Lesson Objectives

  • Define rock and describe what rocks are made of.
  • Know how rocks are classified and described.
  • Explain how each of the three main rock types formed.
  • Describe the rock cycle.

What Are Rocks?

A rock is a naturally formed, non-living Earth material. Rocks are made of collections of mineral grains that are held together in a firm, solid mass (see Figure below).
The different colors and textures seen in this rock are caused by the presence of different minerals.
The different colors and textures seen in this rock are caused by the presence of different minerals.

The different colors and textures seen in this rock are caused by the presence of different minerals.
How is a rock different from a mineral? Rocks are made of minerals. The mineral grains in a rock may be so tiny that you can only see them with a microscope, or they may be as big as your fingernail or even your finger (Figure below).
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A pegmatite from South Dakota with crystals of lepidolite, tourmaline, and quartz (1 cm scale on the upper left).
Rocks are identified primarily by the minerals they contain and by their texture. Each type of rock has a distinctive set of minerals. A rock may be made of grains of all one mineral type, such as quartzite. Much more commonly, rocks are made of a mixture of different minerals. Texture is a description of the size, shape, and arrangement of mineral grains. Are these two samples the same rock type (Figure below) and (Figure below)? Do they have the same minerals? The same texture?
Rock - diorite.
Rock - diorite.

Sample 1.
Rock - andesite.
Rock - andesite.

Sample 2.
Comparing properties of Sample 1 and Sample 2|| Sample || Minerals || Texture || Formation || Rock type ||
Sample 1
plagioclase, hornblende, pyroxene
Crystals, visible to naked eye
Magma cooled slowly
Sample 2
plagioclase, hornblende, pyroxene
One type of crystal visible, rest microscopic
Magma erupted and cooled quickly
These two rocks have the same chemical composition and contain mostly the same minerals, but they do not have the same texture. Sample 1 has visible mineral grains, but Sample 2 has some visible grains in a fine matrix. The two different textures indicate different histories. Sample 1 is a diorite, a rock that cooled slowly from magma (molten rock) underground. Sample 2 is an andesite, a rock that cooled rapidly from a very similar magma that erupted onto Earth’s surface.

CH 3.2 Igneous Rocks

  • there are 3 types of rocks; igneous, sedimentary and metamorphic.
  • In the very beginning of the Earth's evolution, both the interior and exterior were molten, or melted rock.
  • As a result, Igneous Rocks are the 1st rocks, the ancient ancestors from which sedimentary and metamorphic rocks come from.


the root of "igneous" is ignis, which is latin for "fire."
  • Magma is lava that is still inside the Earth.
  • Lava is magma that has erupted.
There are 2 types of Igneous Rocks
1) Intrusive Igneous rocks form from magma which COOLS OFF VERY SLOWLY WHILE IT IS STILL INSIDE (intrusive) the Earth's lithosphere.
2) Extrusive Igneous rocks originate from erupti that erupts onto Earth’s surface is lava (oras seen in Figure below).
Extrusive Igneous Rocks are formed from the lava or other material that is ejected during a volcanic eruption
types of
The type of ROCK that forms from The chemical composition of the magma and the rate at which it cools determine what rock forms as the minerals cool and crystallize.
Flowing lava.
Flowing lava.

This flowing lava is molten rock that will harden into an igneous rock.

2) Sedimentary Rocks

Sedimentary rocks form by the compaction and cementing together of sediments, broken pieces of rock-like gravel, sand, silt, or clay (see Figure below). Those sediments can be formed from the weathering and erosion of preexisting rocks. Sedimentary rocks also include chemical precipitates, the solid materials left behind after a liquid evaporates.
This sandstone is an example of a sedimentary rock.
This sandstone is an example of a sedimentary rock.

This sedimentary rock is made of sand that is cemented together to form a sandstone.

Metamorphic Rocks

Metamorphic rocks form when any existing rock is changed by heat or pressure within the Earth, so that its minerals change. See Figure below for an example of a metamorphic rock.
This quartzite is an example of a metamorphic rock.
This quartzite is an example of a metamorphic rock.

Quartzite is a metamorphic rock formed when quartz sandstone is exposed to heat and pressure within the Earth.
A simple explanation of the three rock types and how to identify them can be seen in this video: htt
This Science Made Fun video discusses the conditions under which the three main rock types form (3c): The Rock Cycle
Rocks change as a result of natural processes that take place all the time. Most changes happen very slowly. Many happen below Earth’s surface so we do not even notice the changes. The physical and chemical properties of rocks are constantly changing in a natural, never-ending cycle called the rock cycle.
In the rock cycle, illustrated in the Figure below, the three main rock types – igneous, sedimentary, and metamorphic are shown. Arrows connect the three rock types showing the processes that change one rock type into another.
The Rock Cycle.

Processes of the Rock Cycle

Several processes can turn one type of rock into another type of rock. The key processes of the rock cycle are crystallization, erosion and sedimentation, and metamorphism.


Magma cools either underground or on the surface and hardens into an igneous rock. As the magma cools, different crystals form at different temperatures, undergoing crystallization. For example, the mineral olivine crystallizes out of magma at much higher temperatures than quartz. The rate of cooling determines how much time the crystals will have to form. Slow cooling produces larger crystals.

Erosion and Sedimentation

Weathering wears rocks at the Earth’s surface down into smaller pieces. The small fragments are called sediments. Running water, ice, and gravity all transport these sediments from one place to another by erosion. During sedimentation, the sediments are laid down or deposited. In order to form a sedimentary rock, the accumulated sediment must become compacted and cemented together.


When a rock is exposed to extreme heat and pressure within the Earth, but does not melt, the rock becomes metamorphosed. Metamorphism may change the mineral composition and the texture of the rock. A metamorphic rock may have a new mineral composition and/or texture. The rock cycle has no beginning and no end. Rocks deep within the Earth right now are becoming other types of rocks. Rocks at the surface are lying in place before they are next exposed to a process that will change them.
The concept of the rock cycle was first developed by James Hutton, an eighteenth century scientist often called the "Father of Geology" (shown in Figure below). Hutton recognized that geologic processes have "no [sign] of a beginning, and no prospect of an end." The processes involved in the rock cycle often take place over millions of years. So on the scale of a human lifetime, rocks appear to be "rock solid" and unchanging, but in the longer term change is always taking place.
James Hutton is considered the "Father of Geology."
James Hutton is considered the "Father of Geology."

James Hutton is considered the "Father of Geology."

Lesson Summary

  • Rocks are collections of minerals of various sizes and types.
  • The three main rock types are igneous, sedimentary, and metamorphic.
  • Crystallization, erosion and sedimentation, and metamorphism transform one rock type into another rock type or change sediments into rock.
  • The rock cycle describes the transformations of one type of rock to another.

Review Questions

  1. Describe the difference between a rock and a mineral.
  2. Why can the minerals in a rock be a clue about how the rock formed?
  3. What is the difference between magma and lava?
  4. What are the three main types of rocks and how does each form?
  5. Describe how an igneous rock changes into a metamorphic rock.
  6. Describe how an igneous rock changes into a sedimentary rock.
  7. Explain how sediments form.
  8. In which rock type do you think fossils, which are the remains of past living organisms, are most often found?


sedimentation Sediments are laid down in a deposit. sedimentary rock A rock that forms from the compaction of sediments or the precipitation of material from a liquid. sediment Small particles of soil or rock deposited by wind or water. rock cycle The never-ending cycle in which one rock type changes into another rock type. precipitate Solid substance that separates out of a liquid to form a solid, usually when the liquid evaporates. molten Something that is melted. metamorphic rock A rock that forms from a previous rock that is exposed to heat and/or pressure. magma Molten rock below Earth’s surface. lava Molten rock at the surface of Earth. igneous rock A rock formed from cooled magma. erosion The transport and deposition of sediments. crystallization The formation of mineral grains from a cooling magma.

Points to Consider

  • If Earth’s interior was no longer hot, but all other processes on Earth continued unchanged, what would happen to the different rock types in the rock cycle?
  • Stone tools were important to early humans. Are rocks still important to humans today?