This video will explain how rocks melt to produce magma in a variety of plate tectonic settings.
We have a companion video on partial melting that might help you understand these processes more fully.
When we think about melting, things like ice or butter or chocolate might come to mind.
These common items melt as they warm up, turning from a solid into in to a runny liquid.
You probably know that Earth’s interior is very hot.
The rate at which temperature increases with depth is represented by the geothermal gradient.
Earth’s internal temperature increases downward,
and is estimated to be greater than 6000 degrees Celsius in the core.
So, there you go. Things melt when they warm up and Earth gets hotter with increasing depth.
Surely, there must be a point below the surface where it just gets so hot that rocks start to melt.
Not so fast, Sherlock. It turns out we are overlooking another key property of Earth’s interior, and that’s pressure.
Just like temperature, pressure increases with depth.
And that’s going to prevent rocks from melting except under very specific types of conditions.
We are going to focus in on the upper few hundred kilometers of Earth.
This is where most of the melting is going to take place.
We are going to use this graph to show how temperature varies with depth.
On this figure, the oceanic crust would be up here, the continental crust would be about this thick,
and the rest of the graph would represent rocks in the upper part of the mantle.
Including parts of the lithosphere and asthenosphere.
The first thing to know is that this graph is drawn to represent melting conditions for peridotite,
the typical composition of the mantle.
There are several lines on this graph, lets explain what they represent before we go any further.
This line, known as the solidus,
indicates the temperature necessary to start melting peridotite at different depths.
Remember that pressure increases with depth,
so it takes much higher temperatures to melt rocks the deeper we go.
The second red line is the liquidus,
this represents the temperature needed to completely melt peridotite at different depths.
Anything between the two lines represents conditions where partial melting will take place.
Technically, any area to the left of the solidus curve
represents conditions where the rocks would remain solid.
However, many of these conditions are actually unrealistic.
For example, we aren’t going to find any deeply buried at relatively low temperatures like these.
Let’s add another curve, this one indicates a typical geothermal gradient for the crust and mantle.
This is the temperature of the rocks at a given depth.
For example, a rock at 100 kilometers would lie near the
top of the asthenosphere below oceanic basins, and would be solid and around 1300 degrees Celsius.
One final point, this graph is drawn for an ultramafic mantle rock with relatively high melting temperatures.
If, instead, we were to generate a similar graph for the continental crust (more about that later)
the relative positions of the liquidus and solidus lines would shift to the left.
OK, enough of the set up. Let’s use this figure to demonstrate how rocks melt at plate boundaries.
First, let’s remember what happens at divergent plate boundaries
that are mostly represented by oceanic ridges.
Plates move apart and the lithosphere is stretched and it thins.
The hot rocks of the asthenosphere rise from deeper levels to fill this growing gap.
The key here is that these mantle rocks start out at around 100 kilometers depth
where pressures are greater, and move upward to a point just below the surface where pressures are much lower.
This change in pressure is sufficient to prompt partial melting of the rising asthenosphere,
a process known as decompression melting.
Let’s see what that looks like on our graph.
So, imagine if the hot asthenosphere was to move upward fast enough
so that it doesn’t lose much of its initial heat.
Somewhere above 50 kilometers it would cross the peridotite solidus line
and partial melting would begin to generate a mafic magma.
As rising continued, a greater proportion of the original rock would undergo melting
but it would never be sufficient to completely melt the peridotite.
A similar process occurs below hot spots like Hawai’i
as mantle rocks rise in hot plumes that begin near the core/mantle boundary.
As they neared the surface, the hot mantle material in these plumes
would undergo decompression melting to form mafic magma.
Next, let’s turn our attention to convergent plate boundaries
where a plate descends into the mantle along a subduction zone.
We find volcanoes on the over-riding plates, typically in the form or island arcs or volcanic arcs on land.
Let’s see where this magma comes from.
Water-rich minerals in the descending oceanic plate are compressed in the subduction zone,
and the water is squeezed out of the minerals and into the much hotter mantle rocks immediately above.
The addition of this water causes these rocks to melt.
This process is known as flux melting.
The presence of water completely realigns the position of the solidus and liquidus for the mantle rocks.
Rocks that would have been solid under normal conditions at around 100 kilometers,
are now well inside the partial melting field.
As this mafic magma rises through the plate
some of it may get trapped at the base of the continental crust,
heating it up and causing it to undergo partial melting to form a felsic magma
by a process known as heat transfer melting.
OK, let’s see what that looks like on the graph.
We are dealing with continental crust now, which starts to melt at lower temperatures.
The base of the crust would be around 35 to 40 kilometers depth, and these rocks would
need to be heated by another 500 degrees or so to start partially melting the crust and generate felsic magma.
So, to summarize, we described three different ways to create magma
by melting rocks in Earth’s mantle and crust.
As arranged here, they are in order of the volume of magma produced.
Decompression melting is the dominant mechanism at oceanic ridges and hot spots,
and also contributes magma to continental rifts.
Flux melting is the main source of magma for island arcs and
volcanic arcs associated with convergent boundaries.
However, some of the magma produced in these settings is also produced by heat transfer melting.
Finally, this latter process is also associated with continental rifts.
Here is our learning objective for this video.
How confident are you that you could successfully respond to this prompt?