Volcanic Rocks
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 Igneous Rocks   Plutonic Rocks 

Volcanic Rocks - also called extrusive igneous rocks

The four most important types of volcanic rocks are: Basalt, Andesite, Dacite and Rhyolite

Mafic is used for silicate minerals, magmas, and rocks which are relatively high in the heavier elements. The term is derived from using the MA from magnesium and the FIC from the Latin word for iron (ferro), but mafic magmas also are relatively enriched in calcium and sodium. Mafic minerals are usually dark in color and have relatively high specific gravities (greater than 3.0). Mafic magmas are usually produced at spreading centers, and represent material which is newly differentiated from the upper mantle. Common mafic rocks include basalt and gabbro.

Felsic, on the other hand, is used for silicate minerals, magmas, and rocks which have a lower percentage of the heavier elements, and are correspondingly enriched in the lighter elements, such as silica and oxygen, aluminum, and potassium. The term comes from FEL for feldspar (in this case the potassium-rich variety) and SIC, which indicates the higher percentage of silica. Felsic minerals are usually light in color and have specific gravities less than 3.0. The most common felsic rock is granite, which represents the purified end product of the earth's internal differentiation process. Rhyolite has the same chemical composition as granite. Rhyolite is the lava form, while granite is intrusive, which means that it formed beneath the Earth's surface.

Acid versus basic or alkaline

Basalt is a hard, black volcanic rock with less than about 52 weight percent silica (SiO₂). Because of basalt's low silica content, it has a low viscosity (resistance to flow). Therefore, basaltic lava can flow quickly and easily move >20 km from a volcanic vent. The low viscosity typically allows volcanic gases to escape without generating enormous eruption columns. Basaltic lava fountains and fissure eruptions, however, still form explosive fountains hundreds of meters tall. Common minerals in basalt include olivine, pyroxene, and plagioclase. Basalt is erupted at temperatures between 1100 to 1250°C.

Basalts occur in every tectonic environment (read more).

Andesite is a fine grained volcanic rock (or lava) that erupts from volcanoes at destructive plate margins (where one plate of the Earth's surface moves beneath another), including the Andes, from which it gets its name. Its colour is medium dark.

Dacite is a volcanic rock (or lava) that characteristically is light in colour. It may be considered a quartz-bearing variety of andesite (lighter in colour due to the higher content of quarts). Dacite is primarily associated with andesite and trachyte and forms lava flows, dikes, and sometimes massive intrusions in the centres of old volcanoes. The word dacite comes from Dacia, a Roman province found between the Danube River and Carpathian Mountains (now modern Romania) where the rock was first described.

Rhyolite is a light-colored rock with silica (SiO₂) content greater than about 68 weight percent. Sodium and potassium oxides both can reach about 5 weight percent. Common mineral types include quartz, feldspar and biotite and are often found in a glassy matrix. Rhyolite is erupted at temperatures of 700 to 850° C.

The word rhyolite comes from the Greek word for stream (rhyax) + the suffix lite.

Less common than basalts and rhyloites are trachytes and phonolites.

Trachyte

Phonolite

Volcanic rocks are classified as lavas or pyroclastic rocks.

Lava flows

Pyroclastic Rocs

Fragmental material ("pyroclasts") classified on the basis of grain size:

> 32 mm - (volcanic) bombs
32 - 4 mm - lapilli
<4 mm - ash
(< 2 mm - dust)

Tephra is a general term for fragments of volcanic rock and lava regardless of size that are blasted into the air by explosions or carried upward by hot gases in eruption columns or lava fountains. Tephra includes large dense blocks and bombs, and small light rock debris such as scoria, pumice, reticulite, and ash.

When the fragmenst become cemented together (lithified), the rocks formed from the smaller fragments are called volcanic tuffs.
Rhose formed from the larger ones are called volcanic breccias.

Scoria Cones

Ash Fall

Ash Flow

Lahar

Hyalocastites

Basalts

In oceanic tectonic provinces two varieties of basalts are generated

1. Tholeiite. Basalts saturated with silica. It is the dominant basalt of the ocean floors and midocean ridges and ocenic volcanic islands.
2. Alkaline basalts. Basalts undersaturated in silica. They occur in oceanic islands.

Basalts occur in every tectonic environment.

A. Ocean floor basalts formed at midocean ridges make up most of the oceanic crust. They are termed MORB (Mid Ocean Ridge Basalt). They are uniformly tholeiitic basalts with very low contents of K, Ba, P, Sr, U, Th and Zr, low Fe₂O₃/FeO and K₂O/Na₂O, and low initial Sr⁸⁷/Sr⁸⁶ ratios.

B. Volcanic islands away from plate margins are commonly alkali basalt associations, but some thoeliitic associations also occur.

C. Andean type volcanism. Lavas are dominantly andesite with subordiante calc-alcaline basalts. Lavas show a characteristically increasing alkalinity continent inwards.

D. Intracontinental volcanoes are dominated by alkali basalt associations, sometimes highly alkalic, but some thoeliitic associations also occur.

In mature island arcs there is a progression from tholeiitic to calc-alkali to alkali series from the oceanic to the continental side.

When the subducted slab sinks into the mantle water is squeezed out of the sinking slab. When water is added to hot mantle rock, one of the main effects is to lower the melting temperature of the rocks. At a depth of around 100 to 200 km this in combination with the high pressure gives rise to partial melting of mantle (consisting mainly of peridotite).When peridotite melts the liquids gather together as magma that rises to the crust where it feeds volcanoes at the surface. On its way up it give off gasses and may be mixed by crust material from partially melting of the crust. Due to crystal fractionation on the way up combined with reaction with the mantle and crust, both of variable thickness, the compostion of the erupted lava will differ from volcano to volcano.

Portions of the basaltic crust can be recycled back into the mantle at subduction zones. The recycled basaltic crust is transformed into pyroxenite.

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