(RM) Metamorphic Rocks Lesson
Metamorphic Rock
When you think of rocks you may think of phrases like;
"solid as a rock"
"hard as a rock"
These phrases are correct when we think of building foundations, materials and tools. Looking up at giant mountains you can get a feeling that those mountains have been there forever and will remain there forever. The truth is, however, if you could see in the past millions of years ago or in the future millions of years from now, you would discover incredible changes have taken place to the rocks we see today.
Every metamorphic rock is a rock that has changed its form. The word metamorphic originates from the Greek language combining the words meta, meaning "change" and morphosis, meaning "form". Put these two words together and we have a word meaning, to change form.
How Metamorphic Rock Forms
Heat and pressure deep beneath Earth's surface can change any rock into metamorphic rock. When a rock changes into metamorphic rock everything about that rock will change; texture, color, mineral composition, luster and crystal structure.
The way rocks change depends on various processes that are always taking place on and under the earth's surface. When you think about metamorphic rock think - heat and pressure!!!
What happens to cookie dough when you put it in the oven? The heat of the oven produces changes in the ingredients that make them interact and combine. Without melting the dough, the heat changes it into a whole new product — a cookie. A similar process happens to rocks beneath the earth's surface. Due to movements in the crust, rocks are frequently pulled under the surface of the earth, where temperatures increase dramatically the farther they descend. Between 100 and 200 kilometers (62 and 124 miles) below the earth's surface, temperatures are hot enough to melt most rocks. However, before the melting point is reached, a rock can undergo fundamental changes while in a solid state — morphing from one type to another without melting. An additional factor that can transform rocks is the pressure caused by tons of other rocks pressing down on it from above; heat and pressure usually work together to alter the rocks under the earth's surface. This kind of change, which results from both rising temperature and pressure, is called metamorphism, and the resulting rock is a metamorphic rock.
Example of three changes
As the forces of mountain building slowly push granite upward, weathering and erosion wear away the granite to form sand. Streams carry the sand to the ocean where layers of sediment pile up on the ocean floor. The sediments get compacted and cemented together to form sandstone. If intense pressure and heat compacts the sandstone particles further and silica replaces the calcite holding the rock together, quartzite will form.
Classifying Metamorphic Rock - It's All in the Grains
Foliated
The incredible amount of pressure and the high temperatures metamorphic rocks undergo can squeeze rocks so tightly the mineral grains may be forced to line up in flat, parallel layers. These rocks are foliated. The term foliated comes from Latin, meaning "leaf". New minerals can actually form during this change process as well.
Nonfoliate
Metamorphic rocks can also be nonfoliate. Nonfoliate rocks are composed of minerals that are arranged randomly. Foliated rocks will split at the folds but nonfoliate rocks do not split into layers.
|
Type/Rock |
Changes to |
Metamorphic Rock |
|---|---|---|
|
Shale/Sedimentary |
|
Slate |
|
Sandstone/Sedimentary |
|
Quartzite |
|
Granite/Igneous |
|
Gneiss |
Putting Metamorphic Rock to use
When limestone is subjected to heat and pressure it will change into marble. Marble is a beautiful fine grained rock that can be cleanly cut into even slabs. These even slabs make good material for building. Marble can be polished and sealed to give it a rich, majestic look.
Marble is used throughout the U.S. Capitol Building, the congressional office buildings, and many other government and commercial buildings for its beauty, durability and relative ease of carving. It forms exterior surfaces and such interior elements as floors, walls, columns and stairways. Marble is also commonly used for statues and other sculpture, both indoors and out.
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