosite crust on the moon.
And so, how does a giant impact lead to an anorthosite crust?
The answer is a magma ocean.
And so here is a photo of what maybe a magma ocean could've looked like.
The idea of a magma ocean, is you have an entire liquid surface of the moon of magma
about 100 kilometers deep.
At first, this magma ocean would have been exposed directly to the cold of space.
So the first ,about 80% or so,
of a magma ocean would have solidified in only 1,000 years.
But once the magma ocean hit about 80% solidification, our favorite mineral,
plagioclase, hit its liquidus.
This means that our plagioclase mineral would start to crystallize
out of the magma, and those crystals being lighter than the rest of the magma,
would float to the surface, creating a flotation crust.
This flotation crust would act as an insulating lid, and
slow the cooling of the moon, so that the rest of the magma ocean, that last 20%,
would take tens of millions of years to solidify.
The moon in particular is unique, that it allows for the stability and flotation
of plagioclase, because it's such a small body, and has such little gravity.
And so this is how anorthosite crust formed on the moon.
We look at the moon today, and we see the light regions,
which are the lunar highlands, made of anorthosite.
Be aware that this is a an oversimplification saying that the moon
is only anorthosite, and only basaltic maria.
There are of course many other rocks on the moon, but
this is how we're explaining the overall surface of the moon.
Like I said, these lunar highlands make up about 75% of the surface, and
their ages cluster around 4.4 billion years old.
Which gives us a good data point for
constraining the actual timeline of moon formation.