I. Metamorphic Rocks (Chp. 2)

A. Introduction

"Metamorphism" - means to "change form"

• metamorphism changes existing rocks in size, shape, texture, and even the minerals they contain ....
• metamorphic rocks can form from igneous, sedimentary, or even from preexisting metamorphic rocks
• heat, pressure, and chemically active fluids are the agents of metamorphism
• the amount of metamorphism varies in degree
  • low-grade metamorphism - rocks are only slightly changed; e.g., shale to slate - samples are hard to distinguish
  • high-grade metamorphism - the transformation is so complete that the identity of the original rock cannot be determined - features of the orginal rock (e.g., fossils, bedding planes) are obliterated as well
  • in the most extreme metamorphic environments, temperatures approach those at which rocks melt. However, during metamorphism, the deformed material must remain solid, for if melting occurs, the rocks would be considered igneous ....
• the formation of metamorphic rocks takes place within the Earth's interior
  • at the elevated temperatures and pressures of the region extending from a few kilometers below the Earth's surface to the crust-mantle boundary
  • since this process is hidden form view, metamorphism is one of the hardest processes for geologists to study

• extensive areas of metamorphic rocks are exposed on every continent
• metamorphic rocks are an important component of many mountain belts - they make-up a large portion of the mountain's core
• even the stable continental interiors, which are covered by sedimentary rocks, are underlain by metamorphic rocks (see Fig. 2.19)

B. Settings of Metamorphism

(1) Regional Metamorphism

• produces the greatest volume of metamorphic rocks
• occurs during mountain building when great quantities of rock are subjected to the intense stresses and high temps associated with large-scale deformation - resulting in extensive areas of of metamorphic rocks

(2) Contact Metamorphism

• occurs when rock is in contact with, or near, a mass of magma
• the changes are caused primarily by the high temperatures of the molton material, which "bake" the surrounding rock

C. Metamorphic Agents - heat, pressure, & chemically active fluids

• rocks are often subjected to all three agents simultaneously
• the degree of metamorphism and the contribution of each agent varies greatly from one environment to another
• additionally, the mineral composition of the parent rock determines to a large extent the amount of metamorphism - e.g., quartz sandstone vs. limestone

(1) Heat

• probably the most important agent of metamorphism
• it provides the energy to recrystallize minerals
• Heat Source:
  • rocks near the surface are heated greatly when they are intruded by magmas originating at depth
  • or, when surface rocks are buried - recall the thermal gradient (20-30 C/km) - e.g., clay becomes unstable and recrystallizes to minerals that are stable at these higher temperatures (e.g., muscovite)

(2) Pressure

• recall that pressure also increases with depth
• as parent rocks are buried, they are subjected to force by the overlying material - "confining pressure" - this force is applied equally in all directions - rocks deform by decreasing in volume (Fig. 2.20a)
• in addition to to the pressure applied by the load of the material above, stress exerted during mountain building - "directional stress" (a vice-like stress) - shortens and deforms rock strata (Fig. 2.20b)
  • it creates intricate bends (folds) - recall that rock behaves in a plastic manner at depth (Fig. 2.21)

(3) Chemically active fluids

• water is trapped in the pores spaces of rock
• with deep burial and heating, water is forced from the pore spaces, and acts as a catalyst by aiding the movement of ions
• minerals recrystallize to form more stable structures;
• or, ion exchange among minerals creates brand new minerals - e.g., at Yellowstone, where hot mineral-rich water creates completely altered rocks in the near surface environment

D. Metamorphism & Textural Changes

• the degree of metamorphism is reflected in the rocks texture and mineralogy
• when subjected to low-grade metamorphism, the parent rock becomes more compact (more dense) - e.g., shale to slate
  • pressure causes the clay minerals to align into a more compact arrangement
• when subjected to higher-grade metamorphism, pressure causes certain minerals to recrystallize - in general, recrystallization encourages the growth of larger crystals ...
  • thus, many metamorphic rocks consist of visible crystals (like phaneritic igneous rocks)
  • the crystals of some minerals will recrystallize with a preferred orientation - perpendicular to the direction of compression
  • the resulting mineral alignment gives the rock a layered/banded appearance - a foliated texture ... (Fig. 2.22)

• not all metamorphic rocks have a foliated texture - "nonfoliated" texture (e.g., marble)
• in general, metamorphic rocks composed of only one mineral (e.g., calcite > limestone > marble), are not visibly foliated

E. Classifying Metamorphic Rocks

To Review - Metamorphism Causes:

• increased density
• growth of larger crystals
• foliation (reorientation of the mineral grains into a layered or banded appearance)
• transformation of low-temperature minerals into high-temperature minerals
• the introduction of ions generates new minerals - some value metallic ore deposits are formed in this manner

(1) Common Foliated Rocks

• slate - a very fine-grained foliated rock comprised of minute mica flakes - (the clay minerals that comprised shale were recrystallized into fine grained mica flakes) - because the mica flakes are so small, the foliation isn't visible
• schists - strongly foliated rocks, formed by regional metamorphism; they tend to be "platy", and can be split into thin flakes or slabs; the parent material for schist is shale; the term "schist" refers to texture and not composition - e.g., schists comprised of mica, are called "mica schists"
• gneiss - a term applied to banded metamorphic rocks; they are formed from high-grade metamorphism; they contain elongated and granular (not platy) minerals

• they exhibit a strong segregation of of the light and dark silicates (most common minerals are quartz and feldspar); they can be deformed into intricate folds

(2) Common Nonfoliated Rocks

• marble - a coarse, crystalline rock; the parent rock is limestone. Marble is composed of large interlocking calcite crystals, formed from the recrystallization of of smaller grains in the parent rock (limestone)...
• quartzite - a very hard metamorphic rock formed from quartz sandstone - formed under moderate-to-high-grade metamorphism. The quartz grains in the sandstone are fused together....