3.4 Metamorphic rocks form as existing rocks change.

·     Remember:  Rocks change into other rocks over time.

Also:     The rock cycle shows how rocks change.

                           Igneous rocks form from molten rock.

                           Sedimentary rocks form from earlier rocks.

·     Vocabulary

1.  metamorphism-  The process in which an existing rock is changed by heat or pressure or both. 

2.  recrystallization-  Heat and pressure break the bonds that join atoms in minerals.  Then the atoms can join together differently as new bonds form. 

3.  foliation-  An arrangement of minerals in flat or wavy parallel bands.

I.  HEAT AND PRESSURE CHANGE ROCKS

     A.  Metamorphism

         1.  The original (parent) rock is sedimentary or igneous.

         2.  Even a metamorphic rock can be a parent of another metamorphic rock.

     B.  Many metamorphic rocks we use were once sedimentary rocks.

         1.  Limestone is the parent rock of marble.

         2.  Shale is the parent of schist (from which we get garnet).

     C.  During metamorphism, rocks undergo many changes.

         1.  Pressure can cause a rock’s minerals to flatten out in one direction. 

         2.  Rocks do not melt under metamorphism (they would be igneous). 

     D.  Recrystallization

         1.  Heat and pressure breaks the bonds joining atoms in minerals. 

         2.  These atoms then join together differently as new bonds form.

         3.  Recrystallization has two main results.

                  a.  Individual mineral crystals can grow larger.

                  b.  New minerals can form in place of older ones. 

II.  METAMORPHIC CHANGES OCCUR OVER LARGE AND SMALL AREAS.

     A.  Change over large areas

1.  Most metamorphic changes occur over large areas in which both temperature and pressure are high. 

2.  This can affect areas hundreds of kilometers wide and tens of kilometers deep.

3.  In these areas, rocks are buried, pressed together, bent and heated.

4.  The deeper the rocks are, the greater the metamorphic changes. 

B.  Change over small areas

    1.  Magma can push into rocks underground, or surface rock can be covered with a flow. 

    2.  Magma or lava heats rock and causes recrystallization. 

    3.  The rocks get toasted but not squeezed. 

    4.  Temperature, but not pressure, is high. 

    5.  This can affect areas tens to hundreds of meters. 

    6.  In other areas, pressure may be high but not temperature. 

III.  MOST METAMORPHIC ROCKS DEVELOP BANDS OF MINERALS.

A.  Foliation

    1.  Foliated  rocks

             a.  Foliation develops when rocks are under pressure. 

             b.  Develops when minerals flatten out or line up in bands.

    2.  Nonfoliated rocks

             a.  Rocks made of mostly one type of mineral tend not to show foliation (marble). 

             b.  Rocks that are not subject to high pressure often show nonfoliation.