<p>Metamorphic rocks are formed when a rock (sedimentary, igneous or a previous metamorphic rock) comes under high pressure and/or temperature. Pressure and temperature forces the atoms to form new minerals and thereby a new kind of rock. It's not necessary fragmented, but the rock rather morph through <em>recrystallization</em> into a new state in response to the pressure and temperature as the material reorganize. </p>

<p><a href="https://en.wikipedia.org/wiki/Recrystallization_(geology)" rel="nofollow noreferrer">Recrystallization</a> doesn't remove any material, it's only a physical reorganization to compact the rock. If some change of the chemical composition occurs, it's called <em>metasomatism</em>. In <a href="https://en.wikipedia.org/wiki/Metasomatism" rel="nofollow noreferrer">metasomatism</a> atoms are actually moved from one part of the rock or formation, often with water involved. </p>

<p>A good <a href="https://en.wikipedia.org/wiki/Petrology" rel="nofollow noreferrer">petrologist</a> can usually determine what kind of parent rock, <em>protolith</em>, that have been metamorphosed and to what degree, but sometimes it can be difficult do recognize the difference. At lower degrees of metamorphism, features of the original rock is preserved. E.g. you can see ripples and even fossils in slates or deformed structures from the source rock, but at higher degrees of metamorpism it's increasingly difficult to imagine how the parent material looked like. Gneiss formed from sandstone and granite can look very similar as the lihology represents the conditions of metamorphism, not the protolith. </p>

<p>It can also be difficult to recognize a metamorphic rock. A weathered slate and shale or even granite and gneiss can appear very similar in the field. However, the processes to form the rocks are different and with a closer look at the minerals it can be possible to understand what kind of process that formed the rock. </p>

<p><a href="https://i.sstatic.net/wcisW.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/wcisW.png" alt="Metamorphic phases"></a>
    <em>(1, Blueschist facies
    2, Eclogite facies
    3, Prehnite-pumpellyite-facies
    4, Greenschist facies
    5, Amphibolite-facies
    6, Granulite facies
    7, Zeolite facies
    8, Albite-epidote-hornfels facies
    9, Hornblende-hornfels facies
    10, Pyroxene-hornfels facies
    11, Sanidinite facies)</em></p>

<p>The temperature and pressure determine what metamorphic rock you get, but the chemical composition is inherited from the protolith. Degrees of metamorphism are called metamorphic <em>facies</em>. This diagram shows at what depth and temperature a particular rock is formed. </p>

<p>Igneous rocks are formed when a melt hardens to crystals and sedimentary rocks are formed from sediments. All rocks weather if exposed to water and air, and weathered material is transported by rivers to oceans where sandstone can be formed from the sand at the beach and shale from the finer sediments further away from the coast. Igneous rocks can be formed as hot magma intrudes the crust and slowly cools down and minerals are formed or form lava at volcanoes. This is known as the rock cycle and is one of the fundamental, but complicated, concepts in geology. You can read more about the rock cycle e.g. <a href="http://www.cotf.edu/ete/modules/msese/earthsysflr/rock.html" rel="nofollow noreferrer">here</a> to learn more about rock types and how they are formed. </p>

CC BY-SA 3.0
2017-01-21T10:32:16.603Z
Nice answer, as far as i understand, metamorphic is any rock which has undergone high pressure change where a previous type of rock from which it came can be determined. the result is mostly a result of viscosity, movement and recrystallyzation, where mafic and felsic minerals tend to coagulate a bit into new crystals and pockets of black and white material.
bandybabboon
CC BY-SA 3.0
2017-01-21T12:32:07.030Z
Thanks @comprehensible. I wouldn't really describe the process that way. I think that the easiest way to think about a metamorphic rock is that its been changed by pressure and/or heat over time and often/always (see 'discussion' in comments above) with fluids. It can be very difficult to determine what kind of protolith it was, but for lower grade metamorphism it's often possible. The new minerals are formed as a respond to the changed environment. Not necessarily in a mafic/felsic order.
user2821
CC BY-SA 3.0
2017-01-22T09:00:26.333Z
Hey cool. I find the chemistry aspect very difficult to imagine, because even chemistry that is seen is so wondrous and enigmatic. Watching ordinary minerals grow is confusing enough, it's difficult when there is a mix of 20 minerals sorting themselves like some kind of a lava lamp inside stones. It's good to compare the stones to working hot glass, when glass is 300 degrees you can stand on it and it will slowly change shape until it's flat, and that can take one minute or 20 minutes depending on the glass transition stage.
bandybabboon
CC BY-SA 3.0
2017-01-09T16:24:03.647Z
2017-01-10T12:54:53.463Z
2017-01-10T12:54:53.463Z
user2821
<p>The existing answers are correct but I think they miss an important aspect of your question, which is the effect of pressure.</p>

<blockquote>
  <p>Do they become fragments of rocks?</p>
</blockquote>

<p>No. Fragmentation of rocks is a relatively low temperature and pressure process. This is the kind of stuff you would see in near-surface environments. Once rocks become hot under pressure, they are no longer brittle but are ductile. This means they can bend and flow. Think of chocolate: put it in the freezer and it's hard as a rock, but if you take it out (when it's still solid) it becomes easier to bend it and shape it with your hands.</p>

<p>Same with rocks. A very common feature of metamorphic rocks is folding:</p>

<p><a href="https://i.sstatic.net/y6aXZ.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/y6aXZ.jpg" alt="enter image description here"></a> <sup>(<a href="https://commons.wikimedia.org/wiki/File:Agiospavlos_DM_2004_IMG003_Felsenformation_nahe.JPG" rel="nofollow noreferrer">source</a>)</sup></p>

<blockquote>
  <p>Doesn't that mean that the difference between sedimentary rocks and metamorphic rocks (when talking about fragment rocks) is that sedimentary rocks undergo weathering, which makes them fragments and metamorphic rocks undergo immense pressure which makes it into fragments? </p>
</blockquote>

<p>No. The immense pressure actually works well to hold everything together instead of fragment them. There is some fragmentation in metamorphic rocks, but it's usually manifested as faulting rather than fragmentation into pieces (also known as brecciation). Here's an example:</p>

<p><a href="https://i.sstatic.net/m7i2Q.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/m7i2Q.jpg" alt="enter image description here"></a><sup>(<a href="http://www.sci.ccny.cuny.edu/~mcesaire/earthhistory.html" rel="nofollow noreferrer">source</a>)</sup></p>

CC BY-SA 3.0
2017-01-21T07:26:52.113Z
2017-01-21T07:26:52.113Z
Gimelist
<p>Metamorphic can have been halfway to the state of lava for a short time, it may have been like quite tough dough of bread. You can imagine the viscosity of it to be the same as ordinary glass which is at 200 degrees, if you have a window at 200 degrees, you can bend it into a U shape in a time ranging from 20 minutes to 20 seconds depending on the temperature it is at, it becomes more malleable. instead of sand, metamorphic is hot limestone/clay/pebbles...</p>

<p>The dough slowly moves and deforms, for example by pressure of buckling tectonics from distant tectonic shocks like rising mountain chains. The chemicals inside the dough disamalgamate and coagulate into chemicals that have easy bond affiliation. Every chemical reacts differently and has more or less strong bonds at a set pressure of metamorphism, different blobs inside the metamorphic rock may be more or less mobile and viscous. You get crystals of different sizes of new materials, more often roundish, sometimes flat (gneiss) indicating the flow/pressure in the dough, the crystals branch out not that differently from ice crystals, they recrystallize re-mineralize. The time and pressure of metamorphosis vary a lot and determine the level of change that happens. </p>

CC BY-SA 3.0
2017-01-21T10:45:13.160Z
2017-01-22T09:08:10.357Z
2017-01-22T09:08:10.357Z
bandybabboon
<p><a href="https://en.wikipedia.org/wiki/Metamorphic_rock" rel="nofollow noreferrer">Metamorphic rocks</a> are changed by transformations deep underground. Being deep underground there is immense pressure and heat. </p>

<p>The transformations can be just crystal size of the particular mineral, or different minerals can be in fact formed. For a particular mineral there are also may be different crystal structures which depend on the pressure and temperature at which the crystal was formed. The different compositions and crystal structures would be shown on geochemical phases diagrams.  <a href="https://en.wikipedia.org/wiki/Geochemical_modeling" rel="nofollow noreferrer">Geochemical modeling</a> could be used to predict the various reactions based on the temperature and pressure profile to which the material is subjected. </p>

CC BY-SA 3.0
2017-01-09T16:31:15.657Z
2017-01-09T16:31:15.657Z
MaxW
<p>When metamorphic rocks undergo pressure they are changed, but how are they changed? Do they become fragments of rocks? Doesn't that mean that the difference between sedimentary rocks and metamorphic rocks (when talking about fragment rocks) is that sedimentary rocks undergo weathering, which makes them fragments and metamorphic rocks undergo immense pressure which makes it into fragments?</p>

CC BY-SA 3.0
2017-01-10T22:06:04.763Z
We had an answer by a user called xxCDxx here noting the requirement of water for metamorphic reactions, and this was downvoted and eventually deleted by xxCDxx. I'd like to say that he was in fact correct, and the role of water is becoming increasingly understood as vital for metamorphic reactions and I'm sorry to see that answer downvoted and deleted. This might be a good question though - "what is the role of water in metamorphism"? If anyone's up for that.
Gimelist
CC BY-SA 3.0
2017-01-20T08:22:03.200Z
@Michael That is news for me, knowing that water is important, I've always though that metamorphism could take place even in dry rock. Can you suggest any good reference?
user2821
CC BY-SA 3.0
2017-01-20T21:20:16.617Z
@Tbb ask a question about it, I'll provide an answer
Gimelist
CC BY-SA 3.0
2017-01-20T21:44:29.163Z
Yeah, this isn't related at all to my question.
Okama Ksakas
CC BY-SA 3.0
2017-01-21T00:47:55.160Z
I'd say it's very related to op's question as we should try our best to give correct answers based on modern science.
user2821
CC BY-SA 3.0
2017-01-21T06:28:56.423Z
@Tbb since you were asking for a reference [Details of the gabbro-to-eclogite transition determined from microtextures and calculated chemical potential relationships](http://dx.doi.org/10.1111/jmg.12220)
Gimelist
CC BY-SA 3.0
2017-01-21T07:14:01.723Z
Thanks @Michael. I'll have a look at it! I've been trying to formulate a clarifying question, but it's difficult as it hard to look for negative correlations. That eclogitization requires fluids is known from many studies, but I couldn't find good resources for the need of water in e.g.retrograde metamorphism or contact metamorphism. As some fluid is almost always present, it would be hard to argue that Hornfels could not be formed without water.
user2821
CC BY-SA 3.0
2017-01-21T07:15:38.983Z
@Tbb It's not 100% required, but it helps heaps, especially if you want a grain size larger than a few microns. Note that most of the fluids does not come from any infiltration event, like the eclogite, but from dehydration of in-situ phases.
Gimelist
CC BY-SA 3.0
2017-01-21T07:24:30.237Z
@Michael OK. We agree completely. I'm well aware of the importance of fluids, but still think that the formulation of the answer was a bit misleading. Probably there are some sloppy simplifications in my answer too that should be questioned and improved. However, if you stumble across any study of dry metamorphism, I'd be interested to know!
user2821
CC BY-SA 3.0
2017-01-21T07:27:45.763Z
@Tbb well we do dry metamorphism in our experimental petrology lab almost every day. Anyway, we're getting offtopic here.
Gimelist
<geology><rocks><metamorphism>
When metamorphic rocks undergo pressure, how are they changed?
Okama Ksakas
# When metamorphic rocks undergo pressure, how are they changed?

(Asked by: username_0 on 2017-01-09)

When metamorphic rocks undergo pressure they are changed, but how are they changed? Do they become fragments of rocks? Doesn't that mean that the difference between sedimentary rocks and metamorphic rocks (when talking about fragment rocks) is that sedimentary rocks undergo weathering, which makes them fragments and metamorphic rocks undergo immense pressure which makes it into fragments?

**Tags:** geology, rocks, metamorphism

> **Comments:**
> **username_1**: We had an answer by a user called xxCDxx here noting the requirement of water for metamorphic reactions, and this was downvoted and eventually deleted by xxCDxx. I'd like to say that he was in fact correct, and the role of water is becoming increasingly understood as vital for metamorphic reactions and I'm sorry to see that answer downvoted and deleted. This might be a good question though - "what is the role of water in metamorphism"? If anyone's up for that.  
> **username_2**: @Michael That is news for me, knowing that water is important, I've always though that metamorphism could take place even in dry rock. Can you suggest any good reference?  
> **username_1**: @Tbb ask a question about it, I'll provide an answer  
> **username_0**: Yeah, this isn't related at all to my question.  
> **username_2**: I'd say it's very related to op's question as we should try our best to give correct answers based on modern science.  
> **username_1**: @Tbb since you were asking for a reference [Details of the gabbro-to-eclogite transition determined from microtextures and calculated chemical potential relationships](http://dx.doi.org/10.1111/jmg.12220)  
> **username_2**: Thanks @Michael. I'll have a look at it! I've been trying to formulate a clarifying question, but it's difficult as it hard to look for negative correlations. That eclogitization requires fluids is known from many studies, but I couldn't find good resources for the need of water in e.g.retrograde metamorphism or contact metamorphism. As some fluid is almost always present, it would be hard to argue that Hornfels could not be formed without water.  
> **username_1**: @Tbb It's not 100% required, but it helps heaps, especially if you want a grain size larger than a few microns. Note that most of the fluids does not come from any infiltration event, like the eclogite, but from dehydration of in-situ phases.  
> **username_2**: @Michael OK. We agree completely. I'm well aware of the importance of fluids, but still think that the formulation of the answer was a bit misleading. Probably there are some sloppy simplifications in my answer too that should be questioned and improved. However, if you stumble across any study of dry metamorphism, I'd be interested to know!  
> **username_1**: @Tbb well we do dry metamorphism in our experimental petrology lab almost every day. Anyway, we're getting offtopic here.  

---

## 4 answers

### Answer by username_4 on 2017-01-09. Score: 3 | Accepted answer

[Metamorphic rocks](https://en.wikipedia.org/wiki/Metamorphic_rock) are changed by transformations deep underground. Being deep underground there is immense pressure and heat.

The transformations can be just crystal size of the particular mineral, or different minerals can be in fact formed. For a particular mineral there are also may be different crystal structures which depend on the pressure and temperature at which the crystal was formed. The different compositions and crystal structures would be shown on geochemical phases diagrams. [Geochemical modeling](https://en.wikipedia.org/wiki/Geochemical_modeling) could be used to predict the various reactions based on the temperature and pressure profile to which the material is subjected.


---

### Answer by username_2 on 2017-01-09. Score: 6

Metamorphic rocks are formed when a rock (sedimentary, igneous or a previous metamorphic rock) comes under high pressure and/or temperature. Pressure and temperature forces the atoms to form new minerals and thereby a new kind of rock. It's not necessary fragmented, but the rock rather morph through *recrystallization* into a new state in response to the pressure and temperature as the material reorganize.

[Recrystallization](https://en.wikipedia.org/wiki/Recrystallization_(geology)) doesn't remove any material, it's only a physical reorganization to compact the rock. If some change of the chemical composition occurs, it's called *metasomatism*. In [metasomatism](https://en.wikipedia.org/wiki/Metasomatism) atoms are actually moved from one part of the rock or formation, often with water involved.

A good [petrologist](https://en.wikipedia.org/wiki/Petrology) can usually determine what kind of parent rock, *protolith*, that have been metamorphosed and to what degree, but sometimes it can be difficult do recognize the difference. At lower degrees of metamorphism, features of the original rock is preserved. E.g. you can see ripples and even fossils in slates or deformed structures from the source rock, but at higher degrees of metamorpism it's increasingly difficult to imagine how the parent material looked like. Gneiss formed from sandstone and granite can look very similar as the lihology represents the conditions of metamorphism, not the protolith.

It can also be difficult to recognize a metamorphic rock. A weathered slate and shale or even granite and gneiss can appear very similar in the field. However, the processes to form the rocks are different and with a closer look at the minerals it can be possible to understand what kind of process that formed the rock.

[![Metamorphic phases](https://i.sstatic.net/wcisW.png)](https://i.sstatic.net/wcisW.png)
*(1, Blueschist facies
2, Eclogite facies
3, Prehnite-pumpellyite-facies
4, Greenschist facies
5, Amphibolite-facies
6, Granulite facies
7, Zeolite facies
8, Albite-epidote-hornfels facies
9, Hornblende-hornfels facies
10, Pyroxene-hornfels facies
11, Sanidinite facies)*

The temperature and pressure determine what metamorphic rock you get, but the chemical composition is inherited from the protolith. Degrees of metamorphism are called metamorphic *facies*. This diagram shows at what depth and temperature a particular rock is formed.

Igneous rocks are formed when a melt hardens to crystals and sedimentary rocks are formed from sediments. All rocks weather if exposed to water and air, and weathered material is transported by rivers to oceans where sandstone can be formed from the sand at the beach and shale from the finer sediments further away from the coast. Igneous rocks can be formed as hot magma intrudes the crust and slowly cools down and minerals are formed or form lava at volcanoes. This is known as the rock cycle and is one of the fundamental, but complicated, concepts in geology. You can read more about the rock cycle e.g. [here](http://www.cotf.edu/ete/modules/msese/earthsysflr/rock.html) to learn more about rock types and how they are formed.

> **Comments:**
> **username_3**: Nice answer, as far as i understand, metamorphic is any rock which has undergone high pressure change where a previous type of rock from which it came can be determined. the result is mostly a result of viscosity, movement and recrystallyzation, where mafic and felsic minerals tend to coagulate a bit into new crystals and pockets of black and white material.  
> **username_2**: Thanks @comprehensible. I wouldn't really describe the process that way. I think that the easiest way to think about a metamorphic rock is that its been changed by pressure and/or heat over time and often/always (see 'discussion' in comments above) with fluids. It can be very difficult to determine what kind of protolith it was, but for lower grade metamorphism it's often possible. The new minerals are formed as a respond to the changed environment. Not necessarily in a mafic/felsic order.  
> **username_3**: Hey cool. I find the chemistry aspect very difficult to imagine, because even chemistry that is seen is so wondrous and enigmatic. Watching ordinary minerals grow is confusing enough, it's difficult when there is a mix of 20 minerals sorting themselves like some kind of a lava lamp inside stones. It's good to compare the stones to working hot glass, when glass is 300 degrees you can stand on it and it will slowly change shape until it's flat, and that can take one minute or 20 minutes depending on the glass transition stage.  

---

### Answer by username_1 on 2017-01-21. Score: 2

The existing answers are correct but I think they miss an important aspect of your question, which is the effect of pressure.

> Do they become fragments of rocks?

No. Fragmentation of rocks is a relatively low temperature and pressure process. This is the kind of stuff you would see in near-surface environments. Once rocks become hot under pressure, they are no longer brittle but are ductile. This means they can bend and flow. Think of chocolate: put it in the freezer and it's hard as a rock, but if you take it out (when it's still solid) it becomes easier to bend it and shape it with your hands.

Same with rocks. A very common feature of metamorphic rocks is folding:

[![enter image description here](https://i.sstatic.net/y6aXZ.jpg)](https://i.sstatic.net/y6aXZ.jpg) ([source](https://commons.wikimedia.org/wiki/File:Agiospavlos_DM_2004_IMG003_Felsenformation_nahe.JPG))

> Doesn't that mean that the difference between sedimentary rocks and metamorphic rocks (when talking about fragment rocks) is that sedimentary rocks undergo weathering, which makes them fragments and metamorphic rocks undergo immense pressure which makes it into fragments?

No. The immense pressure actually works well to hold everything together instead of fragment them. There is some fragmentation in metamorphic rocks, but it's usually manifested as faulting rather than fragmentation into pieces (also known as brecciation). Here's an example:

[![enter image description here](https://i.sstatic.net/m7i2Q.jpg)](https://i.sstatic.net/m7i2Q.jpg)([source](http://www.sci.ccny.cuny.edu/~mcesaire/earthhistory.html))


---

### Answer by username_3 on 2017-01-21. Score: 2

Metamorphic can have been halfway to the state of lava for a short time, it may have been like quite tough dough of bread. You can imagine the viscosity of it to be the same as ordinary glass which is at 200 degrees, if you have a window at 200 degrees, you can bend it into a U shape in a time ranging from 20 minutes to 20 seconds depending on the temperature it is at, it becomes more malleable. instead of sand, metamorphic is hot limestone/clay/pebbles...

The dough slowly moves and deforms, for example by pressure of buckling tectonics from distant tectonic shocks like rising mountain chains. The chemicals inside the dough disamalgamate and coagulate into chemicals that have easy bond affiliation. Every chemical reacts differently and has more or less strong bonds at a set pressure of metamorphism, different blobs inside the metamorphic rock may be more or less mobile and viscous. You get crystals of different sizes of new materials, more often roundish, sometimes flat (gneiss) indicating the flow/pressure in the dough, the crystals branch out not that differently from ice crystals, they recrystallize re-mineralize. The time and pressure of metamorphosis vary a lot and determine the level of change that happens.


---

