Rock is not only the revolutionary genre of music that liberated gritty vocalizations, heartfelt hallucinations, and rebelliously long locks- rocks are also essential to ecosystems:
Rocks provide the parent materials for soil (learn more about soil formation here)
They make the “backbones of hills and mountains, and the foundations of plains and valleys1”
And they provide essential materials for shelter and tools.
As such, understanding rocks is a big part of understanding life on Earth. So, let’s nerd-out by diving into cycles of stone and dirt:
What is a rock?
By its simplest definition, a rock is a combination of minerals that have been packed together to form a hard and solid substance2. That being said, to understand rocks, it’s important to understand minerals:
A mineral is a solid and crystalline substance3 that’s made of one or more chemical elements (such as oxygen, silicon, iron, and magnesium). Minerals form through various natural processes:
High pressure minerals are “formed under extreme pressure conditions4” such as the “downward movement of tectonic plates” or the high pressure and heat of the Earth’s mantle.
Hydrothermal minerals are formed when hot water dissolves the elements of pre-existing minerals, transports their particles, and then forms new minerals when the water slows and cools5.
While most minerals are made of multiple elements that have bonded together, some minerals take solid form without bonding to other elements (such as gold, copper, platinum, graphite and diamonds). Of these native elements, some hail all the way from outer space, having arrived to Earth by asteroids6 (such as gold and platinum7).
Then, when minerals combine (into new solid substances), they’re called rocks. The process that brings the minerals together determines what kind of rock is formed. With that in mind, there are 3 kinds of rocks:
igneous: made of magma or lava
sedimentary: made from sediments that cement together from of plants, animals, and/or other rocks
metamorphic: made by pre-existing rocks that change through a chemical transformation
The formation of these different rocks happens through the “rock cycle.” In this cycle, the three different types of rocks break down, come together, and transform again “from one form to another.8” They do this with the help of high-pressure conditions, rising and cooling temperatures, gravity, chemistry, water, and/or wind.
To better understand the rock cycle, here’s a little more about how each kind of rock is formed:
Igneous Rocks:
These rocks are formed of the molten (or hot liquid) rock that makes up much of the Earth’s mantle9 (known as magma when it’s underground and lava when it’s above ground10). The mantle11 is located directly below the Earth’s crust, reaching a melting point at about 62 to 124 miles below the Earth’s surface12. The crust is “made of dense, solid rock13” that is “extremely thin, cold and brittle compared to what lies below it.14” Because liquid magma is less dense than the solid rock of the crust, magma is buoyant, naturally rising up through the cracks and weakness in the Earth’s crust.15 As the magma rises, gasses are released along the way. These gasses build up pressure, which assists in the magma’s breakthrough to the surface16.
Along the journey, magma may “cool within the crust or erupt at the surface from a volcano’s lava flow.17” How fast the magma -or lava- cools determines what kind of igneous rock is formed. Intrusive rocks form underground by magma cooling slowly. This “slow cooling process allows crystals to grow large, giving the intrusive igneous rock a coarse-grain18” (such as granite19). Extrusive rocks (conversely) are formed above ground by lava cooling very rapidly. This rapid cooling gives it fine-grain crystals20 (such as basalt and obsidian21).
Due to constantly moving plate tectonics that allow new rocks to enter (and melt into) the Earth’s mantle, magma is constantly being replenished22.
Sedimentary Rocks:
Sedimentary rocks are made of pre-existing rocks and/or the bodies of dead organisms that accumulate over time, that eventually compact and cement into new rock.23
The formation of sedimentary rocks happens in 4 stages24:
Weathering. Weathering breaks down preexisting rocks. There are 3 primary types of weathering:
Physical (mechanical) weathering “breaks down rocks into smaller pieces through… wind, water, temperature changes, and pressure.25” “Common [mechanical] processes include:
“Freeze-Thaw Weathering: Water enters cracks in rocks, freezes, and expands, causing the rock to break apart. This is common in climates with frequent temperature fluctuations above and below freezing.
“Thermal Expansion: Rocks expand when heated and contract when cooled. Repeated temperature changes can cause rocks to fracture, particularly in deserts with significant diurnal temperature variations.
“Abrasion: Wind, water, and ice can carry particles that scrape against rocks, wearing them down over time. This is common in riverbeds, coastal environments, and areas with strong winds.26”
Chemical weathering occurs “through chemical reactions with water, oxygen, and other substances in the environment.27”
Biological weathering occurs from the impact of plants, animals, and microorganisms. For example, “plant roots can grow into cracks in rocks and pry them apart, while lichens can produce acids that chemically weather the rock surface.28” Animals can impact biological weathering through burrowing, drilling, foraging, and/or trampling on rocks29.
Erosion: Erosion transports the pieces of the rock (that have been broken down by weathering) to other places by “six main erosion processes, which are often interconnected30”:
Gravity (for example landslides)
Wind: wind can carry sand and dust, both moving it to new places and causing abrasion (which causes more erosion)
Rain: falling rain dislodges pieces of rock. When the rainfall is heavy, it carries pieces of rock into rivers
Rivers: the force of water (and others rocks) and against riverbanks picks up pieces of rock and carries them downstream
Oceans: the force of water from falling and rising tides, and from waves, removes materials from rocks on the shore and carries them into the sea
Glaciers: the enormous weight of these frozen rivers slowly moving over land dislodges and picks up fragments of rock as it moves
Deposition: The fragments of rock carried by wind and water eventually settle into layers of sediments. This occurs “when the forces that transport sediments, such as water, wind, or ice, lose their energy and can no longer carry the particles31” (such as in calm lakes, deep oceans, and swamps)32
Cementation: Overtime, the fragments of rock within sediment bind together to form new solid rocks
With all this in mind, there are three main groups of sedimentary rocks:
Clastic sedimentary rocks are made from pieces of pre-existing rocks [such as clay, silt, sand or pebbles] that are loosened by weathering33 (such as sandstone and shale)34
“Biologic sedimentary rocks form when living organisms die, pile up, and are then compressed and cemented together35” (including coal and some kinds of limestone)
Chemical sedimentary rocks are made of the minerals the are dissolved from others rocks36” (such as rock salt)37 (as opposed to the solid pieces of pre-existing rock that make clastic rocks)
Metamorphic Rocks:
Metamorphic rocks are formed when pre-existing rocks undergo a significant transformation due to:
“Temperature: elevated temperatures activate chemical reactions that alter the mineral composition of the parent rock [Typically exceeding 150-200°C]
“Pressure: pressure compacts the rock, promoting recrystallization and the formation of new structures [often greater than 100 megapascals]
“Chemical Modifiers: Hot fluids carrying dissolved minerals can introduce new chemical components or remove existing ones, further modifying the rock's composition.
“Time: These transformative processes occur over vast timescales, spanning millions of years, allowing for gradual and profound changes.38”
The forces of metamorphism can arise through:
Regional Metamorphism:
“Mountain Building: As tectonic plates collide, immense forces compress and uplift rock layers, subjecting them to high pressure and shearing forces.
“Continental Collisions: When continents collide, the immense pressure and heat generated can lead to extensive regional metamorphism.
“Subduction Zones: When oceanic plates subduct beneath continental plates, the descending plate undergoes intense heating and pressure, resulting in various metamorphic rocks.39”
Contact Metamorphism: when magma or very hot igneous rock “comes into contact with surrounding cooler rocks,40” the cooler rock bakes, which partially melts and recrystallizes it.
Hydrothermal Metamorphism (metasomatism): when hot, chemically-active fluids (such as seawater or the water released from magma) interact with parent rock, they can chemically alter it41.
Once rocks have formed, their journey within the rock cycle can bring them along many paths, including to:
undergo weathering, which breaks them down to become:
parent materials for soil (learn more about soil formation here)
and/or sediment for new rocks
undergo a metamorphosis into different rocks
and/or enter the Earth’s mantle to become magma again, thus starting the cycle anew.
And there ya have it: the cycle of rocks from magma and minerals to rock to dust to soil to sediment to stone to then being buried underground and finally melting into the mantle so to come back around as new rocks.
Check out this graphic for a helpful visual: Modeling the Earth’s Rock Cycle (from Penn State) and be sure to
Rock out, rock star 🤘🪨
Listen to the Song: Hope of The Hard Rocks
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https://www.nps.gov/subjects/geology/rocks.htm
https://www.ebsco.com/research-starters/geology/rock-cycle-geology
https://opengeology.org/textbook/3-minerals/#
https://www.geologyin.com/2014/11/hydrothermal-metamorphism.html#:~:text=Hydrothermal Metamorphism in Ore Formation,Massive Sulfide (VMS) Deposits:
https://www.ebsco.com/research-starters/chemistry/high-pressure-minerals
https://www.livescience.com/15938-earth-precious-metals-space-origin.html#
https://fightingfor.nd.edu/stories/heavy-metals/
https://www.ebsco.com/research-starters/geology/rock-cycle-geology
https://www.snexplores.org/article/scientists-say-magma-and-lava#:~:text=Magma (noun, “MAG-,igneous, sedimentary and metamorphic rock.
https://www.usgs.gov/faqs/what-difference-between-magma-and-lava#:~:text=Scientists use the term magma,breaks through the Earth's surface.
https://www.snexplores.org/article/explainer-earth-layer-layer
Ibid.
https://www.snexplores.org/article/scientists-say-magma-and-lava#:~:text=Magma (noun, “MAG-,igneous, sedimentary and metamorphic rock.
https://www.snexplores.org/article/explainer-earth-layer-layer
https://www.vaia.com/en-us/explanations/environmental-science/geology/volcanic-vent/#:~:text=As the magma makes its way through,to the conditions necessary for vent formation.
https://volcano.oregonstate.edu/faq/what-makes-magma-come-out-chamber-and-how-does-magma-form#:~:text=Magma forms from partial melting,Lava
https://askaboutireland.ie/learning-zone/secondary-students/geography/physical-environment/the-formation-of-the-phys/rock-types/rock-formation/#
https://geo.libretexts.org/Bookshelves/Geology/Book:_An_Introduction_to_Geology_(Johnson_Affolter_Inkenbrandt_and_Mosher)/04:_Igneous_Processes_and_Volcanoes/4.01:_Classification_of_Igneous_Rocks
https://www.nps.gov/subjects/geology/igneous.htm#:~:text=The word igneous derives from,Extrusive (volcanic) rocks
https://geo.libretexts.org/Bookshelves/Geology/Book:_An_Introduction_to_Geology_(Johnson_Affolter_Inkenbrandt_and_Mosher)/04:_Igneous_Processes_and_Volcanoes/4.01:_Classification_of_Igneous_Rocks
https://geology.com/rocks/igneous-rocks.shtml#
https://www.e-education.psu.edu/earth109/node/910#:~:text=If it's buried deeply enough,mantle at a subduction zone.
https://www.nps.gov/subjects/geology/sedimentary.htm#:~:text=NPS photo.-,Introduction,and cemented, forming sedimentary rock.
https://www.geologyin.com/2014/11/sedimentary-rocks-and-processes.html#:~:text=Weathering: making the sediment by,turning the sediment to rock.
https://www.geologyin.com/2024/07/weathering-of-different-rock-types.html#:~:text=Igneous Rocks: Formed from cooled,clay minerals and iron oxides.
https://www.geologyin.com/2024/07/weathering-of-different-rock-types.html#:~:text=Igneous Rocks: Formed from cooled,clay minerals and iron oxides.
Ibid.
Ibid.
https://asutoshcollege.in/new-web/Study_Material/Biological_Weathering.pdf
https://www.bgs.ac.uk/discovering-geology/geological-processes/erosion/
https://www.longdom.org/open-access-pdfs/deposition-geology-understanding-the-processes-and-importance-in-earth-science.pdf
Ibid.
https://www.nps.gov/subjects/geology/sedimentary.htm#:~:text=NPS photo.-,Introduction,and cemented, forming sedimentary rock.
https://www.usgs.gov/faqs/what-are-sedimentary-rocks
https://www.nps.gov/subjects/geology/sedimentary.htm#:~:text=NPS photo.-,Introduction,and cemented, forming sedimentary rock.
Ibid.
https://open.maricopa.edu/geologylab/chapter/chemical-and-organic-sedimentary-rocks/#:~:text=Chemical Sedimentary Rocks&text=This water solution contains dissolved,sodium, chloride, and sulfate.&text=These rocks can form from,perfect cleavage along one plane.
https://www.geologyin.com/2023/12/metamorphic-rocks.html#:~:text=Regional Metamorphism is the dominant,creating the characteristic foliated textures.
Ibid.
Ibid.
Ibid.