Purpose: Rocks found across the world offer clues to geological processes, as well as the history of Earth and the rest of the solar system. In this activity, students will review types of rocks and the rock cycle and will apply that knowledge to interpret data on two rock samples.
Procedural overview: After reviewing types of rocks and the rock cycle as a class, students will use their knowledge and additional research to identify two rock samples and determine how they were made. Students will then draw connections between Earth’s geology and the geology of the moon.
Approximate class time: 1 class period
Computer with Internet access
“Geologist for a Day” student activity guide
Directions for teachers:
As a class, review the different types of rocks — sedimentary, igneous and metamorphic — and the rock cycle using the questions below. During the class discussion, have students create a flowchart that describes the different phases of the rock cycle and how they connect.
1. What is the difference between rocks and minerals?
A mineral is a naturally occurring element or compound that has a distinct chemical composition, a crystalline form and predictable physical properties. A rock is a combination of one or several minerals and can include organic matter.
2. What are the key properties of sedimentary, metamorphic and igneous rocks?
Sedimentary rocks are made up of many types of smaller rock fragments and sediments, sometimes in layers. These rocks often contain organic material including fossils and are often formed in or near water or glaciers.
Metamorphic rocks contain minerals that have melted and recrystallized. The original features of the rocks are typically distorted.
Igneous rocks can have various chemical structures depending on the composition of the magma that cools to form the rock, along with what minerals are picked up along the way. The rocks can have small or large crystals depending on how quickly the magma cools.
3. How are sedimentary, metamorphic and igneous rocks related to each other?
Sedimentary rocks are made up of eroded pieces of igneous, sedimentary and metamorphic rocks that are later compacted and cemented together. Metamorphic rocks are made up of sedimentary and igneous rocks that have undergone extreme pressure and/or heating. Igneous rocks are made up of rocks that melted and formed magma and then cooled and solidified, often underneath Earth’s surface.
4. Limestone and marble are both made of the same mineral, calcite (CaCO3), yet they have different properties. Why do you think this might be the case?
Limestone and marble are different types of rocks. Limestone is a sedimentary rock generally formed in calm waters from crushed shells and skeletal pieces. Because it is made up of compacted pieces of shell, there are some gaps in its structure, making it porous. Marble is a metamorphic rock typically formed from limestone; under immense pressure and heat, the calcite recrystallizes and locks together, making marble less porous than limestone.
5. Describe how one rock can travel through all phases of the rock cycle.
An igneous rock will be weathered and eroded and the resulting sediments will form layers that will eventually be compacted into sedimentary rock. The sedimentary rock will eventually make its way underground. There, it will become metamorphic rock as it faces extreme pressure and temperatures. Metamorphic rock can melt into magma, cooling into igneous rock under Earth’s surface or rising through cracks in the Earth’s surface, such as volcanic vents, and cooling there.
As a class or in small groups, have students discuss what geologists can learn from studying rock samples.
6. Geologists study different rocks found all over the world. What information can be learned from rocks?
Rocks can offer clues about the history of Earth, such as where water or a volcano was a long time ago. Rocks can also help geologists understand Earth more generally, including its size and interior. Types and extents of rock formations can help geologists to determine where earthquakes may be most likely to strike and to estimate their impacts. Geologists can determine the past locations and movements of glaciers. Knowledge of plate tectonics helps scientists understand which land masses were once connected, and how the land masses are moving today and will move in the future.
7. Most of Earth’s surface is covered with sedimentary rock. What can scientists infer from this about the history of Earth?
It is likely that water once covered almost all areas of Earth. That water would have deposited the raw materials for rock and provided the pressure needed to force those particles together. Wind can also create sedimentary rocks, but the scale and extent of the sedimentary rocks on Earth points to water.
8. Metamorphic rocks form deep underneath Earth’s surface, where pressure and temperatures are incredibly high. Describe one way that metamorphic rocks can be brought to the surface.
Metamorphic rocks can be found on the surface when they get pushed upward by forces below and when weathering and erosion expose the metamorphic rock layers.
9. Sedimentary rocks form in layers. How does the depth of a rock layer help reveal its age?
Because newer materials are deposited on top of existing materials, the deeper a rock is found, the older it generally is. (Though other processes can turn rocks sideways and upside down, obscuring this order.)
10. Intrusive rocks form when magma penetrates existing rock, then cools and becomes a solid before it reaches the Earth’s surface. Based on this explanation, which rock is younger, the intrusive rock or existing rock?
Since the magma flows into preexisting rock, the intrusive rock that solidifies must be younger.
Explain to students that during this activity they will pretend to help a geology lab identify two rock samples, as well as determine the events that caused the rocks to form. If a projector is available, project images of the two samples (linked below) for the students to refer to during their investigation. If a projector is not available, provide color images of the two samples for students to share or open the images on a computer so that students can refer to the images as needed. Note that the URLs contain the type of rock, so you should be careful not to reveal the URLs to students.
Sample 2 (granite):
Provide students with the “Geologist for a Day” student activity guide, which includes the geologist’s notes and rock table, as well as access to the Internet for research. The questions below will walk students through identifying the samples’ rock types and making inferences about their formation. In the final set of questions, students will make connections between geology on Earth and on the moon.
You are working in a university geology lab for the summer, helping identify the types of rock in samples from dig sites and figuring out how the sample formed. A field geologist asks for your assistance on two new samples. Review the geologist’s notes (provided by your teacher) and use them to answer the questions that follow. Your lab has been developing a table of rock types and their characteristics that, though not yet complete, will also aid your efforts.
1. Look at the notes and the pictures of the samples. What other information should be requested from the geologist and added to the notes?
The geologist’s notes describe the colors and sizes of the samples but do not include other important information, such as which of the locations marked X sample 2 came from, where the quarry is located, what the surrounding area is like, how the samples were collected or why these samples or sample locations were chosen.
2. Based on the geologist’s notes, determine the volume of each sample in cm3. Then use the mass and volume of each sample to find their densities in g/cm3.
v = 2.3 cm x 3 cm x 1 cm = 6.9 cm3
d = m / v = 17.6 grams / 6.9 cm3 = 2.55 g/cm3
v = 2 cm x 2 cm x 1.8 cm = 7.2 cm3
d = m / v = 19.8 grams / 7.2 cm3 = 2.75 g/cm3
3. One column on the table is for the “hardness” of the rocks. What is hardness and how is it measured?
Hardness is the resistance of rocks to scratching and is typically measured using reference rocks as a test.
4. The word “silicates” appears several times in the “Material/mineral classification” column of the table. What does this term mean?
A silicate is a mineral that contains molecules of silicon covalently bonded to oxygen with an overall negative charge. The ratio of oxygen to silicon atoms in a negatively charged molecule is greater than 2 to 1.
5. How can several rocks be different types but still have the same material/mineral classification?
The material/mineral classification defines what atoms and molecules are found in the material, but the structural arrangement of these atoms and molecules can be different. For instance, diamond and graphite are both made of carbon atoms, but all of the carbon atoms are covalently bonded in diamond, making it very strong, whereas graphite’s carbon atoms are covalently bonded in sheets that are stacked on top of each other, making the sheets able to readily slide off of one another.
6. Before you proceed with your research, you need to fill in some missing details in your table. Research the properties of granite, feldspar, limestone and quartz to add them to the table.
Answers for relevant rows of the table:
|speckled white, gray, tan, red or black
|feldspar, quartz, mica, amphibole (silicates)
|pink or white
|gray or tan
|colorless, white, gray or purple
7. Based on your table, do you think either of your samples is granite? Why? What could you do to further support your hypothesis?
I think that sample 2 is granite because it has a density of 2.75 g/cm3 and is white with specks of black. I could get more evidence by testing the hardness of the sample to show it has a Mohs hardness of 7.
8. Based on your table, what do you think the identity of sample 1 is? Why? What could you do to further support your hypothesis?
I think that sample 1 is feldspar because it has a density of 2.55 g/cm3 and is white. I could get more evidence by testing the hardness of the sample to show it has a Mohs hardness of 6.
9. Does it make sense that these two materials would be found in the same quarry? Why or why not?
It does make sense that these two materials would be found in the same quarry. They are both made of silica, which is a molecule made from silicon and oxygen. Granite is made when a variety of minerals including feldspar and quartz are put under tremendous pressure and temperatures.
10. Based on your knowledge and any necessary research, how did these rocks form?
Feldspar is a mineral formed from quickly crystallizing magma. All types of feldspar are aluminosilicate minerals, meaning they contain silicon, aluminum, oxygen and either potassium, sodium or calcium. Granite is an igneous rock that is made from a variety of minerals in slowly cooling magma, including feldspar and quartz.
11. What can you infer from the relative depths of the two samples? How may this relate to your understanding of how the two samples formed?
Because the granite sample was found at a greater depth, the feldspar sample was presumably deposited after the granite sample was formed. That suggests the granite sample is older than the feldspar sample. The feldspar could have formed as an intrusion after the granite had formed, in which case it would still be younger than the granite.
Geology of the future
After completing your work, you get a new message from the geologist. She notes that the rocks are not from a quarry on Earth, but from a quarry on the moon! Read a bit about the geology of the moon in “Rover peers beneath moon’s farside.” Consider how this new information might change your understanding of the rocks and their formation by answering the following questions.
12. Some quick research suggests that moon rocks brought back from the Apollo missions were igneous. Why might it make sense that the moon rocks were igneous when most of the rocks on Earth’s surface are sedimentary?
Sedimentary rocks are formed by the compaction and cementing of eroded particulate matter together. Unlike Earth, the moon has very little atmosphere, resulting in almost no wind on the surface. The moon also has no liquid water. Since strong forces from moving water and high surface winds are absent on the moon, it is unlikely the moon’s surface has many sedimentary rocks.
13. How do the rocks on the moon’s farside (the side facing away from Earth) and the nearside differ? What might that suggest about the moon’s history?
The farside of the moon is mostly covered in regolith, according to the Science News article, created when smaller impacts break up the rock on the surface. The nearside is covered in smooth floodplains. Perhaps larger impacts melted the rock on the nearside, or magma erupted from beneath the moon’s surface and covered existing craters.
14. Oxygen and silicon are the two most common elements found on both Earth’s and the moon’s surfaces. Based on the new information, could your identification of the samples still be correct? Why or why not?
Yes, my identifications could still be correct. The moon is primarily made from igneous rock and both feldspar and granite are igneous. Both samples are rich in silica, which is made of silicon and oxygen.
15. Why would geologists compare rocks from Earth and the moon? Why might geologists be interested in rocks from other planets?
Geologists study and compare rocks from Earth and the moon to understand the internal structure of the moon and to understand how it was formed. Comparing samples and data from across the solar system can also offer clues to solar system formation.