Why does decompression melting occur

We call this kind of melting adiabatic — or, more commonly, decompression melting. Decompression melting commonly occurs at divergent plate boundaries, where two tectonic plates are moving away from each other. Mid-ocean ridges are the classic example, but adiabatic melting also occurs during continental lithospheric extension and in some mantle plumes. According to the diagram above, at approximately what depth does adiabatic melting begin? Iceland is unique in that many researchers believe that a mantle plume is rising up through the Mid-Atlantic Ridge here.

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Show Summary Details Overview decompression melting. View all related items in Oxford Reference » Search for: 'decompression melting' in Oxford Reference ». To a depth of 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, km 1, 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. 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.

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. 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.

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. Over millions of years, the magma in this subduction zone can create a series of active volcanoes known as a volcanic arc.

Flux melting occurs when water or carbon dioxide are added to rock. These compounds cause the rock to melt at lower temperatures. This creates magma in places where it originally maintained a solid structure.

Much like heat transfer, flux melting also occurs around subduction zones. In this case, water overlying the subducting seafloor would lower the melting temperature of the mantle, generating magma that rises to the surface. Magma leaves the confines of the upper mantle and crust in two major ways: as an intrusion or as an extrusion. An intrusion can form features such as dikes and xenoliths.

An extrusion could include lava and volcanic rock. Magma can intrude into a low-density area of another geologic formation, such as a sedimentary rock structure. When it cools to solid rock, this intrusion is often called a pluton. A pluton is an intrusion of magma that wells up from below the surface.

Plutons can include dikes and xenoliths. A magmatic dike is simply a large slab of magmatic material that has intruded into another rock body. A xenolith is a piece of rock trapped in another type of rock. Many xenoliths are crystals torn from inside the Earth and embed ded in magma while the magma was cooling.

Lava cools to form volcanic rock as well as volcanic glass. This magma solidifies in the air to form volcanic rock called tephra. In the atmosphere, tephra is more often called volcanic ash. As it falls to Earth, tephra includes rocks such as pumice.

In areas where temperature, pressure, and structural formation allow, magma can collect in magma chamber s. Most magma chambers sit far beneath the surface of the Earth. The pool of magma in a magma chamber is layered. The least-dense magma rises to the top. The densest magma sinks near the bottom of the chamber. Over millions of years, many magma chambers simply cool to form a pluton or large igneous intrusion. If a magma chamber encounter s an enormous amount of pressure, however, it may fracture the rock around it.

The cracks, called fissure s or vents, are tell-tale signs of a volcano. Many volcanoes sit over magma chambers. An eruption reduce s the pressure inside the magma chamber. Large eruptions can nearly empty the magma chamber.

The layers of magma may be document ed by the type of eruption material the volcano emits. Gases, ash, and light-colored rock are emitted first, from the least-dense, top layer of the magma chamber. Dark, dense volcanic rock from the lower part of the magma chamber may be released later.