The Rock Cycle: How Sedimentary, Metamorphic, and Igneous Rocks are Formed

Jenna Mellentine, Peter Jaeger
  • Author
    Jenna Mellentine

    Jenna Mellentine is a certified teacher who has relevant experience teaching in the public school system for over 3 years. She has a bachelor's degree in Elementary Education from Western Governors University, and is currently working on a master's degree in Science Education.

  • Instructor
    Peter Jaeger

    Pete currently teaches middle school Science, college level introductory Science, and has a master's degree in Environmental Education.

Learn about the rock life cycle. Identify the differences between igneous, sedimentary, and metamorphic rocks, and learn how the rock cycle works. Learn some examples of each type of rock and how transformations take place. Updated: 04/11/2022

What Is the Rock Life Cycle?

The rock life cycle is the natural Earth process of how rocks form and convert between types. There are three types of rocks. The three types of rocks are igneous rocks, metamorphic rocks, and sedimentary rocks. These three rock types continually form through geological processes, which are slow compared to the human lifetime. Rocks can take anywhere from hundreds to thousands of years, to millions of years to form or change types.

The rock cycle works as a process that can have more than one path and every rock's life can be unique. Follow along the lines and arrows on the diagram to see how the rock cycle works.

  • Magma - It starts with the formation of rock from molten lava or magma, which is liquid rock. Molten magma or lava crystallizes and forms igneous rocks.
  • Igneous rocks - through the process of intense heat and pressure, can form metamorphic rock. Igneous rocks can be weathered and eroded into sediment and cemented into sedimentary rocks. Igneous rocks can also be transformed back into magma.
  • Sedimentary rocks - can be transformed into metamorphic rocks by intense heat and pressure of the weight of the rock itself. Sedimentary can also be transformed into magma, or melted into magma and crystallized as igneous rock. Sedimentary rocks can become other kinds of sedimentary rocks.
  • Metamorphic rocks - can be transformed into sedimentary rock through the weathering and erosion process and cementation, or can be melted into magma, and crystallized as igneous rock.
  • It is important to recognize while the rock formation cycle has a seemingly circular flow, each rock's life cycle will be unique to the geological experiences it has over time. It can go through one or all three rock types, or only two. It can go back and forth between two types. The rock cycle is a model of how every rock can experience every type of being a rock given the right geological circumstances. It is how rocks are created, altered, and destroyed over time.

Introduction

'Welcome to Earth Science TV! We are back once again to everyone's favorite show, What a Great Life! I am your host, Geo Logy. We have an all new participant this week, Ms. Crystal Rockena. Crystal, you have had an eventful life working your way through the rock cycle. You have met some interesting formations and been in some exotic places. Can you start by filling our viewers in on what the rock cycle is?' 'Sure, I'd be happy to!'

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  • 0:09 Introduction
  • 0:37 Rock Cycle
  • 2:23 Igneous Rock
  • 4:00 Sedimentary Rock
  • 5:55 Metamorphic Rock
  • 7:12 Lesson Summary
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Start at any point in the rock cycle to see how any rock can be transformed. Processes are labeled along the arrows.

Rock cycle showing how rocks are created and transformed within the cycle.

Sedimentary Rock

Sedimentary rocks are rocks that form from sediments that are broken off other rocks and cemented back together. Sediments are formed from metamorphic and igneous rocks and even other sedimentary rocks. Weathering is the first step in the sedimentary rock cycle. Weathering is the process through which tiny pieces break off the original rock, but are not moved. It can be caused by processes of the atmosphere, water or ice, biological organisms, or a chemical process.

Erosion moves the broken pieces of rock away and is usually caused by water or wind. Rains and winds are major causes of erosion. Sediments are transported via erosion processes and are then placed somewhere else and accumulate. This process is called deposition.

Compaction and cementation are the processes by which sediments that have accumulated stick together. Compaction and cementation processes are known together as lithification. As layers of sediments accumulate the bottom layer will have the most weight on it, and this makes the particles more tightly packed. This is compaction. Cementation is the infiltration of chemical sediments that act as glue through the remaining pore spaces. These chemical sediments are precipitates, solids that are the chemical product of a water-based solution. Minerals or ions in the water in the pore spaces crystalize out of the solution and cement the sediments together. This is the last step in forming sedimentary rock.

Three Types of Sedimentary Rock

There are three types of sedimentary rocks. There are clastic rocks, organic rocks, and chemical rocks. While these all form by the basic sedimentary process, there are some differences within their formation and composition that render them into different types.

Clastic sedimentary rocks are what are generally thought of when thinking of classic sedimentary rocks. The process described above is the basic process that forms clastic rocks. The particles and pieces that form the sediments of these rocks are called clasts. Bioclastic sedimentary rocks are pieces and particles from what was once living matter. These rocks can contain fossils. Rocks that contain pieces of shells or bones that were once from living organisms are bioclastic sedimentary rocks.

Here are some listed examples of clastic and bioclastic sedimentary rocks.

  • Conglomerate rocks contain pieces of many sizes of other rocks within a fine-grained structure that is cemented together.
  • Shale is layered and lithified fine-grained sediments.
  • Fossiliferous limestones contain pieces of shells or other fragments that are contained in a fine-grained calcium carbonate structure.

A boulder that has many sizes of pebbles and particles within its structure. A fine example of a conglomerate.

A pebble speckled conglomerate boulder specimen.

A piece of limestone has been cut, and fossils of shells can be clearly seen.

Limestone cut wedge with fossils of shells.

Organic sedimentary rocks are the layered and cemented--the lithified decomposed remains of plants and animals. Some examples of organic sedimentary rocks are types of coals, such as peat, lignite, and bituminous coal. These rocks are the remains of plant matter in old wetland bogs, that have lithified over time, and can be used for fuel.

Chemical sedimentary rocks are precipitates. When rocks are eroded in water, ions from the rocks dissolve into the water and form a solution. When the ions in the solutions come into contact with other ions they will form a chemical product, a solid, and precipitate out of the solution as crystals. These are called chemical sedimentary rocks and may be more familiar than most people may realize. These can be formed in salty wet inland areas, where some water has been allowed to evaporate, and the solution is ion rich. Other areas like the desert have lots of salts, and when it rains, a solution forms and rock crystals can form on the surface upon evaporation. Another place where chemical sedimentary rocks form is in caves and caverns. Stalactites and stalagmites are beautiful precipitate formations.

Stalactites are beautiful chemical sedimentary structures precipitated into rocks, generally found in caves.

Stalactite formations in a cave hang from the ceiling.

One of the main examples of a chemical sedimentary rock is halite, otherwise known as table salt. Limestone, made of calcium carbonate, is a precipitate and can form structures of chemical sedimentary rocks. Nodular types of chert, rich in silicon dioxide crystals, are chemical sedimentary rocks that form within the structures of limestone.

Remember:

Sedimentary rocks can become metamorphic rocks through intense heat and pressure.

Sedimentary rocks can become igneous rocks by melting and becoming magma and crystallizing.

Sedimentary rocks can also become other kinds of sedimentary rocks through weathering and erosion.

Igneous Rock

Igneous rocks can form from volcanic activity. This is when hot liquid magma erupts onto the surface of the earth, and it is called lava. Igneous rocks formed this way are extrusive igneous rocks. That means the minerals that form the rocks have crystallized above the surface. Intrusive igneous rocks from beneath the surface, while the magma is still a melt, and the minerals crystallize out of the melt and settle to the bottom, at certain temperatures. This happens beneath the surface in what is called a pluton. A pluton is an intrusive body of magma that solidifies and becomes igneous rock beneath the surface.

Rock Cycle

The rock cycle can take thousands of years to complete
Rock Cycle Diagram

When the earth was first created, the materials that it made were so hot from the contractions that formed the planet and from the heat generated by the core that the outer layers were mostly liquid. Over time, it slowly cooled, and a hard crust formed on the outer surface. This was much like the thin layer of ice that first forms over a pond when the temperature dips below freezing. And just as there is liquid under the layer of ice, there is still liquid magma flowing under the crust. As this happened, a cycle of events began to be put into place that causes changes in the rocks that exist on the earth. This cycle is known as the rock cycle. The rock cycle is a model used to describe the creation, alteration, and destruction of the rocks that form from magma.

It is important to note that new matter is never created on the earth. All the matter that the earth contains was present when the earth formed and still is here today. It may be in different forms, but it is still present. The rock cycle is the earth's way of recycling the matter used to make up rocks from formation to destruction and back to formation.

'And you participate in that cycle - isn't that right, Crystal?' 'Yes, Geo. All rocks, whether solid or liquid, are located at some point on that cycle.' 'How long does the whole cycle take?' 'It can take a long time, sometimes thousands of years.' 'Well, we won't ask your age or anything, but you still look great! So, Crystal, do you remember this voice?' 'Hi Crystal, remember when we both came up and were formed and spent the beginning of the cycle together? Those were some of the best times!' 'Wow! It's my good friend Iggy Neous! We started the cycle at the same time! We were igneous rocks because we were rocks that formed directly from magma or lava.'

Igneous Rock

Granite is a common igneous rock
Igneous Rock Granite

That's right, Crystal. To begin the process, magma cools and hardens either under the ground or on the earth's surface. As the rock cools, the minerals that make up the rock take shape. All magma that solidifies becomes one of two types of igneous rock: either extrusive rocks, rocks that form on the earth's surface, or intrusive rocks, rocks that form under the earth's surface. Hawaii is made up mostly of extrusive rock called basalt that came from the volcanic activity common to the area.

The most common type of intrusive rock is granite, recognized by its pink or grey color flecked with light- and dark-colored mineral specks. Igneous rocks are constantly being formed on the earth whenever magma leaks to the surface through cracks or erupts in volcanoes.

'Ah, those were good times. That is where I got my name, you know, because the flecks of color you often see in igneous rocks are crystals of minerals!' 'That's lovely! After growing up in the relative safety of a boulder of granite, your life was headed for a big change, wasn't it, Crystal?' 'It sure was, Geo. It was a very difficult time. My home was broken down by the constant exposure to the elements, and then one day it happened. I was broken apart from my family and friends in a small bit of rock.' 'How traumatic!' 'Yes it was, but then I was on to the next phase of my life, learning to be sediment.' 'But don't forget about me!' 'My good friend Sandy Grain! If it wasn't for you, I never would have found my home!' 'Yes, we were all a bit gritty, but we learned how to stick together, right?' 'Absolutely!'

Sedimentary Rock

Limestone is a kind of sedimentary rock
Sedimentary Rock Limestone

Yes, Crystal, rocks that are exposed to the weathering nature of the elements can be broken apart. Small grains of exposed rock can be broken off by the action of wind and water or broken apart as water freezes in cracks. This can affect any type of rock, not just igneous. As time passes, these grains of rock accumulate in the slow currents in rivers or river mouths, low areas, or other quiets spots.

As sediment piles up, the first sediments get buried deeper and are subjected to increased heat and pressure. Sometimes water will flow through the sediment, dissolving and redepositing glue-like substances to bind the grains together into layers. Rock made from eroded material broken off other rocks and cemented together is called sedimentary rock. One of the best examples of sedimentary rock is the layers that are seen in the Grand Canyon.

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Video Transcript

Introduction

'Welcome to Earth Science TV! We are back once again to everyone's favorite show, What a Great Life! I am your host, Geo Logy. We have an all new participant this week, Ms. Crystal Rockena. Crystal, you have had an eventful life working your way through the rock cycle. You have met some interesting formations and been in some exotic places. Can you start by filling our viewers in on what the rock cycle is?' 'Sure, I'd be happy to!'

Rock Cycle

The rock cycle can take thousands of years to complete
Rock Cycle Diagram

When the earth was first created, the materials that it made were so hot from the contractions that formed the planet and from the heat generated by the core that the outer layers were mostly liquid. Over time, it slowly cooled, and a hard crust formed on the outer surface. This was much like the thin layer of ice that first forms over a pond when the temperature dips below freezing. And just as there is liquid under the layer of ice, there is still liquid magma flowing under the crust. As this happened, a cycle of events began to be put into place that causes changes in the rocks that exist on the earth. This cycle is known as the rock cycle. The rock cycle is a model used to describe the creation, alteration, and destruction of the rocks that form from magma.

It is important to note that new matter is never created on the earth. All the matter that the earth contains was present when the earth formed and still is here today. It may be in different forms, but it is still present. The rock cycle is the earth's way of recycling the matter used to make up rocks from formation to destruction and back to formation.

'And you participate in that cycle - isn't that right, Crystal?' 'Yes, Geo. All rocks, whether solid or liquid, are located at some point on that cycle.' 'How long does the whole cycle take?' 'It can take a long time, sometimes thousands of years.' 'Well, we won't ask your age or anything, but you still look great! So, Crystal, do you remember this voice?' 'Hi Crystal, remember when we both came up and were formed and spent the beginning of the cycle together? Those were some of the best times!' 'Wow! It's my good friend Iggy Neous! We started the cycle at the same time! We were igneous rocks because we were rocks that formed directly from magma or lava.'

Igneous Rock

Granite is a common igneous rock
Igneous Rock Granite

That's right, Crystal. To begin the process, magma cools and hardens either under the ground or on the earth's surface. As the rock cools, the minerals that make up the rock take shape. All magma that solidifies becomes one of two types of igneous rock: either extrusive rocks, rocks that form on the earth's surface, or intrusive rocks, rocks that form under the earth's surface. Hawaii is made up mostly of extrusive rock called basalt that came from the volcanic activity common to the area.

The most common type of intrusive rock is granite, recognized by its pink or grey color flecked with light- and dark-colored mineral specks. Igneous rocks are constantly being formed on the earth whenever magma leaks to the surface through cracks or erupts in volcanoes.

'Ah, those were good times. That is where I got my name, you know, because the flecks of color you often see in igneous rocks are crystals of minerals!' 'That's lovely! After growing up in the relative safety of a boulder of granite, your life was headed for a big change, wasn't it, Crystal?' 'It sure was, Geo. It was a very difficult time. My home was broken down by the constant exposure to the elements, and then one day it happened. I was broken apart from my family and friends in a small bit of rock.' 'How traumatic!' 'Yes it was, but then I was on to the next phase of my life, learning to be sediment.' 'But don't forget about me!' 'My good friend Sandy Grain! If it wasn't for you, I never would have found my home!' 'Yes, we were all a bit gritty, but we learned how to stick together, right?' 'Absolutely!'

Sedimentary Rock

Limestone is a kind of sedimentary rock
Sedimentary Rock Limestone

Yes, Crystal, rocks that are exposed to the weathering nature of the elements can be broken apart. Small grains of exposed rock can be broken off by the action of wind and water or broken apart as water freezes in cracks. This can affect any type of rock, not just igneous. As time passes, these grains of rock accumulate in the slow currents in rivers or river mouths, low areas, or other quiets spots.

As sediment piles up, the first sediments get buried deeper and are subjected to increased heat and pressure. Sometimes water will flow through the sediment, dissolving and redepositing glue-like substances to bind the grains together into layers. Rock made from eroded material broken off other rocks and cemented together is called sedimentary rock. One of the best examples of sedimentary rock is the layers that are seen in the Grand Canyon.

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Frequently Asked Questions

What are the three types of rocks?

The three types of rocks are igneous, sedimentary, and metamorphic. The three types can be transformed to another type through the rock cycle.

How are the three types of rocks formed?

The three types of rocks are formed through the rock life cycle. The rock cycle is a model and shows how rocks can be created, changed, destroyed, and recreated through geological processes over time.

What happens in the sedimentary rock cycle?

The sedimentary life cycle of a rock starts with weathering and erosion of another rock. Then accumulated sediments are compacted and cemented, which is known as lithification, and a sedimentary rock is created.

What is the life cycle of a rock?

The life cycle of a rock starts with what conditions and composition it is found to be in initially, then what geological processes it undergoes that will create, change, or destroy it. This will detemine what type of rock it will become next.

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