Metamorphism

Summary

Metamorphic rocks are formed through metamorphism, a process involving physical and chemical changes caused by heat, pressure, and fluids. When rocks are buried deep beneath Earth's surface, they undergo transformation depending on the temperature, pressure, and water availability. Different geological settings, such as subduction zones, volcanic centers, and colliding continental plates, influence these changes. Key indicators of metamorphic conditions, such as index minerals like garnet and kyanite, provide clues to a rock’s formation environment. This summary highlights how metamorphic rocks differ from igneous and sedimentary rocks, with a focus on their unique formation processes.

Takeaways

• 🌋 Metamorphic rocks are formed when existing rocks undergo metamorphism due to high temperatures, pressures, or chemically active fluids.
• 🔥 If a rock is subjected to extreme heat and fully melts, it becomes magma and transforms into an igneous rock.
• 🔄 Metamorphic rocks sit between sedimentary and igneous rocks, formed under conditions of medium heat and pressure.
• 💧 The composition of metamorphic rocks depends on the parent rock's chemistry, temperature, pressure, and water availability.
• 🌍 Metamorphism occurs deep within Earth's crust, often due to burial or tectonic activities like subduction and continental collisions.
• 🗻 In subduction zones, high pressure but low temperatures lead to unique metamorphic environments due to the cold, sinking plate.
• ⛏️ Volcanic centers generate heat and water, resulting in chemical changes in rocks through processes like contact and hydrothermal metamorphism.
• 📏 Increased pressure leads to higher rock density and causes minerals with elongated shapes to align, forming foliation patterns.
• 🌡️ Heat can increase crystal size as atoms vibrate, migrate, and combine into larger structures without melting the rock.
• 🔍 Index minerals like garnet and kyanite help geologists identify the conditions under which metamorphic rocks were formed, providing clues about pressures and temperatures in specific geologic settings.

Q & A

What is metamorphism in rocks?

Metamorphism is the process where rocks undergo physical and chemical changes due to exposure to elevated temperatures, pressures, or chemically active fluids beyond what is normally found near Earth's surface.

What happens to a rock if it melts completely during metamorphism?

If a rock melts completely, it becomes magma and transitions into an igneous rock. This means that temperatures in metamorphism must be lower than the melting point.

How do temperature and pressure affect sedimentary and metamorphic rocks differently?

Sedimentary rocks form under lower temperatures and pressures, compacting and cementing sediments. Metamorphic rocks form at higher temperatures and pressures, but not high enough for the rock to melt, sitting between sedimentary and igneous environments.

What factors determine the specific type of metamorphic rock that forms?

The type of metamorphic rock depends on the parent rock's chemistry, the exact temperatures and pressures it was subjected to, and the amount of water available for chemical reactions.

Where is metamorphism likely to occur on Earth?

Metamorphism occurs when surface rocks are buried beneath layers of sediment or lava, sinking deeper into the Earth's crust where pressures and temperatures increase. It also happens in geologic settings like subduction zones and converging continental plates.

What are the pressure and temperature conditions in subduction zones?

In subduction zones, pressures increase significantly as the plate sinks deeper into the mantle, but temperatures rise more slowly because the subducting plate is cold and insulated.

How does water content influence metamorphic processes in subduction zones?

Water content can be high in subduction zones due to water absorbed by sediments and rocks in contact with ocean water over millions of years. This water contributes to chemical reactions during metamorphism.

What is the difference between contact metamorphism and hydrothermal metamorphism?

Contact metamorphism occurs near magma chambers where rocks undergo changes due to high temperatures, while hydrothermal metamorphism happens when hot chemically active fluids migrate through rocks, causing chemical alterations.

What physical changes occur in rocks during metamorphism?

During metamorphism, rocks may experience increased density, and minerals with long or flat axes align to relieve pressure (foliation). Additionally, crystals can grow larger due to increased heat, as atoms migrate across solid structures.

What are index minerals and why are they important?

Index minerals form under specific temperature and pressure conditions and serve as clues to the rock's metamorphic history. For example, garnet indicates higher pressures and temperatures, while kyanite suggests high pressure but relatively low temperature.

Outlines

00:00

🌋 Introduction to Metamorphic Rocks

Metamorphic rocks form when pre-existing rocks undergo metamorphism, which involves physical and chemical changes due to high temperatures, pressures, or chemically active fluids. These conditions are typically found deeper beneath Earth's surface, but not high enough to cause melting, which would form igneous rocks. Metamorphism falls between sedimentary and igneous processes. The type of metamorphic rock produced depends on factors such as the chemistry of the parent rock, the pressures, temperatures, and water availability during the metamorphic process.

05:02

🧭 How and Where Metamorphism Occurs

Metamorphism happens when surface rocks are buried under layers of sediment or lava, leading to increasing pressure and temperature as they sink deeper into the crust. The intensity of the change, or metamorphism, is higher in areas like subduction zones or where continents converge. Pressure in such regions is extreme due to plate collision, and temperature rises with depth. In subduction zones, plates sink into the mantle, experiencing high pressure but slower temperature increases because of the cold, insulated plates. Water plays a role in the metamorphic processes in these environments, being trapped in minerals or sourced from the ocean.

🌋 Volcanic Centers and Contact Metamorphism

Metamorphism near volcanic centers is characterized by high temperatures but low pressures. Water, sourced from surface fluids or magma chambers, facilitates chemical changes in the surrounding rocks. Rocks adjacent to a magma chamber undergo contact metamorphism, while others are affected by hot fluids that move outward from volcanic zones. This process is known as hydrothermal metamorphism, and it results in significant chemical transformations in the rocks.

🔧 Physical Changes During Metamorphism

Increasing metamorphism causes physical changes such as increased density in rocks. Minerals with a flat or elongated axis align themselves due to pressure, a process called foliation. Heat can also cause crystal growth as atoms within the rock gain energy, allowing them to bond with atoms from neighboring crystals, forming larger crystals. These changes occur without melting the rock, as heat causes atomic vibrations that enable the migration of atoms across the solid rock structure.

🔬 Chemical Changes and Index Minerals

Chemical changes occur during metamorphism when atoms in rocks migrate and form new, more stable minerals under new temperature and pressure conditions. If chemically active fluids are present, they can introduce or remove atoms, aiding in the formation of new minerals. Some minerals, like quartz, are stable across all metamorphic conditions, while others, like garnet, are stable only at high pressures and temperatures. These are known as index minerals and help geologists determine the metamorphic history of a rock. For example, kyanite suggests subduction zone conditions, where pressures are high but temperatures remain moderate.

Keywords

💡Metamorphism

Metamorphism refers to the process by which rocks undergo physical and chemical changes due to exposure to heat, pressure, or chemically active fluids. In the video, it's explained that this process does not involve melting but occurs under conditions between those that form sedimentary and igneous rocks. The concept is central to understanding how metamorphic rocks are formed.

💡Pressure

Pressure plays a crucial role in the metamorphic process, as higher pressures can change the structure and density of rocks. In the video, it is highlighted that pressure increases with depth in Earth's crust and can cause minerals within the rock to align, leading to features like foliation. Pressure conditions vary in different geological settings, such as subduction zones or converging continents.

💡Temperature

Temperature is another critical factor in metamorphism, as higher temperatures can cause the atoms in a rock to vibrate and migrate, leading to crystal growth and chemical changes. The video explains that temperatures must be high enough to induce these changes but not so high as to melt the rock, which would result in igneous rock formation instead of metamorphic.

💡Foliation

Foliation is a texture in metamorphic rocks where minerals align perpendicularly to the direction of pressure, creating a layered or banded appearance. The video describes this process as a response to pressure, where minerals with long or flat axes reorient to distribute stress more evenly within the rock.

💡Subduction Zones

Subduction zones are regions where one tectonic plate sinks beneath another into Earth's mantle, creating unique metamorphic conditions. The video notes that rocks in subduction zones experience high pressure but moderate temperatures. Water-rich sediments from the ocean floor also contribute to the metamorphic changes in this setting.

💡Hydrothermal Metamorphism

Hydrothermal metamorphism occurs when hot, chemically active fluids interact with rocks, causing chemical alterations. This process is common around volcanic centers, as described in the video, where fluids from rainwater, groundwater, or magma facilitate changes in the surrounding rocks without significant pressure changes.

💡Index Minerals

Index minerals are specific minerals that form only under certain pressure and temperature conditions, making them valuable clues for geologists to determine the metamorphic history of rocks. The video provides examples like garnet and kyanite, which indicate high pressure and temperature environments such as subduction zones.

💡Burial Metamorphism

Burial metamorphism occurs when surface rocks are buried under layers of sediment, causing gradual increases in pressure and temperature. The video explains that this type of metamorphism lacks large water sources, relying on the moisture trapped within the rock itself, leading to steady but less intense metamorphic changes.

💡Polymorphs

Polymorphs are minerals with the same chemical composition but different crystalline structures. The video mentions minerals like kyanite, andalusite, and sillimanite as polymorphs that change form depending on the pressure and temperature conditions during metamorphism. Their presence in a rock can help geologists reconstruct the rock's metamorphic environment.

💡Contact Metamorphism

Contact metamorphism occurs when rocks are heated by nearby magma or lava, causing changes without significant pressure. In the video, it's described as happening around volcanic centers, where the heat from magma chambers alters the surrounding rocks, often leading to recrystallization and the formation of new minerals.

Highlights

Metamorphic rocks are formed through metamorphism, which occurs under higher temperatures, pressures, or chemically active fluids than normally found at Earth's surface.

Metamorphic temperatures must be lower than the point at which rocks fully melt into magma, distinguishing them from igneous rocks.

Sedimentary rocks form when surface sediments are compacted or cemented with slight heat or pressure, positioning metamorphic rocks between sedimentary and igneous environments.

The metamorphic rock that forms depends on the original parent rock chemistry, the pressures and temperatures, and available water for chemical reactions.

Metamorphism occurs when rocks are buried deeper in the Earth's crust, where pressures and temperatures increase steadily.

At converging continental plates, extreme pressure and temperature conditions occur, causing intense metamorphism deep within the plates.

In subduction zones, pressures rise with depth, but temperatures increase more slowly because of the cold, insulated subducting plate.

Around volcanic centers, high temperatures and chemically active fluids cause chemical changes in rocks, leading to contact and hydrothermal metamorphism.

Increasing metamorphism results in higher rock density and foliation, where minerals align under pressure.

Under higher temperatures, crystals can grow larger as atoms vibrate faster, migrate, and bond with nearby atoms to form larger structures.

Chemical changes occur when atoms react with nearby minerals, creating new minerals that are stable at higher temperatures and pressures.

Chemically active fluids can introduce or remove atoms, further altering the mineral composition of the rock.

Index minerals form under specific pressure and temperature conditions, providing clues to a rock's metamorphic history.

Garnet is a valuable index mineral, indicating high pressure and temperature conditions in the rock’s formation.

Kyanite, andalusite, and sillimanite are polymorphs that change structure based on temperature and pressure, offering insights into a rock's metamorphic environment, particularly in subduction zones.

Transcripts

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Metamorphic rocks are rocks that have undergone  

metamorphism, or change. When rocks are subjected  to temperatures, pressures, or chemically active  

fluids higher than what is normally found  at Earth’s surface or within about the first  

kilometer of Earth’s surface, these rocks will  undergo physical and chemical changes and create  

new metamorphic rocks. How much pressure and  temperature? What kind of changes? If a rock  

is subjected to so much heat that it fully melts,  it becomes a magma and is igneous – so metamorphic  

temperatures have to be lower than that. If  surface sediments are subjected to a little heat  

or pressure or cool fluids, just enough to compact  or cement sediments, or leave crystals in pockets  

or holes, it’s sedimentary. So metamorphic rocks  sit in the zone between these two environments.  

The metamorphic rocks that form under  these changed conditions depend on the  

original parent rock chemistry the exact  pressures and temperatures to which the  

rock was subjected, and the amount of  water available for chemical reactions.  

Where do we find metamorphism  happening on our planet?  

When we bury surface rocks beneath layers of  sediment or lavas, those rocks will sink deeper  

in the crust. The deeper they go, the higher the  pressures and temperatures and hence the higher  

the grade or intensity of change or metamorphism.  These temperature and pressure gradients will  

vary in particular geologic settings such as  subduction zones or between converging continents,  

but where rocks outside these areas are gradually  buried over time, their temperatures and pressures  

will rise steadily together. There’s usually  no large source of water in burial, just  

whatever water happens to be inside the mineral  formulas or is trapped in holes in the rocks..  

Not surprisingly, pressures get quite high where  two plates collide. When the two colliding plates  

are made of continental crust, there will be  no subduction, and the crust will be folded  

and faulted. Pressures will be most extreme deep  in the middle of these two plates. Temperatures  

will also increase especially the deeper  we move into the center of the convergence  

zone and beneath the crust under the plate  boundary. Like with burial metamorphism,  

any water that’s available comes from water  trapped in the rocks or in the mineral formulas.  

What happens at subduction zones? Pressures  are also quite high here – and get higher  

the deeper the plate sinks into the mantle.  But the temperatures do not rise at the same  

rate because the subducting plate is cold and  insulated and sinks faster than temperatures  

can equilibrate. So subduction zone metamorphic  settings are characterized by high pressures but  

low to moderate temperatures. Water content can  be quite high because of the high amount of water  

absorbed by sediment and rocks in contact with  the ocean water for the potentially hundreds  

of millions of years that the subducting  plate was being pushed across the oceans.  

What happens around volcanic centers? These areas  of course have high amounts of heat and also  

water, water that comes from surface fluids (rains  and groundwater) or coming off the magma chamber.  

So temperatures can get quite high, while there  is no added pressure, so pressures remain low.  

The major changes that occur in the rocks around  volcanic centers are chemical changes facilitated  

by hot chemically active fluids interacting  with the original rock material. Rocks around  

the edges of a magma chamber undergo what we call  contact metamorphism. Rocks further away can be  

metamorphosed by hot fluids that migrate upward  and outward from the volcanic zones. We call  

that metamorphism hydrothermal metamorphism. What are the physical changes that can occur  

during increasing metamorphism? With increased  pressure, we get increased density. Also, any  

minerals that have a long or flat axis will align  themselves to equalize the pressure, a process  

called foliation, which we will describe more  in the next video tutorial. Additionally, under  

increased temperatures, crystals can actually  grow larger. How? Adding heat to a rock will make  

atoms vibrate at greater and greater energies or  speeds. This increased energy can allow the atoms  

to migrate across the solid without melting. Atoms  will use this added energy to seek out new bonding  

partners, if nearby. For example, atoms in two  side-by-side crystals of the same material will  

migrate across the boundaries, obliterating  them and growing into a new larger crystal.  

In addition to physical changes, there are  chemical changes that can result from added heat.  

As the atoms increase their energy and migrate,  they can react with atoms in nearby minerals and  

create new minerals that are more stable at the  new temperatures (and pressures). If chemically  

active fluids are present, these fluids can bring  in new atoms and make those available during the  

process. They can also take some away. If the  new minerals that form are stable under narrow  

temperatures and pressures, they become valuable  clues to the story of the rock. Such minerals are  

referred to as index minerals. For example, quartz  is stable under all pressures and temperatures in  

the metamorphic realm, so finding quartz in a  metamorphic rock doesn’t tell us much. However,  

the mineral garnet is stable only at higher  pressures and temperatures, so if we find garnet  

in a rock, we know that it had to be subjected  to those higher pressures and temperatures. As  

discussed in the Inside Minerals video, these  three minerals, kyanite, andalusite, and  

sillimanite are polymorphs – different crystalline  structures, but same chemical formulas,  

so three different minerals. These minerals are  excellent index minerals because when the pressure  

and temperatures change in a metamorphic setting  where one of these minerals exists in the rocks,  

the mineral will transform one into one of the  others. Finding kyanite in a rock, for example,  

indicates that pressures could have gotten  quite high, but temperatures not. We know  

from earlier in this video tutorial that such a  situation exists in subduction zones. So kyanite  

minerals are good clues that a particular  rock metamorphosed in a subduction zone.

Pause now. [music]

For more information and more detail in  regard to the wide range of metamorphic  

rocks found across the planet, continue  on to the next video in this series.

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