Magma: Characteristics, Types, Sources, and Evolution of Magma

Magma

Magma is molten or semi-molten rock beneath the Earth's surface. It forms when rocks melt due to the high temperatures and pressures found deep within the Earth. Magma is less dense than the surrounding solid rock, so it rises to the surface, where it can erupt from volcanoes as lava.

A magma consists mostly of liquid rock matter, but may contain crystals of various minerals, and may contain a gas phase that may be dissolved in the liquid or may be present as a separate gas phase.

Magma can cool to form an igneous rock either on the surface of the Earth - in which case it produces a volcanic or extrusive igneous rock, or beneath the surface of the Earth, - in which case it produces a plutonic or intrusive igneous rock.

Magma: Characteristics, Types, Sources, and Evolution of Magma

Sources of Magma

1- Partial Melting

Melting of solid rocks to form magma is controlled by three physical parameters: its temperature, pressure, and composition.

When rocks melt they do so incrementally and gradually; most rocks are made of several minerals, all of which have different melting points, and the physical/chemical relationships controlling melting are complex.

As a rock melts, its volume changes. When enough rock is melted, the small globules of melt (generally occurring in between mineral grains) link up and soften the rock. Under pressure within the earth, as little as a fraction of a percent partial melting may be sufficient to cause melt to be squeezed from its source. Melts can stay in place long enough to melt to 20% or even 35%, but rocks are rarely melted in excess of 50%, because eventually the melted rock mass becomes a crystal and melt mush that can then ascend en masse as a diapir, which may then cause further decompression melting.

2- Geochemical Implications of Partial Melting

The degree of partial melting is critical for determining what type of magma is produced. The degree of partial melting required to form a melt can be estimated by considering the relative enrichment of incompatible elements versus compatible elements. Incompatible elements commonly include potassium, barium, caesium, rubidium.

Rock types produced by small degrees of partial melting in the Earth's mantle are typically alkaline (Ca, Na), potassic (K) and/or peralkaline (high aluminium to silica ratio). Typically, primitive melts of this composition form lamprophyre, lamproite, kimberlite and sometimes nepheline-bearing mafic rocks such as alkali basalts and essexite gabbros or even carbonatite.

Pegmatite may be produced by low degrees of partial melting of the crust. Some granite-composition magmas are eutectic (or cotectic) melts, and they may be produced by low to high degrees of partial melting of the crust, as well as by fractional crystallization. At high degrees of partial melting of the crust, granitoids such as tonalite, granodiorite and monzonite can be produced, but other mechanisms are typically important in producing them.

3- Magma Usage for Energy Production

The Iceland Deep Drilling Project, while drilling several 5000m holes in an attempt to harness the heat in the volcanic bedrock below the surface of Iceland, struck a pocket of magma at 2,100m. Being only the third time in recorded history that magma had been reached, IDDP decided to invest in the hole, naming it IDDP-1.

A cemented steelcase was constructed in the hole with a perforation at the bottom close to the magma. The high temperatures and pressure of the magma steam were used to generate 36MW of power, making IDDP-1 the world’s first magma-enhanced geothermal system.

Illustration of how a magma evolves as the earliest-formed minerals (those richer in iron, magnesium, and calcium) crystallize and settle to the bottom of the magma chamber, leaving the remaining melt richer in sodium, potassium, and silica (SiO2). A. Emplacement of a magma body and associated igneous activity generates rocks having a composition similar to that of the initial magma. B. As the magma cools, magmatic differentiation (crystallization and settling) change the composition of the remaining melt, generating rocks having a composition quite different than the original magma. C. Further magmatic differentiation results in another, more highly evolved, melt with its associated rock types.

Evolution of Magma

Primary Melts

When a rock melts, the liquid is a primary melt. Primary melts have not undergone any differentiation and represent the starting composition of a magma. In nature it is rare to find primary melts. The leucosomes of migmatites are examples of primary melts.

Primary melts derived from the mantle are especially important, and are known as primitive melts or primitive magmas. By finding the primitive magma composition of a magma series it is possible to model the composition of the mantle from which a melt was formed, which is important in understanding evolution of the mantle.

Parental Melts

Where it is impossible to find the primitive or primary magma composition, it is often useful to attempt to identify a parental melt. A parental melt is a magma composition from which the observed range of magma chemistries has been derived by the processes of igneous differentiation. It need not be a primitive melt.

For instance, a series of basalt flows are assumed to be related to one another. A composition from which they could reasonably be produced by fractional crystallization is termed a parental melt. Fractional crystallization models would be produced to test the hypothesis that they share a common parental melt.

Types of Magma

Types of magma are determined by chemical composition of the magma. Three general types are recognized:

Mafic Magma

Mafic Magma or Basaltic Magma, This type of magma is high in iron, magnesium, and calcium, but low in potassium and sodium. It also has a relatively low silica content (45-55%). Basaltic magma is the most common type of magma and is responsible for forming many of the world's largest volcanoes, such as Kilauea and Mauna Loa in Hawaii.

Silica content: 45-55%
Color: Dark, almost black
Temperature: 1000-1200°C (1832-2192°F)
Viscosity: Low - 10 - 10³PaS
Gas Content: Low
Minerals: Olivine, pyroxene, plagioclase feldspar

Intermediate Magma

Intermediate Magma or Andesitic Magma, This type of magma has an intermediate silica content (55-65%) and mineral composition. It is less viscous than basaltic magma and can produce both explosive and effusive eruptions. Andesitic magma is common in subduction zones, where one tectonic plate is sliding underneath another.

Silica content: 55-65%
Color: Gray
Temperature: 800-1000°C (1472-1832°F)
Viscosity: Intermediate 10³ - 10⁵ PaS
Gas Content: Intermediate
Minerals: Plagioclase feldspar, pyroxene, amphibole

Felsic Magma

Felsic Magma or Rhyolitic Magma, This type of magma is high in silica content (65-75%) and potassium and sodium, but low in iron, magnesium, and calcium. It is also the most viscous type of magma and can produce very explosive eruptions. Rhyolitic magma is common in continental volcanic provinces, such as the Yellowstone Caldera in the United States.

Silica content: 65-75%
Color: Light gray to white
Temperature: 650-800°C (1202-1472°F)
Viscosity: High- 10⁵ - 10⁹ PaS
Gas Content: High
Minerals: Quartz, plagioclase feldspar, potassium feldspar

Magmas can also be classified based on their origin. Primary magmas are formed from the partial melting of the Earth's mantle. Secondary magmas are formed from the melting of existing rocks, such as those in the crust or upper mantle.

The type of magma that erupts from a volcano has a significant impact on the type of eruption that occurs. Basaltic eruptions are typically effusive, meaning that they produce lava flows that spread out over the landscape. Andesitic and rhyolitic eruptions can be more explosive, producing ash falls and pyroclastic flows.

Characteristics of Magma

Gases in Magmas

At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the gas forms a separate vapor phase when pressure is decreased as magma rises toward the surface. This is similar to carbonated beverages which are bottled at high pressure. The high pressure keeps the gas in solution in the liquid, but when pressure is decreased, like when you open the can or bottle, the gas comes out of solution and forms a separate gas phase that you see as bubbles. Gas gives magmas their explosive character, because volume of gas expands as pressure is reduced. The composition of the gases in magma are:

Mostly H₂O (water vapor) with some CO₂ (carbon dioxide)
Minor amounts of Sulfur, Chlorine, and Fluorine gases

The amount of gas in a magma is also related to the chemical composition of the magma. Rhyolitic magmas usually have higher dissolved gas contents than basaltic magmas.

Temperature of Magmas

Temperature of magmas is difficult to measure (due to the danger involved), but laboratory measurement and limited field observation indicate that the eruption temperature of various magmas is as follows:

Mafic or Basaltic magma - 1000 to 1200^oC
Intermediate or Andesitic magma - 800 to 1000^oC
Felsic or Rhyolitic magma - 650 to 800^oC.

Viscosity of Magmas

Viscosity is the resistance to flow (opposite of fluidity). Viscosity depends on primarily on the composition of the magma, and temperature.

Higher SiO₂ (silica) content magmas have higher viscosity than lower SiO₂ content magmas (viscosity increases with increasing SiO₂ concentration in the magma).
Lower temperature magmas have higher viscosity than higher temperature magmas (viscosity decreases with increasing temperature of the magma).

Thus, mafic magmas tend to be fairly fluid (low viscosity), but their viscosity is still 10,000 to 100,0000 times more viscous than water. felsic magmas tend to have even higher viscosity, ranging between 1 million and 100 million times more viscous than water. (Note that solids, even though they appear solid have a viscosity, but it is very high, measured as trillions time the viscosity of water). Viscosity is an important property in determining the eruptive behavior of magmas.

Non-silicate Magmas

Non-silicate magmas are magmas that do not contain silicon (Si) or oxygen (O) as a tetrahedral structure. They are much less common than silicate magmas, which make up the vast majority of magmas on Earth.

Non-silicate magmas can be classified into several different groups, including:

Carbonatite

Carbonatite magmas are rich in carbonate minerals, such as calcite and dolomite. They are thought to form from the partial melting of carbonate-rich rocks in the mantle or crust. Carbonatite magmas are relatively rare, but they are important sources of certain elements, such as niobium and rare earth elements.

Sulfide Magmas

Sulfide magmas are rich in sulfide minerals, such as pyrite and chalcopyrite. They are thought to form from the partial melting of sulfide-rich rocks in the mantle or crust. Sulfide magmas are also relatively rare, but they are important sources of certain metals, such as copper and nickel.

Halide Magmas

Halide magmas are rich in halide minerals, such as halite and sylvite. They are thought to form from the partial melting of salt deposits in the crust. Halide magmas are extremely rare, but they have been found in a few places on Earth, such as in Ethiopia and Eritrea.

Non-silicate magmas can produce a variety of unusual rocks, such as carbonatites, iron-sulfide rocks, and kimberlites (which are the source of diamonds). Non-silicate magmas can also play an important role in the formation of ore deposits.