The phrase “Steady as a rock.” This familiar phrase implies that a rock is permanent, and unchanging over time. However, it is not. In the time frame of Earth’s history, around 4.57 billion years, the atoms that make up one type of rock can rearrange or move to another location, eventually becoming part of another type of rock. Later, the atoms can move again to form a third type of rock, and so on.
Thus, geologists refer to the progressive transformation of Earth’s materials from one type of rock to another as the rock cycle, one of many examples of cycles operating on Earth or on the crust of the earth.
Rock cycle stages
The rock cycle illustrates relationships between basic rock types (igneous, sedimentary, and metamorphic)
By following the arrows in the image above, you can see many paths around and through the rock cycle.
For example, igneous rocks can be exposed to interperism and erode to produce sediments, which later lithify to form sedimentary rocks.
The new sedimentary rock may be buried so deep that it becomes metamorphic rock, which may then partially melt and produce magma.
This magma later solidifies to form new igneous rock.
This path can be symbolized as follows:
From igneous rocks to sedimentary rocks, then to metamorphic rocks, finally to magma.
Alternatively, however, the metamorphic rock could be uplifted and eroded to form new sediment which is then buried and lithified to form new sedimentary rock, bypassing melting and magma generation.
This path symbolizes a shortcut through the loop as follows:
Igneous rocks to sedimentary rocks, then to metamorphic, finally to sediments and sedimentary rocks.
Similarly, igneous rock could undergo metamorphism directly, without going through sedimentary processes.
This metamorphic rock could erode to produce a sediment that would eventually become sedimentary rock, as follows:
Igneous rocks to metamorphic rocks, then to sediments and sedimentary rocks.
To get a better idea of how the rock cycle, the context in plate tectonics will be discussed.
Importance of the rock cycle
The rock cycle It is important above all for the people who are in charge of the study of the dynamics of the planet earth, those scientists are the geologists, the biologists, the paleontologists, the geophysicists, and even the civil engineers who carry out all their construction works on the different types of rock.
It is thanks to rock cycle that we know how planet earth works, what is the composition of its inner layersand what has been its evolution since the planet was formed until today.
All this is due to the fact that rocks have the ability to record all the events that occur on the planet in each of its minerals and also in the way in which it appears in the field and in outcrops in the form of structures and fossils.
Finally, it is thanks to the rock cycle that many theories of how the planet works and its evolution have been created.
The rock cycle and plate tectonics
The rock cycle It can start when magma rises from the mantle.
Suppose volcanic eruptions form, cooling lava forms basalt (an igneous rock) in a continental hot spot volcano (figure 3a).
Weathering factors such as wind, rain, and vegetation gradually wear down the basalt, physically fracturing it into smaller fragments and chemically transforming it to produce clay (weathering).
The water washes away the newly formed clay and carries it downstream.
If you’ve ever seen a river that’s brown in color, you’ve seen clay traveling to a deposition site.
Eventually, the river reaches the sea, where the water slows and the clay settles.
Let’s imagine, for this example, that clay settles along the continental margin and forms a sediment deposit.
Little by little, over time, the sediments are buried and come together to form a new sedimentary rock, in this one a shale.
The shale is buried 6 km below the continental shelf for millions of years, until the subducting oceanic plate causes the continents to collide.
The edge of the continent creates such stresses that it manages to bury the shales deeper and also fractures them.
As mountains grow, shales that had been 6 km below the surface end up 20 km below.
Under the new conditions of pressure and temperature present at this depth, the shales undergo metamorphism and schists (metamorphic rocks) are generated.
Once mountain building stops, erosion destroys the mountain range, and exhumation exposes part of the schists to the surface.
These schists erode and form sediments, which are transported and deposited elsewhere to form another type of sedimentary rock again.
However, another schist sequence is preserved below the surface (figure 3c).
Eventually, continental shift takes place at the site of the old mountain range, and the crust containing the shale begins to separate.
This process is known as Rifting, which causes decompression and expansion of the crust.
The partial melting of the mantle generates the necessary heat in the crust that part of the schists partially melt and a new felsic magma is formed.
This felsic magma rises to the crustal surface and crystallizes into rhyolite, a new igneous rock (figure 3d).
In terms of the rock cycle, we have finished the cycle, once again an igneous rock has been generated.
Not all atoms go through the rock cycle at the same rate, and for that reason we find rocks of many different ages on the Earth’s surface.
Some rocks stay in one form for less than a few million years, while others remain unchanged for most of Earth’s history.
A rock in the Appalachian Mountains has gone through stages of the rock cycle many times over the past hundreds of millions of years, as the eastern margin of North America has been subject to multiple basin-forming, mountain-building events and rifting during the last billion years.
In contrast, some 3-billion-year-old mafic and ultramafic rocks found deep within continents have not yet passed the first stage of the rock cycle.
Studies show, however, that such long-lived rocks represent a very small proportion of the crust: most of the crust has gone through at least a couple of stages in the rock cycle.
Most of the atoms that comprise the continental rocks never return to the mantle, because the continental crust is buoyant and does not subduct. However, a small amount of sediment that erodes a continent ends up in deep oceanic trenches, and some ends up in the mantle by subduction.
Furthermore, recent research suggests that metamorphic and igneous rocks at the base of the continental crust may be scraped off and transported into the mantle by subduction.
Our tour of the rock cycle has focused on continental rocks.
But what about the oceans? The oceanic crust consists of igneous rocks (basalt and gabbro) covered by sediment.
Because a layer of water covers the oceanic crust, the rock in the oceanic crust does not erode and generally does not follow the path to the sedimentary cycle of the rock cycle.
But sooner or later, the oceanic crust subducts. When this happens, the crustal rock undergoes metamorphism, since as it sinks, it is subjected to progressively higher temperatures and pressures.
What drives the rock cycle in the Earth system?
The rock cycle occurs because Earth is a dynamic planet.
The internal heat and the gravitational field of the planet drive the movements of the plates and the generation of hot spots.
Plate interactions cause uplift of mountain ranges, a process that leads to erosion and sediment production.
The interaction between plates also generates situations where metamorphism occurs, where rock melts, and where sedimentary basins develop.
At Earth’s surface, gases initially released by volcanism come together to form the ocean and atmosphere.
Heat (from the Sun) and gravity drive convection in the atmosphere and oceans, creating wind, rain, ice and currents, weathering and erosion.
In the Earth System, life also plays a key role by adding corrosive oxygen to the atmosphere and directly contributing to weathering.
In summary, external energy (solar heat), internal energy (Earth’s internal heat), gravity, and life all play a role in driving the rock cycle by maintaining the mantle, crust, atmosphere, and rocks. oceans in constant motion.