Origin of Metamorphic Rocks

Summary

This educational video script explores the fascinating process of metamorphism, which transforms sedimentary and igneous rocks into metamorphic rocks under intense pressure and heat. It explains how minerals recrystallize, leading to the formation of rocks like slate, phyllite, schist, and gneiss. The script also delves into the classification of metamorphic intensity and the role of tectonic environments in shaping these geological wonders.

Takeaways

• 🔥 Minerals crystallize from magma to form igneous rocks, which can become sedimentary rocks through uplift, weathering, and deposition.
• 🪨 Sedimentary and igneous rocks can undergo metamorphism due to high pressure and temperature, leading to the formation of metamorphic rocks.
• ⚡ Metamorphism involves the recrystallization of minerals without melting, caused by pressure, temperature, or chemically active fluids.
• 🔄 Minerals change when they move outside their stability range, leading to the formation of more stable minerals under new conditions.
• 📉 Low-grade metamorphism starts above diagenesis (above 200°C), with increasing grades as temperature and pressure rise.
• 📏 Metamorphic intensity is classified as low, medium, or high grade, depending on the conditions and tectonic environment.
• 🔨 Slate is a low-grade metamorphic rock formed from clay minerals, showing good cleavage due to aligned minerals.
• 💎 Phyllite is a medium-grade metamorphic rock, with fine-grained recrystallized micas giving it a shiny appearance.
• ✨ Schist forms as metamorphism intensifies, featuring visible mica grains and sometimes garnet or amphibole minerals.
• 🔀 High-grade metamorphism leads to the segregation of mafic and felsic minerals, resulting in banded rocks called gneiss.

Q & A

What is the process by which minerals crystallize from magma to form igneous rocks?

The process involves the cooling and solidification of magma, which leads to the formation of crystals that make up igneous rocks.

How do sedimentary rocks form from sediments?

Sedimentary rocks form when sediments are transported, deposited in a basin, buried, compacted, and cemented together.

What is metamorphism and how does it relate to the formation of metamorphic rocks?

Metamorphism is the process where sedimentary or igneous rocks are altered by pressure and temperature, causing them to recrystallize without melting, leading to the formation of metamorphic rocks.

How is metamorphism defined in terms of rock changes?

Metamorphism is defined as any change in the mineralogy or physical structure of a rock due to natural processes such as increased pressure and temperature, or the introduction of chemically active fluids, without the rock melting into magma.

What causes minerals to break down during metamorphism?

Minerals break down during metamorphism when the physical and chemical conditions move out of the stability range for those minerals.

Why do igneous rocks undergo chemical weathering at the surface?

Igneous rocks undergo chemical weathering at the surface because the minerals that form in an igneous environment are not stable in the water and oxygen-rich conditions found there.

What is the difference between diagenesis and metamorphism?

Diagenesis is a process that occurs at lower temperatures and pressures, causing slight recrystallization of minerals, while metamorphism involves higher temperatures and pressures leading to significant recrystallization.

How is metamorphic intensity classified?

Metamorphic intensity is classified as low grade, medium grade, or high grade, based on the pressure and temperature conditions during metamorphism.

What characterizes a low-grade metamorphic rock?

A low-grade metamorphic rock is characterized by the beginning of cleavage development, often seen in rocks like slate which have good cleavage that causes them to break along planar surfaces.

What minerals typically recrystallize in medium-grade metamorphic rocks?

In medium-grade metamorphic rocks, clay minerals typically recrystallize into other sheet silicates such as muscovite and chlorite.

What features define high-grade metamorphic rocks?

High-grade metamorphic rocks are defined by the segregation of mafic and felsic minerals during recrystallization, which results in alternating bands of light and dark minerals, as seen in gneiss.

Outlines

00:00

🌋 Formation of Metamorphic Rocks

This paragraph discusses the process of metamorphism, which is the transformation of existing rocks into metamorphic rocks due to high pressure and temperature. It explains that minerals in igneous or sedimentary rocks recrystallize under these conditions without melting into magma. The stability of minerals is dependent on physical and chemical conditions, and when these conditions change, minerals break down into more stable forms. The paragraph also introduces the concept of metamorphic intensity, which is classified into low, medium, and high grades based on the pressure and temperature conditions. An example is given with mudstone, illustrating how it transforms into slate at low-grade metamorphism, characterized by the development of foliation or cleavage due to the alignment of platy minerals under stress.

05:03

🔍 Types and Characteristics of Metamorphic Rocks

The second paragraph delves into the classification of metamorphic rocks based on their metamorphic grade and the tectonic environment. It describes the progression from low-grade metamorphism, where rocks like slate are formed with good cleavage, to medium-grade metamorphism, resulting in rocks like phyllite and schist. The paragraph explains that at higher temperatures and pressures, minerals like muscovite and chlorite recrystallize into other minerals, enhancing the rock's cleavage. High-grade metamorphic rocks, such as gneiss, are characterized by the segregation of light and dark minerals into alternating bands. The paragraph sets the stage for a closer examination of different types of metamorphism and their corresponding rock types.

Keywords

💡Metamorphism

Metamorphism is the process by which existing rock undergoes a change in mineralogy or physical structure due to natural processes such as increased pressure and temperature, or the introduction of chemically active fluids, without melting into magma. It is central to the video's theme as it discusses the formation of metamorphic rocks. For instance, the script mentions that when a sedimentary or igneous rock gets squashed between two colliding plates, metamorphism occurs, leading to the recrystallization of the original rock.

💡Igneous Rocks

Igneous rocks are formed from the solidification of molten material called magma. They are mentioned in the script as one of the precursors to metamorphic rocks, illustrating how rocks can change from one type to another through geological processes. The script explains that igneous rocks can undergo chemical weathering at the surface, leading to the formation of more stable minerals.

💡Sedimentary Rocks

Sedimentary rocks are formed from the accumulation and cementation of mineral and organic particles on the Earth's surface. The script discusses how sedimentary rocks, when subjected to high pressure and temperature, can become metamorphic rocks. This keyword is important for understanding the origin of metamorphic rocks and the transformation process they undergo.

💡Recrystallization

Recrystallization is the process where minerals in a rock change their crystal structure due to changes in temperature and pressure. It is a key concept in the video as it explains how the original rock's minerals are altered during metamorphism. The script uses recrystallization to differentiate between different grades of metamorphic rocks, such as slate, phyllite, and schist.

💡Foliation

Foliation is a planar or linear arrangement of minerals in metamorphic rocks, often resulting from the alignment of platy or elongated minerals. It is a significant feature that helps in determining the metamorphic grade of a rock. The script describes how foliation develops in rocks rich in sheet silicates and is used to define low-grade metamorphic rocks like slate.

💡Cleavage

Cleavage refers to the tendency of a mineral or rock to break along certain planes. It is mentioned in the context of slate, a low-grade metamorphic rock, which has good cleavage that allows it to break along planar surfaces. This property makes slate suitable for building materials, as highlighted in the script.

💡Metamorphic Grade

Metamorphic grade is a classification that describes the intensity of metamorphism a rock has undergone, which is related to the pressure and temperature conditions it experienced. The script explains that there are low, medium, and high grades of metamorphism, each with distinct characteristics and examples, such as phyllite and schist for medium grade.

💡Schist

Schist is a medium to high-grade metamorphic rock characterized by a banding of alternating light and dark minerals resulting from the segregation of mafic and felsic minerals. It is mentioned in the script as an example of a rock that forms under higher temperatures and pressures, where individual mica grains become visible to the naked eye.

💡Gneiss

Gneiss is a high-grade metamorphic rock that displays a banding of light and dark minerals, indicative of the segregation of mafic and felsic minerals during recrystallization. The script uses gneiss to illustrate the characteristic foliation of high-grade metamorphic rocks, which is a result of extreme metamorphic conditions.

💡Diagenesis

Diagenesis refers to the physical, chemical, and biological changes that occur in sediments as they become compacted and lithified into sedimentary rocks. The script mentions diagenesis in the context of low-grade metamorphism, where slightly increased pressures and temperatures can cause recrystallization of minerals like calcite.

💡Tectonic Environment

The tectonic environment refers to the geological setting where rocks are subjected to various forces, including those that cause metamorphism. The script discusses how metamorphism is classified based on its intensity and pressure with respect to temperature, which corresponds to the tectonic environment where the metamorphism took place.

Highlights

Minerals crystallize from liquid hot magma to form igneous rocks, which over time are uplifted, broken down into sediments, transported, and deposited in a basin where they are buried, compacted, and cemented to become sedimentary rock.

Metamorphism occurs when sedimentary or igneous rocks get squashed between colliding tectonic plates, causing the original rock to recrystallize due to high pressures and temperatures.

Metamorphism is defined as any change in the mineralogy or physical structure of a rock by natural processes, such as increased pressure and temperature, or the introduction of chemically active fluids, without melting the rock into magma.

Every mineral has a range of physical and chemical conditions, such as temperature and pressure, over which it is stable. When conditions move out of this stability range, the mineral breaks down into more stable minerals.

Igneous rocks undergo chemical weathering at the surface because minerals stable at high temperatures are not stable in water and oxygen-rich conditions, leading them to react and form more stable minerals like clay.

Diagenesis is considered super-low-grade metamorphism, where slightly increased pressures and temperatures in a sedimentary basin cause recrystallization of minerals like calcite.

Metamorphism involves temperatures greater than 200 degrees Celsius, beyond the realm of diagenesis, and is classified based on intensity and pressure with respect to temperature, corresponding to different tectonic environments.

Metamorphic intensity is classified as low-grade, medium-grade, or high-grade, with low-grade starting above diagenesis conditions and high-grade up to the point where melting begins.

A rock's metamorphic grade is determined by the amount of recrystallization and foliation. For example, mudstone metamorphoses into slate, phyllite, schist, and gneiss as temperature and pressure increase.

Low-grade metamorphic rocks, like slate, are defined by the alignment of platy minerals, such as clay, muscovite, and biotite, which create foliation or cleavage.

Medium-grade metamorphic rocks, like phyllite and schist, show increased recrystallization of sheet silicates, giving them a lustrous sheen, with larger mica grains visible to the naked eye in schist.

High-grade metamorphic rocks form at temperatures above 600 degrees Celsius and pressures above 8 kilobars, resulting in the segregation of mafic and felsic minerals into alternating bands, characteristic of gneiss.

The transformation of rocks into gneiss involves the instability of minerals like chlorite and muscovite, which react with quartz to form more stable minerals like potassium feldspar, garnet, and biotite.

Metamorphic rocks provide insights into the tectonic environment where they formed, with different types and grades corresponding to varying pressures and temperatures experienced by the rock.

Understanding metamorphism is essential for interpreting the geological history of an area, as it reveals the processes and conditions that shaped the Earth's crust over time.

Different types of metamorphism include regional, contact, and dynamic metamorphism, each associated with distinct pressure-temperature conditions and resulting in different types of metamorphic rocks.

Transcripts

In this series, we have learned about how  minerals crystallize from liquid hot magma  

to form igneous rocks, which over time,  are uplifted, broken down into sediments,  

transported, and eventually deposited in a basin,  where they are buried, compacted, and cemented to  

become sedimentary rock. In this tutorial, we  will discuss what happens when a sedimentary  

or igneous rock gets squashed in between two  colliding plates, and the tremendous pressures  

and temperatures that result cause the original  rock to recrystallize. We are of course talking  

about metamorphism, and this is the origin of our  third and final type of rock, metamorphic rock. 

Metamorphism is defined as any change in  the mineralogy or physical structure of  

a rock by natural processes, such as increased  pressure and temperature, or the introduction  

of chemically active fluids, without melting it  into magma. To truly understand metamorphism,  

we must understand that every mineral has a range  of physical conditions, such as temperature and  

pressure, as well as chemical conditions, over  which it is stable. When the conditions move  

out of a mineral’s stability range, it will  break down into other, more stable minerals.  

This is why igneous rocks undergo chemical  weathering at the surface. Minerals that  

form in an igneous environment are stable at very  high temperatures where both oxygen and water are  

scarce, but they are not stable in the water and  oxygen rich conditions at the surface. Therefore,  

they react to form more stable minerals  like clay. Diagenesis can also be thought  

of as super-low-grade metamorphism, where the  slightly increased pressures and temperatures  

deep within a sedimentary basin can cause  recrystallization of minerals like calcite.  

However, when geologists talk about metamorphism,  they are referring to rocks that recrystallize  

at temperatures greater than 200 degrees  Celsius, far above the realm of diagenesis. 

Metamorphism is classified based on both its  intensity, and its pressure with respect to  

temperature, which corresponds to the tectonic  environment where metamorphism took place.  

Metamorphic intensity can be classified as either  low grade, medium grade, or high grade, with low  

grade describing the pressure and temperature  conditions above the realm of diagenesis,  

high-grade encompassing the conditions up until  melting begins, and medium grade being in between.  

When examining a metamorphic rock, its  metamorphic grade can be determined by the  

amounts of recrystallization and foliation. To put  this into context, let’s take a mudstone, which  

is a sedimentary rock primarily composed of clay  minerals, and progressively metamorphose the rock.  

As we increase the temperature to around 300  degrees Celsius and add a few kilobars of  

pressure, the platy and elongate minerals will  begin to align perpendicular to the direction  

of greatest stress. This transformation is most  radical in rocks that are rich in sheet silicates,  

like clay, muscovite, and biotite. In rocks,  this alignment of platy minerals is said to  

create foliation, or cleavage, and the beginning  of the development of cleavage is what defines a  

low-grade metamorphic rock, which is called  slate. Slate makes great building stones  

because of its good cleavage, which causes  it to break along nice, planar surfaces. 

As we continue to ramp up the pressure and  temperature to around 400 degrees Celsius  

and 5 kilobars, the clay minerals will begin  to recrystallize into other sheet silicates,  

most commonly muscovite and chlorite, which will  grow with their sheets, or basal cleavage, lining  

up perpendicular to the main stress, further  enhancing the rock’s cleavage. These medium-grade  

metamorphic rocks are distinguished from slate by  the onset of recrystallization, which gives it a  

lustrous sheen. At the lower end of medium-grade  metamorphism, the recrystallized micas are very  

fine-grained, having the appearance of glitter.  This type of rock is called phyllite. Closer to  

the upper limit, recrystallization increases to  the point where the individual mica grains become  

easily visible to the naked eye. This rock is  called schist. In addition to mica, schists may  

contain other visible minerals, such as garnet  dodecahedra or prismatic grains of amphibole.  

Once the temperature and pressure surpass around  600 degrees Celsius and 8 kilobars, the previously  

stable chlorite and muscovite become unstable and  react with quartz to form more stable minerals  

such as potassium feldspar, garnet, sillimanite,  and biotite. The characteristic feature of  

high-grade metamorphic rocks is the segregation  of the mafic and felsic minerals during  

recrystallization, which manifests as alternating  bands of light and dark minerals. Rocks that  

display this type of foliation are called gneiss. Now that we understand how metamorphic rocks form,  

let’s move forward and get a closer look  at the different types of metamorphism,  

so we can better understand the  different types of metamorphic rocks.