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Rocks (Book)

This chapter discusses the rock cycle and each of the three major types of rocks that form on Earth. Separate sections cover igneous, sedimentary, and metamorphic rocks individually.

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Date: Saturday, April 20, 2019, 10:23 AM


Devils Tower Image

Have you ever heard the phrase “rock solid?” Something is rock solid if it does not and cannot change. It will not fail or go wrong. A rock-solid plan is a sure bet. A rock-solid idea is sure to be doable. Devil's Tower in Wyoming looks rock solid. It looks like it would not change or move. Even in a million years it would look just like it does now.

In this chapter you will find out that rocks do change. Rocks can change from one type to another. Rocks can alter to have different characteristics but still be the same type. Most changes in rocks take place over long periods of time. More rarely the changes take only a short time. This rock formation’s days are numbered… and a diamond is not forever.


There are three major rock types. Rock of any of these three rock types can become rock of one of the other rock types. Rock can also change to a different rock of the same type. Rocks give good clues as to what was happening in a region during the time that rock formed.


SCI-MS.ESS2.01 Develop a model to describe the cycling of Earth’s materials and the flow of energy that drives this process.


Lesson Objectives

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

The Rock Cycle

All rocks on Earth change, but these changes usually happen very slowly. Some changes happen below Earth’s surface. Some changes happen above ground. These changes are all part of the rock cycle. The rock cycle describes each of the main types of rocks, how they form and how they change.

The figure shows how the three main rock types are related to each other. The arrows within the circle show how one type of rock may change to rock of another type. For example, igneous rock may break down into small pieces of sediment and become sedimentary rock. Igneous rock may be buried within the Earth and become metamorphic rock. Igneous rock may also change back to molten material and re-cool into a new igneous rock.

The rock cycle image

The rock cycle.

rock clues image

Rocks are made of minerals. The minerals may be so tiny that you can only see them with a microscope. The minerals may be really large. A rock may be made of only one type of mineral. More often rocks are made of a mixture of different minerals. Rocks are named for the combinations of minerals they are made of and the ways those minerals came together. Remember that different minerals form under different environmental conditions. So the minerals in a rock contain clues about the conditions in which the rock formed (Figure)

Rocks contain many clues about the conditions in which they formed. The minerals contained within the rocks also contain geological information.

Three Main Categories of Rocks

Geologists group rocks based on how they were formed. The three main kinds of rocks are:




Rocks can be changed from one type to another, and the rock cycle describes how this happens.

- Igneous...

...Igneous rocks form when magma cools below Earth’s surface or lava cools at the surface (Figure).

 lava will harden into an igneous rock image

Lava is molten rock. This lava will harden into an igneous rock.

- Sedimentary...

...Sedimentary rocks form when sediments are compacted and cemented together (Figure below). These sediments may be gravel, sand, silt or clay. Sedimentary rocks often have pieces of other rocks in them. Some sedimentary rocks form the solid minerals left behind after a liquid evaporates.

sandstone is an example of a sedimentary rock image

This sandstone is an example of a sedimentary rock. It formed when many small pieces of sand were cemented together to form a rock.

- Metamorphic...

...Metamorphic rocks form when an existing rock is changed by heat or pressure. The minerals in the rock change but do not melt (Figure below). The rock experiences these changes within the Earth.

mica schist is a metamorphic rock image

This mica schist is a metamorphic rock. It was changed from a sedimentary rock like shale.

Processes of the Rock Cycle

Any type of rock can change and become a new type of rock. Magma can cool and crystallize. Existing rocks can be weathered and eroded to form sediments. Rock can change by heat or pressure deep in Earth's crust. There are three main processes that can change rock:

Cooling and forming crystals

Weathering and erosion


- Cooling and forming crystals

Deep within the Earth, temperatures can get hot enough to melt rock. This molten material is called magma. As it cools, crystals grow, forming an igneous rock. The crystals will grow larger if the magma cools slowly, as it does if it remains deep within the Earth. If the magma cools quickly, the crystals will be very small.

- Weathering and erosion

Water, wind, ice, and even plants and animals all act to wear down rocks. Over time they can break larger rocks into smaller pieces called sediments. Moving water, wind, and glaciers then carry these pieces from one place to another. The sediments are eventually dropped, or deposited, somewhere. The sediments may then be compacted and cemented together. This forms a sedimentary rock. This whole process can take hundreds or thousands of years.

- Metamorphism

This long word means “to change form.“ A rock undergoes metamorphism if it is exposed to extreme heat and pressure within the crust. With metamorphism, the rock does not melt all the way. The rock changes due to heat and pressure. A metamorphic rock may have a new mineral composition and/or texture.

Interactive rock cycle diagram

An interactive rock cycle diagram can be found here:

The rock cycle really has no beginning or end. It just continues. The processes involved in the rock cycle take place over hundreds, thousands, or even millions of years. Even though for us rocks are solid and unchanging, they slowly change all the time.

Lesson Summary

  • There are three main types of rocks: igneous, sedimentary, and metamorphic.
  • Melting and later cooling, erosion and sedimentation, and metamorphism transform one type of rock into another type of rock or change sediments into rock.
  • The rock cycle describes the transformations of one type of rock to another.


Most of the Earth is made of igneous rock. The entire mantle is igneous rock, as are some areas of the crust. One of the most common igneous rocks is granite (Figure).

Many mountain ranges are made of granite. People use granite for countertops, buildings, monuments and statues.

Pumice is also an igneous rock. Perhaps you have used a pumice stone to smooth your skin. Pumice stones are put into giant washing machines with new jeans and tumbled around. The result is stone-washed jeans!

Image of life-size elephant is carved from granite

This life-size elephant is carved from granite.


SCI-MS.ESS2.01 Develop a model to describe the cycling of Earth’s materials and the flow of energy that drives this process.


Lesson Objectives

  • Describe how igneous rocks are formed.
  • Describe the properties of some common types of igneous rocks.
  • Relate some common uses of igneous rocks.

Forming Crystals

Igneous rocks form when magma cools and forms crystals. These rocks can form at Earth’s surface or deep underground. Figure below shows a landscape in California’s Sierra Nevada that consists entirely of granite.

Image of the Sierra Nevada of California are composed mainly of granite

The Sierra Nevada of California are composed mainly of granite. These rocks are beautifully exposed in the Yosemite Valley.

- Intrusive...

Intrusive igneous rocks cool and form into crystals beneath the surface. Deep in the Earth, magma cools slowly. Slow cooling gives large crystals a chance to form. Intrusive igneous rocks have relatively large crystals that are easy to see. Granite is the most common intrusive igneous rock. The (figure) shows four types of intrusive rocks.

Image of Intrusive igneous rocks

(A) This granite has more plagioclase feldspar than many granites. (B) Diorite has more dark-colored minerals than granite. (C) Gabbro. (D) Peridotite is an intrusive igneous rock with olivine and other mafic minerals.

- Extrusive...

Extrusive igneous rocks form above the surface. The lava cools quickly as it pours out onto the surface (Figure). Extrusive igneous rocks cool much more rapidly than intrusive rocks. They have smaller crystals, since the rapid cooling time does not allow time for large crystals to form. Some extrusive igneous rocks cool so rapidly that crystals do not develop at all. These form a glass, such as obsidian. Others, such as pumice, contain holes where gas bubbles were trapped in the lava. The holes make pumice so light that it actually floats in water. The most common extrusive igneous rock is basalt. It is the rock that makes up the ocean floor. Figure shows four types of extrusive igneous rocks.

Images of Extrusive igneous rocks

(A) Lava cools to form extrusive igneous rock. The rocks here are basalts. (B) The strange rock formations of Chiricahua National Monument in Arizona are formed of the extrusive igneous rock rhyolite.

Image of 4 types of Extrusive igneous rocks

(A) This rhyolite is light colored. Few minerals are visible to the naked eye. (B) Andesite is darker than rhyolite. (C) Since basalt crystals are too small to see, the rock looks dark all over. (D) Komatiite is a very rare ultramafic rock. This rock is derived from the mantle.


Igneous rocks are grouped by the size of their crystals and the minerals they contain. The minerals in igneous rocks are grouped into families. Some contain mostly lighter colored minerals, some have a combination of light and dark minerals, and some have mostly darker minerals. The combination of minerals is determined by the composition of the magma. Magmas that produce lighter colored minerals are higher in silica. These create rocks such as granite and rhyolite. Darker colored minerals are found in rocks such as gabbro and basalt.

There are actually more than 700 different types of igneous rocks. Diorite is extremely hard and is commonly used for art. It was used extensively by ancient civilizations for vases and other decorative art work (Figure).

Image of igneous extrusive rock porphyry

This sarcophagus is housed at the Vatican Museum. The rock is the igneous extrusive rock porphyry. Porphyry has large crystals because the magma began to cool slowly, then erupted.

Lesson Summary

  • Igneous rocks form either when they cool very slowly deep within the Earth or when magma cools rapidly at the Earth's surface.
  • Composition of the magma will determine the minerals that will crystallize forming different types of igneous rocks.


Did you know that the White House, the official home and workplace of the President of the United States of America, is made out of the same material as the rock faces in Figure? This material is a sedimentary rock called sandstone. Sandstone is very porous. Water can easily move through it.

So the sandstone of the White House could have been water damaged. But during construction workers covered the sandstone in a mixture of salt, rice, and glue. This mixture protects the sandstone and is what gives the White House its distinct white color.

Image of Layers of sand turned to rock are seen in the Navajo sandstone

Layers of sand turned to rock are seen in the Navajo sandstone. The geologic feature is a slot canyon called Antelope Canyon.


SCI-MS.ESS2.01 Develop a model to describe the cycling of Earth’s materials and the flow of energy that drives this process.


Lesson Objectives

  • Describe how sedimentary rocks are formed.
  • Describe the properties of some common sedimentary rocks.
  • Relate some common uses of sedimentary rocks.


Most sedimentary rocks form from sediments. Sediments are small pieces of other rocks, like pebbles, sand, silt, and clay. Sedimentary rocks may include fossils. Fossils are materials left behind by once-living organisms. Fossils can be pieces of the organism, like bones. They can also be traces of the organism, like footprints.

Most often, sediments settle out of water (Figure). For example, rivers carry lots of sediment. Where the water slows, it dumps these sediments along its banks, into lakes and the ocean. When sediments settle out of water, they form horizontal layers. A layer of sediment is deposited. Then the next layer is deposited on top of that layer. So each layer in a sedimentary rock is younger than the layer under it. It is older than the layer over it.

Image of Cobbles, pebbles, and sands are the sediments

Cobbles, pebbles, and sands are the sediments that are seen on this beach.

Sediments are deposited in many different types of environments. Beaches and deserts collect large deposits of sand. Sediments also continuously wind up at the bottom of the ocean and in lakes, ponds, rivers, marshes, and swamps. Avalanches produce large piles of sediment. The environment where the sediments are deposited determines the type of sedimentary rock that can form.

Sedimentary Rock Formation

Sedimentary rocks form in two ways.

Clastic Rocks

Particles may be cemented together.

Chemical Sedimentary Rocks

Chemicals may precipitate.

- Clastic Rocks

Over time, deposited sediments may harden into rock. First, the sediments are compacted. That is, they are squeezed together by the weight of sediments on top of them. Next, the sediments are cemented together. Minerals fill in the spaces between the loose sediment particles. These cementing minerals come from the water that moves through the sediments. These types of sedimentary rocks are called “clastic rocks.“ Clastic rocks are rock fragments that are compacted and cemented together.

Clastic sedimentary rocks are grouped by the size of the sediment they contain. Conglomerate and breccia are made of individual stones that have been cemented together. In conglomerate, the stones are rounded. In breccia, the stones are angular. Sandstone is made of sand-sized particles. Siltstone is made of smaller particles. Silt is smaller than sand but larger than clay. Shale has the smallest grain size. Shale is made mostly of clay-sized particles and hardened mud.

- Chemical Sedimentary Rocks

Chemical sedimentary rocks form when crystals precipitate out from a liquid. The mineral halite, also called rock salt, forms this way. You can make halite! Leave a shallow dish of salt water out in the Sun. As the water evaporates, salt crystals form in the dish. There are other chemical sedimentary rocks, like gypsum.

Table below shows some common types of sedimentary rocks and the types of sediments that make them up.

Common Sedimentary Rocks
Picture Rock Name Type of Sedimentary Rock
Conglomerate Clastic
Breccia Clastic
Sandstone Clastic
Siltstone Clastic
Limestone Bioclastic
Coal Organic
Rock Salt Chemical precipitate

Lesson Summary

  • Most sedimentary rocks form from sediments. These sediments are deposited, forming layers.
  • The youngest layers are found on top, with older layers below.
  • Sediments must be compacted and cemented to make sedimentary rock.
  • Chemical sedimentary rocks are made of precipitated minerals.


Metamorphism changes rocks by heat and pressure. These agents create an entirely new type of rock. Metamorphism changes rocks physically and/or chemically.


SCI-MS.ESS2.01 Develop a model to describe the cycling of Earth’s materials and the flow of energy that drives this process.


Lesson Objectives

  • Describe how metamorphic rocks are formed
  • Describe the properties of some common metamorphic rocks.
  • Relate some common uses of metamorphic rocks.


Metamorphic rocks start off as some kind of rock. The starting rock can be igneous, sedimentary or even another metamorphic rock. Heat and/or pressure then change the rock’s physical or chemical makeup.

During metamorphism a rock may change chemically. Ions move and new minerals form. The new minerals are more stable in the new environment. Extreme pressure may lead to physical changes like foliation. Foliation forms as the rocks are squeezed. If pressure is exerted from one direction, the rock forms layers. This is foliation. If pressure is exerted from all directions, the rock usually does not show foliation.

There are two main types of metamorphism:

- Contact metamorphism

Contact metamorphism results when magma contacts a rock, changing it by extreme heat

Image of Contact metamorphism

A) Hornfels is a rock that is created by contact metamorphism. (B) Hornfels is so hard that it can create peaks like the Matterhorn.

- Regional metamorphism

Regional metamorphism occurs over a wide area. Great masses of rock are exposed to pressure from rock and sediment layers on top of it. The rock may also be compressed by other geological processes.

Metamorphism does not cause a rock to melt completely. It only causes the minerals to change by heat or pressure.

Image of Regional metamorphism

A) Hornfels is a rock that is created by contact metamorphism. (B) Hornfels is so hard that it can create peaks like the Matterhorn.

Hornfels is a rock with alternating bands of dark and light crystals. Hornfels is a good example of how minerals rearrange themselves during metamorphism (Figure). The minerals in hornfels separate by density. The result is that the rock becomes banded. Gneiss forms by regional metamorphism from extremely high temperature and pressure.

Uses of Metamorphic Rocks

Quartzite and marble are the most commonly used metamorphic rocks. They are frequently chosen for building materials and artwork. Marble is used for statues and decorative items like vases (Figure). Quartzite is very hard and is often crushed and used in building railroad tracks. Schist and slate are sometimes used as building and landscape materials.

Images of Uses of Metamorphic Rocks

(A) Marble is a beautiful rock that is commonly used for buildings. (B) Many of the great statues of the Renaissance were carved from marble. Michelangelo created this Moses between 1513 and 1515.

Lesson Summary

  • Metamorphic rocks form when heat and pressure transform an existing rock into a new rock.
  • Contact metamorphism occurs when hot magma transforms rock that it contacts.
  • Regional metamorphism transforms large areas of existing rocks under the tremendous heat and pressure created by tectonic forces.