Lesson 4 - Page 1




PROBLEM: How can you tell the difference between plutonic and volcanic rocks?


MATERIALS: Earth Science - Igneous Rocks , Swift GH microscope or Hand Lens

PROCEDURE: General Questions - Answer the following questions.

A. What type of igneous rocks take the longest to cool? Why?

B. Volcanic rocks cool more quickly than plutonic rocks. Name some environments where volcanic rocks are likely to form. (Hint: can volcanoes form underwater?)

C. If plutonic rocks are formed deep inside the crust of the Earth, will we ever see them on the surface of the Earth? Explain your answer.

D. Examine each of the igneous rock samples. Complete the chart below. It is more important to visualize the environment where each rock formed than to memorize the names of each rock.







(Sierra Nevada)



rhyolite tuff


granite (Texas)

(S. Carolina)




Answer the following questions using specimens provided by your instructor. Be sure to examine each specimen, and use the classification charts to help you.


Pumice is a type of pyroclastic material. It has a vesicular texture (it is full of holes). These formed because the magma cooled when it was full of gas bubbles. The gas escaped, leaving the holes behind. Pumice is composed of volcanic glass rich in silica (SiO2).

A. Why is pumice very light?

B. How did pumice form? Was it cooled quickly or slowly?

How can you tell?

C. Does pumice float on water? Why?

2. SCORIA   

This volcanic rock may look similar to pumice. In fact, it formed the same way as pumice. It is a volcanic rock that solidified when it was full of gas bubbles. The main difference between the two rocks is composition. Scoria contains much more iron (Fe) and magnesium (Mg) and much less silica (SiO2) than pumice.

A. Describe scoria.

B. What is the difference between pumice and scoria?


Obsidian is volcanic glass. It is magma that became solid so quickly that there was no time for minerals to form. Obsidian is easy to break and shape. For this reason, native peoples often used obsidian to make arrowheads, knives, and other cutting tools. Be careful, your obsidian specimens might be sharp!

A. What common material does obsidian resemble?

B. Can you see any crystals in obsidian?

C. This obsidian was used by the Clear Lake Indians of California. What do you think they used it for? (Hint: is obsidian sharp?)

D. Clear Lake was a __________________ area at one time.

4. BASALT   

Basalt is the most abundant igneous rock in the Earth’s crust. Most of the ocean floor is underlain by basalt. It is a volcanic igneous rock, and is typically dark in color because it contains large amounts of iron (Fe) and magnesium (Mg) compared to other igneous rocks. Basalt is also very common on islands like Hawaii.

A. Describe this specimen of basalt.

B. Can you see any minerals in the sample?

C. Why is basalt always a dark color? (Hint: Has something to do with minerals.)


Andesite is a volcanic igneous rock. It is named after the Andes Mountains of South America, which contain many active volcanoes. Most volcanoes on land are composed of andesite, in the form of either lava or pyroclastic material.

A. Is andesite lighter in color than basalt?

B. Basalt contains a lot of "dark" elements like iron and magnesium. Notice that andesite is not as dark as basalt. Does andesite thus have more or less "dark" minerals?

C. Are any minerals visible in this andesite?


Tuff is a name for a pyroclastic rock where the magma fragments are small. A rock with big magma fragments is called a breccia. Tuffs sometimes contain mineral crystals that were blown out of the volcano along with the magma. Tuff is very common in the Western United States, especially in Washington and Oregon, where there are many volcanoes.

A. Describe this rock.

B. Why is rhyolite light in color?

7. GRANITE (South Carolina)   

This granite is a plutonic igneous rock. It formed from a magma that cooled slowly underground. Granites are rich in silica (SiO2), which makes them light in color.

A. Are the minerals that make up this granite visible to the naked eye?

B. What is the general percentage of light minerals compared to dark minerals? (Hint: greater or less than 50%)

C. What is the difference between basalt and granite?

D. What can you attribute this to?

8. GRANITE (Texas)   

This is another granite, from a different part of the United States. It looks different from the specimen in Question 7, because it formed from a slightly different magma. Both rocks are called granite because they have the same general texture and composition. It is important to remember that most rock names are not very specific.

A. What is the general percentage of light minerals compared to dark minerals in your sample?

B. Can you see distinct minerals in this specimen?

C. Does it look the same as #7? Why are they both called "granite?"

9. DIORITE    

Diorite is another phaneritic igneous rock. Like granite, it formed by slow cooling inside the crust of the Earth. If the magma which cooled to make a diorite was instead erupted on the Earth’s surface, it would make andesite. This means that diorite and andesite have the same composition, but have very different textures.

A. What is the percentage of dark minerals in this specimen?

What percentage are lighter minerals?

B. Describe the dark minerals

C. What is the difference between diorite and the two granites in questions 7 and 8?

10. GRANODIORITE (Sierra Nevada)   

Granodiorite is not on your identification chart. It is an igneous rock which is between granite and diorite in composition. Granodiorites can form directly by melting inside the Earth, or by the mixing of other kinds of magma. The formation of magma inside the Earth can be very complex.

A. Estimate the percentages of dark and light minerals in this specimen.

B. Describe the size of the minerals. Why do you think the minerals are smaller than the granites or diorite?

11. GABBRO   

Gabbro is another phaneritic igneous rock. Its dark color indicates that it contains lots of iron (Fe) and magnesium (Mg). Gabbro forms from the same kind of magma that creates basalt and scoria. However, to make gabbro, the magma cooled slowly inside the Earth.

A. Estimate the percentages of dark and light minerals in this specimen.

B. Describe the size of the minerals.

C. What is the difference between gabbro and granodiorite, diorite, and the granites?



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