Geology 11 (Metamorphic Rocks)

Earth and Space Sciences X

29 Oct 201549:34

EducationalLearning

32 Likes 10 Comments

TLDRThe video provides an in-depth look at metamorphic rocks, explaining how they form through the alteration of existing rock types when exposed to high heat and pressure. It examines the various agents of metamorphism like magma, subduction, and impacts. It also explores different metamorphic grades and the index minerals used to identify them. Beautiful imagery illuminates the diverse textures and mineral assemblages of metamorphic rocks. Their formation through plate tectonics and the rock cycle is addressed, concluding they may be the most amazing rocks on Earth.

Takeaways

😀 Metamorphic rocks form from existing rocks that undergo changes due to heat, pressure, and chemically active fluids.
🔥 The main agents of metamorphism are heat, confining pressure, differential stress, and chemically active fluids.
🌡️ Metamorphic rocks exhibit new textures like foliation and contain different minerals from their parent rock.
📈 Metamorphic grade describes the degree of metamorphism, from low grade to high grade.
💧 Metasomatism involves chemical alteration of rocks by ion-rich hydrothermal fluids.
🌋 Contact metamorphism occurs when magma heats surrounding country rock.
⏳ Pressure-temperature timepaths show how P and T change during metamorphism.
🪨 Index minerals like garnet, sillimanite and staurolite indicate metamorphic grade.
😮 Metamorphic rocks can be beautiful, from marble to banded gneiss.
🤯 Partial melting of metamorphic rocks produces migmatites, a mix of melted and unmelted material.

Q & A

What are metamorphic rocks formed from?
-Metamorphic rocks are formed when existing rocks are subject to increased heat and pressure causing them to change into new types of rock.
What are some examples of low and high grade metamorphic rocks?
-Low grade metamorphic rocks include slate and phyllite which are formed under lower heat and pressure. High grade rocks include schist, gneiss and migmatite which require higher temperatures and pressures to form.
How can you identify the parent rock of a metamorphic rock?
-You can identify the original parent rock of a metamorphic rock by comparing its chemical composition. Most metamorphic rocks have a similar overall chemical makeup to their parent rocks.
What causes regional metamorphism?
-Regional metamorphism occurs due to increased heat and pressure during mountain building events and continental collisions.
How does contact metamorphism differ from regional metamorphism?
-Contact metamorphism occurs when a heat source like magma comes into direct contact with surrounding rock. Regional metamorphism happens over wider areas when rocks are buried deep during mountain building.
What are index minerals?
-Index minerals are minerals that only form under specific metamorphic conditions. By identifying the index minerals present, geologists can determine the temperature and pressure conditions needed to form that rock.
What causes the foliation texture in metamorphic rocks?
-Foliation textures occur when platey or elongated minerals align perpendicular to the direction of pressure during metamorphism.
Where does subduction zone metamorphism occur?
-Subduction zone metamorphism happens at oceanic subduction zones where ocean crust descends rapidly causing high pressure before temperatures can rise.
What are porphyroblasts in metamorphic rocks?
-Porphyroblasts are large mineral grains that form within a finer grained metamorphic rock matrix. They are similar to phenocrysts in igneous rocks.
How does burial metamorphism differ from shock metamorphism?
-Burial metamorphism occurs when rocks are subjected to increased heat and pressure due to deep burial. Shock metamorphism happens when rocks undergo extreme instant heating and compression from meteorite impacts.

Outlines

00:00

📺 Intro to Metamorphic Rocks

The introduction covers the basics of metamorphic rocks, stating they are the third type after igneous and sedimentary rocks. It notes they are connected in the rock cycle and are formed by the transformation of existing rocks through heat and pressure. The speaker expresses their personal preference for metamorphic rocks due to their beauty.

05:01

😮 Definition and Process of Metamorphism

This paragraph defines metamorphism as the change in form of rocks through temperatures and pressures unlike those in which they originally formed. It causes changes in mineralogy, texture, and sometimes chemical composition. The paragraph explains metamorphic grade as the degree of change, progressing from low to high grade.

10:03

🔎 Factors Influencing Metamorphic Rocks

Key factors influencing metamorphic rocks are discussed, including the mineral makeup and texture of the parent rock and release of chemically active fluids. The ability of fluids to transport ions and aid crystallization over distances is noted.

15:05

🌡️ Metamorphic Environments

Different metamorphic environments are outlined, including contact metamorphism from nearby magma bodies, burial metamorphism under pressure, and regional metamorphism associated with mountain building. Hydrothermal alteration and subduction zone metamorphism are also covered.

20:08

📈 Metamorphic Grades and Index Minerals

This paragraph explains how index minerals help determine metamorphic grade and provides examples. Chlorite indicates low grade while sillimanite is high grade. Assemblages of minerals narrow the grade range.

Mindmap

Keywords

💡Metamorphic rocks

Metamorphic rocks are rocks that have been altered by heat, pressure, and/or chemical processes. They form when existing rocks are subjected to new temperature/pressure conditions that cause mineralogical changes. Metamorphic rocks are the focus of the video, which discusses their characteristics and how they form in different environments.

💡Foliation

Foliation refers to the planar arrangement of minerals in metamorphic rocks that develops due to the aligned growth of platy minerals like mica. The video discusses foliated textures like slatey cleavage in low grade metamorphic rocks like slate and schistosity in higher grade schists.

💡Index minerals

Index minerals are minerals that only form under certain metamorphic conditions, allowing geologists to determine the grade or level of metamorphism. The video lists common index minerals like chlorite, garnet, and sillimanite that indicate different grades from slate to gneiss.

💡Metamorphic environments

Different metamorphic rock types form in different environments based on the temperature, pressure, and chemical conditions present. The video discusses contact, burial, regional, and subduction zone metamorphism environments.

💡Metamorphic facies

A metamorphic facies refers to a set of metamorphic minerals that develop under certain pressure/temperature conditions. The video implies facies like greenschist, amphibolite, and granulite facies that correspond to low, medium, and high grade metamorphism.

💡Protolith

The protolith or parent rock is the pre-existing rock that gets metamorphosed into a new metamorphic rock type. Identifying the protolith helps understand the original composition and texture of the metamorphic rock.

💡Migmatite

A migmatite is a metamorphic rock that contains both metamorphic minerals/textures and igneous-looking melted light & dark bands. Migmatites form under extreme metamorphism when partial melting occurs.

💡Metasomatism

Metasomatism refers to chemical alteration of rocks by hydrothermal fluids/magmatic water. The video mentions metasomatism at mid-ocean ridges as hot, mineral-rich fluids interact with and alter basalt.

💡Regional metamorphism

Regional metamorphism affects large regions of rock during mountain building events when rocks buried deep undergo high pressures and temperatures. The video shows regional metamorphism of shales to slates, schists, and gneisses.

💡Retrograde metamorphism

Retrograde metamorphism occurs when metamorphic rocks decompress and cool as they are uplifted towards Earth's surface. This reverses some mineral changes from the peak metamorphism experienced at depth.

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Transcripts

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