Identifying Metamorphic Rocks -- Earth Rocks!

Summary

This educational script delves into the fascinating world of metamorphic rocks, explaining their formation from parent rocks through processes influenced by temperature, pressure, and chemical fluids. It outlines how different metamorphic conditions result in varied rock types, such as marble and quartzite, and discusses key characteristics like crystal size and foliation. The script also explores specific rock formations like skarn, hornfels, slate, and gneiss, linking their textures and compositions to their geological origins and settings, providing a foundational guide to identifying and understanding these complex geological wonders.

Takeaways

• 🌋 Metamorphic rocks are created from parent rocks through processes involving changes in temperature, pressure, and potentially the addition of chemically active fluids.
• 🔍 Different parent rocks under varying metamorphic conditions can result in a range of metamorphic rocks, providing clues to the original geological setting.
• 📚 By the end of the learning module, one should be able to identify common metamorphic rocks, recognize their textures and compositions, and understand their formation environments and histories.
• 🔬 For certain parent rocks like calcite-rich limestone, the resulting metamorphic rock, marble, maintains the original composition but shows changes in crystal size as an indicator of metamorphic intensity.
• 📏 The size of crystals in metamorphic rocks like marble and quartzite is a key indicator of the degree of metamorphism experienced, without providing specific information about the setting.
• 📈 Foliation, the alignment of crystals within a rock, is a textural feature that indicates the rock has been subjected to increasing pressure.
• 🛠 The presence of certain minerals and their stability under specific temperature and pressure conditions can provide insights into the metamorphic setting and history of a rock.
• 🌡 Minerals like calcite and quartz are stable across a wide range of conditions and do not typically form new minerals unless in the presence of other minerals or chemicals.
• 💧 Metamorphic changes occur without melting the rock, as increased temperatures allow atoms to rearrange and form larger crystals or more stable compounds.
• 🏔 Nonfoliated rocks like marble, quartzite, skarn, and hornfels can form in contact metamorphic settings, with transformations depending on the original parent rock.
• 🌊 Foliated rocks with clear mineral alignment indicate higher pressures, and their mineralogy and textures can reveal the metamorphic grade and environment, such as burial, convergence, or subduction zones.

Q & A

What causes metamorphic rocks to form?

Metamorphic rocks form when parent rocks undergo chemical and/or physical transformations due to increased temperature, pressure, and possibly the addition and interaction of chemically active fluids, typically hot waters rich in dissolved ions.

How do metamorphic rocks provide clues about their original geologic formation setting?

Different metamorphic conditions result in very different metamorphic rocks from the same parent rock, thus the characteristics of the metamorphic rock can offer clues about the original geological setting.

What is the significance of crystal size in metamorphic rocks?

The size of crystals in metamorphic rocks, such as marble and quartzite, is an indication of metamorphic intensity, reflecting how high the temperature and pressure got during the metamorphic process.

What is foliation and how does it form in metamorphic rocks?

Foliation is the alignment of crystals within a rock, which occurs when a rock is subjected to increasing pressure. Highly foliated rocks display a high alignment of crystals, which can indicate the metamorphic setting.

Why can't calcite and quartz crystals form foliation?

Calcite and quartz crystals, which tend to be uniformly shaped without a long axis, are more like rounded balls or pebbles, making it impossible for them to align and form foliation.

How does the presence of foliation in a rock relate to the rock's metamorphic setting?

The presence of foliation, combined with the types of minerals present, can provide information about the metamorphic setting. For example, foliation can only occur under increasing pressure, suggesting specific geological conditions.

What is the difference between skarn and hornfels, and what do they indicate about the rock's formation environment?

Skarns are rocks that form in a contact metamorphic environment with high temperatures and likely high amounts of chemically active fluids but with little to no pressure. Hornfels, also nonfoliated, forms from mudstone or basalt under similar settings but indicates a different parent rock.

How do the mineral compositions in skarns provide information about the temperature and fluid availability during metamorphism?

The mineral compositions and sizes in skarns can indicate the temperature and types of fluids available during metamorphism, with minerals stable at the highest temperatures found closest to the magma chamber.

What is the sequence of metamorphic rock formation from mudstone under increasing metamorphic grades?

The sequence of metamorphic rock formation from mudstone under increasing grades is mudstone to shale to slate to phyllite to schist to gneiss or migmatite, depending on the pressure and temperature conditions.

How does the formation of serpentinite relate to its geological setting and what does it indicate about the parent rock?

Serpentinite forms through hydrothermal metamorphism of peridotite at mid-ocean ridges, indicating that the parent rock was rich in olivine and that the setting was one where seawater interacted with the mantle rock near the surface.

What can the identification of a metamorphic rock tell us about its formation history?

Identifying a metamorphic rock can reveal its formation history, including the parent rock, the geological setting, the temperature and pressure conditions, and the presence of any chemically active fluids during metamorphism.

Outlines

00:00

🌱 Formation of Metamorphic Rocks and Their Identification

This paragraph introduces the concept of metamorphic rocks, which are created through the transformation of parent rocks due to increased temperature, pressure, and the influence of chemically active fluids. It explains that the type of parent rock and the specific metamorphic conditions determine the variety of metamorphic rocks formed. The paragraph emphasizes the importance of recognizing textures, compositions, and the formation environment of these rocks. It also discusses how certain minerals, like calcite and quartz, can indicate the original rock type but not the metamorphic setting, while the presence of foliation and specific minerals can provide clues about the rock's history and the conditions it was subjected to.

05:02

🏞️ Metamorphic Rock Varieties and Their Geological Settings

The second paragraph delves into the different types of metamorphic rocks and their formation under various geological settings. It discusses nonfoliated rocks such as marble, quartzite, skarn, and hornfels, which form in environments with little to no increased pressure, like contact metamorphism and hydrothermal metamorphism. The paragraph also covers foliated rocks, including shale, slate, phyllite, schist, and gneiss, which develop under conditions of increased pressure. It explains how the alignment of minerals, the presence of porphyroblasts, and the types of mica can reveal information about the rock's formation environment. Additionally, it touches upon migmatite, a rock that forms at the highest grades of metamorphism, and the potential for full rock melting leading to igneous rocks.

10:03

🌋 Metamorphic Rock Formation in Specific Geological Environments

The final paragraph focuses on the formation of metamorphic rocks in specific geological settings, such as subduction zones and deep burial environments. It describes the transformation of basalt into greenstone, blueschist, and eclogite under different conditions of pressure and temperature. The paragraph also discusses the formation of serpentinite through hydrothermal metamorphism and the creation of skarn from a mixture of minerals. It concludes by summarizing how the identification and naming of metamorphic rocks can reveal their formation history, providing insights into the parent rock and the metamorphic conditions they experienced.

Keywords

💡Metamorphic rocks

Metamorphic rocks are the result of the transformation of pre-existing rocks, known as parent rocks, due to changes in temperature, pressure, and the influence of chemically active fluids. They are central to the video's theme as they represent the geological processes that lead to the formation of various rock types. The script mentions that these rocks can form under a variety of geologic settings where temperatures and pressures are elevated.

💡Parent rocks

Parent rocks are the original rocks that undergo metamorphism to form metamorphic rocks. They are a fundamental concept in the script as they serve as the starting point for understanding the transformations that lead to different metamorphic rock types. The video explains that the same parent rock can form different metamorphic rocks based on the specific metamorphic conditions.

💡Metamorphic conditions

Metamorphic conditions refer to the specific temperature, pressure, and chemical environment that a rock experiences during metamorphism. These conditions are critical to the video's narrative as they determine the type of metamorphic rock formed from a parent rock. The script illustrates how different conditions can lead to the formation of various rocks, such as marble or quartzite.

💡Marble

Marble is a metamorphic rock formed from calcite-rich parent rocks like limestone under increased pressure or temperature. In the script, marble is used as an example of a rock where the composition remains the same (calcium carbonate) but the crystal size increases, indicating the metamorphic intensity.

💡Quartzite

Quartzite is another type of metamorphic rock that forms from pure-quartz parent rocks, such as cherts and quartz sandstones. The script explains that quartzite's formation is similar to marble in that the rock's composition remains unchanged, but the crystal size increases, reflecting the degree of metamorphism.

💡Foliation

Foliation is a textural feature of metamorphic rocks characterized by the alignment of mineral crystals. It is a key concept in the script as it indicates the presence of directional pressure during metamorphism. The script describes how certain minerals, like micas and prismatic crystals, can align to form foliated rocks.

💡Skarn

Skarn is a type of metamorphic rock that forms in contact metamorphic environments with high temperatures and chemically active fluids but little pressure. The script uses skarn as an example of a rock that can indicate the presence of specific mineral compositions and temperatures in the environment where it formed.

💡Hornfels

Hornfels is a nonfoliated metamorphic rock that forms from mudstone or basalt under high-temperature, low-pressure conditions. The script mentions hornfels as an example of how different parent rocks can transform into different metamorphic products in the same geologic setting.

💡Slate

Slate is a fine-grained, foliated metamorphic rock that develops from the metamorphism of mudstone or shale. The script describes slate as having a slaty cleavage due to the alignment of clay minerals, and it represents an increase in metamorphic grade from shale.

💡Schistosity

Schistosity is a textural term for the scaly appearance of a metamorphic rock due to the presence of large, visible mica crystals. The script explains that schistosity indicates a higher grade of metamorphism than slate or phyllite, where mica crystals are large enough to be seen with the naked eye.

💡Gneiss

Gneiss is a high-grade metamorphic rock characterized by a banded or gneissic texture, where light and dark minerals are separated into bands. The script uses gneiss to illustrate the result of extreme metamorphic conditions, such as deep burial or subduction zones, where increased pressures accompany rising temperatures.

💡Migmatite

Migmatite is a metamorphic rock that forms when high temperatures cause partial melting of gneiss, leading to the formation of bands of granite within the metamorphic rock. The script mentions migmatite as an example of the extreme end of metamorphism where temperatures are high enough to cause melting, but not complete enough to form a new igneous rock.

Highlights

Metamorphic rocks are created through the transformation of parent rocks by temperature, pressure, and chemically active fluids.

Different metamorphic conditions result in various types of metamorphic rocks from the same parent rock.

Studying metamorphic rocks can reveal clues about their original geologic formation setting.

Marble is a metamorphic rock formed from calcite-rich rocks like limestone under increased pressure or temperature.

Quartzite is the metamorphic equivalent of pure quartz rocks, with crystal size indicating metamorphic intensity.

Foliation in metamorphic rocks is characterized by the alignment of minerals like micas and prismatic crystals.

The presence of foliation and mineral types can indicate the metamorphic setting and pressure conditions.

Mineral stability in metamorphic rocks depends on temperature, pressure, and chemical composition.

Metamorphic rocks can form new minerals without melting, through rearrangement of atoms at high temperatures.

Skarn is a nonfoliated metamorphic rock formed in high-temperature, low-pressure environments with active fluids.

Hornfels is a nonfoliated rock resulting from the metamorphism of mudstone or basalt in similar settings to skarn.

Shale, slate, phyllite, schist, gneiss, and migmatite represent increasing grades of metamorphism in mudstone.

Serpentine forms through hydrothermal metamorphism of olivine-rich rocks in mid-ocean ridge settings.

Basalt can transform into greenstone, blueschist, and eclogite under different metamorphic conditions.

The sequence of metamorphic rock formation provides insights into the rock's formation history and geologic setting.

Naming a metamorphic rock can reveal much about its parent rock and the conditions of its formation.

This learning module simplifies the complex field of metamorphic petrology for basic identification of common rocks.

Transcripts

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Metamorphic rocks are formed when other rocks,

known as PARENT ROCKS, are chemically and/or physically transformed by increases in temperature

and/or pressure and possibly by the addition and interaction of chemically active fluids

– usually hot waters rich in dissolved ions.

Metamorphic rocks can form under a number of geologic settings – anywhere in which

temperatures and/or pressures are raised.

Usually a given parent rock will form very different metamorphic rocks based on the metamorphic

conditions, and hence we can look for clues in the rock to help us identify the original

geologic formation setting.

By the end of this learning module, you should be able to list the most common metamorphic

rocks, recognize their textures and compositions, and link them to their formation environment

and history.

For some parent rocks, the same metamorphic rock will form, regardless of environment.

In that case, studying the metamorphic rock will not give us any information on metamorphic

setting, only the original parent rock material.

For example, in this pile of metamorphic rocks, there is one that is made almost entirely

of calcite (or a magnesium-rich related mineral known as dolomite).

When a calcite-rich rock (such as limestone) is exposed to increased pressure or temperature,

the crystals grown larger.

Because the parent rock is homogeneous in composition – mostly Calcium Carbonate –as

long as no new chemicals are added during metamorphism, the composition of the final

rock is unchanged – Calcium Carbonate.

We call this metamorphic variety, MARBLE.

Pure calcite parent rocks produce pure calcite metamorphic marbles.

And the only indication of metamorphic intensity (just how high the temperature and pressure

got) is the size of the crystals.

The same is true of pure-quartz parent rocks, like cherts and quartz sandstones.

The corresponding metamorphic rock is called Quartzite, and the only indication of metamorphic

intensity or GRADE is the size of the crystals.

So if we can identify a marble or a quartzite, what do we know?

We know some likely candidates for the parent rock, but we have little to no knowledge of

the metamorphic setting.

In addition to crystal size, another textural clue found in metamorphic rocks is something

called FOLIATION, which is the alignment of crystals.

Highly foliated rocks display high alignment of crystals within the rock.

Calcite and Quartz crystals tend to be uniformly shaped – with no long axis – more like

rounded balls or pebbles.

As such, there is no way to align these minerals, and foliation is impossible.

On the other hand, micas, with their sheety structure CAN be aligned.

Similarly, long prismatic crystals like kyanite and actinolite, can also be aligned.

IF we see alignment of these minerals in a rock, we call the rock foliated.

The extent of foliation, combined with the types of minerals present can tell us a lot

about metamorphic setting.

For example, foliation can happen only when a rock is subjected to increasing pressure.

Notice this pile pencils and pens – completely unaligned – or unfoliated.

As I start to compress this system along one axis, the pens and pencils that are free to

move will start to align themselves with each other.

The greater the pressure, the greater the alignment or foliation.

Added to the foliation evidence is mineral evidence.

Many minerals have a limited range of temperatures and pressures at which they are stable.

If we change those conditions so that a mineral is out of its stability range, the unstable

mineral will want to rearrange its atoms and bonds to create a new mineral that is more

stable under the new conditions.

Calcite and Quartz are stable under a very large range of pressures and temperatures

– in fact all of the temperatures and pressures found in typical metamorphic settings.

Thus they do not, if present alone, recombine to form new minerals.

However, if calcite is present WITH quartz, and with other minerals, then as temperatures

and pressures rise, the stability of calcite and quartz might end up being LESS stable

than a new mineral with a new chemical composition, and thus the atoms in the calcite and quartz

might rearrange themselves to form new minerals.

For example, Wollastonite, CaSiO3, might be more stable.

So the stability of a mineral depends on temperature, pressure, AND chemical composition of the

parent rock and any introduced chemicals from active fluids.

It’s important to note here that when chemical constituents of a metamorphic rock rearrange

themselves, they do so WITHOUT melting the rock.

How?

Increased temperatures will increase the motion of the atoms in the solid, and if that vibrational

energy is high enough, the atoms can jump around in the solid and do things like clean

up their crystal structures (remove imperfections), combine to form larger crystals, and rearrange

into more stable compounds.

So we can separate metamorphic rocks into three piles: FOLIATED, NONFOLIATED, and WEAKLY

FOLIATED.

Now let’s go to the nonfoliated rocks.

If these contain minerals that “could” foliate, but haven’t, then what do we know?

The rock has NOT been subjected to high pressures.

The only metamorphic settings in which there is little to no increased pressure are contact

metamorphism (heating up rocks around a magma chamber) and hydrothermal metamorphism (hot

waters percolating through cracks or pores in the rock).

When the nonfoliated rock contains a mixture of large crystals, such as this one, with

some calcite, some quartz, some garnet, and some epidote, we call the final rock SKARN.

Skarns are rocks that form in a contact metamorphic environment with high temperatures and likely

high amounts of chemically active fluids, but with little to no pressure.

We would find these environments underground at all volcanic settings, such as hotspot

volcanism, divergent plate boundary volcanism, and subduction zone volcanism.

A sandstone that contains some mineral and rock fragments PLUS some shell fragments would

be a good parent rock for a skarn.

The mineral compositions and sizes that we find in a skarn can further tell us how much

temperature and what kinds of fluids were available.

We would expect to see the minerals stable at the highest temperatures in a zone closest

to the magma chamber and those indicating lower temperature stabilities further away.

This other rock that is dark black with a sugary texture is called HORNFELS.

It is also nonfoliated, so also found in the same settings just described for SKARN, but

it’s what results when the parent rock was a mudstone or basalt.

Let’s pause for a moment and review all our nonfoliated rocks: Marble, Quartzite,

Skarn, and Hornfels.

All of these can form in a contact metamorphism setting, and each is transformed in that same

setting to a different rock product, not because of different temperatures and pressures, but

because the original parent rock differed.

Of course the marble and quartzite could also have formed in another other setting as we’ve

already discussed, so let’s set them aside and return to our foliated rocks.

Rocks with clear foliation show a clear alignment of minerals, either in visible bands or microscopically

as micas that are aligned and give the rock a sheety or slaty cleavage.

Last week we talked about how mud-sized clays can collect at the bottom of lakes or the

ocean floor and get buried and compressed or cemented to form mudstone.

Continued compaction will align the clay minerals, which are sheet silicates, with sheety cleavage,

and that gives the rock a planar fabric.

A little bit of compression means we get a sedimentary rock called SHALE.

A bit more compression moves us into the metamorphic realm.

The clays are beginning to change chemically into other sheet silicates such as chlorite

or muscovite or biotite.

These perfectly aligned microscopic sheet silicates give the rock slaty cleavage, and

the rock is called SLATE.

You can see how dense it is.

In fact, you can even hear a difference among these three rocks –

mudstone is less dense and compacted – shale is more dense and compacted – slate is the

most dense and compacted.

Continued metamorphism leads to larger and larger crystals, like biotite, which as they

grow, begin to give the rock a satiny luster.

Sometimes other minerals will form, as large crystals surrounded by a sea of microcrystalline

biotite.

Those larger crystals are called PORPHYROBLASTS.

They form by the movement of atoms WITHIN a solid – no melt – just migration.

This process allows the crystals to grow quite large and with perfect edges.

Pyrites and garnets are common porphyroblasts in metamorphic rocks.

This satiny phase of foliated rock, with or without porphyroblasts, is called PHYLLITIC

texture, and the rock is called a PHYLLITE.

As metamorphic intensity or GRADE increases, the mica crystals become visible to the naked

eye.

They can grow so large they give the rock a scaly appearance.

We call this texture SCHISTOSITY.

And the rock is called a SCHIST.

This rock in particular is called a garnet schist because of the garnet porphyroblasts.

As metamorphic grade continues to increase, the dark- and light-colored minerals begin

to separate into bands known as GNEISSIC TEXTURE.

And the rock is a GNEISS.

Gneiss form through high grades of metamorphism of mudstone or granites in settings where

pressures increases with temperature, such as deep burial, converging continents, or

subduction zones.

Increasing temperatures at this point can cause the light-colored bands to melt, so

that half or a fraction of the rock is molten, but not all of it.

This situation leads to folding of the bands and a rock displaying contorted, folded alternating

colored bands.

When the molten bits solidify, they create small layers of granite within the metamorphic

rock.

We call this rock a MIGMATITE.

Any further increase of temperature will cause full rock melting and lead to igneous rocks.

Any unusual porphyroblasts or minerals we see in this sequence of rocks can help us

further identify the exact pressures and temperatures to which the rock was subjected.

For example, garnets typify medium to high temperatures and pressures.

Now what about these rocks – which can show some weak foliation, but not the same as shown

in this sequence of increasing grades of pressure of mudstone.

Let’s start with this familiar green rock made entirely of the mineral serpentine.

The rock is called SERPENTINITE, and it forms when olivine is hydrothermally metamorphosed.

A typical geologicl setting for this California State Rock is deep under a mid-ocean ridge,

where seawater penetrates cracks, is heated up by magmas, and then interacts with mantle

rock, rich in olivine – where that rock is closest to the surface because of the thin

crust.

After serpentinite forms in such a setting it takes advantage of any and all opportunities

to rise upward along cracks in the rocks, because its density is so much lower than

the surrounding rocks.

As such, we typically find serpentinite migrating up subduction zones and accreting to the edge

of the continent as terranes accrete.

We also see it migrating up transform faults and fracture zones on the bottom of the seafloor,

creating serpentinite ridges.

These other two rocks represent the metamorphism of basalt.

When basalt is subjected to low grade metamorphism in a burial setting, converging continents,

or subduction zone, small crystals of chlorite, epidote, and/or actinolite begin to form.

This produces a fine-grained, dense green rock called GREENSTONE.

Usually it still retains much of the shape and texture of the original basalt.

If this rock undergoes increasing pressure and temperature in a converging continents

or deep burial setting, the crystals will grow larger and eventually actinolite crystals

will dominate the rock.

We then call it a GREENSCHIST.

If the greenstone is in a subduction zone environment, on the other hand, where pressures

quickly rise while temperatures lag behind (and where water contents are high), this

rock, BLUESCHIST will form, followed by this rock, ECLOGITE, at the highest grade of subduction.

Blueschist is a fine-grained bluish-colored schist, dominated by blue amphiboles and other

silicates.

Eclogite is distinguished by its green groundmass (produced by small green pyroxene crystals,

a different variety than the one we saw in the minerals lab) and scattered red garnets.

It also can contain some kyanite crystals, which only form at the highest pressures in

a subduction zone environment.

So looking at these rocks again, all together, we see that with metamorphic rocks in particular,

the identification and naming process leads directly to the parent rock and metamorphic

formation setting.

Once you name a metamorphic rock, you can learn quite a lot about its formation history.

For example, which of these rocks had basalt as a parent rock?

If basalt is placed in a contact metamorphic setting, it turns to hornfels.

If it’s placed in a subduction zone, it turns to greenstone, then blueschist, then

eclogite.

If it’s buried deeply, it becomes greenstone, then greenschist, then a rock we didn’t

include here called amphibolite.

And a mudstone?

It turns to hornfels in a high temperature low pressure environment, and a slate, phyllite,

schist, gneiss, or migmatite in increasing grade in a high pressure environment.

Serpentinite forms through hydrothermal metamorphism of peridotite at mid-ocean ridge.

And contact metamorphism of mixture of minerals, like a greywacke sandstone, leads to a skarn.

And remember what happens to a limestone or chert?

Limestone turns to marble, and chert turns to quartzite, in any and all metamorphic settings.

In this review of some basic metamorphic rock types, we have greatly simplified the very

complicated field of metamorphic petrology.

Our goal was to simply give you a starter’s guide for identifying the most basic and common

metamorphic rocks – which you are likely to encounter as you travel the world’s outcrops.

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