5.3: Magma Generation
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Magma and lava contain three components: melt, solids, and volatiles. The melt is made of ions from minerals that have liquefied. The solids are made of crystallized minerals floating in the liquid melt. These may be minerals that have already cooled Volatiles are gaseous components—such as water vapor, carbon dioxide, sulfur, and chlorine—dissolved in the magma . The presence and amount of these three components affect the physical behavior of the magma and will be discussed more below.
Below the surface, the temperature of the Earth rises. This heat is caused by residual heat left from the formation of Earth and ongoing radioactive decay. The rate at which temperature increases with depth is called the geothermal gradient. The average geothermal gradient in the upper 100 km (62 mi) of the crust is about 25°C per kilometer of depth. So for every kilometer of depth, the temperature increases by about 25°C.
The depth-temperature graph (see figure) illustrates the relationship between the geothermal gradient (geotherm, red line) and the start of rock melting (solidus, green line). The geothermal gradient changes with depth (which has a direct relationship to pressure) through the crust into the upper mantle. The area to the left of the green line includes solid components; to the right is where liquid components start to form. The increasing temperature with depth makes the depth of about 125 kilometers (78 miles) where the natural geothermal gradient is closest to the solidus.
The temperature at 100 km (62 mi) deep is about 1,200°C (2,192°F). At bottom of the crust, 35 km (22 mi) deep, the pressure is about 10,000 bars . A bar is a measure of pressure, with 1 bar being normal atmospheric pressure at sea level. At these pressures and temperatures, the crust and mantle are solid. To a depth of 150 km (93 mi), the geothermal gradient line stays to the left of the solidus line. This relationship continues through the mantle to the core-mantle boundary, at 2,880 km (1,790 mi).
The solidus line slopes to the right because the melting temperature of any substance depends on pressure. The higher pressure created at greater depth increases the temperature needed to melt rock. In another example, at sea level with an atmospheric pressure close to 1 bar, water boils at 100°C. But if the pressure is lowered, as shown in the video below, water boils at a much lower temperature.
There are three principal ways rock behavior crosses to the right of the green solidus line to create molten magma: 1) decompression melting caused by lowering the pressure, 2) flux melting caused by adding volatiles (see more below), and 3) heat-induced melting caused by increasing the temperature. The Bowen’s Reaction Series shows that minerals melt at different temperatures. Since magma is a mixture of different minerals, the solidus boundary is more of a fuzzy zone rather than a well-defined line; some minerals are melted and some remain solid. This type of rock behavior is called partial melting and represents real-world magmas, which typically contain solid, liquid, and volatile components.
The figure below uses P-T diagrams to show how melting can occur at three different plate tectonic settings. The green line is called the solidus, the melting point temperature of the rock at that pressure. Setting A is a situation (called “normal”) in the middle of a stable plate in which no magma is generated. In the other three situations, rock at a lettered location with a temperature at the geothermal gradient is moved to a new P-T situation on the diagram. This shift is indicated by the arrow and its temperature relative to the solidus is shown by the red line. Partial melting occurs where the red line temperature of the rock crosses the green solidus on the diagram. Setting B is at a mid-ocean ridge (decompression melting) where reduction of pressure carries the rock at its temperature across the solidus. Setting C is a hotspot where decompression melting plus the addition of heat carries the rock across the solidus, and setting D is a subduction zone where a process called flux melting takes place where the solidus (melting point) is actually shifted to below the temperature of the rock.
Graphs A-D below, along with the side view of the Earth’s layers in various tectonic settings (see figure), show how melting occurs in different situations. Graph A illustrates a normal situation, located in the middle of a stable plate, where no melted rock can be found. The remaining three graphs illustrate rock behavior relative to shifts in the geothermal gradient or solidus lines. Partial melting occurs when the geothermal gradient line crosses the solidus line. Graph B illustrates the behavior of rock located at a mid-ocean ridge, labeled X in the graph and side view. Reduced pressure shifts the geotherm to the right of the solidus, causing decompression melting. Graph C and label Y illustrate a hotspot situation. Decompression melting, plus an addition of heat, shifts the geotherm across the solidus. Graph D and label Z show a subduction zone, where an addition of volatiles lowers the melting point, shifting the solidus to the left of the geothermal gradient. B, C, and D all show different ways the Earth produces intersections of the geothermal gradient and the solidus, which results in melting each time.
Magma is created at mid-ocean ridges via decompression melting. Strong convection currents cause the solid asthenosphere to slowly flow beneath the lithosphere. The upper part of the lithosphere (crust) is a poor heat conductor, so the temperature remains about the same throughout the underlying mantle material. Where the convection currents cause mantle material to rise, the pressure decreases, which causes the melting point to drop. In this situation, the rock at the temperature of the geothermal gradient is rising toward the surface, thus hotter rock is now shallower, at a lower pressure, and the rock, still at the temperature of the geothermal gradient at its old location, shifts past the its melting point (shown as the red line crossing over the solidus or green line in example B in previous figure) and partial melting starts. As this magma continues to rise, it cools and crystallizes to form new lithospheric crust.
Flux melting or fluid-induced melting occurs in island arcs and subduction zones when volatile gases are added to mantle material (see figure: graph D, label Z). Flux-melted magma produces many of the volcanoes in the circum-Pacific subduction zones, also known as the Ring of Fire. The subducting slab contains oceanic lithosphere and hydrated minerals. As covered in Chapter 2, these hydrated forms are created when water ions bond with the crystal structure of silicate minerals. As the slab descends into the hot mantle, the increased temperature causes the hydrated minerals to emit water vapor and other volatile gases, which are expelled from the slab-like water being squeezed out of a sponge. The volatiles dissolve into the overlying asthenospheric mantle and decrease its melting point. In this situation the applied pressure and temperature have not changed, the mantle’s melting point has been lowered by the addition of volatile substances. The previous figure (graph D) shows the green solidus line shifting to the left of and below the red geothermal gradient line, and melting begins. This is analogous to adding salt to an icy roadway. The salt lowers the freezing temperature of the solid ice so it turns into liquid water.
Heat-induced melting, transforming solid mantle into liquid magma by simply applying heat, is the least common process for generating magma (see figure: graph C, label Y). Heat-induced melting occurs at the mantle plumes or hotspots. The rock surrounding the plume is exposed to higher temperatures, the geothermal gradient crosses to the right of the green solidus line, and the rock begins to melt. The mantle plume includes rising mantle material, meaning some decompression melting is occurring as well. A small amount of magma is also generated by intense regional metamorphism (see Chapter 6). This magma becomes a hybrid metamorphic-igneous rock called migmatite.
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