This video will consider how magma is made
and how the different types of magma are linked to specific plate tectonic settings.
We're going to assume that you have a working knowledge of the classification of igneous rocks
if not you might want to take a quick look at this video.
Lava flowing on earth's surface is an example of a molten rock, kind of mesmerizing isn't it.
However our own experiences with melting things
may give us a simplified view about how this lava originally formed.
When we think of melting, we often think of substances that melt to produce
a liquid version of the solid material.
In these situations, the composition of the material doesn't change as a result of the melting process.
For example, this water and ice have the same chemical formula.
However, most rocks are made up of several minerals,
each with different compositions and properties.
In particular, these minerals often melt at different temperatures.
Some at around six to seven hundred degrees Celsius and others at over a thousand degrees.
Let's try a different analogy. If we were to leave some vanilla chocolate chip ice cream
out on the counter, the vanilla ice cream gradually melts, but the chocolate chips remain solid.
What we've just managed to do is to partially melt the material.
Part of it is melted while part of it has stayed solid.
If we had way too much time on our hands, we could separate the melted ice cream and the chocolate chips.
If we were to re-freeze the ice cream it would still taste good but it would be different than the original.
So this partial melting process has essentially changed the composition of the ice cream.
This is the gist of how partial melting occurs in igneous rocks.
We're going to melt some of the minerals to generate a new magma with a different composition
The minerals that melt first have the highest silica content and are the lighter colored minerals
in this sample of granite.
Partial melting will form a new magma with these minerals.
while the darker minerals with a lower silica content will remain solid.
Keep in mind that this is all relative, in comparison to other types of rock,
this granite has a lot of high silica minerals with low melting temperatures.
If instead we were to start with a rock like peridotite,
an ultramafic rock that has a composition similar to the mantle, the only minerals present would be
olivine, pyroxene and amphibole. In this situation, some varieties of the amphibole would have a
relatively higher silica content than the rest of the minerals present and therefore would melt first.
So, to summarize, partial melting generates a magma
that has a higher silica content than the original source rock.
Now, let's look to see what this means for different plate tectonic settings.
Let's take that sample of ultramafic peridotite we had earlier.
Partial melting would generate a mafic magma that would solidify to form either
a volcanic or plutonic igneous rock.
This process is characteristic of how magma is generated at oceanic ridges and hot spots.
Most of the magma generated on earth is formed below the oceanic ridges as a result of
partial melting of the mantle. The magma source of hot spots comes from
the mantle too, but from much deeper depths and therefore exhibits
some secondary compositional differences to the magmas we find at the oceanic ridge.
Regardless of whether the magma comes from the ridges or from the hot spots, it will form
the volcanic igneous rock basalt at the surface, and it will form its plutonic equivalent, gabbro, at depth.
We can identify a third location, where partial melting of mantle rocks is occurring
in the plate overlying subduction zones.
If this magma were to rise undisturbed to the surface
it would also solidify to form mafic rocks.
However, as we'll see, these magmas will undergo a variety of changes on their path toward the surface.
As this magma rises upward its composition may change.
Mafic minerals with higher melting temperatures will solidify first,
and may separate out of the magma to leave a more silica-rich melt behind.
Likewise, the rising magma forces its way upward through fractures in the crust
and may incorporate and melt enough chunks of these rocks to alter its composition.
Finally, different magmas can essentially bump into each other on their travels
and end up forming a new mixture.
Some, or all of these processes, may alter the original mafic magma
to yield a more intermediate composition that forms the volcanic rock andesite and its plutonic equivalent diorite.
The composition of these rocks is actually pretty close to the average composition of the continental crust.
So, let's now take a look at what happens if the continental crust were to undergo partial melting.
If we melt this intermediate composition rock it would result in a felsic magma
that would create the plutonic rock granite and the volcanic version, rhyolite.
Some of the mafic magma that forms above the subduction zone may get trapped
at the base of the crust causing, the intermediate crustal rocks to heat up
and undergo partial melting to form a felsic magma.
These magmas are pretty viscous and most won't rise far enough to reach the surface
but instead will solidify underground to form large granitic plutons.
So to summarize, most of the igneous rocks begin by partially melting ultramafic mantle rocks
At the oceanic ridge, partial melting produces a mafic magma that generates new oceanic crust
of basalt near the surface and gabbro at depth.
A similar combination of mafic rocks is characteristic of hot spots like Hawai'i.
Things get a bit fussier in association with volcanic arcs overlying subduction zones.
First, the mafic magma travels at greater distance through the crust and undergoes
a variety of compositional changes that may create a melt with an intermediate composition.
This can produce eruptions of andesite and plutons of diorite.
Second, the mafic magma may get trapped at the base of the crust,
heating it up sufficiently to create a new felsic magma that solidifies to form either rhyolite or granite.
OK, that's it for today how confident are you that you
could complete the two learning objectives associated with this video.