Exercises: Classification of rocks

What are the three main groups of rocks and what process forms each one? State in which part of the planet each process mainly takes place.

What is the texture of a rock and how does it differ from its structure? Give an example of a glassy texture and another of a phaneritic texture (visible crystals).

A rock with fossils cannot be igneous. Explain why, and state which group a fossil-bearing rock normally belongs to.

What is the rock cycle? Name its main processes and explain why it is said that no rock is permanent.

What is porphyritic texture and what two-stage cooling history does it reveal in an igneous rock?

What are the clasts of a detrital rock and what texture do they define? Distinguish between clasts, matrix and cement.

What is the difference between a loose sediment and a consolidated sedimentary rock? What set of processes turns one into the other?

List five characteristics observable with the naked eye or with a hand lens that would help you classify a rock sample.

Below are descriptions of five numbered rock samples based on their observable features:

1. Sample with a granular texture of visible, interlocking crystals.

2. Sample arranged in layers with the presence of clasts and fossil remains.

3. Sample with foliation marked by alternating light and dark bands.

4. Dark, very fine-grained sample with the presence of vesicles or vacuoles.

5. Sample with fine planar foliation that splits easily into sheets.

Assign each number to its group (igneous, sedimentary, or metamorphic) and justify it using the observed feature (interlocking crystals, layers, foliation, etc.).

In the image, locate the granite and the sandstone. Compare how each formed from similar minerals (quartz, feldspar) and explain why sandstone is porous and granite is not.

A black, very fine-grained rock with numerous vesicles (cavities) crops out in a volcanic cone. Identify it and explain what the vesicles indicate about the magma that produced it.

Compare aphanitic and phaneritic textures: what each one looks like, in what cooling environment it forms, and an example rock of each type.

A volcanic rock shows large feldspar crystals embedded in a very fine-grained matrix. Interpret its cooling history and explain why the phenocrysts and the matrix have such different sizes.

A conglomerate of rounded pebbles and a breccia of angular clasts can have the same composition. Explain what different history of transport and deposition each texture tells.

In a schist, the micas are oriented in parallel planes. Explain what stresses create that foliation and why the rock splits easily along those planes.

A limestone with coral fossils and one formed by oolites are both carbonate rocks. Compare the formation environments each one indicates (quiet reef versus shallow, agitated platform).

During a field survey, a student observes the specimen in the image (a banded gneiss) and writes in their notebook: «This is a sedimentary rock with rhythmic lamination (varves), where the dark bands are organic-rich clay layers and the light ones are quartz sands deposited during the summer».

1. a) Refute the student's interpretation by analyzing the fabric visible in the image: what textural and mineralogical criteria allow you to rule out these bands being primary sedimentary laminations and confirm gneissic banding?
2. b) Predict what microstructures you would expect to observe in a thin section of this rock (in relation to intergranular contacts and the preferred orientation of phyllosilicates) that would be impossible to find in the proposed sedimentary rock.
3. c) If you were to find this outcrop crosscut by granitic dikes that are deformed and folded concordantly with the banding, what tectonic and thermal history would you deduce for the region?

During a geological mapping campaign in a structurally complex area, your team collects three pairs of samples that are difficult to distinguish visually in the field:

• Pair 1: Two dark, aphanitic, and hard rocks (a basalt and a tourmaline-rich black quartzite).

• Pair 2: Two fine-grained, dark pelitic rocks (a compacted mudstone and a metamorphic slate).

• Pair 3: Two light-colored carbonate rocks (a pure micritic limestone and a white saccharoidal marble).

1. a) Design for each pair a protocol of simple field tests (using only a 10x hand lens, a steel pocketknife, a quartz scratcher, and 10% HCl) that allows them to be unequivocally distinguished in situ.
2. b) If pair 2 cannot be resolved with certainty in the field, what simple physical test related to water immersion or drying would you design before resorting to a thin section?
3. c) For pair 1, describe which definitive microtextural diagnostic criteria you would expect to observe under the petrographic polarizing microscope to certify the volcanic origin of one and the metamorphic origin of the other.

At a geological workstation, a set of samples is being analyzed and three disagreements arise among the team members:

1. A technician claims that certain rhyolitic lavas and pyroclastic flow deposits exhibit genuine sedimentary stratification.

2. In a sample with alternating millimeter-scale quartz-feldspar and biotite bands, a student argues that it is a sandstone with primary cross-bedding that has preserved its sedimentary layers intact.

3. A clayey sample shows parallel fracture planes that are confused with slaty cleavage.

1. a) Judge the validity of statement 1 by distinguishing between stratification due to sedimentation in fluid media and lava flow banding caused by viscosity or pyroclastic fall/flow deposits.
2. b) Refute statement 2 by explaining the metamorphic differentiation mechanism that generates gneissic banding from a homogeneous protolith subjected to directed stress, and indicate which intergranular fabric under the microscope would reveal its metamorphic nature compared to a detrital one.
3. c) Design a structural criterion to differentiate in the field or lab the diagenetic fissility of shale (statement 3) from metamorphic slaty cleavage, relating the orientation of the fracture planes to tectonic stress and the original bedding.