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Ask A Geologist - Miller Museum of Geology
Generated: 19 June, 2017, 00:11
What kind of rock is the moon made of?
Posted by lucydelia - 2012/02/17 17:10
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Dear Geologist,
I am in Grade 2, and I am doing a science project about the moon. I wanted to know what kind of rock
the moon is made of. Is it made of moonstone?
Thank you!
Lucy D'Elia
(with permission of her mother, Una D'Elia)
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Re:What kind of rock is the moon made of?
Posted by mbadham - 2012/02/21 09:26
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Hi Lucy
Thanks for your question about the moon. As far as we know, from the moon missions and from very
special meteorites that are actually pieces of the moon that have fallen to Earth, the moon is made up
mostly of four minerals: plagioclase, pyroxene, olivine and ilmenite. These minerals make up 98-99% of
the surface of the moon with the most common being plagioclase feldspar. We also know that common
minerals from the Earth's surface such as quartz, calcite, mica and amphibole, are very rare on the
moon.
Because there are only a few minerals on the moon's surface, the rocks are almost always a
plagioclase-rich rock called "anorthosite" A similar rock called gabbro is also found.
There is a great web page that describes the rocks of the moon here:
http://meteorites.wustl.edu/lunar/howdoweknow.htm . It is a bit technical, but maybe you and your mom
can read it and then ask me some more questions if you have any.
The moon is definitely not made of moonstone (a feldspar that has blue reflections in it) nor cheese as is
sometimes claimed :-)
Hope that helps,
Mark
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Re:What kind of rock is the moon made of?
Posted by lucydelia - 2012/02/23 07:45
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Dear Mark,
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Why is the rock of the moon different from the rocks you find on earth, if the moon was formed when a
meteorite hit the earth, and a chunk of the earth fell off and became the moon?
Thank you a lot!B)
Lucy
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Re:What kind of rock is the moon made of?
Posted by mbadham - 2012/02/23 09:43
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Hi Lucy
Great question! It sounds like you already know a lot about the moon and how scientists think it formed
following a large asteroid impact on the Earth. As a recap, scientists theorize that our solar system itself
formed in a big cloud of dust and gases in space called a nebula. In a real simplification of the process,
the particles in the cloud slowly "accreted" (came together and stuck) to form a central star (the sun)with
planets orbiting it.
Scientists think the moon formed when a large asteroid struck the Earth just after it had formed, throwing
a large amount of rock and particles into space around the Earth creating a "mini dust cloud"
surrounding the Earth. In a process similar to the way the whole solar system formed, these particles in
the dust cloud "accreted" to form a single mass orbiting the Earth (the moon).
Here is a hard part to understand, but I need to talk a bit about energy. Energy is something that
scientists talk about that is basically something that lets you "do" some work, or "change" something. It
comes in all different forms - energy of motion, energy of heat, and stored chemical energy like in a
battery. Energy can change from one kind to another, like if you were to run on a treadmill that was
hooked to a generator making electricity. Here, you are changing energy of motion (running) into
electrical energy that could then be stored as energy in a battery that would let you do something later.
The important part is, energy can't just disappear without having an affect on something, or "doing
something". The energy has to change into another form.
All of the moving pieces in the cloud of dust around the Earth originally had energy of motion. Coming
together in one spot though (in the moon) resulted in a hot molten ball of melted rock. This is because
the energy that the pieces had from moving around in outer space had to be converted to heat energy
when they collided and stopped in one spot. This melted all the pieces that came together, and formed a
big molten ball of rock in space that would slowly cool down to become the moon. Anyway, this is how
the moon is thought to have formed around 4.5 billion years ago, and it was originally a big ball of melted
rock in space.
Now, finally, the answer to your question. As the melted ball cooled, the heavy parts (nickel and iron)
sunk to the centre of the ball making a solid nickel-iron core just like the Earth has. Calcium plagioclase
feldspar (the mineral in the rock anorthosite) always crystallizes from a melted rock first, along with the
minerals olivine and pyroxene. This explains why the moon is mostly made up of these minerals as the
rocks anorthosite and gabbro.
The interesting part is that the Earth should be exactly the same because it formed in the same way
(cooled down from a molten ball of rock). But it isn't! The Earth's crust has a lot of the mineral quartz
and a mineral called potassium feldspar, but beneath its crust it is a lot like the moon with a lot of
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plagioclase feldspar, pyroxene and olivine. The different crust formed because the Earth still has active
volcanoes, and water, and an atmosphere that continue to change the rocks through time. The lighter
minerals (quartz, and potassium feldspar) have formed the crust of the Earth over time, and the
presence of water on the surface makes other minerals form that aren't possible on the moon. The
moon on the other hand hasn't had a volcanic eruption in billions of years as far as we can tell, and there
is no water or air to change the rocks.
So, after that long explanation, the moon formed shortly after the Earth did around 4.5 billion years ago,
and it basically hasn't changed since. It is still the same plagioclase-rich anorthosite rock that formed
when the melted rock originally cooled. The Earth would be exactly the same as the moon, if it didn't
have water, air, volcanoes, and life on it, that acts to continually change and recycle the rocks.
Phew, I hope that helps a bit. Keep up the good questions!
Mark
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Re:What kind of rock is the moon made of?
Posted by Heartattackgirl101 - 2015/04/27 12:00
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That is great to know!
I was wondering of there was any new information on the moon that would be valuable for my grade 9
academic science project on the moon.
Thank- You
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Re:What kind of rock is the moon made of?
Posted by mbadham - 2015/04/27 14:06
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Hi Heartattackgirl101
I think the preceding posts give you a good overview of the moon and how it formed. You will have to do
some further research to find out more for your project. One thing you might focus on is the number of
craters that can be seen on the moon today, what they tell us about events during the early formation of
the solar system (google "late heavy bombardment"), and why we don't see the same numbers of
craters on the Earth (even though the Earth would have been hit by even more large crater-forming
asteroids due to its larger size).
Mark
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Re:What kind of rock is the moon made of?
Posted by hhhhhh - 2017/01/17 20:03
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