Kilauea: Dynamics of eruptions; Magma types
- Kilauea erupted on 4 May 2018, from Haumaumau crater on its summit.
- The rift eruption was driven by collapse of the central (shallow) magma chambers
- The 2018 rift eruption had at least three different magmas:
- a highly evolved cool (1110°) viscous lava presumably from sources in the rift system [May 3-9]
- a less evolved hot (1130°) more fluid lava [May 17-18…]
- a very hot (1145°) magma lacking the cargo of low temperature crystals of the previous lavas, but with olvine with high levels of MgO indicating magma > 1250° somewhere in the feeder system
“The first two were the chemically evolved basalt of the initial fissures and the highly viscous andesite. Both are volumetrically minor sources that represent distinct pockets of old residual magma from Kīlauea’s east rift zone that evolved for more than 55 years, cooling and crystallizing at depth. The third and volumetrically more substantial source was less-evolved and hotter basalt of fissure 8. This source was similar in composition to the magma erupted at Kīlauea in the years before 2018 and was ultimately derived from the summit region. Draining and collapse of the summit by this voluminous eruption may have stirred up deeper, hotter parts of the summit magma system and sent mixed magma down the rift..”
Things I’ve learned re eruption dynamics and magmas
- Not all lava from Hawaiian volcanoes is basaltic
- Even that that is basaltic, changes in composition; each eruption features at least one, and often several, unique lava compositions.
- Magma chambers are not homogeneous; this is presumably even more true of rift systems, where greater cooling can generate mushes of crystals
- The 2018 Kilauea rift eruptions were driven by collapse of summit magma chambers.
- The 2018 Kilauea rift eruption exhibited periodicity of 2-3 days (surges that began within minutes of caldera collapses 40 K upslope) and 5-10 minutes (pulses driven by local outgassing changes )
- The dynamics of an eruption can be mapped into several stages
- Lateral injection of magma into a rift zone (which forms, in Hawaii, due to volcano flanks sliding into ocean) leads to initial eruption
- Pressure in the rift system leads to its elaboration – advancing dikes may capture pockets of highly evolved magma with mushes of low temperature crystals.
- Magma injection into rifts, if large enough, can trigger slip on caldera ring faults
- Ring fault slippage can add pressure to rift system and drive eruptive behavior at the rift
- The central magma chamber appears to be vertically zoned. Initial eruptions of the rift zone (after flushing out pockets of magma that have evolved in the rifts) are composed of younger magmas from lower in the chamber; summit eruptions are fed by older, more evolved magma, higher up in the chamber.
- The 2018 Kilauea eruption produced lava at volumes of 100 meters3/sec
- Stages of Hawaiian volcanoes: pre-shield (alkalic basalt & basanite); shield (thoelitic basalt derived from both shallow plumbing system and deep plumbing system adjacent to mantle); post-shield (alkalic basalt from deep plumbing system adjacent to mantle (shallow plumbing has crystalized)); post erosional/rejuvenated (alkalic basalt, basanite & nephelinite from ???)
Order and nature of basaltic mineral & crystals
Common minerals that crystallize from basaltic magma, ordered by the temperatures at which they typically form:
- *Olivine (Ca2(Mg,Fe)4O4): This is one of the first minerals to crystallize at the highest temperatures, typically around 1,200°C to 1,300°C. Olivine is rich in magnesium and iron and is often found in the earliest stages of crystallization in basaltic magmas.
*Olivine crystals are olive-green to yellow-green color. It often has a glassy or vitreous luster, and the crystals can be angular or rounded, with a granular texture when present in volcanic rocks. When olivine crystals are large enough, they often appear as transparent or translucent, sometimes with visible crystal faces, which are usually in a near-rectangular shape.
When olivine is exposed to oxidation, especially under conditions of high temperatures or in the presence of oxygen, it can alter to a yellowish or brownish hue, sometimes developing a reddish or rusty tint due to the formation of iron oxide minerals. - *Pyroxene (e.g., augite, diopside): Pyroxenes crystallize at slightly lower temperatures, generally around 1,100°C to 1,200°C. These minerals are composed of chains of tetrahedra and are rich in iron and magnesium.
*Augite crystals are dark green to black, often with a shiny, almost metallic luster. It crystallizes in short prismatic crystals, which are often rectangular or blocky in shape. Augite crystals are typically larger than many other basaltic minerals and can be quite visible in coarse-grained basalts.
Augite, being rich in iron, may undergo partial oxidation upon exposure to the atmosphere. The oxidation often causes a darkening of the color to a more brownish or reddish tint, though it rarely forms the rusty, reddish color seen in olivine. Augite may also exhibit a duller or more matte luster when oxidized.
*Diopside is another pyroxene mineral, typically appearing as light green to pale green, although it can also be colorless or pale yellow. It forms prismatic crystals that are often transparent or translucent. Diopside crystals have a glassy or vitreous luster and typically display distinct striations or fine parallel lines on their crystal faces. - *Plagioclase feldspar (labradorite, anorthite): Plagioclase forms between 1,000°C and 1,100°C in basaltic magmas. This mineral can range from calcium-rich (anorthite) to sodium-rich (albite) compositions, with the more calcium-rich varieties crystallizing at higher temperatures.
* Plagioclase crystals vary from white to gray, and often have a glassy luster. They are typically tabular or blocky in shape and can show distinctive twin planes (known as albite twinning). - Magnetite (Fe3O4): Magnetite crystallizes at around 1,000°C to 1,100°C and often forms alongside other iron-rich minerals. It is a common accessory mineral in basaltic magmas.
* Crystals not typically visible in lavas - Ilmenite (FeTiO3): Ilmenite forms at slightly lower temperatures, typically around 900°C to 1,000°C. It is a titanium-iron oxide mineral and often occurs in basaltic lavas.
* Crystals not typically visible in lavas - Spinel (MgAl2O4): Spinel crystallizes at lower temperatures, usually around 900°C. It is a common accessory mineral in basaltic rocks, often forming in the lower temperature range of basaltic crystallization.
* Crystals not typically visible in lavas
These minerals crystallize according to Bowen’s reaction series, where early-formed minerals (like olivine and pyroxene) are typically more magnesium- and iron-rich, while later-formed minerals (like plagioclase and spinel) are more silica-rich due to depletion of Mg and Fe.
Other Notes
TBD
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