Altering Magma Composition
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Altering Magma Composition

October 18, 2019

– Alright. So I’m going to very
quickly talk about 4 different processes that cause magmas
to change over time. So, if we have a magma, I’m just going
to draw a nice little diep here; a blog of magma rising
up through some surrounding rock; I’m going to put little x’s,
just to show that it’s been crystallized. So we have
this magma that’s rising up. Magmas don’t necessarily come
up as a big blob, but they might have extensions that kind
of go up; and they’re going to start wrap around trunks
of the pre existing rocks. So let’s say that this one kind
of goes; and it closes up, and it takes a piece of this
rock and it brings it inside. What’s going to start to happen
is it’s going to assimilate that piece of country rock,
that surrounding cool rock; it’s going to melt it and take
that material, all of those chemicals that are inside of
that rock, and those are going to become melted down and
become part of the magma. This actually works to take that
magma and over time make it more and more felsic. So
this is the process of assimilation of that surround
country rock. Another thing that can happen as the magma
is moving up; we’re gonna call this one A. They’re might be
a second blob of magma that’s rising up, and we’ll call it
B, and if they start to merge and join together they’re going
to kind of cross bounds and they’re going to start to mix;
so that B becomes part of A. Now if A, let’s say A were an
intermediate magma, and maybe B was a mafic magma, so we
take an intermediate and mafic and we’re going to combine
those two. We’re going to see sort of a composition between that
mafic and intermediate. So that intermediate is going to
become more mafic over time; or we could look at, and the
other way around would be to take this mafic magma that’s
now mixing in some intermediate magma; that ones going to become
more felsic, so that’s this process of magma mixing. The
next thing that can start to happen is this fractional
crystallization. So if we have that blob of magma, and it
starts to cool and crystallize out different mineral crystals;
those different mineral crystals are denser than the
magma that they’re sitting in. They’re going to start to
sink down in the magma; and eventually they’re going to
accumulate at the bottom; and so this will start to cool and
crystallize and turn into a rock. As those crystallize, and
they’ve been growing, all of the elements that were present
in the magma; the iron, the magnesium, the silicone, the
oxygen; they’re going to start to take some of those elements
out of the magma, so that they’re no longer available
to crystallize into new minerals. So we start to see
this stuff down at the bottom will be more magnesium and
iron rich than the material at the top. This is the process
of fractional crystallization, and it starts to happen as
that magma is cooling and crystallizing the heavier mineral
crystals sink down towards the bottom; and we see this
change, this fractional crystallization change in
chemistry of the rock that’s left behind. And on the last thing
that we can see with magmas changing, is just this idea
of partial melting. Now, if we were to go to a place where
we have a big piece of continental crust, and we’ll
say maybe it’s intermediate in its overall composition, if
we get a mantle plume that’s rising up and it starts to
create a big hotspot; so this is heat sitting underneath
that continental plate, that lithosphere, it acts sort of
like taking a rock and putting it on a hotplate, although
much to a more extreme level; what actual starts to happen
is that this bottom part of that lithosphere plate is
going to start to melt; it partially melts, and its then
going to lead up to some new blobs of magma rising up
through that lithospheric crust. Now these blobs are going to
be more felsic than the rock that it was originally part
of; and that’s because when we look at Bowen’s Reaction
Series, those light colored minerals; those are going to
start to melt back into the magma. Things that are cooling
and crystallizing at the lowest temperatures; those
are going to be the first to start to melt off, and then
rise up through the continental crust to create new material.
This is just one example of partial melting, it can happen
below oceanic lithosphere or continental lithosphere; and
its just changing the material that comes up, so that it
becomes more felsic over time. So in all of these cases, most of
what we’re looking at is the rock is going to change from
one to the other; and usually, we’re going to see that
happening; that they’re becoming more and more felsic over time,
versus more and more methic. Again, the idea of the magma
mixing, that’s where we might see it becoming a little bit
more felsic, depending on what two magma types are going to
be mixing; and the way you’re looking at which ones changing.
But again, we start to see things become more felsic
over time, and these are 4 processes that help us
understand how that’s happening.

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  1. In Rough Hewn Land, Keith Meldahl describes the mafic minerals crystallizing out of a magma plume and foundering (sinking) into the base of the plume. My question: Do we have evidence for this? If we do borings at the base of the Sierra Mountains, for example, do we find the granite is more enriched in the mafic minerals? At locations where much granite has eroded, do we ever find deposits of granite highly enriched in iron and magnesium minerals?

  2. Thanks a lot Dr
    I'm student in faculty of science department geology
    And I really appreciate your simple explanation of geology 😃❤

  3. As an incipient geologist, I have to say, the only thing hotter than that magma is the lady teaching in this video. Haha

  4. MRS lewis you are one simple geology guru ever see ,but what if i add geo-chemical dispertion of element,metasomatics .

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