felsic

Felsic (feldspar-silica) refers to silicate minerals, magmas, and rocks relatively enriched in the lighter elements such as silica, oxygen, aluminium, sodium, and potassium.

The sial is the upper layer of Earth's crust – the continental crust – and is rich in aluminum/silica minerals (granitic layer). Because felsic-sial rocks are lower in density than mafic-sima rocks, the continental crust 'floats' atop the deeper, denser sima crust. The Conrad discontinuity, arbitrarily set at 2800kg/m^3 marks the base of the sial where it grades into the basalts of the sima.

Felsic minerals are usually light in color, with specific gravities less than 3 (2700 - 2800 kg/m^3). Common felsic minerals include quartz, muscovite, hornblende, orthoclase, and the sodium rich plagioclase feldspars. Felsic rocks contain >75% felsic minerals. Granite is the most common felsic rock. Rocks with greater than 90% felsic minerals are also termed leucocratic, meaning 'light-coloured'.

The term acid rock, although sometimes used as a synonym for felsic rocks, refers to volcanic rocks with high silica content (greater than 63% SiO2 by weight) such as rhyolite.

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lava

The silica content of magma, along with amount of dissolved gas, affects the nature of volcanic eruptions as wells as the amount and composition of lava extruded during an eruption.

Volcanic rocks ◙ andesites ◙ basalts ◙ dacites ◙ rhyolites

Lava composition

_ ● felsic > 63% silica = viscous
___ ● dacitic
___ ● rhyolitic
– Rhyolite is named from the Greek for 'steam', and rhyolitic magma often erupts explosively because its high silica content results in extremely high viscosity, which hinders degassing. Effusive eruptions of rhyolite often produce obsidian, which is bubble-free and black. When bubbles form, they can cause the magma to explode, fragmenting the rock into pumice and tiny particles of volcanic ash. Some of the United States' largest and most active calderas formed during eruption of rhyolitic magmas (for example, Yellowstone in Wyoming, Long Valley in California and Valles in New Mexico). Rhyolite is the volcanic equivalent of plutonic granite.
– Even though dacite contains less silica than rhyolite, dacite can be even more viscous, and so just as dangerous as rhyolites. Dacite was erupted from Mount St. Helens 1980-86, Mount Pinatubo in 1991, and Mount Unzen 1991-1996.

_ ● intermediate 52-63% silica
___ ● andesitic
– Andesite magma commonly erupts from stratovolcanoes as thick lava flows, and can also generate strong explosive eruptions to form pyroclastic flows and surges and enormous eruption columns. Andesite, which is common in the Andes mountains, was the main rock type erupted during the great Krakatau eruption of 1883.

_ ● mafic 45 - 52%
___ ● basaltic
– Basalt's low silica content gives it a low viscosity (resistance to flow), so basaltic lava can flow quickly and can easily move >20 km from a vent. The low viscosity of basaltic magma usually allows volcanic gases to escape without generating enormous eruption columns. However, basaltic lava fountains and fissure eruptions still form explosive fountains hundreds of meters tall. Shield volcanoes, such as those in the Hawaiian islands (movies), are composed almost entirely of basalt, which is the commonest rock type in the Earth's crust; huge, ancient outpourings of lava called 'flood basalts' make up large igneous provinces on many continents; and, most of the ocean floor is basalt that has been extruded at spreading mid-ocean ridges. aa, aa active photo, hornito, lava tube, lava lake, pahoehoe, pahoehoe photo, pahoehoe active photo, Pele's hair, Pele's tears, tumulus

_ ● ultramafic ≤ 45%
___ ● komatiites