Three Great Ways to Melt the Mantle

Here is the latest animation from UTD GSS, titled: "Three Great Ways to Melt the Mantle." It explains how the mantle melts using an animated P/T diagram, and relates melting to tectonic setting. Please leave comments, suggestions, criticisms, and questions below.

“Three Great Ways to Melt the Mantle”

Most of us are comfortable with the concept of magma, but how many of us are comfortable with the conditions under which magmas form? Consider a simple cross section of the Earth. Most people understand that the Earth consists of 3 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 magmas 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 km 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 in 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. 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 km 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 Gigapascals. 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, 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 geotherm. The geotherm tells us the temperature at a particular depth in the Earth. Notice how the geotherm’s slope abruptly changes at the lithosphere/asthenosphere boundary? This is because heat is transferred via 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 geotherm in the lithosphere is said to be conductive, while the asthenosphere’s geotherm is 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 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 the geotherm is a function of lithospheric thickness. Notice how the conducting geotherm’s slope changes with lithospheric thickness, while the slope of the adiabatic geotherm doesn’t change. If the adiabatic geotherm extended to the surface, the temperature would be 1400 degrees Celsius. This is called the mantle’s potential temperature. This is not the case because the conducting lithosphere acts as a heat insulator, causing a wide temperature variation between the asthenosphere and the surface.

Ok, so how does all of this help us understand how magmas 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 the diagram by a line called the solidus. At temperatures and pressure on the left side of the line, peridotite will remain solid. At temperatures and pressure on 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 allows 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 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 mantle underneath 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 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 this 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 a 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 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 plates. Mt 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 geotherm to cross the solidus.

Divergent 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 margins introduce water thus replacing the regular solidus with a wet solidus that naturally crosses the geotherm.

We hope you enjoyed this simple explanation of the three great ways that Earth’s solid mantle is able to melt.