World Builders™                                                                            Session Two  --  Geology 




Metamorphic Rocks

Heat and pressure transform  igneous and sedimentary rocks into metamorphic rocks.

We are learning about rocks on earth planet.

We have read about igneous rocks, which result from the cooling of molten rock.

We have read about sedimentary rocks, which are made of reassembled mineral fragments that erosion has taken from igneous and other sedimentary rocks.

There is a third kind of rock, metamorphic rock.

A metamorphosis is a transformational change.  For example, when a caterpillar turns into a butterfly, that is a transformation.

Metamorphic rocks are rocks which were igneous or sedimentary, but which were subjected to so much heat and pressure that they changed into another kind of rock.  They sometimes have stripes or very convoluted patterns in them.  Marble is a metamorphic rock.

 Heat, Pressure, and Rock Type

    up to 200 Celsius  Sedimentary Rocks  lithification 
(becomes rock)
   200 - 320 Celsius  Low Grade Metamorphic  becomes more dense
   320 - 550 Celsius  Medium Grade Metamorphic  structural changes
   550 - 650 Celsius  High Grade Metamorphic  recrystallization
   Above 650 Celsius  Igneous (molten)  molten rock

     Pressure from the weight of the crust above the rocks steadily increases as the metamorphosis takes place at greater and greater depths.  As the rock is buried more and more deeply, temperatures in the earth also ncrease.  If a rock is actually melted, it reforms as an igneous rock, but if it is softened it may be stretched and folded, and its structure may be rearranged somewhat.

     It is interesting that when a rock is changed to a metamorphic rock the results may be different depending on how hot the metamorphosis was.  Sometimes the rocks even soften and recrystallize.  Metamorphic rocks may have distinct layers which are sometimes bent and folded.  Looking at the patterns in  metamorphic rocks, you can sometimes see that they must have been soft as soft taffy candy.

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