Chapter 7

 

METAMORPHISM AND METAMORPHIC ROCK

 

I. Metamorphism- change in the mineralogy and/or texture of a rock as a result of subjecting it to higher pressures and temperatures without melting the rock (in solid state);

 

Much of earth's continental crust is made of metamorphic (along with some igneous) rock that form the "basement" rock that underlies a thin layer of

sedimentary rock in most areas; places where large areas of basement rock are exposed (not covered by sedimentary rocks) are called shields

(e.g., Canadian shield) (fig. 7.2).

 

III. Agents of metamorphism

 

During metamorphism, the original rock undergoes change to achieve equilibrium with the new environment.

 

1) heat- increases rate of chemical reactions that produce new minerals.

Sources of heat:

a)      Earth's geothermal gradient

b) intruding magma body

 

2) pressure- different minerals stable under different pressures; minerals either recrystallize to a different form of the same mineral or a new mineral forms. The two types of pressure are:

a) lithostatic pressure- increases with depth in the Earth; due to weight of overlying rocks; is applied equally in all directions (fig. 7.3).

b) differential pressure- additional pressure not equal on all sides (rock distorted) (fig. 7.4); result of plate tectonic or mountain building processes; get distinctive metamorphic texture (foliation)

3) fluid activity- water and carbon dioxide almost always present in rocks; important because water transports ions in solution.

Fluids enhance metamorphism by increasing the rate of chemical reaction.

IV. Types of metamorphism- 3 main types: contact, dynamic, and regional.

1)      Contact metamorphism- (figs. 7.5, 7.6) occurs when a body of magma heats the surrounding rock; high temperature is the dominant factor;

fluids may be important; pressure may or may not be involved.

 

Two types of contact metamorphism:

1.      Baking

2.      Hydrothermal alteration

 

 

2) Dynamic metamorphism- Associated with fault zones where rocks are subject to high differential pressure

 

3) Regional metamorphism- very large areas of metamorphism usually associated with mountain building at convergent plate boundaries; form

deep in earth and later exposed by erosion.

 

 

Regional Metamorphism is the result of:

a) high temperature (geothermal gradient);

b) high confining pressure (deep burial);

c) high differential pressure (rocks 'squeezed').

 

 

-index mineral- mineral that forms only in specific T & P ranges; allows geologists to recognize areas of low and high metamorphic intensity; (ex. chlorite=low P&T (~200˚C), garnet=medium P&T) (Fig.7-8).

 

V. Classification of Metamorphic Rocks- based on texture and mineralogy (Table 7.1).

Texture foliated (leaf) and nonfoliated.

 

-foliated- minerals arranged in a parallel fashion (figs. 7.9 to 7.13); form in rocks subjected to high differential pressure (regional metamorphism);

named on the basis of grain size:

-in order of increasing grain size and metamorphism:

slate, phyllite, schist, gneiss, migmatite.

 

-nonfoliated- no preferred mineral orientation (figs. 7.15 to 7.17); usually form in absence of differential pressure (contact and regional metamorphism);

named on basis of composition.

-marble (metam. limestone), quartzite (metam. quartz sandstone), hornfels (metam shale), anthracite (metam. coal).

 

VI. Metamorphism and Plate Tectonics

-most common at convergent plate boundaries (figs. 7.20, 7.21).

 

VII. Metamorphism and Natural Resources

 

The most widely used Metamorphic Rocks are Marble and Slate.

 

Ore deposits formed by the migration of hot, iron-rich fluids in the surrounding rock.

 

Asbestos (perspective 7.1)