Earth.Parts #27 - Igneous, Metamorphic & Sedimentary rock types and the rock cycle
Channel: Earth.Parts
Date: 2017-12-12

In which I introduce the concepts of basic rock types: igneous, metamorphic & sedimentary rocks, and what processes cause them to form. Also featured is the Rock Cycle, a schematic overview of how the different rock types transform into each other.

Basic rock types are sorts of rock that form naturally by distinctly different mechanisms or processes:

Igneous - fresh new rock solidified from the cooling of molten lava or magma.

Metamorphic - rocks of any type compressed & heated deep within Earth's crust and transformed into new crystalline shapes.

Sedimentary - accumulation of rocks & mineral bits transported by wind, water and gravity to pile up elsewhere & end up compressed into new solid rock.

Magma is molten rock inside the Earth, beneath the surface in the crust or upper mantle. Partial melting of peridotite in Earth's mantle generates magma that forges the cores of continental and ocean crusts alike. Deep magma chambers can solidify entirely within the crust. Lava is molten rock that erupts onto Earth's surface, either in air or underwater.

Sediment is weathered, eroded or simply transported geologic mass (sand, silt, plants, shells, salt etc...) that is deposited by natural processes and accumulates in place over long ages. Sedimentary rock forms layered bodies of sediment that has been buried deeply in the crust, where sediment grains are squeezed together and hardened into solid rock over geologic time. Coal is rock formed from layers of ancient plant life, compressed & buried into hardened rock consisting mostly of black carbon (C). Limestone forms by the accumulation of dead shell bits. Rock salt (halite, NaCl) forms by evaporation of seawater to form layers of sedimentary crystals. Halite is an example of evaporite rock, originally laid down from drying seas. 

Metamorphic rock is rock reforged; rock of any source or type that becomes subjected to intense pressure and/or heat deep below, recrystallizing parent rock minerals into new crystalline forms natural to the conditions of Earth's deep crust. Deep burial causes rock to undergo escalating pressures & temperatures along the geothermal gradient. In the crust, the temperature gradient is around 25º C per km of depth, while pressure rises by about 300 bars (atmospheres) per km.

All metamorphic rocks have parent rocks, the original source rock prior to becoming metamorphosed. Contact metamorphism can happen when an igneous intrusion exposes surrounding rock to high temperatures at fairly low geologic pressures. Regional metamorphism occurs when tectonic forces bend, fold, compress & stretch rock across wide continental regions when mountains-building occurs. 

Music by Chris Zabriskie: http://freemusicarchive.org/music/Chris_Zabriskie/