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Lava: the story on HearLore | HearLore
Lava
The word lava comes from the Italian word for fire, but its true nature was only understood after centuries of watching it destroy villages and reshape continents. In 1737, Francesco Serao wrote the first account describing a flow of fiery lava as an analogy to water and mud, yet the substance was far more complex than a simple liquid. It is molten rock expelled from the interior of a terrestrial planet, erupting at temperatures ranging from 700 to 1,200 degrees Celsius, depending on its chemical makeup. This molten material can flow great distances before cooling causes it to solidify, because lava exposed to air quickly develops a solid crust that insulates the remaining liquid lava, helping it stay hot and inviscid enough to continue moving. The viscosity of most molten lava is about that of ketchup, roughly 10,000 to 100,000 times that of water, yet it can still travel for miles, carving paths through forests and towns. The solid volcanic rock resulting from subsequent cooling of the molten material is often also called lava, creating a permanent record of the eruption in the Earth's crust.
Chemical Recipes
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. The silica component dominates the physical behavior of silicate magmas, as 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. This polymerization makes the lava viscous, so lava high in silica is much more viscous than lava low in silica. 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 present in the lava. Silicate lavas are divided into four chemical types based on silica content: felsic, intermediate, mafic, and ultramafic. Felsic lavas have a silica content greater than 63 percent and are extremely viscous, ranging from 10 to the power of 8 centipoise for hot rhyolite lava to 10 to the power of 11 centipoise for cool rhyolite lava. Intermediate lavas contain 52 to 63 percent silica and are lower in aluminium and usually somewhat richer in magnesium and iron than felsic lavas. Mafic lavas have a silica content limited to a range of 45 to 52 percent and generally erupt at temperatures of 1,000 to 1,200 degrees Celsius. Ultramafic lavas, such as komatiite, take the composition and temperatures of eruptions to the extreme, with silica content under 45 percent and eruption temperatures thought to have been as high as 1,600 degrees Celsius.
The word lava comes from the Italian word for fire. Its true nature was only understood after centuries of watching it destroy villages and reshape continents.
What temperatures do silicate lavas erupt at?
Mafic lavas generally erupt at temperatures of 1,000 to 1,200 degrees Celsius. Ultramafic lavas such as komatiite have eruption temperatures thought to have been as high as 1,600 degrees Celsius.
Which volcano produces carbonatite lavas?
Ol Doinyo Lengai volcano in Tanzania is the sole example of an active carbonatite volcano. These lavas are extremely fluid with measured temperatures of 500 to 600 degrees Celsius.
When did the Nyiragongo lava flow disaster occur?
The Nyiragongo disaster occurred on the night of the 10th of January 1977 in Zaire, now the Democratic Republic of the Congo. A crater wall was breached and a fluid lava lake drained out in under an hour.
How do lava flows form columnar jointing?
As lava cools, it fractures into five- or six-sided columns in the lower part of the flow called the colonnade. The uppermost parts of the flow show irregular downward-splaying fractures known as the entablature.
Some lavas of unusual composition have erupted onto the surface of the Earth, including carbonatite and natrocarbonatite lavas known from Ol Doinyo Lengai volcano in Tanzania, which is the sole example of an active carbonatite volcano. These lavas are extremely fluid, with viscosities only slightly greater than water, and are very cool, with measured temperatures of 500 to 600 degrees Celsius. Iron oxide lavas are thought to be the source of the iron ore at Kiruna, Sweden, which formed during the Proterozoic, and occur at the El Laco volcanic complex on the Chile-Argentina border. Sulfur lava flows up to 100 meters long and 10 meters wide occur at Lastarria volcano in Chile, formed by the melting of sulfur deposits at temperatures as low as 110 degrees Celsius. 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. These unusual lavas challenge the traditional understanding of volcanic activity, showing that the Earth's interior can produce a wide variety of molten substances beyond the common silicate rocks. The carbonatite 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. These 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.
Flowing Textures
Aa is basaltic lava characterized by a rough or rubbly surface composed of broken lava blocks called clinker, and the word is Hawaiian meaning stony rough lava, but also to burn or blaze. The loose, broken, and sharp, spiny surface of an aa flow makes hiking difficult and slow, yet the clinkery surface actually covers a massive dense core, which is the most active part of the flow. Pahoehoe is basaltic lava that has a smooth, billowy, undulating, or ropy surface, and the Hawaiian word means smooth, unbroken lava. A pahoehoe flow typically advances as a series of small lobes and toes that continually break out from a cooled crust, and it also forms lava tubes where the minimal heat loss maintains a low viscosity. Block lava flows are typical of andesitic lavas from stratovolcanoes, and they behave in a similar manner to aa flows but their more viscous nature causes the surface to be covered in smooth-sided angular fragments of solidified lava instead of clinkers. Pillow lava is the lava structure typically formed when lava emerges from an underwater volcanic vent or subglacial volcano or a lava flow enters the ocean, and the viscous lava gains a solid crust on contact with the water, which cracks and oozes additional large blobs or pillows as more lava emerges from the advancing flow. The rounded texture of pahoehoe makes it a poor radar reflector, and is difficult to see from an orbiting satellite, while the sharp, angled texture of aa makes it a strong radar reflector, and can easily be seen from an orbiting satellite.
Landforms and Lakes
Volcanoes are the primary landforms built by repeated eruptions of lava and ash over time, ranging in shape from shield volcanoes with broad, shallow slopes formed from predominantly effusive eruptions of relatively fluid basaltic lava flows to steeply-sided stratovolcanoes made of alternating layers of ash and more viscous lava flows. A caldera, which is a large subsidence crater, can form in a stratovolcano if the magma chamber is partially or wholly emptied by large explosive eruptions, and the summit cone no longer supports itself and thus collapses in on itself afterwards. Lava domes are formed by the extrusion of viscous felsic magma, and they can form prominent rounded protuberances, such as at Valles Caldera. Lava tubes are formed when a flow of relatively fluid lava cools on the upper surface sufficiently to form a crust, and beneath this crust, which being made of rock is an excellent insulator, the lava can continue to flow as a liquid. Lava lakes are rare, and there are only a few sites in the world where permanent lakes of lava exist, including Mount Erebus in Antarctica, Erta Ale in Ethiopia, Nyiragongo in the Democratic Republic of Congo, and Ambrym in Vanuatu. Lava deltas form wherever sub-aerial flows of lava enter standing bodies of water, and 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. Kipukas are elevated areas such as hills, ridges or old lava domes inside or downslope from an area of active volcanism, and new lava flows will cover the surrounding land, isolating the kipuka so that it appears as a forested island in a barren lava flow.
Destructive Power
Lava flows are enormously destructive to property in their path, and casualties are rare since flows are usually slow enough for people and animals to escape, though this is dependent on the viscosity of the lava. Nevertheless, injuries and deaths have occurred, either because they had their escape route cut off, because they got too close to the flow, or more rarely, if the lava flow front travels too quickly. This notably happened during the eruption of Nyiragongo in Zaire, now the Democratic Republic of the Congo, on the night of the 10th of January 1977, when a crater wall was breached and a fluid lava lake drained out in under an hour. The resulting flow sped down the steep slopes at up to 60 kilometers per hour, and overwhelmed several villages while residents were asleep. As a result of this disaster, the mountain was designated a Decade Volcano in 1991. Towns destroyed by lava flows include the Nisga'a villages of Lax Ksiluux and Wii Lax K'abit in northwestern British Columbia, Canada, which were destroyed by thick lava flows during the eruption of Tseax Cone in the 1700s, and Garachico on the island of Tenerife, which was destroyed by the eruption of Trevejo in 1706. Kalapana, Hawaii, was destroyed by the eruption of the Kilauea volcano in 1990, and Kapoho, Hawaii, was largely inundated by lava in June 2018, with its subdivision Vacationland Hawaii being completely destroyed. Areas of recent lava flows continue to represent a hazard long after the lava has cooled, and where young flows have created new lands, land is more unstable and can break off into the sea.
Cooling Crystals
A cooling lava flow shrinks, and this fractures the flow, with basalt flows showing 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. As lava cools, crystallizing inwards from its edges, it expels gases to form vesicles at the lower and upper boundaries, and 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. Lava flows quickly develop an insulating crust of solid rock as a result of radiative loss of heat, and 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, and after three years, the solid surface crust, whose base was at a temperature of 1,000 degrees Celsius, was still only 10 meters thick, even though the lake was about 100 meters deep.