Most of us are comfortable with the concept of magma
But how many of us are comfortable with the conditions Under Which magma forms
Consider a Simple Cross-Section of The Earth
Most people understand that the Earth consists of three great
compositional Layers, The crust, The mantle and the core
Many people Mistakenly think that the mantle is molten and While it is very hot rock it is also pretty Solid rock
If neither the crust nor the mantle are molten where do the magma that feed earth's volcanoes come from
To answer this question we need to think deeply but not too deeply, we need to think about the outermost
200 kilometers of The Earth, this is the only part of The earth where the mantle can melt. Another way that
geoscientists see the outermost 200 kilometers of The Earth is as upper Lithospheric Mantle and crust
and underlying Asthenospheric Mantle, the lithosphere differs from the asthenosphere and being cooler and stronger
Because It is more rigid, It cannot convect, and only cools By conduction
The asthenosphere is hotter and weaker and is able to convect.
Asthenosphere convection brings the great heat Of Earth's interior to the base of the lithosphere
with a depth scale in place, note that the lithosphere generally extends to 100 Kilometers of depth
A pressure scale can also be added as depths in the earth are easily converted to pressure.
Most geoscientists think of pressure in terms of Giga Pascals.
A Pascal Is the SI unit of pressure and Is equal to one newton per square meter, the intense pressures Inside the earth
require that we add Giga or
One-Billion Prefix. Finally a temperature scale can be added which will allow us to plot a line that describes how temperature
Increases with depth and pressure inside the Earth. This line is called the geothermal. The geothermal
tells us the temperature at particular depth
in the earth. Notice how the geothermal slop abruptly changes at the Lithosphere-Asthenosphere Boundary? This is because heat is transferred via
a conduction in the lithosphere,
leading to widely varying temperatures while heat is well distributed, via convection in the asthenosphere,
leading to nearly constant temperatures with increasing depth.
The geothermal in the lithosphere is said to be conductive while the asthenosphere is geothermal 'Adiabatic'
Let's pause for a minute, what do I mean when I say an Adiabatic geotherm in
thermodynamics. An Adiabatic process is one that occurs without transfer of heat or matter between a thermodynamic system and its surroundings
in the context of The geothermal
Adiabatic refers to a process in which pressure increases or decreases
greatly but temperature Increases or decreases
only slightly Adiabatic temperature
gradients are expected for convecting materials like Earth's asthenosphere. In this case, the asthenosphere
temperature remains almost constant with depth. Changing only a fraction of a degree per kilometer. This is different from the lithosphere as
conductive geotherm where temperatures change several degrees per kilometer of depth. The slope of the geothermal is a function of the lithospheric thickness.
Notice how the conducting Geothermal slope changes with lithospheric Thickness
while the slope of the adiabatic doesn't change
if the adiabetic Geotherm extended to the surface, the temperature would be
1410 degrees Celsius. This is called The Mantles potential temperature, this is not the case because the conducting lithosphere acts
acts a heat insulator causing a wide temperature
variation between the asthenosphere and the surface.
Okay, so how does all of this help us understand how Magma's form. The Mantle is composed chiefly of peridotite and
partial melting of peridotite produces basaltic magma. The temperature of This first melting of peridotite
varies with pressure or depth in the earth and is represented on this diagram by a line called The solidus
At temperatures and pressures on the left side of the line, peridotite will remain Solid, at
temperatures and pressures on the right side of the line, peridotite begins to melt
to generate magma, the Geotherm must cross the solidus.
Notice How the geotherm never crosses the solidus? This is the normal condition, in which the mantle will never melt
we know that basaltic magma does form so some special conditions must exist that allow the
geotherm to cross the solidus to cause partial melting of the Mantle
What are these special conditions
There are three great tectonic settings that enable the special conditions required for the Mantle to melt. These are
Intraplate Mantle Plumes
Divergent Margins and
Convergent Margins. Let's look at each of these
tectonic settings in more depth. The first way to melt the Mantle is by simply making the Mantle hotter
This Increases The Mantle potential temperature and is most often caused by a Mantle plume like that beneath Hawaii.
Notice how the Mantle plume rises through the asthenosphere and ponds at the base of the lithosphere, the Geotherm is deflected to higher temperatures?
When the Geotherm crosses the Solidus
Partial Melting starts to occur in the asthenosphere at the depth indicated on the diagram.
If this magma escapes to the surface it will erupt and form a volcano
This basaltic Lava on Hawaii was formed when a mantle plume partially melted the mantle underneath the Hawaii
Here is the global distribution of Mantle Plumes
The Second Way to partially melt the Mantle is by letting the adiabatic temperature gradient of the asthenosphere
extend to unusually shallow Depths in the Earth
Thus decreasing the pressure on the Mantle.
This requires decreasing Lithospheric Thickness as in the case of Mid-Oceanic Ridges at divergent Margins.
Notice how as the lithosphere thins the geothermal changes response? If the lithosphere thins sufficiently, the geothermal
can cross the solidus and partial melting occurs. The field here on the diagram represents melting that occurs in the melt triangle
This Lava erupting in the Lau Basin was formed by decreasing pressure on the mantle.
Here is the global distribution of divergent Margins
The third way to melt the mantle is by adding water as in the case of the subduction zone at a convergent margin. Consider a
simple cross section of a subduction zone. The plate on the left is subducting beneath the plate on the right.
The subducted Plate releases water as It gets deeper in the Earth.
Water interacts with the peridotite in the Mantle changing its melting point.
This is represented on the diagram by changing the dry peridotite solidus to a wet peridotite solidus
wet prototype melts at much lower temperatures than dry prototype allowing a normal Geothermal to intersect it.
This causes melting in the mantle wedge above the subducted plate.
Mount st. Helens is an example of a volcano fed by magma generated in a subduction zone.
Here is the global distribution of Convergent Margins
Let's recap. Under normal conditions, the Mantle is solid and will never melt. However there are three great
tectonic settings where conditions are right for the mantle to melt: Intraplate Mantle Plumes make the Mantle hotter
causing the regular geothermal across the Solidus
Divergent margins allowed the asthenosphere Mantle to rise to unusually shallow depths
decreasing pressure on the Mantle and causing the adiabatic Geothermic across the solidus
Convergent Margins introduce water thus replacing the regular solidus with a wet solidus that naturally crosses the geothermal
We hope You enjoyed This simple Explanation of The three Great Ways That earth's Solid Mantle is Able to melt