Product Code Database
classification of aphanitic extrusive igneous rocks to their relative alkali (Na2O + K2O) and silica (SiO2) weight contents. Blue area is roughly where alkaline rocks plot; yellow area where subalkaline rocks plot.
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The main effect of extrusion is that the magma can cool much more quickly in the open air or under seawater, and there is little time for the growth of . Sometimes, a residual portion of the matrix fails to crystallize at all, instead becoming a natural glass like obsidian.
If the magma contains abundant volatile components which are released as free gas, then it may cool with large or small vesicles (bubble-shaped cavities) such as in pumice, scoria, or vesicular basalt. Other examples of extrusive rocks are rhyolite and andesite.
Fissure volcanoes pour out low viscosity basaltic magma from to form the extrusive rock basalt.
Composite or Stratovolcano often have andesitic magma and typically form the extrusive rock andesite. Andesitic magma is composed of many gases and melted mantle rocks.
Cinder cone or scoria cones violently expel lava with high gas content, and due to the vapor bubbles in this mafic lava, the extrusive basalt scoria is formed.
are formed by high viscosity lava that piles up, forming a dome shape. Domes typically solidify to form the rich in silica extrusive rock obsidian and sometimes dacite domes form the extrusive rock dacite, like in the case of Mount St. Helens.
are volcanic depressions formed after an erupted volcano collapses. Resurgent calderas can refill with an eruption of rhyolitic magma to form the extrusive rock rhyolite like the Yellowstone Caldera.
Submarine volcanoes erupt on the ocean floor and produce the extrusive rock pumice. Pumice is a light-weight glass with a vesicular texture that differs from scoria in its silicic composition and therefore floats.
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