Igneous rocks

What is magma and what three components make it up? Distinguish between magma and lava.

A mantle peridotite (lherzolite composed of 60% olivine, 20% orthopyroxene, 15% clinopyroxene, and 5% spinel/garnet) undergoes partial melting due to adiabatic decompression. Analyses show that a melting degree of 10 to 15% produces a tholeiitic basaltic liquid (silica content around 49%), leaving harzburgite as a solid residue.

1. a) Justify why the resulting melt has a higher concentration of silica, aluminium, and calcium than the source peridotite (~42-44% SiO2), specifying which mineral phases melt first and which remain in the residue.
2. b) Predict what mineralogical and chemical composition the magma would have if the degree of mantle anatexis theoretically reached 100%, and explain why this type of ultramafic magma was common mainly in the Archaean (komatiites) and is exceptional on modern Earth.
3. c) Explain how repeated episodes of partial melting and plate recycling in subduction zones have led to the formation of an average continental crust of andesitic composition from a peridotitic mantle.

Analyse the following three geological data about the Canary Islands archipelago:

1. There is a general decreasing correlation between the basal ages of submarine complexes/first eruptions and geographic location: Fuerteventura (~20 Ma), Gran Canaria (~15 Ma), Tenerife (~12 Ma), La Palma (~3,5 Ma), and El Hierro (~1,1 Ma).

2. Holocene and historical volcanism is not restricted to El Hierro and La Palma; recent eruptions have been recorded in Tenerife (1909), and extraordinarily voluminous historical eruptions in Lanzarote (Timanfaya, 1730-1736).

3. The orientation of volcanic ridges ('rifts') and the clustering of vent centres across the islands follow preferred NNE-SSW and NE-SW trends, parallel to the transpressional tectonic structures of the Moroccan Atlas mountain range.

1. a) Justify why datum 1 supports the hypothesis of a stationary mantle plume beneath the oceanic lithosphere, but datum 2 contradicts the classic Hawaii-type hotspot model.
2. b) Evaluate how alternative geotectonic models (propagating fracture from the African Atlas or compression-uplifted blocks) explain datum 3, and propose a unified model that reconciles the mantle thermal anomaly with regional tectonic control.
3. c) If the African Plate maintains its current east-northeast displacement vector at a rate of about 1,5 cm/year, predict the relative geographical location and physiographic characteristics of the future island of the Canary archipelago.

Differentiate volcanic, hypabyssal, and plutonic rocks according to where cooling occurs, and give an example of each type.

The cross-section shows the same magma cooling in three environments (1: deep chamber; 2: narrow conduit; 3: surface lava flow), and the magnifying glasses show the appearance of the rock formed in each. Using the diagram, explain why plutonic rocks have large, visible crystals whereas volcanic rocks barely show any.

Match each plutonic rock with its volcanic equivalent: granite, diorite and gabbro. Explain what the rocks of each pair share and how they differ.

What is a batholith and what is a laccolith? How do they differ from a dyke and a sill?

A quartz porphyry dyke cuts through limestones. Explain what type of igneous body a dyke is, what porphyritic texture indicates about its cooling history, and what effect you expect to find in the limestones at the contact.

Some basalts contain peridotite xenoliths. Explain what xenoliths are, where the magma tore them from during its ascent and what direct information they provide about the composition of the mantle.

In a geological mapping study, three metapelitic samples collected along a 400 m transect that perpendicularly approaches the contact of a post-orogenic granitic pluton hosted in slates are studied:

• Sample 1 (350 m from the granite): clayey slate with well-defined cleavage and small millimetre-scale carbonaceous aggregates (spotted slate).

• Sample 2 (at 120 m): rock with prismatic andalusite porphyroblasts of the chiastolite variety oriented randomly, cutting across the pre-existing foliation.

• Sample 3 (at the direct contact): hard, massive hornfels with a granoblastic fabric, composed of cordierite, andalusite, biotite, and potassium feldspar, without any visible schistosity.

1. a) Justify the progressive disappearance of tectonic foliation approaching the pluton and interpret the relative pressure and temperature conditions that governed this metamorphism.
2. b) Predict what mineralogical and textural changes would be observed in Sample 3 if the granitic intrusion had occurred at a depth of 25 km under an active compressive orogenic regime instead of at 3 km in a static, brittle crust.
3. c) A student classifies the aluminium silicate in Sample 2 as kyanite. Using the phase relations of the Al2SiO5 polymorphs, reasonably refute their identification in this geological setting.

Bowen's reaction series has a continuous branch (plagioclases) and a discontinuous branch (olivine, pyroxene, amphibole, biotite). Explain the difference between the two branches and what happens to an olivine crystal that reacts with the residual melt until it becomes isolated from it (reaction rim).

In a borehole drilled into a layered igneous intrusion, two samples were collected at different depths:

• Sample A (deep): ultramafic rock composed of 75% forsteritic olivine and 25% anorthite-rich plagioclase.

• Sample B (shallower): felsic rock rich in quartz, potassium feldspar, and biotite.

1. a) Based on Bowen's reaction series and gravity-driven magmatic differentiation processes, justify how a single homogeneous basaltic parental magma could generate both types of rocks.
2. b) Predict what compositional anomaly Rock B would display if the early olivine crystals had not segregated from the melt and had reacted continuously with it during very slow cooling in a closed system.
3. c) A preliminary petrological report states: «Since olivine is a silicate, its massive precipitation rapidly depletes silica from the residual melt». Refute this statement using stoichiometric comparative reasoning between the silica proportion in olivine and that of the original basaltic magma.

The geochemical and textural study of a granodioritic batholith has revealed the following data:

1. The 87Sr/86Sr isotopic ratio varies gradually from 0,7045 in the centre of the pluton to 0,7110 at the margins, where it is hosted in ancient crustal metasediments with 87Sr/86Sr = 0,7250.

2. Plagioclase phenocrysts exhibiting complex oscillatory zoning and resorption surfaces mantled by anorthite-richer overgrowths (sieve texture) are observed, along with rounded quartz xenocrysts rimmed by coronas of clinopyroxene microcrystals.

1. a) Interpret which magmatic evolution process explains the peripheral isotopic anomaly and deduce whether this mechanism caused an evolution towards more felsic or more mafic compositions.
2. b) Justify which petrogenetic phenomenon accounts for the corrosion and mineralogical disequilibrium textures observed in the plagioclase and quartz.
3. c) A technical report claims that all the variability of this pluton is solely due to fractional crystallisation in a closed system. Refute this statement by providing at least two arguments based on the data provided.

In a Plinian pyroclastic deposit, a 2,5 mm plagioclase phenocryst is sampled and analysed using an electron microprobe along a cross-profile from the core to the rim:

• Core: homogeneous An45 composition (andesine), euhedral.

• Intermediate zone: corroded inner margin with cellular resorption (sieve texture), followed by an abrupt band with an An80 composition (bytownite).

• Outer rim: continuous normal zoning gradually decreasing from An80 to An62 across the outermost 40 micrometres.

1. a) Interpret the thermal and compositional history of the magma chamber recorded chronologically within this crystal.
2. b) Predict what phenomenon likely acted as the physical trigger for the final volcanic eruption and justify your answer considering the contrasts in density, temperature, and volatile content between the magmas involved.
3. c) It is estimated via Fe-Mg diffusion models in plagioclase that the time between the injection of the anorthitic melt and the magma reaching the surface was only 15 days. Justify how this petrological information can be applied to the design and interpretation of seismic and geodetic monitoring networks prior to an eruption.

What is lava and how do rough-surfaced flows (aa) and ropy-surfaced flows (pahoehoe) differ? What controls each morphology?

What are pyroclasts and which rocks do they form when consolidated (tuffs, ignimbrites)? What type of eruption emits them?

Which gases does a volcano emit and which is the most abundant? Why does the gas content influence the explosive character of the eruption?

Relate the silica content of a magma to its viscosity and eruptive style: explain why basaltic magmas produce effusive eruptions and rhyolitic magmas produce explosive eruptions.

Describe the differences between a Hawaiian and a Plinian eruption: type of magma, emitted products, shape of the volcanic structure, and associated hazards.

Basalt flows forming long lava flows; rhyolite, domes and short, thick flows. Relate these morphologies to the viscosity of each lava and to its silica content.

The columnar jointing of basalt forms hexagonal prisms. Explain how they originate by thermal contraction as the flow cools and what their orientation indicates about the cooling direction.

Pillow lavas typically form in submarine eruptions. Explain their formation mechanism and what they indicate when they appear inside a mountain range, far from the present-day sea.

The Teide-Pico Viejo complex (Tenerife) is a stratovolcano with explosive eruptions of phonolitic lavas and viscous domes, whereas the Tajogaite eruption (La Palma, 2021) formed a cinder cone with very fluid basanitic lavas. Considering that both islands share a common origin linked to the Canarian hotspot:

1. a) Justify how it is possible that radically different volcanic edifices and eruptive styles can be generated from similar primary mantle magmas on two islands of the same archipelago.
2. b) Devise a hypothesis regarding the architecture of the magma feeding system (chambers, reservoirs, and residence time) beneath Tenerife versus La Palma that explains the presence of highly evolved magmas in one case and primitive magmas in the other.
3. c) Predict which specific volcanic hazards derived from magma rheology and composition (beyond the advance of basaltic lava flows) must be considered in the hazard map for the surroundings of Teide that were not decisive in the Tajogaite eruption.

During the Permian-Triassic transition (~252 Ma ago), the Siberian Traps erupted more than 3.000.000 km³ of basaltic lavas in less than one million years. This event coincides with the largest mass extinction of the Phanerozoic, a sharp negative excursion in δ13C in global sediments, and global warming of about 8 °C to 10 °C.

1. a) Contrast the timescale and magmatic eruption rate of a Large Igneous Province (LIP) with modern mid-ocean ridge volcanism, explaining why mid-ocean ridges do not trigger global extinctions despite generating enormous volumes of basaltic crust.
2. b) The volume of CO2 directly outgassed by basaltic magma is not sufficient to explain the drastic temperature rise or the δ13C isotopic signature observed in the geological record. Devise a thermodynamic hypothesis involving the effect of basaltic sills and dykes on the host sedimentary basins (rich in coal, oil shales, and evaporites) to resolve this discrepancy.
3. c) Justify the cause-and-effect chain connecting the magmatic pulse to the anoxia and acidification of shallow marine waters that wiped out most benthic invertebrates.