The Earth changes because of its internal and external heat engines. If the Earth’s engine stopped working and tectonic forces stopped operating, the external engine plus gravity would have levelled the continents to sea level long ago. Resulting sediments would have been deposited on the seafloor. Solid Earth would not be changing except when struck by meteorites or other extraterrestrial bodies. Rocks would be at rest, minerals and the atmosphere would be in equilibrium and geology would be a dull subject. But this not the case. The internal and external forces continue to interact, forcing substances out of equilibrium. Therefore, the Earth has a highly varied and ever-changing surface. And minerals and rock change as well.
The rock cycle is a useful aid in visualising these changing relationship.
Rock cycle is a concept used to describe the cyclic movement of rocks from creation of rocks by tectonics down to the destruction of rocks by denudation.
The Mechanism of Rock Cycle
There are generally three types of rocks namely; Igneous rocks, Sedimentary rocks, and Metamorphic rocks.
Igneous rocks form from the cooling of molten magma. If magma is brought to the surface by a volcanic eruption where it flows on the surface as lava, it solidifies as an extrusive or volcanic igneous rock. Magma may solidfy slowly beneath the surface, resulting in an intrusive rock. These rocks may be exposed later after uplifts and the overlying rock removed by erosion.
Exposed Igneous rocks may undergo weathering and erosion and debris produced is transported and ultimately deposited (usually on seafloor) as sediments. Other sediments are deposited atop these sediments, overburden pressure and compaction from overlying sediments leads to lithification (cementation and consolidation into rock). The resulting rock is a sedimentary rock. This process can bury layered rocks in the uppermost crust to depths of several kilometres.
Tectonic forces are required to transport sedimentary (and extrusive igneous rock) to lower levels in the crust. Heat and pressure on these rocks increases with increasing depth of burial (geothermal gradient). If the temperature and pressure becomes high enough as occurs in the middle and lower levels of the continental crust, the original sedimentary rock is recrystallized. The resulting rock is a metamorphic rock. If temperature gets very high the rock partially melts, producing magma and completing the cycle.
The cycle may be repeated. However, may not go through each step in the cycle. For instance, sedimentary rocks might be uplifted and exposed to weathering creating new sediments.
Scholars should note that the rock cycle is a simplified concept deviced to aid students identify common rocks and how they form in perspective and as such does not encompass all geologic processes.