Magmas are hot liquid solutions of silicate rock forming elements and volatiles.

• The major silicate rock forming elements are O, Si, Al, Fe, Mg, K, Na and Ca. These are the elements that make up the rock forming silicate minerals.
• Volatiles are substances, dissolved in the magma, which don't go into minerals as the magma solidifies. They are released in the form of liquids or gasses. H20 is by far the most abundant volatile in magmas. Others are CO2, SO2 and H2S.

Magmas form in the crust and upper mantle, within 200 km (most much shallower) of the surface. Since most of the Earth at these depths is NOT molten, magma formation implies special circumstances of temperature, pressure or material properties.

Magmas form by the melting of pre-existing (parent) silicate rocks. Three major mechanisms can cause a rock to melt... these are:

1. Heating - despite being an obvious cause, this is not usually the most important cause of magmas.
2. Pressure decrease - this is a very important mechanism at divergent boundaries
3. Addition of water - this is a very important mechanism at convergent boundaries

Usually the parent rock does not completely melt. When only a portion of the parent rock melts we call the process partial melting.

• Partial melting always produces a magma that is less mafic than the parent rock.
• Partial melting is thus a way of differentiating material, making new rocks that differ in composition from the parent rock.

Magmas are less dense than the parent rock and so try to rise. Some reach the surface, others solidify at depth in the crust. Lava is magma that reaches the Earth's surface.

Magmas vary in composition (chemical makeup)

• The composition is usually characterized by the amount of SiO2 in the magma.
• Magmas range from about 45% to 75% SiO2.
• Fe, Mg and Al vary along with SiO2.
• Magmas with about 50% SiO2, high amounts of Fe and Mg, and low Al are referred to as mafic in composition.
• Magmas with about 70% or more SiO2, low amounts of Fe and Mg, and high Al are referred to as felsic in composition.
• Magmas with about 60% SiO2 are referred to as intermediate in composition
• Volatile content varies with composition, felsic magmas hold more volatiles than mafic magmas
• Since mafic minerals melt at higher temperature, mafic magmas are generally hotter than felsic magmas
• Because of the linking of SiO4 tetrahedra, felsic magmas are much more viscous than mafic magmas

Magma Properties

Igneous Rocks

Igneous rocks are classified, and named, based on grain size (texture) and composition. Fine grained rocks are volcanic rocks that cooled quickly, at or near the Earth's surface. Coarse grained igneous rocks are plutonic rocks that cooled slowly deep in the Earth's crust.

• At mid-ocean ridges, upwelling mantle elevates the temperature at all depths and the lithosphere is correspondingly thinner.
• Rising ultramafic (peridotite) mantle crosses the solidus as pressure decreases. This results in decompression partial melting . The result of partial melting is always a liquid component (magma) that is more felsic than the parent rock. The partial melting of peridotite under the ridge produces magmas of basaltic composition.
• The basaltic magma rises and pools in a magma chamber that is located in the lower half of the oceanic crust.
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• Oceanic crust is formed at the mid-ocean ridge crest by extrusive and intrusive solidification of basaltic magmas. These processes result in a uniform, layered ocean crust that is approximately 5 km thick throughout the ocean basins. The layers are:

• As the lithosphere moves away from the ridge, it thickens by a process of underplating in which asthenosphere solidifies onto the underside of the plate.
• The lithosphere reaches a steady-state thickness of about 100 km when it is approximately 70 MY in age.
• The thickening lithosphere subsides as it ages. The shape of both the sea-floor and the lower boundary of the lithosphere result from this subsidence.

• A divergent boundary can form within continental crust, driven by upwelling mantle beneath the continent. In the first stages of continental rifting, the continental crust is upwarped by the rising asthenosphere beneath it.
• The continental crust is stretched and thinned with extensional faulting. A rift valley develops.
• Mafic magmas are produced by decompression melting of the peridotite of the asthenosphere. Many of these mafic magmas erupt as the pass quickly through the thinned continental crust.
• Felsic magmas are produced by heating and hydration of the continental crust. Because the region is usually under tension, these felsic magmas can rise and erupt forming rhyolite.
• This mixture of mafic and felsic (with little intermediate) volcanism is called bimodal volcanism.
• When the continental crust is stretched to its limits, new oceanic crust begins to form and a narrow ocean basin is formed. The resulting boundaries between continental and oceanic crust are not plate boundaries. These are passive continental margins.

• A hot spot is a localized source of basaltic magma that is stationary in the mantle, beneath the lithosphere. The basaltic magma is produced by partial melting of mantle in a concentrated zone of the asthenosphere.
• If an oceanic plate moves across a hot spot, basaltic volcanism occurs. This gives rise to large shield volcanoes. Hawaii is the perfect example of such a setting. Since the volcanoes move away from the hot spot after some period of time, they become extinct.
• If a continental plate moves over a hot spot, basaltic and felsic volcanism occur. The felsic magmas form as the rising basaltic magma melts some of the continental crust. Because the region is usually under tension, these felsic magmas can rise and erupt forming rhyolite. Basaltic magmas erupt as well.
• This mixture of mafic and felsic (with little intermediate) volcanism is called bimodal volcanism.
• This type of bimodal volcanism is observed along the Snake River Plain in Idaho which is thought to be the trace of a hot spot. The hot spot is currently under Yellowstone

• The volcanic activity at the Earth's surface, caused by these magmas is called arc volcanism. The name derives from the many arc shaped island chains around the Pacific basin that result from this form of volcanic activity.
• Arc volcanoes on continental crust erupt predominantly intermediate (andesitic) magmas, but mafic (basaltic) magmas are also erupted. When the overriding plate in a subduction zone is a continental plate, the magmas have to rise through continental crust to reach the surface. In this process, rising mafic magmas often incorporate some continental crust through assimilation. These magmas may also reside for extended periods of time in crustal magma chambers and undergo fractional crystallization. Both of these processes make the magma more felsic. Thus the primary volcanism in this setting involves intermediate magmas. These lavas build stratovolcanoes through alternating eruption of lavas and pyroclastic materials. Stratovolcanoes include Mt. Fuji, Mt. St. Helens and the major peaks of the Andes. Some nearly felsic (granitic) magmas can be formed in this oceanic-continental subduction setting. These magmas do not erupt, but solidify beneath the surface forming batholiths such as those that make up the Sierra Nevada.
• Arc volcanoes on ocean crust erupt mafic and intermediate magmas, producing island chains called island arcs. Since the overriding plate in this subduction zone is an oceanic plate, the magmas initially rise only through the thin oceanic crust to reach the surface, thus there is little assimilation or fractional crystalization. These early magmas are thus primarily basaltic. In larger, older island arcs systems such as Japan, the crust is thicker and more diverse, and intermediate (andesitic) magmas are also erupted.

• Continental plate cannot subduct; it is too buoyant.
• Continental collisions thicken continental crust. Within this thickened crust, wet, felsic and intermediate rocks undergo partial melting due to heating and compression. Compressing wet felsic rock can cause melting due to the slope of the wet felsic solidus.
• The resulting felsic magma cannot move quickly due to its viscosity. As it rises, it recrosses the wet felsic solidus, and solidifies deep in the crust forming granite bodies.
• Later, erosion and uplift often expose the granite batholiths at the surface. Examples include Enchanted Rock, Texas and Stone Mountain, Georgia. Both are the result of ancient continental collisions and mountain building.

Magmas and Plate Tectonics Table