Origin of Metamorphic Rocks
Summary
TLDRThis 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
🌋 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.
🔍 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.
Mindmap
Keywords
💡Metamorphism
💡Igneous Rocks
💡Sedimentary Rocks
💡Recrystallization
💡Foliation
💡Cleavage
💡Metamorphic Grade
💡Schist
💡Gneiss
💡Diagenesis
💡Tectonic Environment
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.
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