The ocean floor is almost entirely one kind of rock: basalt
Around 65 percent of Earth's crust by volume is igneous rock, born from cooled magma, and the oceanic crust is 99 percent basalt. Continents lean instead on granite and its relatives. Yet geologists admit that rock types have no sharp borders; names simply mark chosen points along a continuous spectrum.
A rock, or stone, is any natural solid mass of minerals or mineral-like matter. Rocks make up the crust and most of the planet's interior, apart from the liquid outer core and pockets of magma. Most are built from mineral grains, orderly crystals of bonded atoms, though some include mineraloids like volcanic glass that have no crystal structure. Silicates dominate, accounting for about a third of known mineral species and roughly 95 percent of the crust, and silica content largely determines a rock's name and properties.
Geologists sort rocks into three families, and the rock cycle can turn any one into another. Igneous rock, from the Latin for fire, forms as magma or lava solidifies after heating, falling pressure or a change in composition melts older rock. Slow cooling underground makes plutonic rock such as granite; eruption at the surface yields volcanic rock like basalt or pumice. Rising magma gets richer in silica as low-silica minerals crystallise out and it absorbs surrounding crust. Of all igneous rock, 66 percent is basalt and gabbro and 16 percent granite.
Sedimentary rock forms at the surface from fragments of older rock, minerals and organisms, or from chemicals precipitating out of water, then compacting and cementing under moderate heat and pressure. Metamorphic rock arises when existing rock is subjected to intense heat and pressure that transform it without substantial melting.
People have relied on stone since the Stone Age, first for tools, then for buildings, while mining extracted metals from within. Modern technology has added artificial rock-like materials such as concrete. Rock study became a formal science in the 19th century, when plutonism emerged as a theory. The discovery of radioactive decay in 1896 made radiometric dating possible, and plate tectonics took shape in the second half of the 20th century.
Source: Rock (geology)
