Volcanic Rock Types
B (Basalt)—Use normative mineralogy to subdivide.
O1 (Basaltic andesite)
O2 (Andesite)
O3 (Dacite)
R (Rhyolite)
T (Trachyte or Trachydacite)—Use normative mineralogy to decide.
Ph (Phonolite)
S1 (Trachybasalt)—*Sodic and potassic variants are Hawaiite and Potassic Trachybasalt.
S2 (Basaltic trachyandesite)—*Sodic and potassic variants are Mugearite and Shoshonite.
S3 (Trachyandesite—*Sodic and potassic variants are Benmoreite and Latite.
Pc (Picrobasalt)
U1 (Basanite or Tephrite)—Use normative mineralogy to decide.
U2 (Phonotephrite)
U3 (Tephriphonolite)
F (Foidite)—When possible, classify/name according to the dominant feldspathoid. Melilitites also plot in this area and can be distinguished by additional chemical criteria.
(*)Sodic as used above means that Na2O – 2 is greater than K2O, and potassic that Na2O – 2 is less than K2O. Yet other names have been applied to rocks particularly rich in either sodium or potassium—as are ultrapotassic igneous rocks.
Lava flows from Mauna Loa
Palagonite
About Palagonite
Palagonite is an alteration product formed from basaltic glass (tachylite); concentric bands of it often surround kernels of unaltered tachylite, and are so soft that they are easily cut with a knife. In the palagonite the minerals are also decomposed and are represented only by pseudomorphs.
Palagonite soil is a light yellow-orange dust, comprising a mixture of particles ranging down to sub-micrometer sizes, usually found mixed with larger fragments of lava. The color is indicative of the presence of iron in the +3 oxidation state, embedded in an amorphous matrix.
Palagonite tuff is a tuff composed of sideromelane fragments and coarser pieces of basaltic rock, embedded in a palagonite matrix. A composite of sideromelane aggregate in palagonite matrix is called hyaloclastite.
Formation of Palagonite
Phreatomagmatic. Palagonite can be formed from the interaction between water and basalt melt. The water flashes to steam on contact with the hot lava and the small fragments of lava react with the steam to form the light-colored palagonite tuff cones common in areas of basaltic eruptions in contact with water.
Weathering. Palagonite can also be formed by a slower weathering of lava into palagonite, resulting in a thin, yellow-orange rind on the surface of the rock. The process of conversion of lava to palagonite is called palagonitization.
Tachylite
About Tachylite (tachylyte)
Tachylite (from ταχύς, meaning “swift”) is a form of basaltic volcanic glass formed by the rapid cooling of molten basalt. It is a type of mafic igneous rock that is decomposable by acids and readily fusible. The color is a black or dark-brown, and it has a greasy-looking, resinous luster. It is often vesicular and sometime spherulitic. Small pheoncrysts of feldspar or olivine are sometimes visible. Fresh tachylite glass often contains lozenge-shaped crystals of plagioclase feldspar and small prisms of augite and olivine, but all these minerals occur mainly as microlites or as skeletal growths with sharply-pointed corners or ramifying processes.
All tachylites weather easily and become red to brown as their iron oxidizes.
Formation
Three modes of occurrence characterize this rock. In all cases they are found under conditions which imply rapid cooling, but they are much less common than acid obsidians. (Alkaline rocks have a stronger tendency to crystallize (i.e. not form glass), in part because they are more liquid and the molecules have more freedom to arrange themselves in crystalline order.)
Scoria
The fine scoria (aka cinders) thrown out by basaltic volcanoes are often spongy masses of tachylite with only a few larger crystals or phenocrysts imbedded in black glass. Basic pumices of this kind are exceedingly widespread on the bottom of the sea, either dispersed in the pelagic red clay and other deposits or forming layers coated with oxides of manganese precipitated on them from the sea water. These tachylite fragments, which are usually much decomposed by the oxidation and hydration of their ferrous compounds, have taken on a dark red color (scoria is from σκωρία, skōria, Greek for rust.); this altered basic glass is known as “palagonite.” [see above]
Lava flows
In the Hawaiian Islands volcanoes have poured out vast floods of black basalt, containing feldspar, augite, olivine, and iron ores in a black glassy base. They are highly liquid when discharged, and the rapid cooling that ensues on their emergence to the air prevents crystallization taking place completely. Many of them are spongy or vesicular, and their upper surfaces are often exceedingly rough and jagged, while at other times they assume rounded wave-like forms on solidification. Great caves are found where the crust has solidified and the liquid interior has subsequently flowed away, and stalactites and stalagmites of black tachylite adorn the roofs and floors. On section these growths show usually a central cavity enclosed by walls of dark brown glass in which skeletons and microliths of augite, olivine and feldspar lie embedded
Dikes and Sills
A third mode of occurrence of tachylite is as margins and thin offshoots of dikes or sills of basalt and diabase. They are often only a fraction of an inch in thickness, resembling a thin layer of pitch or tar on the edge of a crystalline diabase dike, but veins several inches thick are sometimes found. In these situations tachylite is rarely vesicular, but often shows pronounced fluxion banding* accentuated by the presence of rows of spherulites that are visible as dark brown rounded spots. The spherulites have a distinct radiate structure and sometimes exhibit zones of varying color. The non-spherulitic glassy portion is sometimes perlitic, and these rocks are always brittle. Common crystals are olivine, augite and feldspar, with swarms of minute dusty black grains of magnetite. At the extreme edges the glass is often perfectly free from crystalline products, but it merges rapidly into the ordinary crystalline diabase, which in a very short distance may contain no vitreous base whatever. The spherulites may form the greater part of the mass, they may be a quarter of an inch in diameter and are occasionally much larger than this.
Fluxion banding
See: https://en.wikipedia.org/wiki/Flow_banding
Flow banding is caused by friction of the viscous magma that is in contact with a solid rock interface, usually the wall rock to an intrusive chamber or the earth’s surface.
The friction and viscosity of the magma causes phenocrysts and xenoliths within the magma or lava to slow down near the interface and become trapped in a viscous layer. This forms laminar flow, which manifests as a banded, streaky appearance.
Flow banding also results from the process of fractional crystallization that occurs by convection if the crystals that are caught in the flow-banded margins are removed from the melt. This can change the composition of the melt in large intrusions, leading to differentiation.
From GPT:
Fluxion banding results from shear forces within a moving magma body. This can happen in several ways:
1. Differential Flow in Lava. As lava moves, its viscosity varies due to cooling and crystallization. The outer layers, which cool faster, may develop a plastic or solid crust, while the inner material remains fluid. This difference in viscosity causes layers of magma to stretch and deform, forming elongated bands.
2. Crystal Sorting and Alignment. “ As magma flows, mineral crystals within it may become aligned due to shear stress. This is common in silicic lavas like rhyolite and dacite, where feldspar and quartz can form parallel bands.
3. Magma Mixing and Compositional Banding. If two magmas of different compositions mix, they may not completely homogenize, leading to streaks of contrasting compositions that appear as bands.
4. Intrusive Settings. In some plutonic rocks, fluxion banding may form as a result of late-stage magmatic flow, where crystals and melts segregate due to convection or deformation.
Kilauea: Dynamics of eruptions; Magma types
- over the last 4 decades Kilauea has been very active, erupting both from Haumaumau crater on its summit and various rifts on its east side.
- in 1983 Kilauea longest and most voluminous outpouring of lava from Kīlauea’s East Rift Zone in over 500 years. It resulted in the creation of the Pu‘u ‘Ō‘ō cone and extensive lava flows that covered significant areas, destroyed numerous structures, and added new land to the island.
- Kilauea erupted on 4 May 2018 — it was an east rift zone eruption following the collapse of the Pu’u O’o vent. 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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