Introduction to Geological Sciences

ROCK PROPERTIES LABORATORY

Let him heed who can and will;
Enchantment fixed me here
To stand the hurts of time, until
In mightier chant I disappear.
If thou trowest
How the chemic eddies play,
Pole to pole, and what they say;
And that these gray crags
Not on crags are hung,
But beads are of a rosary
On prayer and music strung;
And, credulous, through the granite seeming,
Seest the smile of Reason beaming;—
Can thy style-discerning eye
The hidden-working Builder spy,
Who builds, yet makes no chips, no din,
With hammer soft as snowflake's flight;—
Knowest thou this?
O pilgrim, wandering not amiss!
Already my rocks lie light,
And soon my cone will spin.

From “Monadnock” by Ralph Waldo Emerson

Rocks are the basic materials that geologists study to understand earth’s history and the processes by which earth changes, including the formation of mineral resources and the occurrence of hazards. Rocks are characterized in terms of composition and texture.

Composition refers to what materials the rock contains (its minerals, fossils, rock fragments, pores).

Texture refers to how the materials are assembled: the sizes of mineral grains or rock fragments; bedding and foliation.

Together composition and texture help us determine the geological environment in which the rock formed (e.g., intrusive igneous, extrusive igneous, sedimentary, metamorphic). In lab today you will practice using rock characteristics to infer how the rock formed.

STATION 1.

Use your lab manual to study the textures of igneous rocks:

Igneous rocks that cool slowly:

Intrusive rocks: magma crystallizes far below earth’s surface

Phaneritic texture: large grain size (easily visible to the eye) (Fig. 2.2)

Igneous rocks that cool quickly:

Extrusive rocks: magma is erupted onto earth’s surface as lava or ash

Aphanitic texture: individual mineral grains are invisible (Fig. 2.3)

Glassy texture: grains absent; glassy appearance (Fig. 2.11A)

Pyroclastic texture: shards of glassy materials welded together (Fig. 2.11B)

Igneous rocks with complicated cooling histories

Extrusive rocks: magma contains some large crystals (phenocrysts) at the time it erupts

Porphyritic texture: two distinctly different sizes of mineral grains (Fig. 2.4)

Question 1. For each igneous rock sample, determine the history of mineral growth and depth of crystallization (shallow or deep).

Texture	Depth of crystallization (surface, deep, or both)	Cooling history (fast, slow, complicated)	Rock name
1.
2.
3.
4.
5.

Question 2. Rocks that crystallize at depth in Earth are called plutonic rocks. Which of the rocks are plutonic?

Question 3. Which rocks most clearly shows evidence of rapid cooling? Explain.

Question 4. The specimens include igneous rocks names: andesite, basalt, gabbro, granite, and obsidian. Using the information in your lab manual, write the names of each sample in the last column of the table.

STATION 2.

Different magmas contain different amounts of gasses (H₂0, CO₂), depending where they form. If gas content of the magma is very low, it might be erupted as lava containing little evidence of the gases. If the magma contains more gas, it might form a rock that contains bubbles, the porosity indicating the amount of gas. If gas content is very high, the magma might be erupted explosively as ash (partially molten fragments of rock and magma), and the rock that forms would be a porous collection of ash.

Question 5. Look at the specimens and rank them according to the amount of gas in the magma (not necessarily in the rock!) Write down what you observe that leads you to your answer.

Rock	Rank (1=high; 3= low)	Observations
Basalt
Pyroclastic rock
Scoria

Question 6. In 1980 Mt. St. Helens in the Cascade Mountains exploded violently, erupting nearly 1 km3 of ash. Which rock type would you expect to find in the vicinity of Mt. St. Helens?

Explain.

STATION 3.

For rocks that do not contain easily visible material grains we can’t say much about their composition from examining hand specimens. However, texture can give us important clues about how such rocks formed. Igneous rocks often are homogeneous (i.e., they contain no layers). Sedimentary rocks may contain bedding (i.e., layers of particles with different sizes or compositions; See Fig. 3.6). Metamorphic rocks may contain foliation (i.e., layers of platy mineral grains aligned by stress)

Of the specimens at this station, one rock is igneous, one sedimentary, and one metamorphic.

Question 7. Look for a homogeneous texture (igneous), bedding (sedimentary) and foliation (metamorphic) to determine how each rock formed. Describe what you see in each specimen.

Rock	Observation
Basalt
Shale
Slate

STATION 4.

In clastic (made of particles) sedimentary rocks, particle size is an important aspect of texture.

Question 8. Rank the rocks in terms of grain size.

Rock	Rank (1=largest, etc.)	Sorting (well-sorted or poorly sorted)
Breccia
Conglomerate
Sandstone
Shale

Question 9. Determine what “sorting” means (p.39). Record the degree of sorting for each rock type in the table.

Question 10. Sorting usually increases the farther sediments are transported. Which rocks contain sediments that were not transported very far?

Question 11. One of the rocks contains large particles that are rounded, another has large particles that are angular. Explain what these different shapes mean in terms of transportation distance. In what type of sedimentary environment could rounding of large particles occur? (See p. 38).

STATION 5.

Question 12. Two of these rocks are composed predominantly of quartz: sandstone and quartzite. Sandstone is metamorphosed into quartzite if the sand particles grow to form larger, interlocking grains. Describe the appearance and feel of each rock and explain how you can tell the differences between sandstone and quartzite.

Question 13. Two of these rocks are metamorphic and both have similar appearances. However, one is mostly quartz (quartzite) and the other is mostly calcite (marble). Explain how you can tell them apart using the tools available. (Check your lab manual to find out the physical properties of the materials quartz and calcite.)

STATION 6.

All of these rocks contain fossil evidence (clam-like shells, a coiled shell, plants, worms tubes).

Question 14. What rock type (igneous, sedimentary, or metamorphic) do you think characteristically contains fossils? Explain.

Question 15. Find the fossils in each and explain in which environments they might have formed. (Think where you might find similar organisms today; see Fig. 3.15)

Sample	Environment
1.
2.
3.
4.

STATION 7.

When metamorphism occurs, minerals are often oriented by stress, forming a foliation , Which is parallel orientation of the minerals within the rock (see Fig. 4.4).

Questions 16. Which of the rocks were metamorphosed under stress and which are not (based on the presence of oriented mineral grains)? What type of orientation can you see?

Rock	Evidence of stress (Y/N)	Foliation (Y/N)
Gueiss
Granite
Mica schist
Quartzite

Question 17. One of the rocks is metamorphic but not foliated. Which one is it?

(refer to Figure 4.8).

STATION 8.

This station shows four samples of organic material at various stages of alteration. Alteration arises form both pressure from burial and the addition of heat. Examine each sample and note the presence of organic material vs. pure carbon.

Rank each sample in order of increasing alteration:

Which sample looks most like the dead plant material from which it arose?

With increasing alteration comes higher carbon content, lower levels of impurities such as sulfur, and generally higher heat output. Which of the samples would therefore make the best fuel? Which is the second best?

STATION 9

This station is divided into three substations. Each have two thin sections of rock prepared in the lab, along with a dissecting microscope for observation.

9a. Observe both slides. One is siltstone and one is sandstone. (Identify by the number on the slide.)

Siltstone: __________

Sandstone: __________

(cont. on next page)

How were you able to determine this?

9b. Observe both slides. One is obsidian and one is basalt. Which is which?

Obsidian: __________

Basalt: __________

How were you able to determine this?

9c. Observe both slides. Both are metamorphic rocks; one is schist and the other phyllite. What characteristic of some metamorphic rocks do both share?

Which is which?

Schist: ________

Phyllite: __________

After observing all three stations, what aspects of rocks were more easily determined with the study of thin section compared with hand samples?

What aspects were identifiable that would be impossible to observe in hand samples?

Reflection. Write a one-page reflection on this laboratory. Look at the big picture: how would you use some combination of these rock characteristics to identify a rock you stumble upon? How does understanding rock properties help you understand the nature of rocks in general? How did working collaboratively help or hinder the successful completion of this assignment?