Most of us have a basic understanding of what magma is – molten rock below the Earth’s surface,
which becomes lava when it erupts. What is less understood is how this
rock becomes molten in the first place. Consider a simple cross-section of the Earth.
Most people know that the Earth consists of three great layers: the crust, the mantle, and the core.
Many people imagine the mantle as an underground molten sea, but in reality, it’s just a very hot
solid. If neither the crust nor the mantle is molten, then where do the magmas that
feed Earth's volcanoes come from? And what does magma generation have to do with plate tectonics?
To answer these questions, 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 conditions are right for the mantle to melt.
Another way that geoscientists see this outermost 200 kilometers is as crust,
lithospheric mantle, and underlying asthenospheric mantle.
The lithosphere is rigid, and thus can only cool by conduction. This cooling style differs
from the asthenosphere, which is able to convect due to being hotter and weaker.
Asthenospheric 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
deep. A pressure scale can also be added, as pressure varies with depth in Earth’s interior.
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 the “Giga,” or one-billion, prefix.
Finally, a temperature scale can be added, and now we can plot a line
that describes how temperature increases with depth and pressure inside the Earth.
This line is called the geotherm. The geotherm identifies the expected temperature at a
given depth in the Earth, based on these pressure, depth, and temperature relations.
Notice how the geotherm’s slope abruptly changes at the lithosphere-asthenosphere boundary?
Since heat is transferred by conduction in the lithosphere, there
will be widely varying temperatures. Heat is well distributed via convection in the asthenosphere,
leading to nearly constant temperatures with increasing depth. The geotherm in the
lithosphere is said to be conductive, while the asthenosphere’s geotherm is 'adiabatic.'
Let’s pause and discuss what the term adiabatic means. In thermodynamics, an adiabatic process is
one that occurs without transfer of heat between the system and its surroundings.
If heat transfer is observed, it is a negligible amount in comparison to the other measured changes
in the system. In the context of Earth’s geotherm, 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’s temperature remains almost constant with depth, changing
only a fraction of a degree per kilometer. This is different from the lithosphere’s conductive
geotherm, where temperatures change several degrees per kilometer of depth.
The slope of this geotherm is a function of lithospheric thickness. If the asthenosphere’s
geotherm were to extend to surface depths, its temperature would reach 1410 degrees Celsius.
This is called the mantle potential temperature. Such high surface temperatures not observed due
to the lithosphere’s presence. The conductive lithosphere acts as an insulating blanket,
causing a wide temperature variation between the asthenosphere and the surface.
Okay, so how does all of this help us understand how magmas form?
The mantle is composed chiefly of peridotite, and partial melting of this material produces
basaltic magma. The melting point of peridotite varies with pressure and depth in the Earth
and is represented on this diagram by a line called the solidus.
At temperature and pressure conditions to the left of the solidus, peridotite and thus the mantle
will remain solid. At temperature and pressure conditions to the right of the solidus, peridotite
begins to melt. For magma to be generated from the mantle, its geotherm must cross the solidus.
Under normal conditions, the mantle’s geotherm doesn’t cross this solidus. However, from past
and present volcanic activity, we know that basaltic magmas exist more than just in theory.
This means that some special conditions must exist that allow the geotherm to
cross the solidus, and thus cause partial melting of the mantle.
So, what are these conditions, and which are related to plate tectonics?
There are three situations that enable the solidus to cross the geotherm and allow the mantle to
melt. These are: intraplate mantle plumes, divergent margins, and convergent margins.
Let's look at each of these a little more closely.
The first way to melt the mantle is by simply making the mantle hotter. This increases the
mantle temperature and is most often caused by a mantle plume like that beneath Hawaii.
Notice how as 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 depths 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 underlying mantle.
Here is the global distribution of mantle plumes.
Note that other planets like Mars and Venus which don’t have plate tectonics have mantle plumes.
Mantle plumes then are not obviously related to plate tectonics.
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. This requires
thinning the lithosphere, as in the case of mid-oceanic ridges at divergent plate margins.
Notice how as the lithosphere thins, the geotherm changes in response?
If the lithosphere thins sufficiently, the geotherm 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.
This style of melting is associated with plate tectonics.
The third way to melt the mantle is by adding water as in the case of subduction zones at
convergent plate margins. Let’s look at 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. This water rises
and is incorporated into the overlying mantle, lowering its melting temperature.
This is represented on the diagram by changing the dry peridotite solidus to a wet peridotite
solidus. Wet peridotite melts at much lower temperatures than dry peridotite allowing a normal
geotherm 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.
This style of melting only happens with plate tectonics.
Let's recap. Under normal conditions, the mantle is solid and doesn’t melt.
However, there are three great situations where conditions are right for the mantle to melt:
rising mantle plumes heat the mantle, causing the geotherm to cross the solidus.
Divergent plate margins allow the asthenospheric mantle to rise to unusually shallow depths,
decreasing pressure on the mantle and causing the adiabatic geotherm to cross the solidus.
Convergent plate margins introduce water, thus replacing the regular solidus with a wet solidus
that naturally crosses the geotherm. Plumes happen on all convecting Earth-like planets,
but decompression melting beneath spreading ridges and flux melting
beneath arcs above subduction zones only happens with plate tectonics.
We hope you enjoyed this deep dive into the Earth’s mantle
and how magmas form as a result of plate tectonics and other processes.