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:
- The asthenosphere
- The 400 km transition of olivine (the major component of the mantle rock peridotite) to wadsleyite.
- The 670 km transition of wadsleyite (the major component of the mantle rock peridotite) to ringwoodite.
- The core-mantle boundary
For now we focus on Earth's upper 200 km - down to the zone of partial melting in the asthenosphere. The following graphs track:
- The terrestrial geotherm only to the depths of the asthenosphere (right).
- The melting curve for peridotite (below). The melting curve shows the boundary of temperature and pressure beyond which peridotite melts.
How do magmas form? Three factors influence melting point:
- Temperature. All other things being equal, every mineral has a distinct melting point. In the mantle, heat is brought upward by convection. As hot rocks convect upward they transfer heat to cooler rocks lying above them, which may melt.
- Pressure: All other things being equal, the greater the pressure, the less likely materials are to melt. (This explains why the asthenosphere is limited to a shallow region of the mantle and the inner core is solid despite being hotter than the liquid outer core.) When rocks experience decompression without losing their heat, they can experience decompression melting. Consider the fate of hot rocks rising through the mantle from a hot spot.
- Volatile substances: Generally, the addition of substances like water or CO2 to a mineral lowers its melting point. When this happens to peridotite, the shape of its melting curve actually changes.
Where does magma form on Earth?
- Mid-ocean ridges: Rising rocks in mantle convection cell bring heat near the surface, transfering heat to overlying rocks. At the same time, the hot rising mantle rocks experience decompression melting. The motion of lithospheric plates away from the mid-oceanic ridge further diminishes pressure yielding more melting.
- Mantle plumes: Those enigmatic localized upwellings of hot mantle rock from hot spots very deep in the mantle, expressed on the surface as hot spots. As in mid-ocean ridges, hot spot rocks transfer heat to overlying rocks and experience decompression as they come up.
- Subduction zones: As oceanic crust sits at bottom of ocean, it becomes charged with sea water. Subducting slabs, although relatively cold, dive into hot surrounding rock. The slab acts as conveyors drawing water into the hotter, drier asthenosphere. When the water percolates into the surrounding hot rocks, melting due to the infusion of volatiles occurs. This leads to some interesting consequences:
- Subduction zone magmas tend to be low temperature magmas compared to those from the other regions. Because they are, they are compositionally enriched in SiO4. These magmas are the ultimate origin of continental crust.
- We said earlier that Venus apparantly has no subduction zones. Even if it did, this kind of melting would not occur there because the subducting slab would be carrying no water downward. If we ever discover continental crust on Venus it will strongly suggest that subduction zones and oceans were previously present.
Peridotite from University of Pittsburgh
- Composition and melting point: Generally speaking, minerals rich in Fe and Mg have much higher melting points (~1470 K) than those that are rich in silica (SiO2) (~1070 K). Thus when they are in a liquid state, they are hotter. These compositions are end members of a spectrum of compositions. Basic terminology:
- Mafic: Rich in iron and magnesium, poor in silica (< 50%) (E.g. gabbro, right.)
Granite from Wikipedias
- Felsic: Rich in silica (> 65%), poor in iron and magnesium. (E.g. granite, right.)
Gabbro from Eastern Illinois University
Note: On Earth, oceanic crust is always mafic, continental crust is mostly felsic.
- Mafic: Rich in iron and magnesium, poor in silica (< 50%) (E.g. gabbro, right.)
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:
- Composition: Magmas with different chemistries will solidify into different minerals at different temperatures.
- Emplacement process: Magma chambers may solidify slowly underground or rapidly upon reaching the surface. Resulting igneous rocks will be radically different in texture, even if their composition is similar.
Common minerals in igneous rocks:
Olivine from Wikipedia
Pyroxene from Wikipedia
Amphibole from MoneralTown.com
Plagioclase from Wikipedia
Muscovite from Wikipedia
Biotite from Geology.com
Quartz from Wikipedia
Orthoclase from Collector's Corner
Process differences in igneous rocks:
- Intrusive rocks: An igneous rock that is formed by the cooling of magma that has forced its way into surrounding rock but not reached the surface. Typically displays:
- Large interlocking crystals
- Crystals usually have no preferred orientation - i.e. a piece of intrusive rock looks the same no matter which way you turn it.
Basalt - Santiago - Galapagos Islands
- Extrusive rocks: An igneous rock that is formed by the cooling of magma that has erupted onto the surface. Crystals are typically small.
Diorite - Saguaro National Park, AZ
Chemical and Mineral composition: The actual names of igneous rock types reflect both:
- Texture and crystal size
- Granite (Intrusive). Common component of continental crust.
Rhyolite from Rocks Rocks!
- Rhyolite (Extrusive). Tends to erupt on continents, often above subducting plates.
- Diorite (Intrusive). Common component of continental crust.
- Andesite (Extrusive). Tends to erupt on continents, often above subducting plates.
Diorite from Geology.com
- Gabbro (Intrusive). Major component of oceanic crust.
Basalt from Georgia SouthWestern State University
- Basalt (Extrusive). The principle surface rock of oceanic crust and probably the most common rock on the surface of the terrestrial worlds. The basalt of the oceanic crust formed at mid-oceanic ridges, so it is known as "MORB" - Mid-Ocean Ridge Basalt.
Gabbro from Georgia SouthWestern State University
- Peridotite (Intrusive). The dominant rock of the mantle.
Komatiite from Wikipedia
- Komatiite (Extrusive). No extrusive ultramafics exist in the recent world. Komatiite, an ultramafic extrusive rock, was erupted commonly during roughly the first half of Earth's history, in the Archean and early Proterozoic eons. In the last 600 my, only two occurrences of komatiite are known, the youngest of which is 90 ma from the island of Gorgona in Colombia.
Peridotite from Belmont Secondary
Key concepts and vocabulary:
- Igneous rocks
- The geotherm
- Peridotite melting curve
- Factors effecting melting:
- Volatile content
- Magma forming regions:
- Sea-floor spreading centers
- Hot spots
- Subduction zones
- Felsic (SiO2 poor)
- Mafic (SiO2 rich)
- Fractional melting
- Fractional crystallization
- Intrusive rocks
- Extrusive rocks
- Granite (intrusive)
- Rhyolite (extrusive)
- Diorite (intrusive)
- Andesite (extrusive)
- Gabbro (intrusive)
- Basalt (extrusive)
- Peridotite (intrusive)
- Komatiite (extrusive)