Volcanism I: Sources and Composition of Magma


Puu O'o eruption, HI from Kilauea Mitigation & Preparedness

Up until now we have treated volcanoes as simple facts of life. Now we explore them in greater depth.

Definitions:

Igneous Rocks: Rocks that form through the solidification of magma.

Volcanism: The eruption of magma onto a planet's surface.

The geotherm - a graph of the relationship of temperature and depth, is a useful means of visualizing the processes the cause rocks to melt. At right, a schematic geotherm tracks the big patterns of Earth's temperature gradient all the way to its center. Temperature does not increase evenly with depth. Rather, there are sharp discontinuities at:


For now we focus on Earth's upper 200 km - down to the zone of partial melting in the asthenosphere. The following graphs track:

How do magmas form? Three factors influence melting point:


In detail:

Where does magma form on Earth?

Composition:


Peridotite from University of Pittsburgh
Here's a paradox: Even though peridotite (right) is typically what melts to make magma, hardly any magma ever solidifies into peridotite. Why not? Five factors drive magmas to differ chemically from their parent rocks and, eventually, from one another:

Fractional melting: Mantle rocks like peridotite consist of several different minerals (mostly olivine and pyroxene), each with its own melting point. As the rock heats, decompresses, or is infused with water, the minerals with the lowest melting point (more felsic) melt first and begin to move away from the source rock, so a magma is always somewhat more felsic than its source. Consider that the magma erupting at mid-ocean ridges has moved maybe a mere ten km from its source, but whereas that source was Fe and Mg rich peridotite, the magma is slightly less.

Compatible and incompatible: Add to this the fact that some elements such as potassium (K) and sodium (Na) are "incompatible" - they prefer to inhabit a liquid phase, and will migrate from a solid crystal into an adjacent pocket of magma. "Compatible" elements like magnesium (Mg) tend to remain in the solid phase.

Fractional crystallization: Reverse process of fractional melting. The most Mg and Fe rich minerals in a melt (i.e. those with the highest melting point) will be the first to crstallize out, leaving an increasingly felsic magma.


Assimilation: The vast majority of intrusives we see on the continents are felsic, like granite. Fractional crystallization can't account for this. Remember, in general, continental crust is much more felsic than oceanic. As ultramafic magmas encounter the felsic rocks of the continental crust, they cause the most felsic minerals in those felsic rocks (the ones with the lowest melting point) to melt. Thus, felsic material is added to the magma as mafic material is lost to fractional crystalization. The result is that magmas that have passed through thick layers of continental crust represent highly refined concentrations of felsic materials.

Size matters: The puzzle remains as to why we might have felsic and mafic magmas erupting adjacent to one another on the continents. A fourth parameter is the size of the magma chamber. Remember that surface area scales as the 2/3 power of volume. Thus, a small magma chamber has a proportionally larger surface across which it can interact with adjacent rock. A large one may work its way to the surface and, despite its having assimilated continental crust, retain something of its original composition while a smaller one will be significantly altered by passage through the same thickness of crust.

Big message:

Remember, magmas start out ultramafic, but all of the above processes drive them toward the felsic end of the compositional continuum. This trend is strongest in subduction zone volcanism. Without this (and the plate tectonics that drive it) there would be no differentiated continental crust!

How does magma behave?

When melting first occurs, it happens at the peripheries of individual crystal grains (b - right), yielding minute pockets of magma. When these pockets grow to the point that they interconnect (a - right) the magma is able to move.

Being liquid, magma tends to be lighter than surrounding material from which it has melted. Thus, it tends to percolate upward by any available means. As this happens, droplets coalesce, eventually forming large magma chambers that can be relatively small (Enchanted Rock, TX) to very large (Sierra Nevada Batholith).

The rocks, themselves:

Rocks formed from the solidification of magma are termed igneous. Igneous rocks differ widely depending on:

Common minerals in igneous rocks:

Process differences in igneous rocks:


Chemical and Mineral composition: The actual names of igneous rock types reflect both:

The chart below gives a taxonomy of the most common igneous rocks. Its x axis shows the percentage of silica in the rock, the y axis shows the relative abundance of different minerals in the rock. Remember, for each composition there are intrusive and extrusive versions.

For example, we see that granite might have 70% silica and be composed of 50% orthoclase, 25% quartz, and 25% plagioclase, muscovite, biotite, and amphibole. Its extrusive equivalent is rhyolite.

  • Felsic: High quantities of orthoclase and quartz, small amounts of plagioclase, muscovite, biotite, and amphibole.

  • Intermediate:

  • Mafic (Mafic = Ma + Fe): High quantities of olivine and pyroxene, smaller amounts of plagioclase.

  • Ultramafic: Very rare on surface, often found as mantle xenoliths. Low silica content, with rocks primarily made up of olivine with some pyroxene.


    Key concepts and vocabulary: