Lava

(An explosive eruption, by contrast, produces a mixture of volcanic ash and other fragments called tephra, not lava flows.)

[2][3] An early use of the word in connection with extrusion of magma from below the surface is found in a short account of the 1737 eruption of Vesuvius, written by Francesco Serao, who described "a flow of fiery lava" as an analogy to the flow of water and mud down the flanks of the volcano (a lahar) after heavy rain.

[4][5] Solidified lava on the Earth's crust is predominantly silicate minerals: mostly feldspars, feldspathoids, olivine, pyroxenes, amphiboles, micas and quartz.

[10] Other cations, such as ferrous iron, calcium, and magnesium, bond much more weakly to oxygen and reduce the tendency to polymerize.

[16] 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.

Because of their lower silica content and higher eruptive temperatures, they tend to be much less viscous, with a typical viscosity of 3.5 × 106 cP (3,500 Pa⋅s) at 1,200 °C (2,190 °F).

[20] Higher iron and magnesium tends to manifest as a darker groundmass, including amphibole or pyroxene phenocrysts.

[24] Ultramafic lavas, such as komatiite and highly magnesian magmas that form boninite, take the composition and temperatures of eruptions to the extreme.

[26] 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.

These include: The term "lava" can also be used to refer to molten "ice mixtures" in eruptions on the icy satellites of the Solar System's giant planets.

[13] Lava flows quickly develop an insulating crust of solid rock as a result of radiative loss of heat.

For instance, geologists of the United States Geological Survey regularly drilled into the Kilauea Iki lava lake, formed in an eruption in 1959.

Likewise, regular vertical patterns on the sides of columns, produced by cooling with periodic fracturing, are described as chisel marks.

[48] As lava cools, crystallizing inwards from its edges, it expels gases to form vesicles at the lower and upper boundaries.

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.

ʻAʻā (also spelled aa, aʻa, ʻaʻa, and a-aa, and pronounced [ʔəˈʔaː] or /ˈɑː(ʔ)ɑː/) is one of three basic types of flow lava.

The word is Hawaiian meaning "stony rough lava", but also to "burn" or "blaze";[52] it was introduced as a technical term in geology by Clarence Dutton.

[53][54] The loose, broken, and sharp, spiny surface of an ʻaʻā flow makes hiking difficult and slow.

[55] The sharp, angled texture makes ʻaʻā a strong radar reflector, and can easily be seen from an orbiting satellite (bright on Magellan pictures).

[53][54] A pāhoehoe flow typically advances as a series of small lobes and toes that continually break out from a cooled crust.

The surface texture of pāhoehoe flows varies widely, displaying all kinds of bizarre shapes often referred to as lava sculpture.

[63] The rounded texture makes pāhoehoe a poor radar reflector, and is difficult to see from an orbiting satellite (dark on Magellan picture).

They behave in a similar manner to ʻaʻā flows but their more viscous nature causes the surface to be covered in smooth-sided angular fragments (blocks) of solidified lava instead of clinkers.

[64] Because it is formed from viscous molten rock, lava flows and eruptions create distinctive formations, landforms and topographical features from the macroscopic to the microscopic.

The top and side margins of an inflating lava dome tend to be covered in fragments of rock, breccia and ash.

Beneath this crust, which being made of rock is an excellent insulator, the lava can continue to flow as a liquid.

The lava cools and breaks up as it encounters the water, with the resulting fragments filling in the seabed topography such that the sub-aerial flow can move further offshore.

On the night of 10 January 1977, a crater wall was breached and a fluid lava lake drained out in under an hour.

Diverting a lava flow is extremely difficult, but it can be accomplished in some circumstances, as was once partially achieved in Vestmannaeyjar, Iceland.

[85] The optimal design of simple, low-cost barriers that divert lava flows is an area of ongoing research.