THE ROCK CYCLE

Unit Overview

As you explore the rocks and minerals of the earth using their properties, you will discover that they can change over time. This unit walks you through the cycle rocks follow as they change and become igneous, sedimentary, or metamorphic rocks. You will learn how plate tectonics factors into this ongoing cycle of change.

Labs - In this course, there may be some lab activities that require normal household items for completion. If you do not have the items required for a particular lab activity, please contact your online teacher for accomodation.

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Rocks

Did you know that there is no such thing as a plain old rock? In fact every rock tells a story. And at least one part of every rock has been on Earth since the earth formed. You could say any rock is a piece of history. Rocks can tell you about where it formed, what it is made of and where it has been. Rocks even give clues as to what’s going on below the Earth’s rocky surface.

Rocks can be divided into three groups: igneous, sedimentary, and metamorphic. Remember from the last unit that all rocks are made up of minerals. Without minerals, rocks wouldn’t exist.

Rocks are the most common material on Earth. They are naturally occurring combinations of one or more minerals. Rock divisions occur in three major families based on how they formed: igneous, sedimentary, and metamorphic. Each group contains a collection of rock types that differ from each other on the basis of the size, shape, and arrangement of mineral grains. The rock cycle is an illustration that is used to explain how the three rock types are related to each other and how Earth processes change a rock from one type to another through geologic time.

Plate tectonic movement is responsible for the recycling of rock materials and is the driving force of the rock cycle. This planet is rockin'!

WORDS TO KNOW
Erosion	Wearing down of rocks by such forces as wind, water, heating and gravity
Fossils	Remains or outline of ancient life preserved in rock.
Igneous	one of three major rock types. Igneous rocks form from a molten state.
Lava	hot liquid rock that flows from a volcanic vent. Lava flows are responsible for making extrusive igneous rocks including basalt, obsidian, pumice
Magma	liquid rock inside the Earth which either erupts to form lava or slowly cools to form intrusive igneous rocks such as granite. Magmas are heated by radioactive elements concentrated in Earths crust and mantle.
Ore	mineral or rock that contains a significant amount of a useful element.
Metamorphic	rock that forms as a result of heat and pressure. Metamorphic rocks often show a mineral alignment from the pressure.

Let’s see how scientists categorize rocks. As we explore how rocks are formed and reformed we can understand the basics behind the three broad categories of rocks: Sedimentary, Igneous, and Metamorphic.

How Sedimentary Rock Is Formed

For thousands, even millions of years, little pieces of our earth have been eroded--broken down and worn away by wind and water. These little bits of our earth are washed downstream where they settle to the bottom of the rivers, lakes, and oceans. Layer after layer of eroded earth is deposited on top of each. These layers are pressed down more and more through time, until the bottom layers slowly turn into rock.

Some Examples of sedimentary rock are SANDSTONE, SHALE, LIMESTONE, CONGLOMERATE, and GYPSUM.

Have you seen any rocks that look like this around your house and yard? These are sedimentary rock. Most of the rocks found in Ohio are sedimentary rock

How Igneous Rock Is Formed

Igneous rocks are called fire rocks and are formed either underground or above ground. Underground, they are formed when the melted rock, called magma, deep within the earth becomes trapped in small pockets. As these pockets of magma cool slowly underground, the magma becomes igneous rocks.

Igneous rocks are also formed when volcanoes erupt, causing the magma to rise above the earth's surface. When magma appears above the earth, it is called lava. Igneous rocks are formed as the lava cools above ground.

How Metamorphic Rock Is Formed

Metamorphic rocks are rocks that have "morphed" into another kind of rock. These rocks were once igneous or sedimentary rocks. How do sedimentary and igneous rocks change? The rocks are under tons and tons of pressure, which fosters heat build up, and this causes them to change. If you exam metamorphic rock samples closely, you'll discover how flattened some of the grains in the rock are.

METAMORPHIC ROCK

Let’s review the main points of the rock cycle

· Rocks can be made over and over again and again, going back and forth from one type to another in a never ending process called the rock cycle.

· Minerals are heated to extremely high temperatures and then cooled forming igneous rocks such as granite. There crystal grains are large because the rock cools under ground slowly.

· When rocks are exposed at the Earth’s surface, their mineral structure changes because they erode and break down into smaller grains. These grains are transported by wind or water and deposited as sediments such as sand and pebbles.

· Sediments are compacted over time and cemented forming sedimentary rocks. You can usually find sedimentary rocks in and near river beds. Sedimentary sometimes contain traces of life, fossils, that can give scientists clues to what was in the past

· Igneous or sedimentary rocks that are heated or put under pressure can be turned into metamorphic rocks. Some or all of the minerals in the original rock are changed into new minerals. Minerals are replaced atom by atom to form new minerals. Metamorphic rocks are often squished, twisted, smeared out, and folded from the heat and the pressure.

· At higher temperatures over time metamorphic rocks can again melt. That changes the crystals of the rocks and creates igneous rocks. What happens now? Start over at step one.

· What drives the rock cycle? The rock cycle of the Earth is basically being driven by the tectonic plate action. As the plates on the Earth’s crust move around the outside of the planet, they cause mountains to form, create volcanoes, and squeeze rocks at incredibly high pressures.

JUST FOR FUN “ROCKY RIDDLES”

1. What does a rock want to be when it grows up?

2. What do you do to a baby rock?

3. What is a rock’s favorite kind of music?

4. Where do rocks sleep?

5. How do rocks wash their clothes?

6. What is a rock’s favorite transportation?

7. What is a rock’s favorite cereal?

ANSWERS:

1. A Rock Star

2. Rock it!

3. Rock’N Roll

4. Bedrock

5. On the Rock Cycle

6. A Rocket

7. Cocoa Pebbles

A mineral is a naturally occurring, inorganic, solid which possesses a characteristic internal atomic structure and a definite chemical composition.

If we take the definition of a mineral term by term, it becomes easier to understand.

• Minerals must occur naturally. This means man-made substances such as steel would not be considered to be minerals.
• Inorganic substances are those substances that are not living and are not formed by living processes.
• Crystalline solids are those solids in which the atoms composing the solid have an orderly, repeated pattern.
• Minerals will have definite chemical compositions, but these compositions may vary within given limits.

Substances that have these features will also have distinctive physical properties such as color, crystal form, cleavage, luster, streak, etc. We can identify the minerals in rocks we find by comparing the mineral’s physical properties with a mineral identification chart. Links at the end of this lesson will take you to a variety of mineral charts. Check the library or book story for a copy of a rock guide. This is very helpful if you want to start a rock collection of your own. Realize some charts are much more detailed and reflect many physical properties. Other charts are simpler and good for beginners.

Here are some brief descriptions of the physical properties you will use to identify rocks and minerals.

Rocks are made of two or more minerals. So when you try to identify rocks, you are actually identifying the minerals that the rock is made of.

Now that you understand what a mineral is, let’s discuss the methods we can use to identify them.

Minerals have certain physical properties (characteristics) that help us to identify them, such as:

Luster is the way the surface of a mineral reflects light. Luster should be observed on a cut or freshly broken, untarnished surface. There are two general types of luster -- metallic and non-metallic. The terms used to describe luster are:

Ø Metallic -- example: gold

Ø Vitreous (glassy) -- example: quartz, tourmaline

Ø Adamantine (brilliant) -- example: diamond

Ø Resinous (like resin or sap from a tree) -- example: sphalerite

Ø Greasy or waxy -- example: turquoise

Ø Pearly -- example: talc

Ø Silky -- example: asbestos

Ø Dull or earthy -- example: bauxite

Hardness

Hardness is the resistance of a mineral to scratching. It does NOT refer to how easily the mineral is broken. Hardness is a measure of the bond strength between atoms. If these bonds are strong, the mineral is not easily scratched. Minerals with weaker bonds are more easily scratched. Pencil "lead" is softer than paper, so it writes. We determine the relative hardness of minerals using a scale devised by mineralogist Friedrich Mohs. The scale assigns hardness to ten common index minerals, and is based on the ability of one mineral to scratch another.

Color

Although color is the most obvious, it is the least reliable. Notice the picture above. Usually, we notice the color of a mineral first. Some minerals are easily identified by color because they are never any other color. Keep in mind, however, that color by itself isn't enough to identify a mineral.

Breakage Pattern

How a mineral breaks is determined by its internal structure, and is there very important. There are two (2) major subdivisions: fracture and cleavage.

Ø Fracture: The mineral just breaks, leaving an uneven surface.

Ø Cleavage: When a mineral sample is broken with a hammer, it breaks along planes of weakness that are part of its crystalline structure. These breaks are cleavages. Some minerals break only in one direction. Others break in two or more directions.

Streak
When a mineral is rubbed firmly across an unglazed tile of white porcelain (a streak plate); it leaves a line of powder. This is called the streak. The color of the streak is always the same, whether or not the mineral has impurities. For example, quartz leaves a white streak, whether it's violet (amethyst), pink (rose quartz), or brown (smoky quartz).

Transparency
Transparency describes how well light passes through a mineral sample. There are three degrees of transparency: transparent, translucent, and opaque. You can see objects through a transparent mineral. You can see light, but no objects through a translucent mineral. You can't see anything through an opaque mineral. The quartz to the right is transparent.

Specific Gravity
Defined as "the weight of a specific volume of a mineral divided by the weight of an equal volume of water (at 4°C.)" Since water is always 1.0, it's the same number as density.

Effervescence (the Fizz test)
Minerals containing calcium carbonate (CaCO3) will generally react when exposed to hydrochloric acid (HCl). Carbon dioxide (CO2) is released and the mineral or rock literally "fizzes." Some may need to be powdered (increases surface area) before any reaction can take place.

Magnetism
Some minerals are magnetic.

Taste
Some minerals have a distinctive taste. Notable examples include Halite (rock salt), and Chalcanthite.

Science Safety

Do Not Taste Any Science Item. It can be harmful.

Smell

Some minerals have a distinctive odor. Sulphur is a good example. This is what you smell when you strike a match.

There is a great difference in the way different minerals look. Some minerals sparkle in the light while others are dull and boring!! Some minerals are so hard that they can scratch steel while other minerals are so soft that they feel powdery and can be scratched easily by a fingernail. There are many ways that scientists classify or group minerals. These are properties of minerals. A property is a characteristic of a mineral. Properties help scientists to better understand how the mineral was formed and also to help identify a mineral. With this beginning information you can continue to learn more on your own as you collect and observe rocks all around you.

ACITIVITY: SHAKE RATTLE AND ROLL

Part of the rock cycle is weathering of rocks on earth’s surface. In this experiment you will speed up the natural process of weathering to see how this works on different rocks.

Materials

· Clean empty coffee can with plastic lid

· 2 inch samples of granite, sandstone chalk, basalt or rocks from your yard

· Clear 7oz plastic cup of water

· Watch or timer

· Metric ruler

Other Rock Sample Sources: Science supply houses, teacher stores, masonry supply companies that provide rock for construction (may give you broken stones they can’t use) Monument manufactures, the people who make tombstones, make chips of the stones and are usually free for the asking.

1. Gather some rock samples from around your yard. Try to find different types of rocks.

2. In your science notebook make a chart. Measure each rock and make a drawing of it. Write down as many physical characteristics as you can think of.

3. Predict which rock sample will be the most resistant to weathering.

4. Place each sample in the coffee can.

5. Pour in the water.

6. Shake vigorously for 5 minutes.

7. Take out the sample and note any changes in size and shape o the rocks. Record your findings

Repeat with each rock specimen. Repeat with each rock two more times until each rock has had 15 minutes of weathering. Pour the water into a clear container. How has the water changed? Record in science notebook.

Additional Resources:

Watch the following videos to discover how igneous, sedimentary, and metamorphic rocks are formed. In addition, you will learn how fossils are important tools for interpreting how plants and animals lived long ago, as well as providing evidence of how the environmental conditions of the Earth have changed over time. The second part explains why the study of rocks and minerals is important. From everyday uses, such as building materials or as fuel sources, to our fascination with precious minerals and gems, students begin to realize how a detailed understanding of this science can become a fascinating career or life-long hobby.

Rock Cycle Video

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