Rock Cycle:A Fundamental Concept of Geology

The rock cycle is one of the fundamental concepts in geology that describes the dynamic transitions through geologic time among the three main rock types: sedimentarymetamorphic, and igneous. Each of the types of rocks are altered or destroyed when it is forced out of its equilibrium conditions. An igneous rock such as basalt will break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and are forced to change as they encounter new environments. The rock cycle is an illustration that explains how the three rock types is related to each other, and how processes change from one type to another over time.

When it comes to Earth’s surface and cools there, magma is called lava. On Earth’s surface, wind and water break rock into pieces. They  also carry rock pieces to another place. Usually, the rock pieces, called sediments, drop from the wind or water to make a layer. The layer can be buried under other layers of sediments. After a long time the sediments can be cemented together to make sedimentary rock. Through this process, igneous rock changes into sedimentary rock.

All rocks can be heated. But where does the heat come from? Inside Earth there is heat from pressure. There is heat from friction . There is also heat from radioactive decay .

So, what does the heat do to the rock? It bakes the rock.

Baked rock does not melt, but it can change. It forms crystals. If it has crystals already, it forms larger crystals. Because this rock changes, it is called metamorphic.  Metamorphosis can occur in rock when they are heated to 300 to 700 degrees Celsius.

When Earth’s tectonic plates move around, they produce heat. When they collide, they build mountains and metamorphose (met-ah-MORE-foes) the rock.

The rock cycle continues. Mountains made of metamorphic rocks can be broken up and washed away by streams. New sediments from these mountains can make new sedimentary rock.

It never stops.

The Concept

The original concept of the rock cycle is usually attributed to James Hutton, from the eighteenth century father of geology. The rock cycle was a part of Hutton’s uniformitarianism and his famous quote: no vestige of a beginning, and no prospect of an end, applied in particular to the rock cycle and the envisioned cyclical nature of geologic processes. This concept of a repetitive non-evolutionary rock cycle remained dominant until the plate tectonics revolution of the 1960s. With the developing understanding of the driving engine of plate tectonics, the rock cycle changed from endlessly repetitive to a gradually evolving process. The Wilson cycle (a plate tectonics based rock cycle) was developed by J. Tuzo Wilson during the 1950s and 1960s.

Forces that drive the rock cycle

 Plate tectonics

In 1967, J. Tuzo Wilson published an article in Nature describing the repeated opening and closing of ocean basins, in particular focusing on the current Atlantic Ocean area. This concept, a part of the plate tectonics revolution, became known as the Wilson cycle. The Wilson cycle has had profound effects on the modern interpretation of the rock cycle as Plate tectonics became recognized as the driving force for the rock cycle.

 Spreading Ridges

The  cycle starts at the mid-oceandivergent boundaries where new magma is produced by mantle upwelling and a shallow melting zone. This  juvenile basaltic magma is the first phase of the igneous portion of the cycle. Least dense magma is more likely to be erupted. As the ridge spreads and the new rock is carried away from the ridge, the interaction of heated circulating seawater through crevices starts the initial retrograde metamorphism of the new rock.

(Read here in detail about Earthquake and Plate Tectonics)

 Subduction zones

Subduction process of the Juan de Fuca Plate i...

Subduction process of the Juan de Fuca Plate in Oregon, USA as a cutaway graphic (Photo credit: Wikipedia)

New basaltic oceanic crust eventually meets a subduction zone as it moves away from the spreading ridge. This crust is pulled back into the mantle, the increasing pressure and temperature conditions cause a restructuring of the mineralogy of the rock, this metamorphism alters the rock to form eclogite. As the slab of basaltic crust and some included sediments are dragged deeper, water and other more volatile materials are driven off and rise into the overlying wedge of rock above the subduction zone which is at a lower pressure. The lower pressure, high temperature, and now volatile rich material in this wedge melts and the resulting buoyant magma rises through the overlying rock to produce island arc or continental margin volcanism. This volcanism includes more silicic lavas the further from the edge of the island arc or continental margin, indicating a deeper source and a more differentiated magma.

At times some of the metamorphosed downgoing slab may be thrust up or obducted onto the continental margin. These blocks of mantle peridotite and the metamorphic eclogites are exposed as ophiolite complexes.

The newly erupted volcanic material is subject to rapid erosion depending on the climate conditions. These sediments accumulate within the basins on either side of an island arc. As the sediments become more deeply buried lithification begins and sedimentary rock results.

 Continental collision

On the closing phase of the  Wilson cycle, two continental or smaller terranes meet at a convergent zone. As the two masses of continental crust meet, neither can be subducted as they are both low density silicic rock. As the two masses meet, tremendous compressional forces distort and modify the rocks involved. The result is regional metamorphism within the interior of the ensuing orogeny or mountain building event. As the two masses are compressed, folded and faulted into a mountain range by the continental collision the whole suite of pre-existing igneous, volcanic, sedimentary and earlier metamorphic rock units are subjected to this new metamorphic event.

 Accelerated erosion

The high mountain ranges produced by continental collisions are immediately subjected to the forces of erosion. Erosion wears down the mountains and massive piles of sediment are developed in adjacent ocean margins, shallow seas, and as continental deposits. As these sediment piles are buried deeper they become lithified into sedimentary rock. The metamorphic, igneous, and sedimentary rocks of the mountains become the new piles of sediments in the adjoining basins and eventually become sedimentary rock.

 An evolving process

The plate tectonics rock cycle is an evolutionary process. Magma generation, both in the spreading ridge environment and within the wedge above a subduction zone, favors the eruption of the more silicic and volatile rich fraction of the crustal or upper mantle material. This lower density material tends to stay within the crust and not be subducted back into the mantle. The magmatic aspects of plate tectonics tends to gradual segregation within or between the mantle and crust. As magma forms, the initial melt is composed of the more silicic phases that have a lower melting point. This leads to partial melting and further segregation of the lithosphere. In addition the silicic continental crust is relatively buoyant and is not normally subducted back into the mantle. So over time the continental masses grow larger and larger.

 The role of water

The presence of abundant water on Earth is of great importance for the rock cycle. Most important  are the water driven processes of weathering and erosion. Water in the form of precipitation and acidic soil water and groundwater is quite effective at dissolving minerals and rocks, especially  igneous and metamorphic rocks and marine sedimentary rocks that are unstable under near surface and atmospheric conditions. The water carries away the ions dissolved in solution and the broken down fragments that are the products of weathering. Running water carries vast amounts of sediment in rivers back to the ocean and inland basins. The accumulated and buried sediments are converted back into rock.

A less obvious role of water is in the metamorphism processes that occur in fresh seafloor volcanic rocks as seawater, sometimes heated, flows through the fractures and crevices in the rock. All of these processes, illustrated by serpentinization, are an important part of the destruction of volcanic rock.

The role of water  in the melting of existing crustal rock in the wedge above a subduction zone is a most important part of the cycle.

Links and Sources: (Earth Hour)      (wikipedia)


About Rashid Faridi

I am Rashid Aziz Faridi ,Writer, Teacher and a Voracious Reader.
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4 Responses to Rock Cycle:A Fundamental Concept of Geology

  1. Pingback: Metamorphism and metamorphic rocks « Rashid's Blog

  2. Pingback: Plate Tectonic , Plate Boundaries and Mid Oceanic Ridge « Rashid's Blog

  3. Pingback: Weathering | Rashid's Blog

  4. Pingback: Pumice: Solidified Frothy Lava | Rashid's Blog

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