Unit E Lesson E8 Rock Classification
Completion requirements
Lesson E8: Rock Classification
Video Lesson
Unless you see an erupting volcano or a rock crusher break larger stones, you probably never see rocks change. We might move rocks or use them to build structures, but they almost always stay in the same form. Over long periods, however, all rocks change.
They can change from one rock type to another, too. We classify rocks into three types: igneous, metamorphic, or sedimentary.
Lesson E8: Rock Classification
The Three Classes of Rock
Rocks change from one form to another, but they can be classified into three main types: igneous, metamorphic, or sedimentary. Each rock type is defined by how it formed.
Assume you are a rockhound. What do you do the try to identify an unknown rock? If possible, you need to find a sample of your rock that is still attached to Earth’s crust. Also, you need a nice, clean, and clear view of the rock. Put on your safety glasses and use your rock hammer to break a piece from the unknown rock. If you did not collect the rock yourself, try to find out where it was collected. The best rockhounds collect as much information as possible!
Rocks change from one form to another, but they can be classified into three main types: igneous, metamorphic, or sedimentary. Each rock type is defined by how it formed.
Assume you are a rockhound. What do you do the try to identify an unknown rock? If possible, you need to find a sample of your rock that is still attached to Earth’s crust. Also, you need a nice, clean, and clear view of the rock. Put on your safety glasses and use your rock hammer to break a piece from the unknown rock. If you did not collect the rock yourself, try to find out where it was collected. The best rockhounds collect as much information as possible!
Reading for This Lesson
Science in Action 7
Materials:
Science in Action 7
Reading: Pages 377–383
Materials:
No other materials are required for this lesson.
Figure E.2.8.1 – Amateur geologists sometimes like to search for and collect rocks. These “rockhounds” might hunt for precious gemstones, or they might want simply to collect various rocks. Photo by
Hannes Grobe
Figure E.2.8.2 – Rockhounds find and identify rocks. Here’s your first rockhound sample. As you read through this lesson, try to identify clues that will help you determine the type of rock this is.
Figure E.2.8.3 – Basalt is an extrusive igneous rock that forms above Earth’s crust.
Figure E.2.8.4 – Gabbro is an intrusive igneous rock that forms below Earth’s crust.
Intrusive Igneous Rocks
Igneous rocks form when molten hot liquid rock cools to form solid rock. Material deep beneath Earth’s surface is incredibly hot – so hot that it is molten and minerals mix and flow together. This hot liquid rock is magma. It is so hot that it can melt other rocks that it contacts. Large chambers of magma can form in Earth’s crust. Deeper down, Earth’s mantle is composed almost entirely of magma.
Igneous rocks form when molten hot liquid rock cools to form solid rock. Material deep beneath Earth’s surface is incredibly hot – so hot that it is molten and minerals mix and flow together. This hot liquid rock is magma. It is so hot that it can melt other rocks that it contacts. Large chambers of magma can form in Earth’s crust. Deeper down, Earth’s mantle is composed almost entirely of magma.
Igneous rocks can form anywhere that magma cools. If this happens below Earth’s surface, the rocks that form are called intrusive igneous rocks. You can remember this because intrusive igneous rocks form inside Earth’s crust. Intrusive igneous rocks cool
very slowly; they can take many years to become solid rocks. The longer the intrusive rocks take to become solid rocks, the longer the minerals in the cooling rock have to join together to form grains and crystals. This means that an igneous rock
with large grains and crystals must have cooled very slowly and is likely an intrusive igneous rock.
Extrusive Igneous Rocks
Igneous rocks can form above Earth’s surface, too. Magma can rise and flow from volcanoes and other cracks in Earth’s crust. When the magma reaches the surface, it is called lava. Lava cools more quickly above Earth’s surface than magma does below the surface. Lava that flows into water cools even more quickly.
Extrusive Igneous Rocks
Igneous rocks can form above Earth’s surface, too. Magma can rise and flow from volcanoes and other cracks in Earth’s crust. When the magma reaches the surface, it is called lava. Lava cools more quickly above Earth’s surface than magma does below the surface. Lava that flows into water cools even more quickly.
Figure E.2.8.5 – Igneous rocks form from molten rock either below or above Earth’s crust.
Rocks that form from cooling lava above Earth’s surface are called extrusive igneous rocks. You can remember this because extrusive igneous rocks form after the lava has exited Earth’s crust. Extrusive igneous rocks can still take months to become completely
solid, but they cool much more quickly than intrusive igneous rocks do. This does not give the minerals time to clump together very well. The grains and crystals in an extrusive igneous rock are much smaller, and sometimes they cannot be seen at all.
When you look closely at an igneous rock, the first thing you will notice is what is not there – layers. Igneous rocks rarely have any layers in them. When a rockhound sees a rock with no layers, he or she can be quite sure it is an igneous rock. Then, the rockhound looks for crystals and grains in the rock. Large crystals mean the rock is intrusive, and small grains mean it is extrusive. Examples of intrusive igneous rocks include granite, gabbro, and diorite. Examples of extrusive igneous rocks include basalt and obsidian.
When you look closely at an igneous rock, the first thing you will notice is what is not there – layers. Igneous rocks rarely have any layers in them. When a rockhound sees a rock with no layers, he or she can be quite sure it is an igneous rock. Then, the rockhound looks for crystals and grains in the rock. Large crystals mean the rock is intrusive, and small grains mean it is extrusive. Examples of intrusive igneous rocks include granite, gabbro, and diorite. Examples of extrusive igneous rocks include basalt and obsidian.
Figure E.2.8.6 – Notice the large grains in this sample of the igneous rock granite. Does this mean that this is an intrusive or an extrusive igneous rock?
Connections
Connections: First Nations
>> Obsidian
Many hundreds of years ago, a rare black rock that looks like glass was valued highly by many First Nations groups throughout Alberta and British Columbia. This rock could be chipped until it formed sharp tools and weapons. The rock is called obsidian, an extrusive igneous rock.
The Blackfoot First Nations were one of the groups that prized the qualities of obsidian. Early trade among First Nations included the valuable obsidian. Where do you think the original obsidian was found? How do you think the obsidian formed
Obsidian found in Alberta was unlikely to have come from Alberta. Instead, it likely came from the volcanoes of British Columbia, Washington, and Oregon.
Figure E.2.8.7 – Obsidian is an extrusive igneous rock that cools so quickly that no grains form. Because of this, it looks almost like glass. In fact, obsidian often is referred to as “volcanic glass”.
For example, the obsidian found in tools and weapons of First Nations of northern Alberta often can be traced to Mount Edziza where the Tahltan First Nations lived in British Columbia. The Blackfoot and other First Nations in southern Alberta likely obtained
their obsidian through trade with the Squamish First Nations who lived near Mount Garibaldi in British Columbia. Both Mount Edziza and Mount Garibaldi are dormant volcanoes.
Figure E.2.8.8 – Sandstone layers that were deposited at different times are angled slightly different than others around them are.
Figure E.2.8.9 – The layers in the sedimentary rock limestone can be difficult to see. When tested with a weak acid such as vinegar, the limestone fizzes and bubbles.
Sedimentary Rocks
Sedimentary rocks form when bits of weathered rocks are eroded into a place where they are deposited as layers of sediment. Some of the sedimentary layers can become very thick, and many layers can pile on top of one another. The weight of all the materials and layers compresses the materials. Over hundreds, thousands, and millions of years, the layers become solid sedimentary rock. If the sediments were mostly sand, sandstone has formed. The sediments can be the shells and skeletons of ocean life, which form the sedimentary rock limestone.
Sedimentary rocks form when bits of weathered rocks are eroded into a place where they are deposited as layers of sediment. Some of the sedimentary layers can become very thick, and many layers can pile on top of one another. The weight of all the materials and layers compresses the materials. Over hundreds, thousands, and millions of years, the layers become solid sedimentary rock. If the sediments were mostly sand, sandstone has formed. The sediments can be the shells and skeletons of ocean life, which form the sedimentary rock limestone.
Figure E.2.8.10 – Splitting shale layers can reveal amazing fossils trapped in the sedimentary rock.
Figure E.2.8.11 – Sedimentary rocks can be made of other rocks. Conglomerates are made of rounded gravel, and breccia are made of angled gravel.
Watch More
Watch this video to see how a special rock layer containing space sediment may help to explain what happened to the dinosaurs.
Figure E.2.8.12 – Marble is the metamorphic rock that forms from limestone. Often, it is used for sculptures and construction of buildings and monuments.
Figure E.2.8.13 – Gneiss (pronounced “nice”) is a metamorphic rock with layering. These are not the same as the layers in sedimentary rock. Rather, they are layers of minerals that join together when the rock was heated.
Metamorphic Rocks
Metamorphic rocks are rocks that change. The word “metamorphism” means “change in form”. Metamorphic rocks result when rock is exposed to high heat and extreme pressure. This can occur near magma, under layers of heavy rock, or where tectonic plates meet. Igneous, sedimentary, and even other metamorphic rocks can become new metamorphic rocks under these intense conditions.
Metamorphic rocks are rocks that change. The word “metamorphism” means “change in form”. Metamorphic rocks result when rock is exposed to high heat and extreme pressure. This can occur near magma, under layers of heavy rock, or where tectonic plates meet. Igneous, sedimentary, and even other metamorphic rocks can become new metamorphic rocks under these intense conditions.
An interesting example of metamorphic rock is schist. Schist is a hard metamorphic rock that has long, flat grains that often look like scales. Schist forms when slate undergoes large amounts of heat and pressure. Slate is also a metamorphic rock. Slate
has very fine grains, and it can be split into wide sheets. It sometimes is used for the flat surface of billiard tables or floor tiles, or you might have seen it as shingles on roofs of some old buildings. Slate was the sedimentary rock shale. Shale
forms when thick layers of mud are buried under pressure for long periods. Therefore, schist forms in the following way:
-
Layers of mud form the sedimentary rock shale.
-
Heat and pressure cause shale to become slate by metamorphosis.
- Heat and pressure cause slate to become schist by metamorphosis.
Figure E.2.8.14 – Schist forms when slate undergoes large amounts of heat and pressure.
Figure E.2.8.15 – Quartz sandstone is a sedimentary rock. It is formed from layers of sand that are cemented together and then buried for long periods.
Figure E.2.8.16 – Obsidian is an igneous rock that formed when a rock such as sandstone or quartzite melts below ground and then turns solid after flowing out of the ground as lava.
Figure E.2.8.17 – Quartzite is a metamorphic rock that used to be quartz sandstone until it was changed by high heat and pressure into its new form.
Classifying Rocks
To identify a rock can be very challenging. To see where the rock comes from and what it is surrounded by is helpful. A rock by itself does not have anyone else around to help tell its story. However, we often will not have a chance to see the source of a rock. Therefore, we must use other clues. The following checklist is helpful when trying to identify an unknown rock:
To identify a rock can be very challenging. To see where the rock comes from and what it is surrounded by is helpful. A rock by itself does not have anyone else around to help tell its story. However, we often will not have a chance to see the source of a rock. Therefore, we must use other clues. The following checklist is helpful when trying to identify an unknown rock:
1. Does it look pure? Does it look like a crystal?
2. Does it have layers or large chunks in it?
3. Are the layers long, flat, and evenly spaced? Are the chunks actual rocks or fossils? Does the rock chip away easily?
- Yes: It is probably a mineral.
- No: It is probably a rock. Go to step 2.
2. Does it have layers or large chunks in it?
- Yes: It could be a sedimentary or metamorphic rock. Go to step 3.
- No: It is probably an igneous rock.
3. Are the layers long, flat, and evenly spaced? Are the chunks actual rocks or fossils? Does the rock chip away easily?
- Yes: It is probably a sedimentary rock.
- No: It is probably a metamorphic rock.
At this point, you will know the class of rock, but you will have to go through a more specific process to determine the name of your unknown rock. Many more questions must be asked and minerals in the rock must be identified before you can classify
and identify the rock accurately.
Figure E.2.8.18 – Can you identify which rock group our unknown sample is from? It has layers that appear short and spaced randomly. It is likely a metamorphic rock.
Think • Interpret • Decide
Identifying Rocks and Minerals
Perhaps your inner rockhound has been awakened, and you are starting to notice the rocks around you. Exploring places such as river valleys, stream banks, gravel beds, and other places where you can find rocks is the best way to learn more about them.
Because in this course we cannot put rocks in your hand, we are providing some beautiful photos of some rocks and minerals, supplied by Julian Gray of Rice NW Museum of Rocks and Minerals.
Using the simple identification key provided above, try to match the rocks and minerals in the following pictures with their descriptions.
Photos by Julian Gray, Rice NW Museum of Rocks and Minerals
Perhaps your inner rockhound has been awakened, and you are starting to notice the rocks around you. Exploring places such as river valleys, stream banks, gravel beds, and other places where you can find rocks is the best way to learn more about them.
Because in this course we cannot put rocks in your hand, we are providing some beautiful photos of some rocks and minerals, supplied by Julian Gray of Rice NW Museum of Rocks and Minerals.
Using the simple identification key provided above, try to match the rocks and minerals in the following pictures with their descriptions.
Photos by Julian Gray, Rice NW Museum of Rocks and Minerals
Matching Exercise: Match the photos on the left with their proper description on the right.
Photos
Descriptions
1.
2.
A.
B.
Mineral: Gypsum
Description: Large, flat crystals that often are white or colourless.
Description: Large, flat crystals that often are white or colourless.
Sedimentary: Fossiliferous Sandstone
Description: Fossilized remains of plants or animals trapped in layers or a background of sandstone.
Description: Fossilized remains of plants or animals trapped in layers or a background of sandstone.
3.
4.
C.
D.
Mineral: Agate
Description: Bands of very tiny grains and bright colours instead of crystals.
Description: Bands of very tiny grains and bright colours instead of crystals.
Metamorphic rock: Gneiss
Description: Dark bands of mineral grains on a light background.
Description: Dark bands of mineral grains on a light background.
5.
6.
E.
F.
Igneous rock: Granite
Description: Light-coloured minerals in background with dark speckles of other minerals.
Description: Light-coloured minerals in background with dark speckles of other minerals.
Igneous rock: Gabbro
Description: Dark coloured with green hues. Coarse grained.
Description: Dark coloured with green hues. Coarse grained.
7.
8.
G.
H.
Mineral: Fluorite
Description: Large, colourful crystals form in cube shapes, often on a bed of another mineral.
Description: Large, colourful crystals form in cube shapes, often on a bed of another mineral.
Mineral: Microcline Feldspar
Description: This mineral often forms large, blue-green crystals that are roughly the shape of rectangular prisms.
Description: This mineral often forms large, blue-green crystals that are roughly the shape of rectangular prisms.
9.
10.
I.
J.
Sedimentary: Conglomerate
Description: Large round particles of gravel and rock cemented together.
Description: Large round particles of gravel and rock cemented together.
Sedimentary: Breccia
Description: Small angular particles of gravel and rock cemented together.
Description: Small angular particles of gravel and rock cemented together.
11.
12.
K.
L.
Mineral: Barite
Description: White or colourless crystals.
Description: White or colourless crystals.
Metamorphic rock: Schist
Description: Scaly texture of mineral grains; sometimes has embedded garnet gemstones.
Description: Scaly texture of mineral grains; sometimes has embedded garnet gemstones.
13.
14.
M.
N.
Igneous rock: Basalt
Description: Gray or black with possible rusting; fine grained.
Description: Gray or black with possible rusting; fine grained.
Mineral: Quartz
Description: Clear to colourful crystals that are often seen as six-sided prisms topped with 6-sided pyramids.
Description: Clear to colourful crystals that are often seen as six-sided prisms topped with 6-sided pyramids.
15.
16.
O.
P.
Sedimentary: Sandstone
Description: Layers of sand particles of slightly different colours (often reddish).
Description: Layers of sand particles of slightly different colours (often reddish).
Mineral: Mimetite
Description: Yellow, brown, orange, or colourless clusters of crystals that resemble round or angular grapes.
Description: Yellow, brown, orange, or colourless clusters of crystals that resemble round or angular grapes.
Photo
Description
Photo
Description
1.
K.
2.
G.
Mineral: Barite
Description: White or colourless crystals.
Description: White or colourless crystals.
Mineral: Fluorite
Description: Large, colourful crystals form in cube shapes, often on a bed of another mineral.
Description: Large, colourful crystals form in cube shapes, often on a bed of another mineral.
3.
M.
4.
A.
Igneous rock: Basalt
Description: Gray or black in colour, with possible rusting; fine grained.
Description: Gray or black in colour, with possible rusting; fine grained.
Mineral: Gypsum
Description: Large, flat crystals that are often white or colourless.
Description: Large, flat crystals that are often white or colourless.
5.
O.
6.
F.
Sedimentary: Sandstone
Description: Layers of sand particles of slightly different colours (often reddish).
Description: Layers of sand particles of slightly different colours (often reddish).
Igneous rock: Gabbro
Description: Dark coloured with green hues; coarse grained.
Description: Dark coloured with green hues; coarse grained.
7.
I.
8.
L.
Sedimentary: Conglomerate
Description: Large round particles of gravel and rock cemented together.
Description: Large round particles of gravel and rock cemented together.
Metamorphic rock: Schist
Description: Scaly texture of mineral grains; sometimes has embedded garnet gemstones.
Description: Scaly texture of mineral grains; sometimes has embedded garnet gemstones.
9.
B.
10.
N
Sedimentary: Fossiliferous Sandstone
Description: Fossilized remains of plants or animals trapped in layers or a background of sandstone.
Description: Fossilized remains of plants or animals trapped in layers or a background of sandstone.
Mineral: Quartz
Description: Clear to colourful crystals that are often seen as six-sided prisms topped with 6-sided pyramids.
Description: Clear to colourful crystals that are often seen as six-sided prisms topped with 6-sided pyramids.
11.
E.
12.
P.
Igneous rock: Granite
Description: Light-coloured minerals in background with dark speckles of other minerals.
Description: Light-coloured minerals in background with dark speckles of other minerals.
Mineral: Mimetite
Description: Yellow, brown, orange, or colourless clusters of crystals that resemble round or angular grapes.
Description: Yellow, brown, orange, or colourless clusters of crystals that resemble round or angular grapes.
13.
D.
14.
C
Metamorphic rock: Gneiss
Description: Dark bands of mineral grains on a light background.
Description: Dark bands of mineral grains on a light background.
Mineral: Agate
Description: Bands of very tiny grains and bright colours instead of crystals.
Description: Bands of very tiny grains and bright colours instead of crystals.
15.
J.
16.
H.
Sedimentary: Breccia
Description: Small angular particles of gravel and rock cemented together.
Description: Small angular particles of gravel and rock cemented together.
Mineral: Microcline Feldspar
Description: This mineral often forms large, blue-green crystals that are roughly the shape of rectangular prisms.
Description: This mineral often forms large, blue-green crystals that are roughly the shape of rectangular prisms.
Understanding the differences is quite difficult until you have compared many samples.
Probably the most important clues are the qualities mentioned in the simplified questions:
- Do you see a pure mineral and/or crystals?
- Do you see layers or chunks?
- Do the layers and chunks look like whole objects or formed crystals?
Other tests that would help are to see where the rock came from, to use a magnifying glass to see the grains, to chip some of the sample, and to perform mineral tests such as hardness, streak, etc. These tests would help to provide more evidence to determine
each sample.
Figure E.2.8.19 – Notice the black line in the mountain face in the upper-left corner of this photo of Mt. Siyeh in Waterton Lakes National Park. That is a band of igneous rock. The white bands beside it are metamorphic rock.
Figure E.2.8.20 – Layers of sedimentary rock can be seen throughout Alberta in underground caverns as well as in valleys and mountains.
The Rocks around You
The seas that covered Alberta for millions of years were rich in plants and animals, and many layers of sediment were deposited. For this reason, most rocks in Alberta are sedimentary.
The seas that covered Alberta for millions of years were rich in plants and animals, and many layers of sediment were deposited. For this reason, most rocks in Alberta are sedimentary.
In only two significant locations can igneous and metamorphic rocks be found in Alberta. The Canadian Shield extends into the northeast corner of the province. The Canadian Shield is a huge area of igneous and metamorphic rocks that was formed billions of years ago.
The other location to find igneous and metamorphic rocks is near Waterton Lakes National Park in southwestern Alberta. A thick black layer of igneous rock and thin layers of white metamorphic rock can be seen in several mountains in that area. Likely, a large lava flow covered the bottom of the sea at this location more than 500 million years ago.
A few igneous and metamorphic rocks are found scattered throughout Alberta. Can you guess how they got there? As the glaciers moved from the north, they scraped the Canadian Shield and brought portions of these igneous and metamorphic rocks far south. Often, these rocks are found in gravel pits with other eroded rock, but farmers find them scattered in their fields, too.
Figure E.2.8.21 – Did you remember to identify our unknown rock? It is a metamorphic rock called gneiss.
Make sure you have understood everything in this lesson. Use the Self-Check below, and the Self-Check & Lesson Review Tips to
guide your learning.
Unit E Lesson 8 Self-Check
Instructions
Complete the following 6 steps.
Don't skip steps – if you do them in order, you will confirm your
understanding of this lesson and create a study bank for the future.
- DOWNLOAD the self-check quiz by clicking here.
- ANSWER all the questions on the downloaded quiz in the spaces provided. Think carefully before typing your answers. Review this lesson if you need to. Save your quiz when you are done.
- COMPARE your answers with the suggested "Self-Check Quiz Answers" below. WAIT! You didn't skip step 2, did you? It's very important to carefully write out your own answers before checking the suggested answers.
-
REVISE your quiz answers if you need to. If you answered all the questions correctly, you can skip this step. Revise means to change, fix, and add extra notes if you need to. This quiz is NOT FOR MARKS, so it is perfectly OK to correct
any mistakes you made. This will make your self-check quiz an excellent study tool you can use later.
- SAVE your quiz to a folder on your computer, or to your Private Files. That way you will know where it is for later studying.
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Self-Check Time!
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Self-Check Quiz Answers
Click each of the suggested answers below, and carefully compare your answers to the suggested answers.
If you have not done the quiz yet – STOP – and go back to step 1 above. Do not look at the answers without first trying the questions.
Rocks have hidden clues that help to identify them and that help us to understand what has happened around them. If you know exactly where a rock comes from, you might learn much about its history.
Igneous rocks are either intrusive or extrusive. Intrusive form below Earth’s surface, cooling slowly and forming large grains or crystals. Extrusive form above Earth’s surface, cooling quickly and forming small grains or crystals.
Sedimentary rocks form from layers of eroded sediment deposited over very long periods. These layers can contain fossils and materials that help us understand Earth’s past better.
Metamorphic rocks form when rocks are exposed to enormous heat and pressure. Many metamorphic rocks are found near igneous rocks because the igneous rocks have provided the heat.
Both metamorphic and sedimentary rocks can have layers, or bands, in them. Sedimentary rocks tend to be softer and flakier than metamorphic rocks are, so if your rock chips and flakes easily, it is probably sedimentary. Metamorphic rocks form under heat and pressure; therefore, they tend to be hard, and chipping pieces from them is difficult. Metamorphic layers can be very squiggly and twisted, but sedimentary rocks usually have long, flat layers. Looking at the surroundings in this case probably will not help because streams and rivers can erode rocks from far away.