The protolith could also be a previously metamorphosed rock. Ultimately however, if you go far enough back into the history of a metamorphic rock you would find that the first protolith was either a sedimentary or igneous rock. Because all metamorphic rocks form below the surface, for them to become exposed at the surface, they must undergo tectonic uplift and removal of the overlying material by erosion.
These materials known as sediment consist of sand, mud, bodies of animals, shells, and other materials. Over millions of years, these sediments are covered by other particles and the layers are pressed down by the weight of the sediments and water above. Gradually, the sediments are hardened into sedimentary rock.
Scientists determine the age of a sedimentary rock by its thickness, the mineral layers, and the plant and animal remains it may contain. Nearly 75 percent of the land area of the Earth is covered with sedimentary rock. The mouth of the Mississippi River, in the Gulf of Mexico, has layers of sedimentary rock that measure 12, meters thick. The sediments there were carried by the Mississippi River and its tributaries from the interior of North America.
The bluffs above the Jordan River are made of chalky limestone formed from fossil remains. Utah has large amounts of sedimentary rock. Most of it was formed at the bottom of different prehistoric lakes that covered parts of the state. Sedimentary rocks include limestone, shale, sandstone, and breccia. Metamorphic rocks are formed from igneous, sedimentary, or other metamorphic rocks which contain minerals that have been changed by heat, pressure, or chemical action.
Strong heat and pressure inside the Earth can cause minerals in rocks to change. For example, when shale is changed to slate, the fine quartz crystals of the shale become broken, flattened, and reoriented.
The clay particles of shale recrystallize to form tiny flakes of mica. This realignment of the bits of quartz and mica result in a rock that splits easily into fine, thin sheets with smooth surfaces.
This property makes slate ideal for chalkboards, roofing tiles, and paving tiles. Observe objects. Use a classification system. Know science information. As a class, discuss the difference between rocks and minerals. Rocks are made of minerals. See background information. In their science journals, have each student describe the differences between minerals and rocks. Assign the students to groups. Give each group a collection of rock samples.
The collection should include several examples of each of the three types of rocks; igneous, metamorphic, and sedimentary. Instruct students to observe the rocks using a magnifying glass and draw the shapes and colors of the minerals.
Sometimes the magma cools inside the earth, and other times it erupts onto the surface from volcanoes in this case, it is called lava. When lava cools very quickly, no crystals form and the rock looks shiny and glasslike. Sometimes gas bubbles are trapped in the rock during the cooling process, leaving tiny holes and spaces in the rock. Examples of this rock type include basalt and obsidian. Here's a chart of some of the key characteristics that can help you identify the rocks within these three main classes.
Crystals Small, flat surfaces that are shiny or sparkly, like tiny mirrors. Fossils Imprints of leaves, shells, insects, or other items in the rock. Gas bubbles "Holes," like Swiss cheese, in the rock.
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