7.3: Melting
Melting
What Is a Melt?
A “melt” refers to molten rock, typically formed at high temperatures (~2000°F/1090°C). Melts are characterized by where they are located. Lava Links to an external site. exists at or very near the surface of the Earth, whereas magma Links to an external site. forms at depth within the crust and lithosphere Links to an external site.. Although the term “melt” indicates a rock that is completely molten (i.e. a liquid), most melts actually contain all three states of matter. The liquid component is the actual molten rock. The solid components are minerals that have crystallized or glasses that have formed. Lastly, the gaseous components are dissolved within the melt, like carbon dioxide, water vapor, and others. These gases will be an important driver in volcanic eruptions.
Where Do Melts Form?
There are a number of myths surrounding melts. The first often perpetuated myth is that melts come from the outer core. This is FALSE. A melt can only originate in the mantle or lithosphere. The second often perpetuated myth is that melts exist everywhere beneath the crust. This is also FALSE. Melts only form in specific tectonic settings, including continental volcanic arcs and volcanic island arcs (subduction zones), hot spots, and divergent boundaries (mid-ocean ridges and rifts).
How Do Rocks Melt?
Melting is an interplay between temperature and pressure. Under normal or low pressures, like on the surface of the Earth, you need only 1) add heat, 2) meet or exceed the melting temperature of the object you are trying to melt, and 3) a melt will be produced. For example, chocolate has a relatively low melting temperature, around 90°F, while our body temperature is around 98°F; this is why chocolate will melt in your hand (and in your mouth!). Earth’s “body” temperature runs much hotter, and temperatures increase with depth moving towards the core; however, pressure also increases with depth. Increasing pressure counteracts increasing temperature, preventing the breaking of mineral bonds and disruption of crystal lattices that are required to melt rocks. In Figure 7.2, the red lines represent the geothermal gradient Links to an external site., which tracks how temperature increases with depth; the gray areas on the charts represent the temperature and pressure conditions under which rock remains solid, whereas the pink regions represent the temperature and pressure conditions under which rock will melt into liquid. Notice how Earth is not molten throughout; instead melts typically only exist within the crust and upper lithosphere, usually fewer than 50 miles (80 km) from the surface, and form only in unique tectonic settings where specific conditions are met (i.e. where the geothermal gradient crosses the solidus Links to an external site. curve).
Figure 7.2: Schematic diagram illustrating the physical processes within Earth’s upper mantle that lead to the generation of magma. Each graph shows the geothermal gradient (red) and the solidus (blue). When the two curves cross each other, magma is generated by partial melting. A) the curves do not cross; rock does not melt; B) at mid-ocean ridges, rock undergoes decompression melting; C) at a hotspot (mantle plume), rock undergoes decompression melting; D) at a subduction zone, volatiles are added and rock undergoes flux melting. (CC-BY-SA 3.0; Woudloper via Wikimedia Links to an external site.)
One way to surpass the melting temperature of a rock and create a melt is through a process called decompression melting Links to an external site. (Figure 7.3). This process relieves pressures on the rock, which shifts the geothermal gradient. Decompression melting occurs at divergent boundaries (MORs and rifts) and hot spots (like Hawaii) (Figure 7.2B and C). Here, rock is decompressed at a constant temperature. Convection brings hot mantle from depth and moves it towards the surface. The mantle material stays hotter than the surrounding rocks and, as pressure is relieved, melt is produced. This is why basalt commonly forms at MORs and at hot spots below oceanic plates.
Figure 7.3: An illustration depicting decompression melting in the asthenosphere at mid-ocean ridges (MORs). (CC-BY 4.0; Chloe Branciforte)
Earth has additional processes at play, which help to lower the melting temperature of rock. Typically, this occurs through the addition of volatiles, in a process called flux melting Links to an external site. (Figure 7.4). At subduction zones, hydrated minerals in the oceanic crust release water as they subduct and heat up. This addition of water into the mantle rock causes the mantle to melt at a lower temperature than its normal melting temperature (Figure 7.2D). This process is similar to the use of salt on roadways during the winter months; the salt causes the ice to melt at a lower temperature and helps reduce ice development on roadways. For rock in subduction zones, water behaves like the salt.
Rock can also melt through conduction Links to an external site., a simple way to transfer heat. Melt produced through decompression or flux melting is buoyant and will begin to rise toward Earth's surface. As the rising melt interacts with lithospheric rock, it may transfer enough heat to the surrounding rock to melt it. This is why volcanic arcs (both continental and island) form at subduction zones.
Figure 7.4: An illustration depicting the addition of volatiles, primarily water, at a subduction zone, resulting in melting. (CC-BY 4.0; Chloe Branciforte)