1.6 - Rock Identification Exercise | Soil Genesis and Development, Lesson 1 - Rocks, Minerals, and Soils

Your parents are cleaning out the attic and find your ancestor’s rock collection. All you know about him is that he came over from Germany in 1812, starting the family farm soon thereafter. It seems that he was quite the naturalist, carefully studying the rocks and soils on and around his property. Too bad that all his notes are in German, and you can’t read any of it (should have taken German in high school.) So, at the request of your parents, you put your college education to good use, pick up the tray of rocks, and head off to the geology department to do some rock identifications. Here is his rock collection. Use your new knowledge of rocks to identify the specimens.

View your old class notes to help you determine the mineral content.

Quiz

Question

This rock consists of thin platy shards and breaks easily. It is...

A. Igneous

Incorrect: Igneous rocks have randomly oriented crystals; they are rarely layered.

B. Metamorphic

Incorrect: This rock is too brittle to be a slate. It has not been subjected to high enough temperatures and pressures to be turned into a metamorphic rock.

C. Sedimentary

Correct: This rock is a shale.

Looks Good! Correct: This rock is a shale.

Quiz

Question

A soil developed from this grainy, quartz-rich rock may have problems because it will......

A. have low fertility.

Correct: The coarse particles in many sandstones have few nutrients for plants.

B. be very acidic.

Incorrect: While some sandstones may form acidic soils, we can't say that all will, at least soon after exposure.

C. have excessively high clay content

Incorrect: Sandstones will form coarse-textured soils with little clay.

Looks Good! Correct: The coarse particles in many sandstones have few nutrients for plants.

Quiz

Question

This light colored rock contains nice black swirls of biotite. It is...

A. Igneous

Incorrect: Igneous rocks have randomly oriented crystals; these black swirls are not random!

B. Metamorphic

Correct: The black swirls formed as the rock was subjected to high temperatures and differential pressure.

C. Sedimentary

Incorrect: Look at a close-up of the rock and you'll see interlocking, preferentially aligned crystals - not the hallmark of a sedimentary rock.

Looks Good! Correct: The black swirls formed as the rock was subjected to high temperatures and differential pressure.

Quiz

Question

This rock is black to dark gray and contains a lot of bubbles. It is...

A. Igneous

Correct: It is a fine crystalline rock that cooled quickly on the surface of the earth. So quickly, indeed, that gasses in the lava had no time to escape and froze into a pattern of black Swiss cheese.

B. Metamorphic

Incorrect: The rock looks rather homogenous, no shiny surfaces or any other sign of preferred crystal orientation.

C. Sedimentary

Incorrect: Can you see any layers, large rock particles or fossils? These would be signs of sedimentary rocks.

Looks Good! Correct: It is a fine crystalline rock that cooled quickly on the surface of the earth. So quickly, indeed, that gasses in the lava had no time to escape and froze into a pattern of black Swiss cheese.

Quiz

Question

This light gray rock contains an abundance of fossils. It is...

A. Igneous

Incorrect: Fossils are not found in rocks that form from magma.

B. Metamorphic

Incorrect: Fossils would not survive exposure to the high temperature and pressure needed to metamorphose a rock. They would recrystalize.

C. Sedimentary

Correct: This rock is a limestone. The fossils are a dead giveaway for this kind of sedimentary rock.

Looks Good! Correct: This rock is a limestone. The fossils are a dead giveaway for this kind of sedimentary rock.

Quiz

Question

A soil developed from this rock will likely be...

A. alkaline

Correct: The carbonate minerals in limestone will keep resulting soils alkaline for many years.

B. infertile

Incorrect: The calcium and magnesium found in limestone help to maintain high fertility as the soil develops.

C. sandy

Incorrect: Even though limestone is a sedimentary rock and so was formed from all sizes of particles, limestone is rarely dominated by coarse inorganic particles like sands.

Looks Good! Correct: The carbonate minerals in limestone will keep resulting soils alkaline for many years.

Quiz

Question

This light-colored rock consists of small, randomly oriented crystals. It is...

A. Igneous

Correct: Randomly oriented crystals visible in the image suggest that the rock cooled slowly deep underground.

B. Metamorphic

Incorrect: Metamorphic rocks are generally characterized by crystals that display preferred orientation.

C. Sedimentary

Incorrect: No layers, no fossils, no clastic fragments - no sedimentary rock.

Looks Good! Correct: Randomly oriented crystals visible in the image suggest that the rock cooled slowly deep underground.

Quiz

Question

This crumbly rock displays nice brown and green layers. It is...

A. Igneous

Incorrect: Igneous rocks are rarely layered this nicely.

B. Metamorphic

Incorrect: Metamorphic rocks are rarely layered this nicely (metamorphic fabric, caused by preferential alignment of newly formed crystals, tends to form swirly or crinkly layers).

C. Sedimentary

Correct: The nice parallel layers and the crumbly nature of the specimen suggest a poorly cemented sedimentary rock.

Looks Good! Correct: The nice parallel layers and the crumbly nature of the specimen suggest a poorly cemented sedimentary rock.