Exercises: Metamorphic rocks and the rock cycle

What is metamorphism? Differentiate it from diagenesis and melting, stating the range of conditions in which it takes place.

Name the three agents of metamorphism and explain how each acts on rock minerals.

What is foliation and which type of pressure (lithostatic or directed) produces it? Give an example of a foliated rock.

What is the recrystallization of minerals during metamorphism and how does it differ from the melting of the rock?

Distinguish slate and schist by their appearance and their metamorphic grade. What happens to the size of the micas when passing from one to the other?

What is contact metamorphism and where does it occur? What structure does it form around an igneous intrusion?

What are hornfels and what characteristics do they show (fine grain, hardness, absence of foliation)? From which rocks are they generated?

What is a mylonite and in what type of zones does it form? Which deformation process produces it?

Order the following series in increasing metamorphic grade from a mudstone: gneiss, phyllite, slate, and schist. Explain what textural and mineralogical changes accompany the increase in grade.

Distinguish regional, contact, and dynamic metamorphism, and relate each to a specific plate tectonic setting.

Which metamorphic rock comes from the transformation of granite and which from limestone? Why does marble rarely display foliation even though it forms under directed pressure?

Regional metamorphism accompanies orogenies. Explain why compression and the burial of large thicknesses of rocks raise pressure and temperature at the same time.

With an average geothermal gradient of $30\ \text{°C/km}$, estimate at what depth the $300\ \text{°C}$ typical of low-grade metamorphism are reached, and comment on the limitations of that calculation (variable gradients, fluid circulation).

In the aureole of a granite, slates pass into hornfels with andalusite and, near the contact, with cordierite. Explain the concentric zone arrangement of the metamorphic aureole and what each mineral represents.

Serpentinites come from the hydration of ultramafic rocks such as peridotites. Explain what ultramafic rocks are, where they originally form, and what geological context their presence at the surface indicates.

Distinguish slaty cleavage and schistosity: at what metamorphic grade each one appears, which minerals define them, and how they are recognized in hand specimen.

A geology student makes the following claim while looking at the rock cycle diagram: "In order for a metamorphic rock (3) to form, it is essential that the rock first pass through the sedimentation stage (2) starting from an igneous rock (1); furthermore, every magma (4) originates directly from the melting of a prior metamorphic rock (3)".

1. a) Based on plate dynamics in a collisional orogen, reasonably refute both claims by designing "shortcuts" or alternative pathways between the lithological reservoirs that demonstrate that the cycle is neither a closed nor a unidirectional sequence.
2. b) If metamorphic rock 3 originates directly from igneous rock 1 without passing through sedimentary state 2 (ortho-derived rock), what petrological, geochemical, or fabric criteria would allow you to distinguish it in the field from a para-derived rock (originated from 2)?
3. c) In the transition from 3 to 4 by crustal anatexis, the melt generated fails to reach the surface and crystallizes at depth, giving rise to 1. Predict the resulting plutonic rock if the protolith was an aluminum-rich metapelite and explain what texture it will develop based on its cooling rate.

The rock cycle is not a closed, balanced loop, but a dissipative system coupled to Earth's thermal and tectonic engine. Consider the following global observations:

1) Basaltic oceanic crust has densities of 2.9 to 3.0 g/cm³ and is almost completely recycled into the mantle, never exceeding 200 Ma in age.

2) Continental crust has an average density of 2.7 g/cm³ and preserves rocks older than 3800 Ma.

3) In active collisional belts, the rate of erosional denudation can reach 2 to 5 mm/year.

1. a) Based on these data, justify why continental crust acts as a cumulative reservoir and undergoes irreversible geochemical differentiation toward compositions richer in silica and incompatible elements over geological time, instead of being destroyed in the mantle.
2. b) Model the coupled feedback mechanism between orogenesis, climate, and the rock cycle: explain how the chemical weathering of silicates in young orogens alters global atmospheric composition and how superficial erosional unloading triggers isostatic rebound and the exhumation of deep crustal rocks.
3. c) In a pure continental transform fault (with no convergent or divergent component), predict which lithological transformations of the cycle will be dominant (in terms of metamorphic type and rock deformation) and which will be practically absent compared to a convergent boundary, justifying your answer according to the prevailing thermal and stress regime.

Unit challenge. Lithological biography of your surroundings.

What geological history do the materials of your everyday environment tell? You will act as an environmental consultant preparing a «petrological biography» of your area in four stages:

1. Digital mapping and sampling: Access the GEODE cartographic viewer of the IGME (Geological and Mining Institute of Spain). Locate your municipality and identify two lithologies present in the geological substrate. Add a third rock used for ornamental or construction purposes photographed in your street or building.

2. Petrographic classification: Design a comparative table where you classify the three samples (they must represent the igneous, sedimentary, and metamorphic groups), detailing their observable texture, estimated mineralogical composition, and genetic environment of formation.

3. Rock cycle modeling: Create a flow chart relating your three rocks within the petrogenetic cycle, justifying the thermodynamic transformations ($P$ and $T$) or exogenic processes necessary for some to evolve into others.

4. Sustainability assessment: Draft a reasoned proposal (maximum $1.5$ pages) evaluating the suitability and environmental impact of using local rocks versus imported prefabricated materials.

Assessment criteria:

• Accuracy in extracting and interpreting cartographic data from the official USGS viewer.

• Conceptual correctness in the textural and genetic classification of the three lithologies.

• Scientific coherence in integrating and relating the selected rocks within the rock cycle.

• Robustness of the arguments in the environmental and territorial sustainability proposal.