Types of Rocks

Types of Rocks

Middle School

​

Learning Objectives

• Identify the three main types of rocks and learn how each one is formed.

• Describe the features like composition and texture used to classify different rocks.

• Explain how the rock cycle changes rocks from one type into another over time.

• Learn the differences between the subtypes of igneous, sedimentary, and metamorphic rocks.

Key Vocabulary

​

Minerals

Natural, non-living, solid substances that are the building blocks of all rocks on our planet.

​

Rock Cycle

A nonstop process where rocks are created, transformed from one type to another, and then reformed.

​

Lithification

The process where loose sediments are pressed and glued together to form solid sedimentary rock layers.

​

Magma

Hot, melted rock material found beneath the Earth’s surface that forms certain types of igneous rocks.

​

Foliated

The texture of a metamorphic rock where mineral grains are arranged in parallel layers or bands.

​

Weathering

The process where rocks on the Earth's surface are broken down into smaller pieces called sediments.

​

​

What Are Rocks?

• Rocks are natural solids made of one or more minerals.

• Minerals, like Quartz and Calcite, are made from elements.

• Rocks are classified by their composition and texture.

• Texture is the size, shape, and arrangement of the grains.

​

Multiple Choice

What two main characteristics do scientists use to classify rocks?

• A rock's hardness and market value

• A rock's composition and texture

• A rock's location and age

• A rock's color and weight

The Rock Cycle

• The rock cycle shows how rocks change from one type to another.

• The three main rock types are igneous, sedimentary, and metamorphic.

• These changes happen very slowly over a long period of time.

Multiple Choice

What is the primary concept demonstrated by the rock cycle?

• That all rocks are formed from volcanic activity.

• That the three major rock types can convert from one form to another over time.

• That the rock cycle is a rapid, overnight process.

• That rocks are static and never change.

​

​

Igneous Rocks: Formed from Fire

Intrusive Rocks

• Forms when magma cools slowly beneath the Earth’s surface over many years.

• Slow cooling allows large, visible crystals to grow inside the rock.

• This gives the rock a coarse-grained texture, like in granite.

Extrusive Rocks

• Forms when lava erupts and cools quickly on the Earth’s surface.

• Rapid cooling results in small or no crystals, creating a fine-grained texture.

• Examples include obsidian, which is glassy, and pumice, which is porous.

Multiple Choice

What is the key difference in the formation of intrusive versus extrusive igneous rocks?

• There is no difference in their formation.

• Intrusive rocks cool quickly to form small crystals, while extrusive rocks cool slowly to form large crystals.

• Intrusive rocks cool slowly to form large crystals, while extrusive rocks cool quickly to form small crystals.

• Intrusive rocks form from lava, and extrusive rocks form from magma.

Types of Sedimentary Rocks

​

Clastic Rocks

• ​Made from cemented pieces or fragments of other rocks.

• ​​These rock fragments are what we call sediments.

• ​Sandstone is a very common type of clastic rock.

​

Chemical Rocks

• ​Form when minerals are dissolved in a body of water.

• ​​The water evaporates, leaving the minerals behind to harden.

• ​Halite, also known as rock salt, is a chemical rock.

​

Organic Rocks

• ​Made from the remains of once-living plants or animals.

• ​​Limestone can be formed from the shells of sea creatures.

• ​Coal is an organic rock that is formed from plants.

Multiple Choice

Which example correctly shows how living organisms can contribute to the formation of sedimentary rocks?

• Magma cooling to form granite.

• Shells of sea creatures building up to form limestone.

• Lava hardening after a volcanic eruption.

• Minerals recrystallizing under high pressure to form marble.

Metamorphic Rocks: Changed by Heat & Pressure

• Metamorphic rocks form from other rocks under intense heat and pressure.

• Foliated rocks have minerals arranged in striped or wavy layers.

• Non-foliated rocks do not have layers or bands.

Multiple Choice

What characteristic distinguishes a foliated metamorphic rock, like gneiss, from a non-foliated one, like marble?

• The presence of fossils.

• Its color and shine.

• The arrangement of its mineral grains in parallel layers or bands.

• The size of its crystals.

​

Common Misconceptions About Rocks

Misconception	Correction
Rocks are permanent and do not change.	The rock cycle continuously changes rocks from one type to another.
Fossils can be found in any type of rock.	Fossils are found in sedimentary rocks, as others would destroy them.
Magma and lava are the same thing.	Magma is molten rock below Earth's surface; lava is on the surface.
All rocks are formed in the same way.	Rocks form differently: igneous (cooling) and sedimentary (cementing).

Multiple Choice

A rock is discovered with a coarse-grained texture, meaning it has large, visible crystals. What does this texture tell you about how the rock was likely formed?

• It cooled slowly deep within the Earth.

• It was formed from compacted sand and cooled quickly.

• It cooled rapidly on the Earth's surface.

• It was changed by extreme pressure but not by heat.

Multiple Choice

How does the formation of a clastic sedimentary rock like sandstone differ from the formation of an organic sedimentary rock like coal?

• Sandstone is an intrusive rock, while coal is an extrusive rock.

• Sandstone forms from evaporated sea water, while coal forms from cooled lava.

• Sandstone is made of cemented rock fragments, while coal is formed from the remains of ancient plants.

• Sandstone is formed from heat, while coal is formed from pressure.

Multiple Choice

A piece of slate, a metamorphic rock, is pushed deep into the Earth where it melts completely into magma. If that magma then cools and hardens, what new type of rock will be created?

• The rock will remain slate.

• Another metamorphic rock, like marble.

• A sedimentary rock, like limestone.

• An igneous rock, like granite or basalt.

Multiple Choice

A geologist finds a rock sample that contains fossilized shells. Based on this evidence, what is the most logical conclusion about the rock's origin?

• It is an intrusive rock that cooled inside the Earth.

• It is a sedimentary rock formed in a marine environment.

• It is a metamorphic rock changed by intense pressure.

• It is an igneous rock that erupted from a volcano.

​

Summary

• Rocks are classified into three types based on formation: igneous, sedimentary, and metamorphic.

• Igneous rocks form from cooled magma or lava; cooling speed determines crystal size.

• Sedimentary rocks form from compacted sediments and are the only rocks containing fossils.

• Metamorphic rocks are formed when existing rocks are changed by heat and pressure.

• The rock cycle is the continuous process that transforms rocks between types.

• All rocks are classified by their composition (minerals) and texture (grain size).

Poll

On a scale of 1-4, how confident are you about identifying the three main types of rocks and their formation?

• 1

• 2

• 3

• 4