Earth Materials and Geology

Mineralogy and Crystallography are the study of minerals and the way their tiny particles are arranged. Minerals are the basic materials that make up rocks. Learning about minerals helps us understand Earth and the materials we use every day.

A mineral is a special kind of Earth material. Scientists use four important rules to decide if something is a mineral.

• It is naturally occurring, which means it is made by nature, not by people.
• It is inorganic, which means it does not come from living things.
• It is a solid, so it keeps its own shape.
• It has a definite chemical composition and an ordered crystalline structure.

Let’s look more closely at those last two ideas, because they are the heart of this lesson.

Definite chemical composition means a mineral is made of the same kinds of matter in a set pattern. For example, the mineral halite is made of sodium and chlorine in a fixed combination. This is why a mineral has its own special properties.

Ordered crystalline structure means the tiny particles inside the mineral are lined up in a repeating pattern. Even if we cannot see the pattern with our eyes, it is there. This repeating pattern helps give crystals their shapes.

A crystal is a solid in which particles are arranged in a repeating pattern. When a mineral has room to grow, it can form flat sides and straight edges. That is why many crystals have beautiful geometric shapes.

Not all solids are minerals. A material must fit all four rules to be a mineral.

1. Naturally occurring: It must form in nature.
2. Inorganic: It must not come from once-living material.
3. Solid: It must not be a liquid or gas.
4. Definite composition and crystal structure: It must have a set makeup and a repeating inside pattern.

Here are some examples of things that are minerals:

• Quartz — a common mineral found in many rocks.
• Halite — the mineral form of salt.
• Diamond — a very hard mineral made of carbon.
• Calcite — a mineral found in limestone and seashell areas.

Here are some examples of things that are not minerals:

• Plastic — it is made by people, so it is not naturally occurring.
• Coal — it forms from living things, so it is not inorganic.
• Water — liquid water is not a solid.
• Wood — it comes from living things.

Minerals have properties that help scientists identify them. These include:

• Color — what color the mineral looks like.
• Luster — how it shines, such as glassy or dull.
• Hardness — how easily it can be scratched.
• Streak — the color of its powder when rubbed on a streak plate.
• Crystal shape — the form made by its crystal structure.

The crystal shape on the outside is related to the repeating pattern on the inside. If the inside particles are arranged in an orderly way, the mineral can grow into a crystal with flat faces and angles. This is why crystallography, the study of crystal shapes and patterns, is important.

Think of crystal structure like stacking blocks in rows. If you place the blocks in the same pattern again and again, you get a neat shape. Minerals do something similar, but with tiny particles too small to see without special tools.

Minerals are the building blocks of rocks. Most rocks are made of one or more minerals. For example, granite is made of minerals such as quartz, feldspar, and mica.

This means that when we study minerals, we learn more about the rocks that make up Earth’s crust. That also helps us understand Earth’s surface and how it changes over long periods of time.

Worked Example 1: Is table salt a mineral?

Question: Table salt comes from the mineral halite. Does halite count as a mineral?

Step 1: Is it naturally occurring? Yes, halite forms in nature.

Step 2: Is it inorganic? Yes, it does not come from living things.

Step 3: Is it a solid? Yes.

Step 4: Does it have a definite composition and crystal structure? Yes.

Answer: Yes, halite is a mineral.

Worked Example 2: Is coal a mineral?

Question: Coal is found in the ground, so is it a mineral?

Step 1: Is it naturally occurring? Yes.

Step 2: Is it inorganic? No. Coal forms from ancient plants, which were once living things.

Answer: No, coal is not a mineral because it is not inorganic.

Worked Example 3: Why do crystals have shapes?

Question: A quartz crystal has flat sides and points. Why?

Step 1: Quartz is a mineral.

Step 2: Minerals have particles arranged in a repeating pattern.

Step 3: That repeating pattern helps the crystal grow in an orderly shape.

Answer: Crystals have shapes because their particles are arranged in an ordered crystalline structure.

Worked Example 4: Mineral or not?

Question: A student finds ice in nature. Could it be a mineral?

Step 1: Is it naturally occurring? Yes, if it formed in nature.

Step 2: Is it inorganic? Yes, it does not come from living things.

Step 3: Is it a solid? Yes.

Step 4: Does it have a definite composition and crystal structure? Yes, frozen water has a set makeup and crystal pattern.

Answer: Yes, natural ice can be a mineral.

Here is a simple way to test whether something is a mineral. Ask these questions:

• Did nature make it?
• Did it come from nonliving material?
• Is it a solid?
• Does it have a set chemical makeup?
• Are its particles arranged in a repeating crystal pattern?

If the answer is yes to all of them, then it is a mineral.

Important idea to remember: A rock and a mineral are not the same thing. A mineral is one Earth material with a definite makeup and crystal structure. A rock is usually made of one or more minerals together.

For example:

• Quartz is a mineral.
• Granite is a rock made of several minerals.

Scientists study minerals to learn about Earth’s crust, where materials come from, and how rocks form and change. Because minerals are the parts that build rocks, they are an important part of Earth science.

Summary

A mineral is a naturally occurring, inorganic solid with a definite chemical composition and an ordered crystalline structure. The repeating pattern inside a mineral forms its crystal structure and helps create crystal shapes. Minerals are different from rocks because rocks are made of one or more minerals. By learning the rules for minerals, we can tell which Earth materials are minerals and which are not.

Mineral Identification Techniques

Minerals are the natural materials that make up rocks. Scientists study minerals to learn about Earth. Because many minerals can look alike, we need special identification techniques to tell them apart.

In this lesson, you will learn how to identify minerals by testing their hardness, streak, luster, cleavage, and specific gravity. These are clues that help us figure out what mineral we have.

What is a mineral?

A mineral is a natural, nonliving solid found in Earth. It has a definite chemical makeup and forms in a regular pattern called a crystal structure. You do not need to memorize all of that right now, but it helps explain why minerals have different properties.

Why can we not identify a mineral by color alone?

Color can be helpful, but it is not always reliable. The same mineral may come in different colors. For example, quartz can be clear, white, pink, or purple. That is why scientists use several tests instead of just looking at color.

Main Mineral Identification Properties

1. Hardness

Hardness is how easily a mineral can be scratched. A harder mineral can scratch a softer mineral. Scientists often compare hardness using the Mohs scale, which goes from 1 to 10.

• 1 = very soft
• 10 = very hard

Here are a few common hardness examples:

• Talc = 1
• Gypsum = 2
• Calcite = 3
• Fluorite = 4
• Apatite = 5
• Feldspar = 6
• Quartz = 7
• Topaz = 8
• Corundum = 9
• Diamond = 10

You can also use common objects to estimate hardness:

• Fingernail: about 2.5
• Copper coin: about 3
• Steel nail: about 5
• Glass: about 5.5

If a mineral is scratched by a fingernail, it is very soft. If it scratches glass, it is harder than glass.

2. Streak

Streak is the color of a mineral in powdered form. To test streak, rub the mineral across an unglazed porcelain streak plate.

A mineral's outside color may vary, but its streak is often more dependable. For example, pyrite looks gold on the outside, but its streak is dark greenish-black or brownish-black.

3. Luster

Luster is how a mineral reflects light. In simple words, it tells how the surface looks when light shines on it.

• Metallic luster: looks shiny like metal
• Nonmetallic luster: does not look like metal

Some kinds of nonmetallic luster include:

• Glassy: like glass
• Pearly: like a pearl
• Dull: not shiny

Quartz often has a glassy luster. Pyrite often has a metallic luster.

4. Cleavage

Cleavage is the way a mineral breaks along flat surfaces. This happens because of the way the particles are arranged inside the mineral.

Some minerals break into smooth, flat pieces. Others do not. If a mineral does not break along flat surfaces, it may break in a rough or curved way instead.

For example:

• Mica splits into thin, flat sheets.
• Halite breaks into cubes.
• Quartz does not show cleavage well and breaks unevenly.

5. Specific Gravity

Specific gravity tells how heavy a mineral feels for its size. Some small minerals feel surprisingly heavy. Others feel light.

You do not always need to calculate a number in 5th grade. It is often enough to compare two minerals of the same size and ask, “Which one feels heavier?”

If two mineral samples are about the same size, but one feels much heavier, that heavier mineral has a greater specific gravity.

How Scientists Identify a Mineral

Scientists do not usually use just one test. They gather several clues and put them together.

1. Look at the mineral's color and shape.
2. Test its luster.
3. Check its hardness by seeing what can scratch it.
4. Test its streak on a streak plate.
5. Observe how it breaks to check cleavage.
6. Compare how heavy it feels for its size.

Using many properties makes identification more accurate.

Important Safety Reminder

Mineral testing should be done carefully and with an adult or teacher. Some minerals can have sharp edges after breaking. Never taste a mineral, and do not rub powders near your eyes or mouth.

Worked Example 1: Using Hardness

A student has a mineral sample. The student's fingernail cannot scratch it, but a copper coin can scratch it.

Question: What can we say about the mineral's hardness?

Step 1: A fingernail has hardness about 2.5.

Step 2: A copper coin has hardness about 3.

Step 3: If the fingernail cannot scratch the mineral, then the mineral is harder than 2.5.

Step 4: If the coin can scratch it, then the mineral is softer than 3.

Answer: The mineral's hardness is between 2.5 and 3.

Worked Example 2: Using Streak and Luster

A mineral looks shiny and gold-colored. A student thinks it is gold. But when the student rubs it on a streak plate, it leaves a dark streak.

Question: What does this tell us?

Step 1: The gold color alone is not enough to identify the mineral.

Step 2: The shiny look means the mineral may have metallic luster.

Step 3: A dark streak is a clue that it may be pyrite, not real gold.

Answer: The mineral is probably not gold. The streak test gives a better clue than color alone.

Worked Example 3: Using Cleavage

Two minerals are hit gently and break. Mineral A breaks into thin flat sheets. Mineral B breaks into rough pieces with no flat sides.

Question: Which mineral shows cleavage more clearly?

Step 1: Cleavage means breaking along flat surfaces.

Step 2: Mineral A breaks into flat sheets.

Step 3: Mineral B breaks into rough pieces.

Answer: Mineral A shows cleavage more clearly.

Worked Example 4: Putting Clues Together

A student tests an unknown mineral and finds these clues:

• It scratches glass.
• It has a glassy luster.
• It does not leave a strong colored streak.
• It breaks unevenly, not in flat sheets or cubes.

Question: What common mineral might this be?

Step 1: If it scratches glass, it is harder than glass, so its hardness is more than about 5.5.

Step 2: Glassy luster is a clue.

Step 3: Uneven breaking means it does not show clear cleavage.

Step 4: A common mineral with these properties is quartz.

Answer: The mineral may be quartz.

Tips for Remembering the Tests

• Hardness = Can it be scratched?
• Streak = What color is its powder?
• Luster = How does it shine?
• Cleavage = How does it break?
• Specific gravity = How heavy does it feel for its size?

Why These Tests Matter

Mineral identification helps geologists understand rocks, mountains, soil, and Earth's history. Minerals are also used in everyday life. They are found in buildings, pencils, electronics, jewelry, and even toothpaste.

When we identify minerals correctly, we learn more about the materials that make up our planet.

Lesson Summary

Minerals can be identified by their properties, not just by their color. The most useful tests include hardness, streak, luster, cleavage, and specific gravity.

A scientist looks at all the clues together. By testing carefully and comparing results, we can identify many common minerals and better understand Earth materials.

Mechanical and Chemical Weathering

The ground under our feet is made of rock, soil, sand, and water. Earth’s surface does not stay the same forever. Over time, wind, water, ice, plants, and even air can change rocks.

One important way rocks change is called weathering. Weathering is the breaking down of rock into smaller pieces. Some weathering happens by pushing, cracking, or scraping. Some weathering happens when rocks change because of chemicals in water or air.

In this lesson, you will learn about two kinds of weathering: mechanical weathering and chemical weathering.

What Is Mechanical Weathering?

Mechanical weathering is when rock is broken into smaller pieces without changing what the rock is made of. The rock gets smaller, cracked, or broken, but it is still the same kind of rock.

You can think of it like breaking a cracker into crumbs. The cracker pieces are smaller, but they are still cracker.

Mechanical weathering can happen in many ways.

• Ice wedging: Water gets into a crack in a rock. When the water freezes, it expands and pushes the crack wider. After this happens many times, the rock can break apart.
• Plant roots: A tiny root can grow into a small crack in a rock. As the root gets bigger, it pushes on the rock and makes the crack larger.
• Rubbing and scraping: Rocks can bump and scrape against each other in rivers, on hills, or at the beach. This can wear them down into smaller, smoother pieces.

What Is Chemical Weathering?

Chemical weathering is when rock changes because of a chemical reaction. The rock does not just get smaller. It also changes into something a little different.

You can think of it like when an apple slice turns brown after it sits out. It changes because of a reaction with the air.

Chemical weathering can happen in these ways.

• Acid rain: Rainwater can mix with gases in the air and become a little acidic. This weak acid can slowly wear away some rocks.
• Oxidation: Some rocks have iron in them. When iron reacts with oxygen in air or water, it can form rust. Rust is a sign that the rock has changed.
• Water reactions: Water can react with some minerals in rocks and slowly change them.

How Are They Different?

The big difference is this:

• Mechanical weathering breaks rock into smaller pieces, but the rock stays the same kind of material.
• Chemical weathering changes the rock into a different material because of a chemical reaction.

Why Does Weathering Matter?

Weathering helps make smaller pieces of rock called sediment. Sediment includes pieces like sand, tiny rock bits, and clay.

These smaller pieces can be moved by water, wind, or ice. Over a long time, weathering helps shape mountains, hills, riverbeds, and beaches.

Weathering also helps form soil. Soil is important because plants grow in it.

Mechanical Weathering Examples

Here are some simple examples of mechanical weathering:

1. A puddle of water seeps into a crack in a rock.
2. The weather gets very cold, and the water freezes.
3. Frozen water takes up more space and pushes on the rock.
4. The crack gets bigger.
5. After many freezes and melts, a piece of rock breaks off.

In this example, the rock is still rock. It was broken by force, not changed into a new material.

Another example is a tree root growing in a sidewalk crack or rock crack. The root grows thicker over time and pushes the crack wider. This breaks the rock apart.

Chemical Weathering Examples

Here are some simple examples of chemical weathering:

1. Rain falls through the air and picks up gases.
2. The rain becomes a little acidic.
3. The acid in the rain touches certain rocks.
4. Very slowly, the rock begins to change and wear away.

Another example is a rock with iron in it. When the iron reacts with oxygen and water, rust can form. This is chemical weathering because the material changes.

Worked Example 1

Question: Water freezes in a crack in a rock and the rock breaks. Is this mechanical weathering or chemical weathering?

Step 1: Ask what happened to the rock. It cracked and broke into pieces.

Step 2: Ask if the rock changed into a new material. No. It is still the same rock, just broken.

Answer: This is mechanical weathering.

Worked Example 2

Question: A plant root grows into a crack and pushes the rock apart. What kind of weathering is this?

Step 1: The root is using force to push the rock.

Step 2: The rock is breaking into pieces.

Step 3: The rock did not change into a different material.

Answer: This is mechanical weathering.

Worked Example 3

Question: Rainwater reacts with rock and slowly changes it. Is this mechanical or chemical weathering?

Step 1: Look for a reaction. The rainwater reacts with the rock.

Step 2: A reaction means the rock is changing, not just breaking.

Answer: This is chemical weathering.

Worked Example 4

Question: A rock contains iron and starts to rust. What kind of weathering is this?

Step 1: Rust forms when iron reacts with oxygen and water.

Step 2: That means a chemical change happened.

Answer: This is chemical weathering.

Helpful Clue Words

If you are trying to tell the two kinds apart, these clue words can help.

• Mechanical weathering clue words: crack, break, scrape, rub, freeze, roots, push
• Chemical weathering clue words: react, change, acid rain, rust, oxygen

Let’s Compare

• Ice in cracks → mechanical weathering
• Roots pushing rocks apart → mechanical weathering
• Rocks rubbing in a river → mechanical weathering
• Acid rain wearing away rock → chemical weathering
• Iron in rock making rust → chemical weathering

Quick Check

Ask yourself these questions:

1. Did the rock just break into smaller pieces? If yes, it is probably mechanical weathering.
2. Did the rock change because of a reaction with water or air? If yes, it is probably chemical weathering.

Summary

Weathering is the process that breaks down rock. Mechanical weathering breaks rock into smaller pieces by forces like ice, roots, and rubbing. Chemical weathering changes rock through reactions, such as acid rain and oxidation.

Both kinds of weathering are important because they help make sediment and soil. Over time, they help shape Earth’s surface.

The Dynamic Rock Cycle

Rocks may look hard and unchanging, but Earth is always at work. Over very long periods of time, rocks can break apart, melt, cool, get squeezed, and change into new kinds of rocks.

This repeating set of changes is called the rock cycle. It is called a cycle because rock material is reused again and again. It is called dynamic because it does not always follow one single path.

That means a rock can change in many different ways. Any rock type can become another rock type if the right Earth processes happen over time.

What are the three main rock types?

• Igneous rock forms when melted rock cools and hardens.
• Sedimentary rock forms when bits of rock, sand, mud, shells, or other material are pressed and stuck together.
• Metamorphic rock forms when rock changes because of heat and pressure, but does not fully melt.

Let’s look at each rock type more closely.

Igneous rocks begin as melted rock. Melted rock under Earth’s surface is called magma. Melted rock that reaches the surface is called lava.

When magma or lava cools, it becomes solid rock. This makes igneous rock. Examples include granite and basalt.

Sedimentary rocks form from sediments. Sediments are small pieces of rock, minerals, and once-living material.

Wind, water, ice, and gravity can move sediments from one place to another. Over time, layers of sediments build up. The lower layers get pressed together and stick into rock. Examples include sandstone and shale.

Metamorphic rocks start as other rocks. Deep inside Earth, strong heat and pressure can change a rock’s shape, texture, or mineral pattern.

The rock does not melt completely. Instead, it changes into a new kind of rock. Examples include marble and slate.

The main processes in the rock cycle

The rock cycle happens because of several Earth processes. These processes can happen in different orders.

• Weathering: breaking rock into smaller pieces.
• Erosion: moving those pieces by water, wind, ice, or gravity.
• Deposition: dropping sediments in a new place.
• Compaction and cementation: pressing sediments together and gluing them into sedimentary rock.
• Heat and pressure: changing a rock into metamorphic rock.
• Melting: turning rock into magma.
• Cooling: hardening magma or lava into igneous rock.

Why is the rock cycle called non-linear?

Non-linear means it does not move in just one straight line. A rock does not have to go through the same steps every time.

For example, an igneous rock might:

• break into sediments and become sedimentary rock,
• get buried and changed by heat and pressure into metamorphic rock, or
• melt and then cool again into a new igneous rock.

A sedimentary rock might stay sedimentary for a long time, or it might become metamorphic if it is buried deep underground. Later, it could melt and become igneous.

A metamorphic rock might be lifted up to the surface, weather into sediments, and become sedimentary rock. Or it might melt and cool into igneous rock.

This is why scientists often draw the rock cycle with many arrows, not just one path.

How Earth’s energy drives the rock cycle

Energy from the Sun and from inside Earth helps keep the rock cycle moving.

• The Sun helps drive weather, wind, rain, and flowing water. These cause weathering and erosion.
• Heat from inside Earth helps melt rock and creates the heat and pressure that form metamorphic rocks.

The rock cycle takes a very long time

The rock cycle usually happens over millions of years. Rocks do not usually change overnight.

Even though the changes are slow, they are always happening. Mountains rise, rocks crack, rivers carry sediments, and deep heat changes buried rocks.

A simple way to model the rock cycle

You can think of the rock cycle like a map with several places and many roads between them. The “places” are the three rock types and magma. The “roads” are the processes that change one into another.

• Magma --cooling--> Igneous rock
• Any rock --weathering, erosion, deposition, compaction, cementation--> Sedimentary rock
• Any rock --heat and pressure--> Metamorphic rock
• Any rock --melting--> Magma

This model shows that any rock type can change into another. The path depends on what happens to the rock.

Worked Example 1: From magma to igneous rock

A volcano erupts and lava flows onto Earth’s surface. The lava cools and becomes solid.

Question: What kind of rock forms?

Answer: Igneous rock forms.

Why? Igneous rock forms when melted rock cools and hardens. Since lava is melted rock, cooling changes it into igneous rock.

Worked Example 2: From rock pieces to sedimentary rock

A cliff is broken into small pieces by wind and rain. A river carries the pieces away. The pieces settle in layers at the bottom of a lake. Over time, the layers are pressed together and stuck together.

Question: What kind of rock forms at the end?

Answer: Sedimentary rock forms.

Why? The rock first went through weathering and erosion. Then the sediments were deposited, compacted, and cemented. Those steps form sedimentary rock.

Worked Example 3: From sedimentary rock to metamorphic rock

Layers of sedimentary rock are buried deep underground. They do not melt, but they are exposed to strong heat and pressure for a very long time.

Question: What kind of rock do they become?

Answer: Metamorphic rock.

Why? Heat and pressure can change existing rock into metamorphic rock, as long as the rock does not fully melt.

Worked Example 4: More than one possible path

Suppose you start with an igneous rock deep underground.

Path A: It is pushed upward, breaks apart at the surface, becomes sediments, and later turns into sedimentary rock.

Path B: It stays underground and is changed by heat and pressure into metamorphic rock.

Path C: It melts into magma and later cools into a new igneous rock.

Question: Which path is correct?

Answer: All of them can be correct.

Why? The rock cycle is dynamic and non-linear. A rock can take different paths depending on the conditions around it.

Important ideas to remember

• Rocks are always changing over long periods of time.
• There are three main rock types: igneous, sedimentary, and metamorphic.
• Weathering and erosion help make sediments.
• Compaction and cementation form sedimentary rock.
• Heat and pressure form metamorphic rock.
• Melting makes magma, and cooling makes igneous rock.
• The rock cycle is not one straight path.
• Any rock type can become another rock type over geologic time.

Brief Summary

The dynamic rock cycle explains how Earth’s rocks change again and again over very long times. Igneous, sedimentary, and metamorphic rocks form in different ways, but any of them can change into another when Earth processes like weathering, erosion, heat, pressure, melting, and cooling occur.

When you build a model of the rock cycle, be sure to show many possible pathways, not just one circle. That is what makes the rock cycle dynamic.

Weathering Mechanics is the study of how rocks break down at Earth’s surface. Weathering happens slowly over time, but it is always at work. Wind, water, ice, plants, and even air can change rocks.

There are two main kinds of weathering:

• Mechanical weathering: rocks break into smaller pieces, but the rock’s material stays the same.
• Chemical weathering: rocks are changed into new materials because of chemical reactions.

Learning the difference is important because both kinds of weathering help shape Earth’s surface.

What is weathering?

Weathering is the process that breaks down rock in place. This means the rock is changed or broken where it is found. Weathering is different from erosion. Erosion is when rock pieces or soil are moved by wind, water, ice, or gravity.

For example, if a rock cracks apart on a hillside, that is weathering. If rainwater carries the broken pieces downhill, that is erosion.

Mechanical Weathering

Mechanical weathering is also called physical weathering. It changes the size or shape of a rock, but not what the rock is made of.

Imagine breaking a cracker into smaller crumbs. The crumbs are smaller, but they are still cracker. In the same way, a large rock can be broken into smaller rocks, but each piece is still the same kind of rock.

Causes of mechanical weathering include:

• Ice: Water can seep into cracks in rocks. When the water freezes, it expands and pushes the crack wider.
• Plant roots: Roots grow into small cracks. As roots get bigger, they push the rock apart.
• Wind and water: Moving sand, pebbles, and water can rub against rocks and wear them down.
• Temperature changes: In some places, heating and cooling can make rocks expand and shrink, which may cause cracking over time.

Ice wedging is a common kind of mechanical weathering. Water enters a crack, freezes, and expands. Then the crack gets bigger. If this happens many times, the rock may split apart.

Chemical Weathering

Chemical weathering happens when a rock’s minerals are changed into different substances. The rock is not just broken into smaller pieces. Instead, the material itself changes.

This kind of weathering often happens when rocks react with water, oxygen, or weak acids in nature.

Causes of chemical weathering include:

• Water: Water can dissolve some minerals in rock.
• Oxygen: Oxygen in the air can react with minerals, especially iron, causing rust.
• Acid rain: Rainwater can mix with gases in the air and become slightly acidic. This can slowly wear away some rocks.
• Plant acids: Some plants and decaying matter add weak acids to the soil, which can react with rock.

A good example is a rock that contains iron. When iron reacts with oxygen, it can form rust. Rust is a new material, so this is chemical weathering.

Another example is limestone. Limestone can slowly dissolve when it comes in contact with weak acids in rainwater. The rock changes because some of its material is carried away in the water.

How are mechanical and chemical weathering different?

• Mechanical weathering changes a rock’s size, shape, or pieces.
• Chemical weathering changes a rock’s materials.

A simple way to remember it is:

• Mechanical = break apart
• Chemical = change into something new

How they work together

Mechanical and chemical weathering often happen together. First, mechanical weathering may crack a rock into smaller pieces. Then chemical weathering can act on those smaller pieces more easily.

Why? Smaller pieces have more exposed surface. That means more of the rock can touch air and water.

For example, think about a whole apple and an apple cut into slices. The slices have more surface exposed to air. In the same way, broken rock has more surface exposed to weathering.

Weathering changes Earth’s surface

Weathering helps make soil, sand, and sediment. It helps wear down mountains and cliffs. Over long periods of time, weathering can change the shape of landforms.

Weathering is part of the larger story of Earth materials. Rocks break down, the pieces move, and later they may become part of new rocks. This connects weathering to the rock cycle.

Clues to tell which kind of weathering is happening

You can ask these questions:

1. Did the rock only break into smaller pieces?
   If yes, it is probably mechanical weathering.
2. Did the rock’s minerals change or form new materials?
   If yes, it is probably chemical weathering.

Look for clues such as cracks, broken pieces, rust-like color changes, or rock that seems dissolved.

Worked Example 1

A tree root grows into a small crack in a sidewalk rock. Over time, the crack gets wider and pieces of rock break apart.

Question: Is this mechanical or chemical weathering?

Step 1: Look at what happened. The root pushed the rock apart.

Step 2: Ask whether the rock became a new material. No. It only broke into pieces.

Answer: This is mechanical weathering.

Worked Example 2

Rainwater slowly dissolves part of a limestone statue over many years.

Question: Is this mechanical or chemical weathering?

Step 1: Notice that the rock material is being dissolved.

Step 2: The rock is changing because of a reaction with weak acid in rainwater.

Answer: This is chemical weathering.

Worked Example 3

Water fills a crack in a rock during the day. At night, the water freezes and expands. After many freezes and thaws, the rock splits.

Question: What kind of weathering is this?

Step 1: Freezing water pushes on the crack.

Step 2: The rock breaks, but it does not turn into a new substance.

Answer: This is mechanical weathering, especially ice wedging.

Worked Example 4

A rock with iron in it is exposed to air and water. After a long time, reddish-brown rust forms on the rock.

Question: What kind of weathering is this?

Step 1: Rust is forming, which means a new material is being made.

Step 2: The rock’s minerals are changing because of a reaction with oxygen.

Answer: This is chemical weathering.

Compare the two kinds

• Mechanical weathering examples: ice cracking rock, roots splitting rock, wind rubbing rock, moving water wearing rock down
• Chemical weathering examples: rusting, dissolving limestone, minerals reacting with water or weak acids

Why weathering matters

Without weathering, Earth’s surface would look very different. Weathering helps create soil for plants. It helps form sand on beaches and sediment in rivers. It also helps break old rock into pieces that can later become new sedimentary rock.

Weathering is slow, but over deep time it causes big changes. Small changes repeated again and again can wear down even very large rocks.

Quick check

• If a rock is smashed into smaller bits, is that mechanical or chemical weathering?
  Mechanical
• If a rock reacts with oxygen and forms rust, is that mechanical or chemical weathering?
  Chemical
• If freezing water widens a crack, is that mechanical or chemical weathering?
  Mechanical
• If weak acid dissolves part of a rock, is that mechanical or chemical weathering?
  Chemical

Summary

Weathering is the breakdown of rock at Earth’s surface. Mechanical weathering breaks rock into smaller pieces without changing what it is made of. Chemical weathering changes the rock’s minerals into new materials. Both kinds of weathering work together to shape Earth’s surface over time.

Agents of Erosion and Deposition

Earth’s surface is always changing. Mountains wear down, rivers carve valleys, beaches grow and shrink, and new landforms are built over time. Two important processes that cause these changes are erosion and deposition.

Erosion is the movement of rock, soil, and sediment from one place to another. Sediment means small pieces of rock, sand, soil, or clay. Deposition happens when those sediments are dropped in a new place.

You can think of it like this: erosion is the carrying away, and deposition is the dropping off.

The main agents, or causes, of erosion and deposition are:

• Water
• Wind
• Glacial ice
• Gravity

These agents move sediment and shape Earth’s surface over long periods of time.

1. Water as an Agent of Erosion and Deposition

Water is one of the most powerful agents of erosion. Moving water in rivers, streams, rain runoff, and ocean waves can pick up and carry sediment.

When rain falls, some water flows over the ground. This moving water can wash away loose soil. Over time, it can cut small channels in the land. Rivers and streams can carry rocks and sand for many miles.

Water can also cause deposition. When water slows down, it can no longer carry as much sediment. The sediment falls and builds up in a new place.

Water creates many landforms through erosion and deposition, including:

• Canyons from rivers cutting through rock
• Deltas where rivers drop sediment at their mouths
• Sandbars where water slows and drops sand
• Beaches where waves deposit sand

Example: A river flowing down a mountain moves quickly and carries pebbles, sand, and soil. When the river reaches flatter land, it slows down and drops some of that sediment. This can form a wide, rich plain.

2. Wind as an Agent of Erosion and Deposition

Wind can pick up and move tiny particles such as sand, dust, and loose soil. Wind erosion happens most easily in dry places with little plant cover, like deserts or bare fields.

As wind blows, it can wear away rock and move sediment from one place to another. Strong winds can carry dust high into the air and move it far away.

Wind also causes deposition. When the wind slows down, it drops the sediment it was carrying.

Wind can create landforms such as:

• Sand dunes formed when wind deposits sand
• Loess deposits, which are piles of fine dust and soil

Example: On a beach, wind blows dry sand inland. When the wind meets an obstacle, like grass or a fence, the sand drops and begins to pile up. Over time, a sand dune forms.

3. Glacial Ice as an Agent of Erosion and Deposition

A glacier is a large, slow-moving mass of ice. Glaciers move very slowly, but they are powerful. As they move, they scrape and pluck rocks from the ground. This causes erosion.

Glaciers can carry huge amounts of rock and soil. When a glacier melts, it drops the sediments it was carrying. This is deposition.

Glacial ice can create landforms such as:

• U-shaped valleys carved by moving ice
• Moraines, which are piles of sediment left by glaciers
• Lakes formed in places glaciers carved out

Example: A glacier moves through a mountain valley and grinds against the rock. It widens and deepens the valley. Later, when the glacier melts, it leaves behind piles of rock and soil.

4. Gravity as an Agent of Erosion and Deposition

Gravity is the force that pulls objects toward Earth. Gravity causes erosion when rocks, soil, or mud move downhill. This movement is called mass movement.

Gravity can act alone or help other agents. For example, rain can loosen soil, and then gravity pulls it down a hill.

Some examples of gravity-caused movement are:

• Landslides
• Mudslides
• Rockfalls
• Creep, which is very slow downhill movement of soil

Gravity also leads to deposition because the material that falls or slides downhill collects at the bottom.

Example: After heavy rain, wet soil on a steep hill may slide downward. The soil and rocks collect at the bottom of the slope.

How Erosion and Deposition Work Together

Erosion and deposition are connected. First, sediment is broken loose and moved. Then, it is deposited somewhere else.

A simple pattern is:

1. Rock and soil are loosened.
2. An agent like water, wind, ice, or gravity moves the sediment.
3. The agent slows down or stops.
4. The sediment is deposited in a new place.

In general, faster-moving water or wind can carry more sediment. Slower-moving water or wind carries less, so it drops what it was carrying.

Size of Sediment Matters

Not all sediments are the same size. Some are large, like pebbles. Some are tiny, like clay.

Larger and heavier sediments usually drop first. Smaller and lighter sediments can travel farther before being deposited.

For example:

• A fast river may carry pebbles, sand, and mud.
• When the river slows, the pebbles drop first.
• Sand may settle next.
• Tiny mud particles may travel the farthest.

Landforms Made by Deposition

Deposition builds new landforms. Some important examples are:

• Delta: a fan-shaped area of deposited sediment where a river enters a lake or ocean
• Alluvial fan: a fan-shaped deposit formed when a stream leaves a steep area and slows down
• Beach: sand or pebbles deposited by waves
• Sand dune: a hill of sand formed by wind deposition
• Moraine: a ridge or pile of sediment left by a glacier

Worked Example 1: Water in a River

A river rushes quickly through the mountains. Later, it reaches a flat plain and slows down. What is most likely to happen?

Step 1: Identify the agent. The agent is water.

Step 2: Think about speed. Fast water can carry more sediment. Slow water carries less.

Step 3: Decide what happens. When the river slows, it will deposit some of the sediment.

Answer: The river will drop sediment on the flat plain. This can help form a floodplain or delta if it is near an ocean or lake.

Worked Example 2: Wind and Sand

Strong winds blow across a dry desert. The winds move sand for many days. Then the wind becomes weaker near a group of bushes. What landform may begin to form?

Step 1: Identify the agent. The agent is wind.

Step 2: Ask what happens when wind slows down. Slower wind drops sediment.

Step 3: Name the deposit. Sand will pile up near the bushes.

Answer: A sand dune may begin to form.

Worked Example 3: Glacier Change

A glacier slowly moves through a valley. It scrapes rock from the ground and carries it. Later, the climate gets warmer and the glacier melts. What are two things that happened?

Step 1: While the glacier moved, it scraped and carried rock. That is erosion.

Step 2: When the glacier melted, it dropped the rock and soil. That is deposition.

Answer: The glacier caused erosion by scraping and moving sediment, and it caused deposition by leaving sediment behind when it melted.

Worked Example 4: Gravity on a Hill

After several days of rain, part of a steep hillside collapses and slides downward. Rocks and mud collect at the bottom. Which agent caused this, and where did deposition happen?

Step 1: The downhill movement tells us the agent is gravity.

Step 2: The material moved from high ground to low ground. That movement is erosion.

Step 3: The place where the rocks and mud collect is where deposition happened.

Answer: The agent is gravity, and deposition happened at the bottom of the hill.

How Plants Can Help

Plants can slow erosion. Their roots hold soil in place, and their stems help block wind and water.

When there are fewer plants, soil can be carried away more easily by water and wind. That is why grasses, trees, and other plants are important for keeping soil in place.

Important Ideas to Remember

• Erosion moves sediment from one place to another.
• Deposition drops sediment in a new place.
• Water, wind, glacial ice, and gravity are the main agents of erosion and deposition.
• When an agent slows down, deposition is more likely to happen.
• These processes create and change landforms over time.

Brief Summary

Earth’s surface changes because sediment is constantly being moved and dropped. Water, wind, glacial ice, and gravity are the main agents that cause erosion and deposition. These processes wear down old landforms and build new ones, such as deltas, dunes, beaches, and moraines.

Soil Horizons and Pedogenesis are big words for a simple idea: soil forms slowly over time, and it has different layers.

When rocks break into tiny pieces and dead plants and animals decay, they mix together to make soil. This slow process of soil forming is called pedogenesis. You can think of pedogenesis as the making of soil.

Soil is very important. Plants grow in it, many animals live in it, and it helps hold water. Soil is not all the same from top to bottom. If you dig down into the ground, you may see different layers. These layers are called soil horizons.

In 5th grade, the main soil horizons to learn are O, A, B, and C. Each layer has different materials and looks a little different.

How soil forms

Soil begins with parent material. Parent material is the rock or broken rock pieces that soil comes from. Over time, wind, water, ice, plant roots, and changes in temperature help break rock into smaller pieces.

At the same time, leaves, dead plants, and dead animals decay. This decayed material becomes humus, which is dark, rich organic matter. Humus mixes with small rock pieces and helps make the upper soil layers.

Soil forms very slowly. It can take many years to make even a small amount of soil. A simple way to think about soil formation is:

$$\text{rock pieces} + \text{decayed organic matter} + \text{time} = \text{soil}$$

The soil horizons

The horizons are usually found from top to bottom in this order:

1. O horizon
2. A horizon
3. B horizon
4. C horizon

Let’s learn what each one is made of.

O Horizon

The O horizon is the top layer. The letter O stands for organic. This layer is made mostly of dead leaves, twigs, pine needles, and other decaying plant material.

This layer is often dark and soft. It may be thick in a forest, where many leaves fall to the ground. In some places, the O horizon may be very thin or hard to see.

A Horizon

The A horizon is often called topsoil. It is one of the most important layers for plants. It contains minerals from broken rock mixed with humus.

Because it has humus, the A horizon is usually darker than the layers below it. Many plant roots, worms, insects, and tiny living things are found here.

B Horizon

The B horizon is the layer below topsoil. It is often called subsoil. This layer has less humus than the A horizon.

The B horizon often collects minerals and clay that move down from the layers above. It is usually lighter in color than topsoil and may feel more packed together.

C Horizon

The C horizon is made of partly broken parent material. It has large rock pieces and not much organic matter.

This layer looks more like rock than rich soil. It is close to the original material from which the soil formed.

A simple way to remember the layers

• O = organic material like leaves
• A = topsoil with humus and minerals
• B = subsoil with clay and minerals
• C = broken parent material

How the layers form over time

First, rock breaks into smaller pieces. Then plants begin to grow. Leaves and other living things die and decay. This adds organic matter to the top.

Rainwater can carry tiny bits of minerals and clay downward. Because of this movement, different layers slowly become different from one another.

Over a long time, the soil develops clear horizons. This is why older soil often has more noticeable layers than younger soil.

What affects soil formation?

Several things can change how soil forms:

• Parent material: Different rocks make different soils.
• Water: Rain helps break down rock and move materials through the soil.
• Plants and animals: Living things add organic matter and mix the soil.
• Climate: Warm, wet places may form soil faster than cold, dry places.
• Time: Soil needs a long time to develop layers.

Why soil horizons matter

Soil horizons help scientists understand how soil formed and what is in it. Farmers and gardeners also care about soil layers because plants grow best in healthy topsoil.

If topsoil washes away, it can be hard for plants to grow well. That is one reason why protecting soil is important.

Worked Example 1: Identifying the top layer

Question: A student sees a dark layer made mostly of dead leaves and twigs on the forest floor. Which horizon is it?

Step 1: Look for clues. The layer is made mostly of decaying plant material.

Step 2: Match the clue to a horizon. The O horizon is made of organic material.

Answer: It is the O horizon.

Worked Example 2: Finding topsoil

Question: Which soil horizon is best described as a mix of minerals and humus where many roots grow?

Step 1: Remember that topsoil has both mineral particles and humus.

Step 2: Think about where most small roots grow. They grow in topsoil.

Answer: The layer is the A horizon.

Worked Example 3: Comparing two layers

Question: How is the B horizon different from the C horizon?

Step 1: Recall the B horizon. It is subsoil and often has clay and minerals that moved down from above.

Step 2: Recall the C horizon. It is made of partly broken parent material and has larger rock pieces.

Answer: The B horizon has more subsoil materials like clay and minerals, while the C horizon is closer to broken parent rock.

Worked Example 4: Putting the layers in order

Question: Put these horizons in order from top to bottom: B, O, C, A.

Step 1: Remember the usual order of horizons.

$$O \rightarrow A \rightarrow B \rightarrow C$$

Step 2: Rewrite the list in that order.

Answer: O, A, B, C

Common mistakes to avoid

• Do not confuse O horizon with topsoil. The O horizon is mostly organic material, while the A horizon is topsoil.
• Do not think all places have thick, easy-to-see layers. Some soils have very thin horizons.
• Do not forget that soil comes from both parent material and organic decay.
• Do not mix up B horizon and C horizon. B is subsoil; C is broken parent material.

Quick review

• Soil forms slowly through pedogenesis.
• Parent material is the starting rock material.
• Humus is decayed organic matter.
• The main soil horizons are O, A, B, C.
• O is organic material.
• A is topsoil.
• B is subsoil.
• C is broken parent material.

Summary

Soil is made when broken rock mixes with decayed organic matter over a long time. This process is called pedogenesis. Soil has layers called horizons, and the main ones are O, A, B, and C. Each layer has different materials, from dead leaves at the top to broken parent rock lower down.