<Li> Volcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental ejecta, forming minerals such as pumice or basalt . </Li> <P> The chemical abundance and the rate of cooling of magma typically forms a sequence known as Bowen's reaction series . Most major igneous rocks are found along this scale . </P> <P> About 64.7% of the Earth's crust by volume consists of igneous rocks; making it the most plentiful category . Of these, 66% are basalts and gabbros, 16% are granite, and 17% granodiorites and diorites . Only 0.6% are syenites and 0.3% peridotites and dunites . The oceanic crust is 99% basalt, which is an igneous rock of mafic composition . Granites and similar rocks, known as meta - granitoids, form much of the continental crust . Over 700 types of igneous rocks have been described, most of them having formed beneath the surface of Earth's crust . These have diverse properties, depending on their composition and the temperature and pressure conditions in which they were formed . </P> <P> Sedimentary rocks are formed at the earth's surface by the accumulation and cementation of fragments of earlier rocks, minerals, and organisms or as chemical precipitates and organic growths in water (sedimentation). This process causes clastic sediments (pieces of rock) or organic particles (detritus) to settle and accumulate, or for minerals to chemically precipitate (evaporite) from a solution . The particulate matter then undergoes compaction and cementation at moderate temperatures and pressures (diagenesis). </P>

The most common type of rock in earth's crust is