Lava

Lava is the name given to eruptiertes magma that has escaped to the surface. The name comes from the fact that it flows, as long as its temperature is correspondingly high. Both on the surface and the solidified lava flowing out of the movement lava is called lava.

Lava is a volcanic conveyor product and belongs to the group of volcanic rocks. Other volcanic conveyor products are the pyroclastics and the gaseous and thus volatiles (volatile ) such as carbon dioxide, water, sulfur dioxide, ammonia, noble gases, which has lost the magma by pressure relief. Magmas occur comparatively rarely directly to the surface, there to considerable forces are required. More common are so-called intrusions, where magma solidifies within the crust.

Composition

Lavas are usually silicate melts at a weight fraction of 45-70 % SiO2, however, there is rarely occurring lavas that contain smaller amounts of silicates, such as the carbonatite lavas of Ol Doinyo Lengai. In addition to the silicates of magnesium and iron compounds may be contained. We distinguish between acidic or rhyolitic lava ( SiO2 content > 65%, high viscosity ) and basic or basaltic lava ( SiO2 content < 52%, low viscosity ). In between, the intermediate or andesitic lavas find ( SiO2 content between 52 % and 65 %). During the ascent of the magma various processes take place, the impact on the composition of the emerging lava have ( Magmatic differentiation), so that these can be different from that of the primary magma. As lava cools quickly when exiting to the surface, its texture is usually fine-grained or glassy. By the escape of gases through the pressure relief during ascent smaller or larger gas bubbles can form in the lava. The temperature of lava is at the outlet between 800 ° C ( rhyolitic lava) and 1200 ° C ( basaltic lava). Solidified lava forms volcanic rock.

Manifestations

Depending on the conditions during ascent and cooling lava forms different manifestations that depend critically on the viscosity of the lava. The most common forms are:

Lava flow

Pāhoehoe and AA lava on Hawaii

Pillow lava, Askja, Iceland

Pāhoehoe Lava

Pāhoehoe Lava [ ˌ pə hoʊehoʊe ] is a low-viscosity basaltic lava (= low viscosity), the downslope flows as lava flow. It forms glassy surfaces. Manifestations of pahoehoe lava are ropy, Fladenlava or Schollenlava.

Aa- lava

Aa- lava [ ʔɑʔɑ ː ] - also Brockenlava - is a viscous basaltic lava, which hardens into sharp-edged chunks and lumps. A lava flow may exist in the upper part of pahoehoe lava, while in its lower part aa- lava dominates ( due to the increasing viscosity by cooling and degassing).

Flood basalts

Flood basalts result from extremely low-viscosity basaltic lava that forms powerful low level terrain volcanic ceilings. In individual cases, however, the delivery rates are also sufficient to generate powerful charts that are earlier today also called Trapp, rather than igneous United Province. Examples are the 160,000 km ² Columbia River Plateau (Oregon, Washington and Idaho) in the U.S., the more than 250,000 km ² vast Karoo basalts of South Africa or 500,000 km ² Deccan Plateau in India ( Deccan Traps ).

Pillow or pillow lava

Pillow or of pillow consists of clusters of round or elliptical in cross-section, tubular basalt structures of approximately 1 m diameter or more. It results in the very rapid cooling of lava in the water. By uplift of rock bodies that were originally under the ocean, pillow lavas can also be found on the mainland.

Block lavas

Typical of viscous andesitic lavas and dacitic is the formation of short, stubby lava flows, the surface of which has been dissolved by Autobrekziierung in compact, low-porosity blocks with diverse surface forms.

Bread crust bomb

Bread crust bombs are made of lava that cooled down yet during exit in the flight phase and takes the form of loaves of bread. Bombs with a diameter less than 64 mm as indicated lapilli (Italian stones ).

Lava lakes

A special appearance are lava lakes, as at the Erta Ale or Nyiragongo.

Some lava lakes created by craters are filled by above-ground lava lava, they are then called secondary lava lakes. Such lava lakes can have depths of up to 100 meters. The lava cools slowly in such a lake from ( several hundred days) and thus offers the possibility to investigate the solidification processes of lava.

Lava caves and Lavagräben

Upon cooling of lava flows large voids can arise because the cooled surface already solidified, while inside the lava flow, the liquid lava can continue to flow. The cavities of these lava tubes can also collapse and there are Lavagräben. (see also Cave)

Lava domes

Is the lava when leaving already so tough that they can not drain, then a lava dome. This refers to a short and thick, often pancake -like lava flow. Frequently lava domes fill in finger-like form a crater on throat, which had previously discharged by an explosive eruption.

The formation of Lavadomen takes place over a longer period and is therefore a well-documented by scientists operation. Famous are about the lava domes at Mount St. Helens, where did before the outbreak of a 1980 made ​​and even since then begun the formation of a new lava dome. Other well-known examples are the lava domes of the ounces and the Usu on the island of Hokkaido in Japan, where it has ever followed well the first time, how a volcano swells and deforms before an outbreak. An example of European lava domes, the Puy de Dome in the South of France dar.

Lava dome at Mount St. Helens

Lava cave in West Iceland

Pāhoehoe - lava fountains

Lava pillars

Basalt columns ( much less common, there are also Rhyolithsäulen ) formed during cooling of the lava under certain conditions. Lava contracts and shatters during the cooling process.

When cool quickly v. a pahoehoe lava flows, the lava pillars are not so striking and are irregularly shaped. However, explained by the fact that the lava fields A., are very susceptible to erosion.

Beautifully trained Basaltlavasäulen however, form at slower cooling. The columns are always perpendicular to the cooling surface. Therefore, can be found in lava flows and flat-lying intrusions vertical alignment of the columns, such as in Gerðuberg in Hnappadalur in Iceland, the Giant's Causeway in Bushmills in Ireland or at St. Flour in the Auvergne region in southern France; however lava pillars are aligned horizontally in the steep transitions.

Lava does not cool off evenly, but more quickly to the surface and more slowly in the deep; therefore upper columns are often thinner than lower.

Fan pattern and rosettes are formed, however, in Lavagängen and caves. Like formations are found, for example, in the Barranco de Agaete on Gran Canaria or at Hljóðaklettar in Jökulsárgljúfur National Park in Iceland.

Most of these basalt columns are hexagonal, it can be found but also pentagonal such as the Dvergasteinn at Kirkjubæjarklaustur in South Iceland, and heptagonal.