Rock classification

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.

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

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.

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

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.

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 originate that foliation and why the rock splits easily along those planes.

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?

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

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).

A construction company purchases slabs of "black granite" for the exterior cladding of a building. When analyzed in the laboratory, a density of 3,0 g/cm³, a total absence of free quartz and alkali feldspar, and a content of calcic plagioclase (labradorite) and clinopyroxene exceeding 85% by volume are determined. The construction company files a claim alleging that the material is not granite.

1. a) As a geological expert, justify petrologically why the sample is not a granite, locating its modal composition on the Streckeisen QAPF diagram and deducing which plutonic rock it is.
2. b) Refute the commercial supplier's argument that "in geology, anything with a granular crystalline texture is a type of granite", explaining the difference between phaneritic texture and modal mineralogical classification.
3. c) Predict and justify how this cladding will respond chemically to weathering by urban acid rain rich in SO₂ compared to a true granitoid rich in quartz and potassium feldspar.

For the foundation of a dam and the supply of railway ballast, a geotechnical company analyzes two macroscopically similar siliceous rocks (samples A and B), both consisting of more than 95% SiO₂.

The tests yield the following results:

• Sample A: When struck with a sledgehammer, the fracture surface exhibits a dull lustre and a rough feel ("like sandpaper"); its effective porosity is 4,2% and point contacts between rounded clasts with microcrystalline silica cement are observed under the microscope.

• Sample B: Exhibits a clean conchoidal fracture that cuts indiscriminately through grains and matrix with a continuous vitreous lustre; its effective porosity is 0,3% and under the microscope it shows a granoblastic texture with sutured and interdigitating contacts.

1. a) Reasonably identify which of the samples corresponds to an orthoquartzite (sedimentary quartz sandstone) and which to a metamorphic metaquartzite, justifying the mechanical difference on the fracture surface based on the crystalline fabric of each.
2. b) Justify which of the two rocks is more suitable as ballast against freeze-thaw cycles (gelifraction) and continuous mechanical abrasion.
3. c) If you subject sample A to regional metamorphism in greenschist facies, predict how its porosity, the shape of the grains, and its intercrystalline boundaries will evolve.

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-coloured 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 pocket knife, 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.

Three independent geological reports describe the same rock formation in a sedimentary basin using three different designations:

• Report A (lithostratigraphic): "Micritic limestone with fossils".

• Report B (laboratory petrological according to Folk/Dunham): "Biomicrite" or "Foraminifera and bivalve wackestone".

• Report C (palaeoenvironmental): "Carbonate deposit of a protected tidal flat or shallow lagoon".

1. a) Interpret the information provided by each nomenclature and justify, based on it, the relative proportion of clasts to micritic mud and the hydrodynamic energy level of the environment at the time of deposition.
2. b) In a regional hydrogeological project to assess the potential of karst aquifers, reason which of the classifications is more operative and what drawbacks arise from basing the mapping exclusively on the environmental interpretation of report C.
3. c) If in the immediately overlying layer the rock turns into a "Grainstone or biosparite" with ooids, well-sorted bioclasts, and sparitic cement without mud matrix, deduce what change in relative sea level or in the hydrodynamics of the basin has taken place.

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

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

In a marine stratigraphic series composed of fossiliferous micritic limestones, a granitic pluton is emplaced. At 600 m from the contact, the rock is a grey limestone with well-preserved foraminifera; at 80 m from the contact, it turns into a fine-grained, white rock with a saccharoidal appearance and no trace of fossils; and at the sharp contact with the granite, a rock with calcite crystals larger than 3 mm appears alongside calcium silicates (wollastonite and grossular).

1. a) Justify the textural changes observed as we approach the pluton: why do fossils disappear and the calcite grain size progressively increase?
2. b) Design a simple field test that can prove that both the original limestone and the marble retain the same major mineral phase, and explain why their mechanical strength and porosity are markedly different.
3. c) Deduce which mineralogical reaction explains the formation of wollastonite (CaSiO₃) at the contact and which chemical component must have been provided by the protolith or the hydrothermal magmatic fluid for it to take place.

In a deep borehole in the root of a collisional mountain range, a banded rock with a heterogeneous morphology is extracted: tortuous and folded felsic bands (leucosomes) of quartz and feldspar with a granular phaneritic texture, intimately interwoven with mafic bands rich in biotite and sillimanite (melanosomes) with intense foliation.

1. a) Justify why this rock (a migmatite) lies on the genetic boundary between metamorphism and magmatism, deducing the physical state of the leucosome and the melanosome during the thermal peak.
2. b) Drawing upon the concept of anatexis and the solidus curve of hydrated continental crust, predict what would have happened to the rock if the temperature had increased by another 150 °C and how the appearance of the resulting rock body would have changed upon cooling.
3. c) A student states: "If the leucosome managed to melt, the rock must have exceeded 1700 °C, which is the melting point of pure quartz." Refute this statement, justifying the behaviour of eutectic mixtures and the influence of water on crustal melting.

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 millimetric 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 laboratory 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.