The three main layers of the Earth are the crust, mantle, and the core. Each layer varies greatly in size, composition, temperature, and pressure. The crust makes up Earth's skin. It is about 5-70 km thick depending on where on Earth it is and it is a layer of solid rock that includes both dry land and the ocean floor. The mantle is made up of rock that is very hot, but solid. Scientists have divided the mantle into layers based on physical characteristics. They are the lithosphere, asthenosphere and lower mantle. It is nearly 3000 km thick. The core is made mostly of metals iron and nickel. It consists of two parts a liquid outer core and a solid inner core. Together they are 3,486 km thick and the outer core enables the Earth to have a magnetic field. (3)
The lithosphere is broken into tectonic plates. These plates carry not only the oceans but the continents. The theory of plate tectonics explains the formation, movement, and subduction (or sinking) of Earth's plates. As these plates move and shift they produce Earth's awesome looking landforms including volcanoes, mountain ranges, ocean trenches, and more importantly the rocks that we will come to learn about. (2)
The first group project of the unit will introduce students to the different types of rocks that are involved in the rock cycle: igneous rocks, metamorphic rocks, and sedimentary rocks.
Students will learn basic information on the rock that they are assigned. This will include colors, shapes, textures, and the group of rock that their specific rock belongs to. They also will learn what makes up their rock and where we can find these other minerals that make up these rocks.
The rock cycle is a general model that describes how various geological processes create, modify, and influence rocks. This model suggests that the origin of all rocks can be ultimately traced back to the solidification of molten magma. Magma consists of a partially melted mixture of elements and compounds commonly found in rocks. Magma exists just beneath the solid crust of the Earth in what we call the mantle.
The first rocks to form on Earth were probably igneous rocks. All igneous rocks form from lava that has erupted onto Earth's surface or magma that hardens beneath Earth's surface.
All rock types can be broken down by weathering on Earth's surface. Those same rocks that were broken down are moved thanks to erosion. Some examples of erosion are streams, waves, glaciers, wind, and gravity. When these pieces of rocks are deposited they then go through the processes of compaction, which is the pressing down of rocks together (similar to pressing down a freshly made sandwich) and cementation (the gluing of rocks together by mud) which change these pieces of rock over long periods of time to produce sedimentary rocks. (See Figure 2.)
Many geologic processes, such as mountain building, or plate movement can exert heat and pressure on both igneous and sedimentary rocks causing them to be altered physically or chemically. Rocks formed in this way are called metamorphic rocks.
All of the three rock types described above can be returned back to the inside of the Earth. Once in the Earth's interior, great pressures and temperatures melt the rock back into magma to begin the process of the rock cycle all over again. (1, 3)
The word igneous comes from the Latin for fire, and all igneous rocks begin as a hot, fluid material. This material may have been lava erupted at the Earth's surface, or magma at shallow depths, or magma in deep bodies. Rock formed of lava is called extrusive; rock from shallow magma is called intrusive. (3)
The different types of igneous rocks are mostly divided apart by their texture and mineral composition. Texture describes the overall appearance of an igneous rock based on the size, shape, and arrangement of its interlocking crystals. Texture is a very important characteristic because it reveals a lot of information about where that rock had formed. Rapid cooling creates small crystals, where slow cooling creates large crystals. The rate of cooling is very slow in magma chambers deep within the Earth compared to the faster rate of cooling close to the Earth's surface. Igneous rocks that form rapidly at the surface have a fine-grained texture, where grains are too small to be seen by the naked eye. (1)
Coarse-grained textured rocks are formed when large masses of magma solidify below the surface. These have an appearance of a mass of inter-grown crystals, which are pretty much equal in size and large enough to be seen. A good example of a coarse-grained igneous rock is your common granite. (See Figure 1.)
Figure 1. A pink and black colored granite rock used in my classroom.
The third type of texture is created when there are different cooling rates going on within the rock. If magma that already contains some large crystals suddenly erupts at the surface of the Earth then the remaining portion of lava will then cool quickly. This rock will then have large crystals mixed in with small crystals and given the name porphyritic. A good example of this is andesite.
The last texture is created when there is such rapid cooling at the Earth's surface that the crystals don't have any time to grow. These are glassy textured igneous rock, and are created when there is not enough time for any crystalline structure to form. Obsidian is a common glassy textured rock that appears to be a dark piece of glass, also pumice resembles fine shards of glass but because it is vesicular it actually can float in water.
The mineral composition of an igneous rock depends on the chemicals of the magma from which it crystallizes. There are actually various eruptive stages of volcanoes and the same magma can actually produce many different types of igneous rocks. Since there are so many various ways igneous rocks can be broken down, geologists decide to put them into three categories; light, dark, and medium colored minerals.
Light colored rocks are referred to being as felsic. This is because there is mostly feldspar and silica found in these rocks and so they are very light colored. Granite is the best known intrusive igneous rock from this category and is great because it has a variety of uses, such as making tombstones, monuments, building stones, and kitchen countertops. When students are told of all the uses they will be amazed because granite is found practically everywhere! Dark colored rocks are referred to as being mafic. This is because there is mostly an abundance of magnesium and ferrum (Latin for iron) in them. They are very dark colored igneous rocks and basalt is a great example of this. Many volcanic islands are made from basalt which is the most common extrusive igneous rock. The final group has a composition falling somewhere in between light and dark colored, so it is said to have a medium or intermediate composition. (1, 2)
Figure 2. A basic outline of the sedimentary process. http://education.usgs.gov/schoolyard/RockSedimentary.html
When weathering and erosion break down rocks, they leave behind sediments which are then carried away by forces such as gravity, water, or wind and then become compacted, cemented and deposit them into another areas turning them into a sedimentary rock. Scientists classify sedimentary rocks by how they were formed and how their texture is. These rocks can be placed into three groups; clastic, chemical, or organic.
Clastic sedimentary rocks are created when large pieces of gravel are cemented together creating one rock. Usually, these types of rock are very coarse, or large grained. Two examples of this type are conglomerate and breccia. Both of these rocks look like they are just made from pieces of other rocks. The only difference is, is that conglomerate will have rounded edges while breccia has sharp edges.
Chemical sedimentary rocks are created when their sediments come from material that is in a form of a solution and brought to lakes, or another body of water. Eventually these chemicals sediments harden and create rocks. These rocks can have a range of fine texture to coarse texture but all are composed with some kind of chemical sediment, whether it is halite or calcite. Some familiar chemical sedimentary rocks include rock salt (what plows uses when cleaning up the snow covered streets in the winter), chalk (what teachers use on the chalkboard!), and flint (when out in the woods this is good to have to start a fire).
The last group of sedimentary rocks is organic. These rocks are made from living plants and other organisms that were compacted and cemented together to form a rock. A popular sedimentary rock students will be familiar with is coal. Coal forms from dead plans that decompose, usually areas rich in moisture and soil such as swamp areas. This type of rock also goes through several stages in order for it to form.
All three types of sedimentary rocks have an appearance to them that can clearly be seen so they aren't confused with igneous or metamorphic rocks. Many sedimentary rocks have a clearly distinguished layered appearance, such as this sandstone shown (See Figure 3.)
Figure 3. A piece of sandstone rock. Notice the distinct layers in the rock.
(picture courtesy of: republicans.resourcescommittee.house.gov)
And others have fossils in them, whether they are from shells or stems and leaves, such as limestone. (See Figure 4.) (1, 2, 3)
Figure 4. A limestone rock with pieces of shells in it. (picture courtesy of: republicans.resourcescommittee.house.gov)
Metamorphic rocks are rocks that have been changed with the help of heat and pressure. During the process of metamorphism, rocks are usually affected by these two processes at the same time.
The most important agent out of the two is heat. It allows chemical reactions to occur and change minerals within the rock or it can make new ones. Rocks can receive heat, in two ways. They can heat up from inside the Earth or when rocks are moved on Earth's surface toward warmer temperatures.
Pressure increases with depth just like heat does inside the Earth. When there is great pressure it creates any space between minerals to be reduced or even changes the minerals from one type to another. Pressure and heat have changed this igneous granite rock into a metamorphic gneiss rock. (See Figure 5.)
Figure 5. Left is an igneous rock, granite. On the right is a metamorphic rock, gneiss.
(picture courtesy of: http://www.nvcc.edu/home/cbentley/shenandoah/index.htm)
Metamorphic rocks are categorized into two main groups: foliated and non-foliated. Foliated rocks have a banded appearance, while non-foliated rocks do not have a banded appearance. A commonly used foliated metamorphic rock is slate. Students will be able to relate to slate because they can actually see it in their classrooms when a teacher writes on the chalkboard. Since slate can easily be cut into large, flat slabs it is useful for chalkboard, and roof and floor tiles. A commonly used non-foliated rock is marble. It has many interlocking crystals so there is no layered appearance but it found in many monuments and statues. A popular marble statue that students will be familiar with is the statue of Abraham Lincoln in the Lincoln Memorial in Washington, D.C. (2, 3)