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The Rock Cycle

Rocks

What exactly is a rock? Great question! I'm so glad you asked.

 

At their simplest, rocks are defined as assemblages of two or more minerals bound together.

 

The majority of rocks are composed of multiple minerals. For instance, granite (left) is composed mainly of quartz and feldspar with smaller amounts of mica, amphiboles, and other minerals. However, monomineralic rocks contain 90% or more of just one mineral. (The prefix 'mono-' means 'one'!) For example, quartzite (right) is composed nearly entirely of fused quartz grains.
The majority of rocks are composed of multiple minerals. For instance, granite (left) is composed mainly of quartz and feldspar with smaller amounts of mica, amphiboles, and other minerals. However, monomineralic rocks contain 90% or more of just one mineral. (The prefix 'mono-' means 'one'!) For example, quartzite (right) is composed nearly entirely of fused quartz grains.

 

Rocks are identified by the minerals they contain, their chemical composition, and how they are formed. There are three main categories of rocks, determined by how they are formed: Igneous, Sedimentary, and Metamorphic. 

Now we have to ask - what exactly is a mineral?

 

Minerals

Minerals are naturally occurring, inorganic (non-living), solid materials. Each mineral has a specific chemical composition and  ordered internal structure (i.e. crystalline structure). individual minerals also have their own characteristic density (how hard it is), color, texture, and crystal shape. (Interestingly, ice fits the definition of a mineral, but water does not!)

 

Minerals come in a variety of colors, textures, and crystalline structures.
Minerals come in a variety of colors, textures, densities, and crystalline structures.

 

Minerals are the building blocks of rocks – remember, an assemblage of two or more minerals together makes the majority of rocks. There are over 4200 known minerals, but only 30 are the most common components of rocks, known as rock-forming minerals. These minerals make up close to 90% of all rocks in Earth's crust.

 

To be considered a common rock forming mineral, a mineral must be one of the most abundant minerals in Earth's crust, be one of the original minerals present when a rock forms, and be an important mineral in determining a rock's classification.
To be considered a common rock forming mineral, a mineral must be one of the most abundant minerals in Earth's crust, be one of the original minerals present when a rock forms, and be an important mineral in determining a rock's classification.

 

Some rocks are made of mineraloids - a naturally occurring substance that resembles a mineral but does not meet all of the criteria listed above. For instance, they may have different chemical compositions or not have a crystalline structure. Some materials considered to be gemstones are actually mineraloids, not true minerals. For instance, opals are made of an amorphous form of silica. The crystalline form of silica is considered a mineral and makes up much of the world's sand, as well as the rock quartz. Other examples include amber, jet, and pearls.

 

Jet is a mineraloid gemstone that is actually a type of coal, formed from decomposed and compressed wood. Jet is either black or dark brown and was commonly used as mourning jewelry in Europe, particularly the United Kingdom, in the late 1800s and early 1900s. This image shows a French jet mourning necklace from this time.
Jet is a mineraloid gemstone that is actually a type of coal, formed from decomposed and compressed wood. Jet is either black or dark brown and was commonly used as mourning jewelry in Europe, particularly the United Kingdom, in the late 1800s and early 1900s. This image shows a French jet mourning necklace from this time.

 

Rock Cycle

Rocks may appear to be stable and unchanging – but they are not!

Rocks are always in the process of becoming other types of rocks.

The rock cycle depicts the continuous alteration of Earth materials from one rock type to another. For instance, igneous rocks formed from magma might break down into sediment through weathering and erosion, then lithify (compress and cement) into sedimentary rock. Sedimentary rock might become buried, then eventually exposed to heat and pressure deep within Earth, turning it into metamorphic rock.

 

The purple arrows show the cycling of Earth materials into different types of rocks: Igneous rocks are formed from melted Earth materials deep beneath the surface, which later cool and crystalize into a solid rock. Those rocks eventually undergo weathering, erosion, transportation, and deposition before being cemented into sedimentary rocks. Eventually, sedimentary rocks are buried and subject intense heat and pressure, creating metamorphic rocks. The blue arrows show 'shortcuts' within this cycle - for instance, igneous rocks can also become metamorphic rocks under Earth's surface, or metamorphic rocks can be uplifted and transformed to sedimentary rocks.
The purple arrows show the cycling of Earth materials into different types of rocks: Igneous rocks are formed from melted Earth materials deep beneath the surface, which later cool and crystalize into a solid rock. Those rocks eventually undergo weathering, erosion, transportation, and deposition before being cemented into sedimentary rocks. Eventually, sedimentary rocks are buried and subject intense heat and pressure, creating metamorphic rocks. The blue arrows show 'shortcuts' within this cycle - for instance, igneous rocks can also become metamorphic rocks under Earth's surface, or metamorphic rocks can be uplifted and transformed to sedimentary rocks.

 

Two other cyclic systems drive the rock cycle:

  • The hydrologic cycle, which is fueled by solar energy and drives the exogenic (outer) processes of the rock cycle above Earth’s surface
  • The tectonic cycle, which is powered by Earth’s internal heat and drives the endogenic (inner) processes of the rock cycle below Earth’s surface and within the crust.

There are three rock types associated with the rock cycle, each of which is associated with specific processes:

  • Igneous (Melting and Crystallization)
  • Sedimentary (Erosion and Sedimentation)
  • Metamorphic (Burial and Metamorphism)
This simplified rock cycle shows how each kind of rock can become another kind of rock through the processes associated with each category: igneous rocks are formed through melting and cooling, sedimentary rocks through weathering and sedimentation, and metamorphic rocks through heat and pressure.
This simplified rock cycle shows how each kind of rock can become another kind of rock through the processes associated with each category: igneous rocks are formed through melting and cooling, sedimentary rocks through weathering and sedimentation, and metamorphic rocks through heat and pressure.

 

The next three pages will go over each individual rock type.