Metamorphic Petrology- Types of Metamorphism, Protoliths, Textures, & Classification | GEO GIRL

Summary

This video delves into metamorphic petrology, explaining the process of metamorphism as recrystallization under high temperature and pressure. It distinguishes between types of metamorphism, such as regional and contact, and discusses how to determine a rock's protolith. The video also explores the classification of metamorphic rocks based on both structural characteristics, like foliation and schistosity, and compositional aspects, including examples like marble and quartzite. It aims to provide a foundational understanding of the subject for viewers.

Takeaways

• 🔥 Metamorphism in geology refers to the recrystallization of a rock under high temperature and pressure conditions.
• 🌡️ Different types of metamorphism, such as regional and contact metamorphism, involve varying degrees of heat and pressure.
• 🏔️ Regional metamorphism occurs on a large scale, often due to subduction zones or plate boundaries, while contact metamorphism is caused by the heat from magma intrusions.
• 🌋 The line between metamorphism and other processes like diagenesis and igneous activity can be blurry, especially at low and high temperatures, respectively.
• 💠 The goals of metamorphic petrology include determining the protolith of a rock, the conditions of metamorphism, and the structural history of the rock.
• 🗝️ Determining the protolith of a metamorphic rock can be straightforward in cases like marble (limestone) and quartzite (sandstone), but it can be complex for rocks like basalt.
• 📏 Metamorphic textures can be primary, preserved from the original rock, or secondary, formed during metamorphism, and include static and tectonic textures.
• 📚 The script introduces various types of metamorphic rocks, such as gneiss, schist, and slate, each with distinct characteristics and textures.
• 📖 Metamorphic rocks can be classified based on their structure, like granofels and hornfels, or composition, like green schist and eclogite.
• 🔍 The script emphasizes the importance of understanding metamorphic textures and rock types for interpreting the geological history and environment of rock formation.

Q & A

What is metamorphism in geology?

Metamorphism in geology is the recrystallization of a rock under high temperature and pressure, where different types of metamorphism have varying amounts of pressure and temperature acting on the rock.

How does contact metamorphism differ from regional metamorphism?

Contact metamorphism is dominated by heat from intrusions like magma chambers or plutons coming into contact with country rock, causing metamorphism primarily due to heat rather than pressure. In contrast, regional metamorphism occurs on a larger scale, often due to subduction zones or plate boundaries, and involves both high temperatures and pressures.

What is the difference between metamorphism and diogenesis?

The line between metamorphism and diogenesis can be distinguished based on the participation of clastic grains in reactions with pore water and the precipitates from that water. Diogenesis involves the cementation of clastic grains in a sedimentary environment, while metamorphism occurs at higher temperatures and pressures where these grains react with the cement or pore water.

What are the three major goals of studying metamorphic petrology?

The three major goals of studying metamorphic petrology are to determine the protolith of the metamorphic rock, the conditions of the metamorphism such as temperature, pressure, and fluid composition, and the structural history of the rock including whether it was deformed before, during, or after metamorphism.

Why are metamorphic rocks harder than their sedimentary protoliths?

Metamorphic rocks are harder than their sedimentary protoliths because they become densely packed and recrystallized under pressure, which causes the grains to be much harder.

How can you determine the protolith of a metamorphic rock?

The protolith of a metamorphic rock can often be determined by its composition. For example, marble is the metamorphic rock that forms from limestone, quartzite typically has a sandstone protolith, and gneiss is often the metamorphic version of granite.

What are primary textures in metamorphic rocks?

Primary textures in metamorphic rocks are those that form during deposition of sedimentary rocks or during crystallization of igneous rocks. These can sometimes be preserved in metamorphic rocks if the metamorphism was relatively low grade or the original rock was sturdy enough to preserve some of its primary texture.

What are static and tectonic textures in metamorphic rocks?

Static textures in metamorphic rocks refer to minerals that grew or recrystallized without deformation or after deformation, resulting in equant or equal-sized grains. Tectonic textures, on the other hand, develop during or before deformation, showing foliation, schistosity, and lineation due to the preferred orientation of minerals.

How are metamorphic rocks classified structurally and compositionally?

Metamorphic rocks can be classified structurally using terms like granofels, hornfels, slate, phyllite, schist, and gneiss, which describe the rock's texture and structure. Compositionally, they can be classified based on their mineral assemblages, such as green schist, amphibolite, granulite, eclogite, marble, quartzite, and serpentine.

Why is it sometimes difficult to determine the protolith of a basalt-derived metamorphic rock?

It is sometimes difficult to determine the protolith of a basalt-derived metamorphic rock because basalt can have many different compositions, structural varieties, and a lot of variety that makes its final metamorphic equivalent hard to determine. The metamorphic grade also plays a role, as high-grade metamorphism results in different rock types than low-grade metamorphism.