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The Earth was formed about 4.6 billion years ago. The planet was so hot that the entire Earth was molten or liquid. As the Earth cooled, the lightest materials floated to the top and the heaviest materials sank to the center. The outer part of the Earth, the crust, consists of the lightest rock.
The lightest rocks form the continents, which are made mostly of the rock granite. Most of the granite on the continents has, over millions of years, been broken down, transported, and deposited into sedimentary rock. These layers of sedimentary rock vary from 8-9 miles thick to nothing in some areas like the Canadian Shield of North America. The Canadian Shield has huge outcroppings of granite right on the surface. Under the thick layers of sedimentary rock lies the denser granite.
The granitic continents ride on a much denser rock called basalt. These basalts form the bottom of our continents and the bottoms of our great oceans. This layer of rock extends down to 40 miles from the surface of the earth.
The crust is very thin in comparison to the other layers of the earth. The crust is only 3 miles thick under the oceans and about 40 miles thick under the highest mountain chains. The layer of the earth under the crust is called the mantle. It is over 1800 miles thick!! The crust and the upper level of the mantle form a layer of the earth that moves very slowly (1-4 inches per year). This layer that moves and causes earthquakes and volcanoes is called the Lithosphere.
Rocks are made of two or more different minerals that have been:
1. cemented together, or
2. squeezed and heated together, or
3. melted and cooled together.
Rocks make up the majority of the Earth's crust. One of the most common rock is granite. The four minerals that make up granite are feldspar, quartz, mica, and hornblende. Granite was formed when magma cooled slowly forming crystals of the four minerals that make up the rock granite.
Look at the photo above of granite. Notice the different mineral crystals that make up the rock, granite.
Most of the Earth's surface rocks are covered by soil or clay. Soil contains very small crushed pieces of rock and organic (plant and animal remains) material. Plants such as grass and trees grow in this region of the crust.
The photo above shows a recent eruption of ash that has covered and burned an area of dense vegetation in Hawaii. The lava has cooled and is now a volcanic rock called basalt. The weathering process will break the basalt down into small, finer pieces of rock called soil. This process can take a few years or thousands of years to produce soil fine enough for plants to grow well in. The soil will become fertile when bacteria decomposes plant and animal material adding nutrients for living plants.
Rocks are produced in a variety of ways and have been cycled in some area many times. This cycling of the rocks is called the rock cycle.
The rock cycle shows how the earth's rocks are changed again and again. The rocks can be changed at times to another type of rock. The rock cycle can begin anywhere in the cycle. Lets start with igneous rocks. Igneous rocks start as magma. The magma (molten rock under the surface) and lava (molten rock on the surface) hardens into igneous rock. The igneous rock then breaks apart over time through the process of weathering. These bits of broken rock are washed away by rains and deposited in a river. These pieces of igneous rocks are cemented together with other bits of rock and form a sedimentary rock called conglomerate. Over time sedimentary rocks can be buried by earthquakes or other geologic processes. Being buried deep under the surface in areas of high temperatures and pressures or coming in contact with magma can cause these sedimentary rocks to change to metamorphic rocks.
Diagram 1 shows layers of rocks around and under a volcano. The white blocks are a sedimentary rock called limestone. Limestone forms on the bottom of the ocean over many, many years. The fish and shelled sea animals decompose and their bones and shells break down into a chemical called calcium carbonate (CaCO3). This is the cementing agent that binds the sediments that fall to the sea floor into the rock called limestone. Magma has pushed its way to the surface and is now coming into contact with the surrounding rock layers.
Diagram 2 shows the limestone being heated by the magma and changing to the metamorphic rock called marble (Yellow). Marble is a beautiful rock that is used by humans as building material and for decorative uses as in sink tops or monuments. Artists have sculpted marble into some of the greatest works of art in the world.
The three main rock classifications are Igneous, Sedimentary, and Metamorphic. Rocks are classified into these groups by the way they were formed.
Rocks that formed from magma are called igneous rocks. Igneous comes from the Latin word ignis which means "fire". Rocks that are formed from heat and pressure are called metamorphic rocks. Rocks that are formed from the cementing together of small pieces of rocks or shells are called sedimentary rocks. We will discuss these three types of rocks more in depth later in this chapter.
This map of the United States and parts of Mexico and Canada shows what type of rock makes up the surface of these regions. The majority of the surface rocks on the North American continent are sedimentary. The mountainous regions of the west and southwest are made of igneous rocks. The Appalachian Mountain region of the eastern U.S. and most of eastern Canada are made of metamorphic rocks.
Granite is an igneous rock that is composed of four minerals. These minerals are quartz, feldspar, mica, and usually hornblende. Granite forms as magma cools far under the earth's surface. Because it hardens deep underground it cools very slowly. This allows crystals of the four minerals to grow large enough to be easily seen by the naked eye. Look at the photo of granite above, notice the different crystals in the rock.
Granite is an excellent material for building bridges and buildings because it can withstand thousands of pounds of pressure. It is also used for monuments because it weathers slowly. Engravings in the granite can be read for hundreds of years, making the rock more valuable.
Granite is quarried in many places in the world including the United States. The state of New Hampshire has the nickname "Granite State" because of the amount of granite in the mountains of that beautiful state. The Canadian Shield of North America contains huge outcroppings (surface rocks) of granite.
Milky quartz is a common mineral that is found in many different types of rocks. The chemical formula is Silicon oxide (SiO2). One type of quartz is easily identified by its hexagonal crystals, but quartz can also be found in a large mass. Quartz can be broken or weathered into the tiny pieces we know as sand. Quartz is a very hard mineral and in fact is the hardest of the common minerals. Quartz is number seven on the Mohs hardness scale. Quartz is also chemically stable, which means that it weathers very slowly.
Quartz can be colored yellow, milky white, rose, smoky (brown or black), and the best known of the colored crystals amethyst, which is purple. Impurities in the rock at the time of formation causes the quartz crystal to have these different colors.
Quartz is used by humans in producing optical instruments and electical devices. It is also used to make sandpaper and grinding tools.
Feldspar is the most abundant mineral in rocks that are located at or near the earth's surface. Feldspar can have a glassy white, blue, green, or red crystals. All feldspars contain silica and aluminum.
When feldspars are exposed to the atmosphere they break down or weather easily. When they are broken down, feldspar forms other minerals, many of which are clay minerals. Feldspars also contain potassium which is a major nutrient for plant growth.
The clays formed by weathered feldspar are used by pottery manufacturing plants. Kaolinite is the highest quality of the feldspar clays used by potters.
Feldspar is number 6 on the Mohs hardness scale.
Hornblende is a mineral that contains magnesium, iron, silica and aluminum. Hornblende is black, brown and green in color. It occurs in crystals of many igneous rocks.
Mica is a mineral that can be split into very thin sheets. These sheets can be so thin that 1000 can be layered into mica 1 inch high. Mica can be clear, black, green, red, yellow, and brown. Clear mica is called Muscovite because it is found near Moscow, Russia and was used as window glass in the Muscovite's homes. Muscovite contains water which helps to make it clear. Biotite mica is dark green to black in color because it contains iron, magnesium.
Mica is mined in Brazil, India, many parts of Africa, Canada, and the United States. It is used in the manufacturing of electronic and electrical devices.
Write the answers to the following questions in complete sentences on a piece of paper.
In your own words write a definition for rock.
What is soil composed of?
Describe in your own words how the rock cycle works.
Name the four minerals that granite is made from and a human use for each of the four minerals.
Name the three classifications for rocks.