Metamorphic Rocks PART 1

OBJECTIVES Define Metamorphism Classifying Metamorphic Rocks Metamorphic Process Metamorphic Environments

DEFINING METAMORPHISM

Metamorphism … is the transformation of rock by temperature and pressure Metamorphic rocks are produced by transformation of: Sedimentary and Igneous rocks, and by the further alteration of other metamorphic rocks Protolith: The source rock.

Origin of pressure in metamorphism Confining pressure aka “lithostatic” (due to burial) (Convergent Margin)

CLASSIFYING METAMORPHIC ROCKS

CLASSIFICATION Texture Foliated: Parallel arrangement of minerals. Nonfoliated (Granoblastic): Nonparallel arrangement of minerals.

What are metamorphic textures? Texture refers to the size, shape, and arrangement of mineral grains within a rock Foliation – planar arrangement of mineral grains within a rock

Metamorphic textures Foliation can form in various ways: Rotation of platy or elongated minerals Recrystallization of minerals in a preferred orientation Changing the shape of equidimensional grains into elongated and aligned shapes

Texture Foliated Classification Size of Crystals Nature of Foliation Degree of segreation of light & dark bands. Metamorphic Grade Nonfoliated (Granoblastic) Classification Mineral Compostion

Development of foliation due to directed pressure

Metamor-phism occurs between about 10 and 50 km of depth Sedimentary 0 km rock Metamorphic rock Igneous Sediment rock 10 km Metamor-phism occurs between about 10 and 50 km of depth ~200ºC Sedimentary rock Metamorphism Increasing depth and temperature The rocks don’t melt 50 km Melting ~800ºC

Originally buried deep, metamorphic rocks are seen when erosion Glaciers exposed the Rocky Mountains Canadian Shield North Cascades Black Hills Appalachian Mountains Best US exposures in New England and the South Grand Canyon Llano Uplift Originally buried deep, metamorphic rocks are seen when erosion removes covering rocks, and in the cores of mountains

METAMORPHIC PROCESS

Metamorphism Metamorphism progresses from low to high grades Rocks remain solid during metamorphism Metamorphism occurs above 50km melting depth for felsic minerals

What causes metamorphism? 1. Heat Most important agent Heat drives recrystallization - creates new, stable minerals Increasing Heat with Depth

Temperature Increase with Depth “Geothermal Gradient” due to: Radioactive Isotopes Intruding Magma Friction Between Moving Bodies of Rock

What causes metamorphism? 2. Pressure (stress) Increases with depth Pressure can be applied equally in all directions or differentially All Directions = “Confining Pressure” also called “lithostatic pressure” Differential = “Directed Pressure”

Origin of pressure in metamorphism Confining pressure aka “lithostatic” (due to burial) (Convergent Margin)

Confining Pressure

Directed Pressure

Source: Kenneth Murray/Photo Researchers Inc. Directed Pressure causes rocks to become folded, and minerals to reorient perpendicular to the stress: “foliation” Source: Kenneth Murray/Photo Researchers Inc.

Foliation Minerals Recrystallize Perpendicular to the Directed Pressure If the minerals are flat, such as sheet-like Micas, their parallel orientation gives a layered look; layering unrelated to the original bedding in the parent rock.

Role of Parent Rock Metamorphic rocks typically have the same chemical composition as the rock they were formed from. New minerals are formed in the rock due to the change in heat and pressure with the same composition.

METAMORPHIC SETTINGS

Metamorphic Settings Five types of metamorphic settings: Contact metamorphism – due heat from adjacent rocks Hydrothermal metamorphism – chemical alterations from hot, ion-rich water Regional metamorphism -- Occurs in the cores of mountain belts and converging margins. Includes: Burial Metamorphism – e.g. Burial of sediments deeper than 10 km – non-foliated Dynamothermal Metamorphism – Directed pressure in Plate Tectonic Processes – foliated Cataclasis metamorphism – Occurs at shearing Plate Boundaries. Shock metamorphism – Meteor/asteroid collision.

1. Contact Metamorphism

Contact Metamorphism Baking due to nearby Magma Effect strongest in rocks in immediate contact

Contact Metamorphism Metamorphic Aureole

2. Hydrothermal Metamorphism

Hydrothermal Metamorphism Due to circulation of water near Magma Important at mid-ocean ridge

Hydrothermal Metamorphism

3. Regional Metamorphism

Regional Metamorphism – “Burial” Variable T, mod-high differential P; characteristic of over-thickened plates (i.e., mountain belts) above convergent boundaries; affects large areas

Regional Metamorphism “Dynamothermal” Most Dynamothermal metamorphism occurs along convergent plate boundaries

Dynamothermal Metamorphism, Felsic continental materials and sediments are buoyant, they have low density They float, cannot be subducted, so they get squashed.

4. Cataclasis Metamorphism

Cataclasis Metamorphism Variable T, very high directed P; typically localized to narrow zones of intense mechanical deformation (shear zones).

5. Shock Metamorphism

Shock Metamorphism Meteor/Asteroid Collision

THE END