Earth Science Essentials: Igneous Rock Lab
Name:________________________
Gently dump the rocks from the tennis ball container. Find rocks labeled 1 through 8 and put them in order.
SET ROCKS A AND B ASIDE UNTIL ASKED FOR. Use your magnifying glass to examine the rock’s color
and texture.
1. Complete the table below by placing the number of the rock in the appropriate space below:
* NOTE: Obsidian is the only exception to the color rule. It’s dark even though it is felsic!
ONLY USE ROCKS 1 THROUGH 8
Texture
Felsic
Mafic
Intrusive
________Granite
________Gabbro
Extrusive
________Rhyolite
________Basalt
________Andesite
Porphyry
Glassy
________Obsidian
Vesicular
________Pumice
________Scoria
STOP! Do not continue until chart has been checked by your teacher______________________________
Use rock names, not numbers, in all answers and use COMPLETE SENTENCES:
2. Explain how granite forming deep within the Earth’s crust results in large crystals? ____________________
_______________________________________________________________________________________
_______________________________________________________________________________________
a. How does the formation of gabbro compare to the formation of granite? __________________________
_______________________________________________________________________________________
b. How is it different? ___________________________________________________________________
_______________________________________________________________________________________
3. Using a magnifying glass, look at the crystal size of basalt. Explain the environment that would form these
crystals? _______________________________________________________________________________
_______________________________________________________________________________________
4. Which of the rocks are extrusive (use the name of the rock not the number)? ___________________,
___________________, ____________________, ___________________&___________________
5. Near what type of geologic feature are extrusive igneous rocks found? _____________________________
6. Explosive gases and liquids are an important part of lava. Which two of the rocks show evidence of gases
present when they cooled? _________________________________________________________________
What is your evidence? ___________________________________________________________________
7. Using a magnifying glass, look at obsidian. It cooled so quickly that no crystals were able to form. Explain a
possible scenario where this could occur? _____________________________________________________
_______________________________________________________________________________________
8. Dark minerals in rocks usually indicate higher concentrations of iron and magnesium and give the rock a
higher density. Which are the mafic rocks? (Do not include obsidian in this group) __________________,
___________________, ___________________ & ____________________
9. Light colored igneous rocks usually indicate higher concentrations of (notes) _______________ ,
_________________ and _______________________. They have a _________________ density than dark
colored rocks. Light colored igneous rocks are called ________________ or _____________. Which are the
felsic rocks? _Obsidian_, ____________________, ___________________, and __________________.
10. Rocks that make up the continents are usually felsic, intrusive, lower density rocks. Which rock best fits that
description? ___________________________________________
11. Rocks that make up the ocean floor are usually mafic, extrusive and have a higher density. Which rock best
fits that description? _______________________________________.
12. Using a magnifying glass, take a look at Rock A. Rock A is called Porphyritic Rhyolite. Classify it by its
texture and color and explain how it received these characteristics. _____________________________
_______________________________________________________________________________________
_______________________________________________________________________________________
13. Using a magnifying glass, take a look at Rock B. Rock B is called Granite Pegmatite. Pegmatite rocks have
HUGE crystals. Provide a possible reason why this piece of granite has larger crystals than the other? _____
_______________________________________________________________________________________
_______________________________________________________________________________________
14. Label the diagram below with the following terms. Basalt, mafic, extrusive, granite, felsic, intrusive, HIGH
density, LOW density. Draw in the ocean.
15. Why are oceans over basaltic/mafic rock? _____________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________