Composition Solidified lava on the
Earth's crust is predominantly
silicate minerals: mostly
feldspars,
feldspathoids,
olivine,
pyroxenes,
amphiboles,
micas and
quartz. Rare nonsilicate lavas can be formed by local melting of nonsilicate mineral deposits or by separation of a magma into
immiscible silicate and nonsilicate liquid
phases.
Silicate Silicate lavas are molten mixtures dominated by
oxygen and
silicon, the
most abundant elements of the Earth's crust, with smaller quantities of
aluminium,
calcium,
magnesium,
iron,
sodium, and
potassium and minor amounts of many other elements.
Petrologists routinely express the composition of a silicate lava in terms of the weight or
molar mass fraction of the oxides of the major elements (other than oxygen) present in the lava. The silica component dominates the physical behavior of silicate magmas. Silicon ions in lava strongly bind to four oxygen ions in a tetrahedral arrangement. If an oxygen ion is bound to two silicon ions in the melt, it is described as a bridging oxygen, and lava with many clumps or chains of silicon ions connected by bridging oxygen ions is described as partially polymerized. Aluminium in combination with alkali metal oxides (sodium and potassium) also tends to polymerize the lava. Other
cations, such as ferrous iron, calcium, and magnesium, bond much more weakly to oxygen and reduce the tendency to polymerize. Partial polymerization makes the lava viscous, so lava high in silica is much more viscous than lava low in silica. Because of the role of silica in determining viscosity and because many other properties of a lava (such as its temperature) are observed to correlate with silica content, silicate lavas are divided into four chemical types based on silica content:
felsic,
intermediate,
mafic, and
ultramafic.
Felsic Felsic or
silicic lavas have a silica content greater than 63%. They include
rhyolite and
dacite lavas. With such a high silica content, these lavas are extremely viscous, ranging from 108
cP (105 Pa⋅s) for hot rhyolite lava at to 1011 cP (108 Pa⋅s) for cool rhyolite lava at . For comparison, water has a viscosity of about 1 cP (0.001 Pa⋅s). Because of this very high viscosity, felsic lavas usually erupt explosively to produce
pyroclastic (fragmental) deposits. However, rhyolite lavas occasionally erupt effusively to form
lava spines,
lava domes or "coulees" (which are thick, short lava flows). The lavas typically fragment as they extrude, producing block lava flows. These often contain
obsidian. Felsic magmas can erupt at temperatures as low as . Unusually hot (>950 °C; >1,740 °F) rhyolite lavas, however, may flow for distances of many tens of kilometres, such as in the
Snake River Plain of the northwestern United States.
Intermediate Intermediate or
andesitic lavas contain 52% to 63% silica, and are lower in aluminium and usually somewhat richer in
magnesium and
iron than felsic lavas. Intermediate lavas form andesite domes and block lavas and may occur on steep
composite volcanoes, such as in the
Andes. They are also commonly hotter than felsic lavas, in the range of . Because of their lower silica content and higher eruptive temperatures, they tend to be much less viscous, with a typical viscosity of 3.5 million cP (3,500 Pa⋅s) at . This is slightly greater than the viscosity of smooth
peanut butter. Intermediate lavas show a greater tendency to form
phenocrysts. Higher iron and magnesium tends to manifest as a darker
groundmass, including amphibole or pyroxene phenocrysts.
Mafic Mafic or
basaltic lavas are typified by relatively high magnesium oxide and iron oxide content (whose molecular formulas provide the consonants in mafic) and have a silica content limited to a range of 52% to 45%. They generally erupt at temperatures of and at relatively low viscosities, around 104 to 105 cP (10 to 100 Pa⋅s). This is similar to the viscosity of
ketchup, although it is still many orders of magnitude higher than that of water. Mafic lavas tend to produce low-profile
shield volcanoes or
flood basalts, because the less viscous lava can flow for long distances from the vent. The thickness of a solidified basaltic lava flow, particularly on a low slope, may be much greater than the thickness of the moving molten lava flow at any one time, because basaltic lavas may "inflate" by a continued supply of lava and its pressure on a solidified crust. Most basaltic lavas are of aā or pāhoehoe types, rather than block lavas. Underwater, they can form
pillow lavas, which are rather similar to entrail-type pahoehoe lavas on land.
Ultramafic Ultramafic lavas, such as
komatiite and highly magnesian magmas that form
boninite, take the composition and temperatures of eruptions to the extreme. All have a silica content under 45%. Komatiites contain over 18% magnesium oxide and are thought to have erupted at temperatures of . At this temperature there is practically no polymerization of the mineral compounds, creating a highly mobile liquid. Viscosities of komatiite magmas are thought to have been as low as 100 to 1000 cP (0.1 to 1 Pa⋅s), similar to that of light motor oil. Most ultramafic lavas are no younger than the
Proterozoic, with a few ultramafic magmas known from the
Phanerozoic in Central America that are attributed to a hot
mantle plume. No modern komatiite lavas are known, as the
Earth's mantle has cooled too much to produce highly magnesian magmas.
Alkaline Some silicate lavas have an elevated content of
alkali metal oxides (sodium and potassium), particularly in regions of
continental rifting, areas overlying deeply
subducted plates, or at intraplate
hotspots. Their silica content can range from ultramafic (
nephelinites,
basanites and
tephrites) to felsic (
trachytes). They are more likely to be generated at greater depths in the mantle than subalkaline magmas. Olivine
nephelinite lavas are both ultramafic and highly alkaline, and are thought to have come from much deeper in the
mantle of the
Earth than other lavas. }} }}
Non-silicate Some lavas of unusual composition have erupted onto the surface of the Earth. These include: •
Carbonatite and
natrocarbonatite lavas are known from
Ol Doinyo Lengai volcano in
Tanzania, which is the sole example of an active carbonatite volcano. Carbonatites in the geologic record are typically 75% carbonate minerals, with lesser amounts of silica-undersaturated silicate minerals (such as
micas and olivine),
apatite,
magnetite, and
pyrochlore. This may not reflect the original composition of the lava, which may have included
sodium carbonate that was subsequently removed by hydrothermal activity, though laboratory experiments show that a calcite-rich magma is possible. Carbonatite lavas show
stable isotope ratios indicating they are derived from the highly alkaline silicic lavas with which they are always associated, probably by separation of an immiscible phase. Natrocarbonatite lavas of Ol Doinyo Lengai are composed mostly of sodium carbonate, with about half as much calcium carbonate and half again as much potassium carbonate, and minor amounts of halides, fluorides, and sulphates. The lavas are extremely fluid, with viscosities only slightly greater than water, and are very cool, with measured temperatures of . •
Iron oxide lavas are thought to be the source of the
iron ore at
Kiruna,
Sweden which formed during the
Proterozoic. •
Sulfur lava flows up to long and wide occur at
Lastarria volcano, Chile. They were formed by the melting of sulfur deposits at temperatures as low as .
Rheology on the east rift zone of
Kīlauea Volcano in
Hawaii, United States The lava's viscosity mostly determines the behavior of lava flows. While the temperature of common silicate lava ranges from about for felsic lavas to for mafic lavas, its viscosity ranges over seven orders of magnitude, from 1011 cP (108 Pa⋅s) for felsic lavas to 104 cP (10 Pa⋅s) for mafic lavas. Lava viscosity is mostly determined by composition but also depends on temperature and shear rate. Lava viscosity determines the
kind of volcanic activity that takes place when the lava is erupted. The greater the viscosity, the greater the tendency for eruptions to be explosive rather than effusive. As a result, most lava flows on Earth, Mars, and Venus are composed of basalt lava. On Earth, 90% of lava flows are mafic or ultramafic, with intermediate lava making up 8% of flows and felsic lava making up just 2% of flows. Viscosity also determines the aspect (thickness relative to lateral extent) of flows, the speed with which flows move, and the surface character of the flows. When highly viscous lavas erupt effusively rather than in their more common explosive form, they almost always erupt as high-aspect flows or domes. These flows take the form of block lava rather than aā or pāhoehoe. Obsidian flows are common. Intermediate lavas tend to form steep stratovolcanoes, with alternating beds of lava from effusive eruptions and tephra from explosive eruptions. Mafic lavas form relatively thin flows that can move great distances, forming shield volcanoes with gentle slopes. In addition to melted rock, most lavas contain solid crystals of various minerals, fragments of exotic rocks known as
xenoliths, and fragments of previously solidified lava. The crystal content of most lavas gives them
thixotropic and
shear thinning properties. In other words, most lavas do not behave like Newtonian fluids, in which the rate of flow is proportional to the
shear stress. Instead, a typical lava is a
Bingham fluid, which shows considerable resistance to flow until a stress threshold, called the yield stress, is crossed. This results in
plug flow of partially crystalline lava. A familiar example of plug flow is toothpaste squeezed out of a toothpaste tube. The toothpaste comes out as a semisolid plug, because shear is concentrated in a thin layer in the toothpaste next to the tube and only there does the toothpaste behave as a fluid. Thixotropic behavior also hinders crystals from settling out of the lava. Once the crystal content reaches about 60%, the lava ceases to behave like a fluid and begins to behave like a solid. Such a mixture of crystals with melted rock is sometimes described as
crystal mush. Lava flow speeds vary based primarily on viscosity and slope. In general, lava flows slowly, with typical speeds for Hawaiian basaltic flows of and maximum speeds of on steep slopes. Lava is most fluid when first erupted, becoming much more viscous as its temperature drops. Lava flows quickly develop an insulating crust of solid rock as a result of radiative loss of heat. Thereafter, the lava cools by a very slow conduction of heat through the rocky crust. For instance, geologists of the
United States Geological Survey regularly drilled into the Kilauea Iki lava lake, formed in an eruption in 1959. After three years, the solid surface crust, whose base was at a temperature of , was still only thick, even though the lake was about deep. Residual liquid was still present at depths of around nineteen years after the eruption. A cooling lava flow shrinks, and this fractures the flow. Basalt flows show a characteristic pattern of fractures. The uppermost parts of the flow show irregular downward-splaying fractures, while the lower part of the flow shows a very regular pattern of fractures that break the flow into five- or six-sided columns. The irregular upper part of the solidified flow is called the
entablature, while the lower part that shows
columnar jointing is called the
colonnade. (The terms are borrowed from Greek temple architecture.) Likewise, regular vertical patterns on the sides of columns, produced by cooling with periodic fracturing, are described as
chisel marks. Despite their names, these are natural features produced by cooling, thermal contraction, and fracturing. As lava cools, crystallizing inwards from its edges, it expels gases to form vesicles at the lower and upper boundaries. These are described as
pipe-stem vesicles or
pipe-stem amygdales. Liquids expelled from the cooling crystal mush rise upwards into the still-fluid center of the cooling flow and produce vertical
vesicle cylinders. Where these merge towards the top of the flow, they form sheets of vesicular basalt and are sometimes capped with gas cavities that sometimes fill with secondary minerals. The beautiful
amethyst geodes found in the flood basalts of South America formed in this manner. Flood basalts typically crystallize little before they cease flowing, and, as a result, flow textures are uncommon in less silicic flows. On the other hand,
flow banding is common in felsic flows. ==Morphology==