Lava is molten material from the Earth's interior (magma) that has escaped to the surface.
It is not identical to magma, its composition is slightly different. It consists mainly of molten oxides of silicon, sodium, iron, potassium, calcium and other metals, and is free of volatile substances. The solidifying lava forms effusive magma rocks, the most common of which are basalt, trachyte, riolite, andesite, and obsidian or pumice.
The hot, molten mass of silicates and aluminosilicates, mixed with oxides and sulfides, with large amounts of water and gases, formed lava as it rose to the surface.
It is formed in the Earth's upper mantle, a layer of the crust that extends to a depth of about 400 kilometers and is characterized by considerable plasticity.
The upper liquid part of the upper mantle, just below the lithosphere, is called the asthenosphere (a layer about 250 to 300 kilometers thick). It can lie from 10 to 100 kilometers below the Earth's surface.
It has also been found on other rocky planets (Mercury, Venus, Mars) and some natural satellites (moons).
It then forms intrusions, bodies of rock that have penetrated between older rock formations. In the shallower regions of the Earth's crust, magma rises in the form of diapirs (geological structures that take the form of chimneys, mushrooms, and walls) and pushes into the surrounding rocks.
Diapirs that reach close to the Earth's surface give rise to volcanic phenomena. However, most of these rocks, due to slow cooling, solidify 5 to 30 kilometers below the surface, forming rock bodies called plutons.
Beyond Earth, lava can reach even higher temperatures; the record holder is Io, Jupiter's moon, where satellites have recorded lava flows with an average temperature of 1,300 °C (2,370 °F).
Lava is similar in composition to magma, but is depleted of volatile components. It consists mainly of molten oxides of silicon, iron, sodium, potassium, calcium, and other metals.
Lavas are distinguished by their acidity:
These volumes can vary, sometimes being very large, such as during the eruption of the Laki rift volcano in southern Iceland, where the volume was 12 km3. Volcanoes in Iceland, known as the land of fire and ice, can spew lava at a rate of 75 tons per second.
In 1783, the Skaptar volcano exploded in Iceland, destroying crops and food supplies across the island. Iceland was hit by a wave of famine that killed one-fifth of the island's population.
Common forms are aa, pahoehoe and pillow lava.
Aa is basaltic lava and is characterized by a rough, jagged surface. It forms rare but large gas bubbles which, as the lava solidifies during flow, promote its fracturing into large blocks. The temperature of this type of lava is usually between 1000 and 1100 °C (1830 and 2000 °F).
The name aa (ʻaʻā) was originally applied only to Hawaiian lavas. However, it has been adopted to refer to all lavas of this type. In Iceland this type of lava is called apalhraun.
It is also known as "smooth, unbroken lava". It is characterized by low viscosity, a large amount of gas contained in small bubbles, and is usually basaltic in composition. As this type of lava cools, it forms a glassy coating as its exterior cools rapidly. Its interior, however, is still moving, causing the surface of the lava flow to wrinkle, resembling tightly bound strings. These are usually twisted and contorted, never forming a smooth surface.
Tunnels and caves formed within pahoehoe lavas are common.
It cools very quickly and splits into ellipsoidal, usually flattened lobes resembling pillows or loaves.
The individual pillows have a spongy structure in the center and a glassy structure on top, and are characterized by a concentric structure. The average size of a lobe is 30-60 cm, but they can also reach several meters.
It is a volcanic magma rock composed of porous volcanic glaze (more than 50% pores), formed from highly gassy, frothy rhyolitic lava. Pumice is usually of a light, whitish color.
The largest deposits of pumice are found in Armenia and the Aeolian Islands in the Tyrrhenian Sea.
However, in situ magma has been encountered in geothermal drilling on three occasions - twice in Iceland and once in Hawaii.