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http://www.classzone.com/books/earth_science/terc /content/visualizations/es0607/flash/es0607_metamorphic.swf |
Metamorphic rock is rock that is changed by extreme heat and/or pressure underground. Heat is extreme near magma and pressure is extreme deep underground or where the earth is being squeezed together from ground movements. Metamorphic rock can go through stages, changing more with more heat or pressure, or as time passes, to change into other forms of metamorphic rock. Click on the player to watch a video on how an igneous rock is transformed into metamorphic rock... |
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Gneiss (say "nice") is an example of metamorphic rock. It often forms in light and dark layers which may be bent from the movement of the earth underground. Boxborough's bedrock is primarily gneiss. This picture shows a sample of gneiss in a road cut on the same road as Blanchard Elementary School. This is such a good example of gneiss that geologists come from miles away to look at it! |
Picture courtesy of Gwen Wilbert |
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Marble is another example of metamorphic rock. It is formed when limestone is heated and squeezed. It has layers that may be colored and can be carved smooth and shiny. It was often used to make statues. Like limestone, marble is made primarily of the mineral calcite. Calcite reacts with acid. Rain is mildly acidic from pollution and natural gases. So marble statues that have been left outside for a long time like this one will melt away from the acid rain. |
http://gardenofeaden.blogspot.com/ 2012/07/effects-of-acid-rain.html |
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Slate is also a metamorphic rock. It is formed from shale and splits easily into thin flat sheets. Because of this, it was often used to make roof tiles or flat stepping stones. Click on the player to see a video of a worker splitting slate to make tiles. |
Metamorphic rocks can also change into other metamorphic rocks with more heat and/or pressure. The following chart shows some of these progressions. Shale (a sedementary rock) can change into slate (a metamorphic rock). Slate can change into phyllite, which can change into schist, which can change into gneiss, which can change into migmatite. These are all metamorphic rocks. As the chart shows, gneiss can also be formed directly from granite (an igneous rock). Other transformations shows how limestone can change into marble and sandstone can change into quartzite.
http://store.schoolspecialty.com/OA_HTML/ ibeCCtpItmDspRte.jsp?minisite=10029&item=38292§ion=107628
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