Analyzing How Different Types of Rocks are Formed

BACKGROUND:
Studying the environments where rocks form is a more creative way of teaching rocks than simply identifying rocks. Students should be able to visualize the different environments of rock formation. For example, if a child picks up granite, they should think that this rock cooled slowly inside the crust of the Earth. The pictures below can help you illustrate the different environments. Igneous is melted and hot; sedimentary is wet and cool; and metamorphic is full of pressure and heat.

The distinct characteristics that separate igneous, sedimentary, and metamorphic rocks are not easily observed. The objective of this exercise is for the students to recognize common characteristics shared by rocks belonging to the same group. Some general characteristics that your students may observe include:

igneous rocks are often composed of minerals that are visible, especially rocks that have cooled slowly;

metamorphic rocks look squished, are very dense and sometimes shiny; and

sedimentary rocks usually are made up of pieces of rocks cemented together, resulting in a grainy texture.

PROCEDURE:

Explain the three ways that rocks form.

Draw the three types of rock-forming environments on the board. Label each diagram with the appropriate name (igneous, metamorphic or sedimentary).

Instruct students to use the microscope or hand lens to look for details of each of different types of rocks. The following are suggested answers, but remember students can be more creative in their descriptions

IGNEOUS ROCKS (obsidian, granite, scoria, basalt) - all samples seem to be hard and not breakable.

GRANITE: Granite is an igneous rock that cooled very slowly. We know this because the minerals in granite are very large. It takes a long time (hundreds of thousands of years) for rocks to grow to visible size.

BASALT: Basalt cooled more quickly than granite. Geologists know this because the minerals are not visible with the naked eye, but are visible with a specialized microscope. The quicker an igneous rock cools the smaller the crystals.

OBSIDIAN: Obsidian, or volcanic glass, cooled very quickly giving minerals no time to form. Obsidian cooled faster than granite or basalt. The chemical composition is silicon dioxide (the same as quartz and glass).

SCORIA: Scoria is partly composed of pre-existing rock and new lava being cooled. It usually is reddish brown, with many gas pockets throughout the rock. Scoria is denser than pumice, but still relatively light in weight.

SEDIMENTARY ROCKS (shale, sandstone, conglomerate, diatomite). These samples are soft and more breakable than igneous or metamorphic rocks. These also seem to be less dense than the other types.

CONGLOMERATE: Conglomerate is composed of fairly large rock fragments and minerals. Students will identify these as "pebbles" or "rocks". The composition of these particles varies quite a lot between samples.

SANDSTONE: The gritty feel of the surface of sandstone hints that this rock was once sand that has been cemented together. Depending on the specimen, individual sand grains may be visible. Sandstones have quite varied compositions; some are composed entirely of quartz, and others are mixtures of rocks, crystals and fossils. Almost any combination is possible. Sandstones thus come in a wide array of colors.

SHALE: Shale is composed of very small particles of mud, which have been compacted and cemented together. Individual mud grains are very small; they will rarely be visible. Shales are quite variable in color.

DIATOMITE: Grains are very fine, smaller than sand size. In the case of diatomaceous shale many of the grains are skeletons of one celled plants called diatoms other grains are clays. The particles are so small that they rub off easily. It can be used as chalk to write on the board. Diatomaceous shale (sometimes called diatomite) is used for many purposes because of its fine grained nature. It is used in filters, fertilizers and many manufacturing items.

METAMORPHIC ROCKS (marble, schist, shale, gneiss) these samples are dense, but all of them seem to have different characteristics

SCHIST: Schist is composed of visible minerals, mostly micas. Schists form under moderately high pressure conditions; this causes the naturally platy mica crystals to line up, giving the rock a platy look. This is a good example for illustrating the characteristic "squished" look of metamorphic rocks to your students. This is called a foliated texture. This It is a new word for most students, so it is important that you have a good sample when illustrating schist.

MARBLE: Marble is composed exclusively of large commonly visible crystals of calcite. The gray/white bands in some of the samples are due to impurities within the calcite. Marble actually comes in a variety of colors, including black, gray, white, and pink. Marble, like all rocks that have calcite in them, fizz if you put a weak acid on it (usually 10% solution of hydrochloric acid). Marble forms when a rock containing calcite in it (such as limestone) was put under high temperature and pressure conditions.

GNEISS: Gneiss is composed of visible minerals that have a "layered" look. They are unusually formed under high pressures. The minerals separate into layers.
SLATE: Slate used to be a mudstone or shale before it was put under high temperature and pressure. Slate is denser than mudstone or shale and has a characteristic "ting" sound when it hits a hard surface. Mudstone and shale will have a "thud" type of sound. This illustrates how geologists use other than their sense of sight, they can also use their hearing sense.


OBJECTIVES: Analyzing how different types of rocks are formed. Interpreting the characteristics of the three types of rocks. VOCABULARY: igneous metamorphic mineral rock sedimentary MATERIALS: Rock Cycle - Rocks (4A) Swift-GH microscopes or hand lens	Students determine the different types of rocks.
	Metamorphic texture