Exercises: Igneous rocks

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

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.

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?

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 dike and a sill?

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.

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

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).

Basalt forms long lava flows; rhyolite forms 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.

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 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 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 dikes 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.

Analyze 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 centers 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.