Igneous Rocks

Igneous Rocks

Middle School

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Learning Objectives

• Define minerals and understand they are the building blocks that make up rocks.

• Describe how atoms and molecules arrange in a repeating pattern to form crystals.

• Explain how magma or lava's cooling speed determines the crystal size in igneous rocks.

• Use crystal size to tell the difference between intrusive and extrusive rock formations.

Key Vocabulary

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Mineral

A naturally occurring substance that is an ingredient of all rocks.

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Crystal

An orderly arrangement of atoms and molecules that forms a definite geometric shape.

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Lattice

The regular, repeating internal structure of atoms that forms within a crystal.

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Atom

The essential and smallest particles of a substance that cannot be broken down chemically.

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Molecule

A structure formed when two or more atoms are chemically bonded together.

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Magma

Molten rock found beneath the Earth's surface where atoms can move freely.

Key Vocabulary

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Intrusive Rock

Igneous rock formed from slowly cooling magma, resulting in very large crystals.

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Extrusive Rock

Igneous rock formed from rapidly cooling lava, resulting in very small crystals.

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Minerals: The Building Blocks of Rocks

• All rocks are made of minerals, which are their essential ingredients.

• Some rocks have one mineral, while others like granite contain several.

• Minerals are identified by unique properties like hardness, color, and cleavage.

• A crystal's ordered atomic pattern, a lattice, creates its geometric shape.

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Multiple Choice

What are the essential building blocks that make up all rocks?

• Minerals

• Water

• Sand

• Lava

Multiple Choice

What is the direct cause of a mineral crystal's specific geometric shape?

• The ordered pattern of its atoms

• The color of the mineral

• The rock it was found in

• The hardness of the mineral

Multiple Choice

If a rock like granite contains several different minerals, what would be the best method to identify them?

• By testing their unique properties like hardness and color

• By counting how many crystals there are

• By measuring the size of the entire rock

• By identifying the location where the rock was found

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From Atoms to Crystals

• Minerals are made of tiny particles called atoms, which bond to form molecules.

• A quartz molecule (SiO₂) has one silicon and two oxygen atoms.

• In a liquid, like molten rock, atoms and molecules can move about freely.

• As it cools, atoms form a fixed, organized pattern called a crystal lattice.

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Multiple Choice

What is a crystal lattice?

• A fixed, organized pattern of atoms.

• A collection of freely moving molecules.

• A liquid state of molten rock.

• A mixture of different types of atoms.

Multiple Choice

What causes atoms in a molten liquid to form a crystal lattice?

• The liquid cools, causing the atoms to slow down and lock into a fixed pattern.

• The atoms are compressed by high pressure, forcing them together.

• The liquid heats up, causing the atoms to move faster and collide.

• The atoms randomly bump into each other until they stick together.

Multiple Choice

If a mineral with a defined crystal lattice is heated until it becomes a molten liquid, what will happen to its atoms?

• They would break out of their fixed pattern and move about freely.

• They would form an even more organized and rigid crystal lattice.

• They would bond together to form much larger, heavier atoms.

• They would be destroyed and cease to exist.

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Crystal Size and Rock Formation

Intrusive Rocks

• Formed from magma that cools slowly deep beneath the Earth's surface.

• The slow cooling process allows large crystals time to grow.

• Crystals are large enough to be easily seen without a microscope.

Extrusive Rocks

• Formed from lava that erupts onto the surface and cools rapidly.

• The quick cooling process leaves no time for large crystals to form.

• Crystals are very small and can only be seen with a microscope.

Multiple Choice

Which factor has the greatest influence on the size of crystals in an igneous rock?

• The rate at which the molten rock cooled

• The color of the molten rock

• The age of the molten rock

• The weight of the molten rock

Multiple Choice

What best explains why intrusive and extrusive rocks have different crystal sizes?

• Intrusive rocks form from magma cooling slowly, while extrusive rocks form from lava cooling quickly.

• Intrusive rocks are found on the surface, while extrusive rocks are found deep underground.

• Intrusive rocks are made of heavier materials than extrusive rocks.

• Extrusive rocks are exposed to more pressure, which shrinks the crystals.

Multiple Choice

A geologist discovers a piece of granite with large, interlocking crystals that are easy to see. What is the most likely conclusion about how this rock formed?

• It formed from magma that cooled slowly deep beneath the Earth's surface.

• It formed from lava that cooled rapidly after a volcanic eruption.

• It was created by the rapid compression of sediments.

• It was found in a riverbed, where water polished it smooth.

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Common Misconceptions

Misconception	Correction
Rocks and minerals are the same thing.	Rocks are made of one or more minerals.
All crystals are rare and valuable gems.	Crystals are defined by their ordered structure, not value.
Faster cooling creates bigger crystals.	Slow cooling is needed to form large, well-defined crystals.

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Summary

• All rocks are composed of one or more minerals.

• Minerals form crystals, which have an ordered internal structure called a lattice.

• The size of crystals in igneous rocks is determined by the cooling rate.

• Slow cooling forms large crystals, while rapid cooling creates microscopic crystals.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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• 3

• 4