Igneous Rocks Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock may form with or without crystallization, either below the surface as intrusive rocks or on the surface as extrusive rocks.

Diorite is the name used for a group of coarse-grained igneous rocks with a composition between that of granite and basalt. It usually occurs as large intrusions, dikes, and sills within continental crust. These often form above a convergent plate boundary where an oceanic plate subducts beneath a continental plate.
Scoria is a dark-colored igneous rock with abundant round bubble-like cavities known as vesicles. It ranges in color from black or dark gray to deep reddish brown. Scoria usually has a composition similar to basalt, but it can also have a composition similar to andesite.
Pumice is a light-colored, extremely porous igneous rock that forms during explosive volcanic eruptions. It is used as aggregate in lightweight concrete, as landscaping aggregate, and as an abrasive in a variety of industrial and consumer products. Many specimens have a high enough porosity that they can float on water until they slowly become waterlogged.

Credits:

Created with images by James St. John - "Zinciferous marble (Franklin Marble, Mesoproterozoic, 1.03-1.08 Ga; zinc mine in Franklin, northern New Jersey, USA) 1" • James St. John - "Orbicular monzodiorite (knoll just south of Rt. 64 & NNW of Plantation Lake, west of the Yadkin River, eastern Davie County, northwest-central North Carolina, USA) 2" • James St. John - "Dark gray scoria" • James St. John - "Pumice 3"

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