Metamorphic Rocks, Part II

GL101 - General Geology I

Lecture of Monday, 11/17/25
Topic: Metamorphic Rocks, Part II

Upcoming Schedule

Today: Metamorphic Rocks (concluded)
Wednesday, 11/19: Review for Hour Exam II
Friday, 11/21: Hour Exam II

Reading

Text: Chapter 6

Outline

Overview of the Major Metamorphic Rocks

Overview of the Major Metamorphic Rocks

Metamorphic rocks form through the transformation of pre-existing rocks under conditions of elevated temperature and pressure. This transformation occurs as a result of processes that include weathering, erosion, burial, high temperatures, and pressures. Initial rock types can be igneous, sedimentary, or even other metamorphic rocks.

The formation sequence begins with processes such as slow uplift to the surface, transport and deposition of sediment, sedimentation, compaction, and cementation, leading eventually to sedimentary rock. If subjected to melting, they may become magma, contributing to intrusive or extrusive igneous rocks. Following burial under increasing pressure and temperature, these rocks undergo metamorphosis into metamorphic rock. This transformation is often influenced by magma emanating from the molten crust and mantle.

Classification of Metamorphic Rocks

Metamorphic rocks can be classified based on texture into two main categories: foliated (layered) and non-foliated (non-layered).

Foliated Metamorphic Rocks

Foliation refers to the development of a preferred orientation of minerals within rocks, which is primarily a result of directed compressional pressures associated with regional metamorphism. During this process, increasing heat and pressure, often associated with geological settings such as ocean basins, subducting oceanic plates, and continental collision zones, gives rise to foliation. The metamorphic texture develops as minerals, particularly platy minerals like micas, align due to this directed pressure.

An example of this metamorphic transformation is depicted where original rock contains randomly oriented mineral grains that after metamorphism produce a layered rock known as foliated rock.

Examples of Foliated Rocks

  • Slate: Fine-grained and exhibits slaty cleavage.

  • Phyllite: Fine-grained with a characteristic satiny sheen, recognized for its phyllitic cleavage.

  • Schist: Medium-grained, showing schistosity.

  • Gneiss: Coarse-grained with distinct alternating layers of light (felsic) and dark (mafic) minerals, known as gneissic banding.

  • Migmatite: Displays granite-like zones but generally resembles gneiss.

In the context of foliation, there is a clear relationship between crystal size and the coarseness of foliation, which directly correlates with the intensity (grade) of metamorphism. Generally, increasing crystal size and foliation coarseness indicate higher metamorphic grades.

Grade of Metamorphism

The metamorphic transformation process progresses through a sequence which includes:

  1. Low Grade: Slate, formed from shale (sedimentary rock).

  2. Intermediate Grade: Phyllite to Schist.

  3. High Grade: Gneiss and Migmatite.

As metamorphic grade increases from shale to slate, phyllite, schist, and gneiss, the textural and mineralogical complexities enhance with exposure to increasing temperature and pressure.

Non-Foliated Metamorphic Rocks

In contrast to foliated metamorphic rocks, non-foliated metamorphic rocks lack a preferred orientation of minerals, resulting in no layering. These rocks can include hornfels, quartzite, marble, argillite, greenstone, amphibolite, and granulite.

Examples of Non-Foliated Rocks

  • Hornfels: Formed under high heat but low pressure.

  • Quartzite: A metamorphosed form of sandstone composed mostly of quartz.

  • Marble: Derived from limestone and composed predominantly of calcite or dolomite.

  • Argillite: Can be derived from clay-rich sedimentary rocks.

  • Greenstone: Typically formed from basalt protoliths.

  • Amphibolite: Derived from basalt or gabbro with significant hornblende and plagioclase.

  • Granulite: A high-grade metamorphic rock that can feature a range of mineral compositions.

Paleothermometers and Paleobarometers

Foliated metamorphic rocks often contain index minerals, which are valuable in geological studies. These minerals are stable only within certain temperature-pressure (P-T) conditions and thus allow geologists to infer the metamorphic conditions during the formation of the rocks. They act as paleothermometers and paleobarometers.
Key index minerals include: - Chlorite - Used to identify low-grade metamorphism. - Muscovite - Commonly found in intermediate-grade metamorphic rocks. - Biotite - Indicative of medium-pressure conditions. - Garnet, Staurolite, Sillimanite - Indicators of high-pressure and high-temperature conditions found deep within the crust.
This relationship assists geologists in creating index mineral maps to define metamorphic zones and grade boundaries, known as isograds.

Summary of Key Points

  1. Metamorphic rocks form under heat and pressure from pre-existing rocks.

  2. Foliated and non-foliated are the two primary texture classifications of metamorphic rocks.

  3. A series of stages of metamorphic rocks relate to grade, from slate to gneiss.

  4. Index minerals serve as indicators for temperature and pressure conditions during metamorphism.

Questions to Consider

  1. What is the highest grade foliated metamorphic rock?

  2. How does pressure and temperature affect the characteristics of metamorphic rocks?

  3. How do index minerals assist in determining the geological history of an area?