Exercises: Magmatism and igneous rocks

Define magma and explain the three factors that can cause rock inside the planet to melt: an increase in temperature, a decrease in pressure, and the addition of water.

Explain why most of the planet's active volcanoes are aligned with lithospheric plate boundaries, and cite a volcanic region associated with a divergent boundary and another with a convergent boundary.

Match each magmatic setting — mid-ocean ridge, subduction zone, and hotspot — with the type of igneous rocks it produces, choosing between basic rocks (basalt, gabbro) and intermediate or felsic rocks (andesite, granite), and cite a famous archipelago formed by a hotspot.

Distinguish between magma and lava, and between plutonic (intrusive) and volcanic (extrusive) rock, giving an example of each rock and explaining where each one cools.

Explain what the viscosity of a magma is, which three factors it depends on (silica content, temperature and dissolved gases) and how viscosity influences whether an eruption is gentle or explosive.

Name the three major groups of products a volcano emits (lava flows, pyroclasts and gases) and associate a specific risk for nearby populations with each one.

What is a volcanic caldera and how does it form? Distinguish it from a crater and name an example in Spain or in the world (Las Cañadas del Teide, Santorini, Yellowstone).

Explain what the volcanic explosivity index (VEI) measures, which parameters it is calculated from (volume of material emitted, height of the eruptive column) and why it is an open scale similar to the earthquake magnitude scale.

The cross-section diagram shows three settings where magmas are generated (1 to 3). a) Identify each one. b) Explain the melting mechanism operating in each setting: pressure reduction, addition of water, or thermal mantle anomaly. c) State what type of igneous rock results in each case.

A magma of the same composition can give rise to either a plutonic or a volcanic rock. Explain, based on their cooling history, the difference between a granite and a rhyolite, and what feature of their texture allows them to be distinguished with the naked eye.

Hotspots generate chains of volcanic islands aligned by age, such as the Hawaiian archipelago. Explain how this chain forms as the plate moves over the hotspot and why the age of the islands serves as a record of plate motion.

"Columnar jointing" splits some basalt lava flows into perfect hexagonal prisms (Los Órganos on La Gomera). Explain which physical process produces them as the lava cools (thermal contraction and fracturing) and why the hexagonal geometry is the most efficient.

At ridges, magma is generated by decompression melting, and at subduction zones by the addition of water. Explain what each mechanism consists of and discuss why the water released by the sunken plate melts the mantle above it even though the temperature does not rise.

As it rises, a magma changes composition through fractional crystallisation. Explain what this process consists of (which minerals crystallise first, what happens to the residual magma) and why evolved magmas end up richer in silica.

In 2021, the lava from the Tajogaite volcano entered the sea and built a new lava delta. Describe the processes that occur when lava comes into contact with ocean water (sudden cooling, clouds of acidic vapour, fragmentation) and the added risks for the population and for fishing.

Distinguish between a stratovolcano and a shield volcano: the shape of the edifice, the composition and viscosity of their lavas, the dominant type of eruption and an example of each (Teide or Vesuvius versus Hawaii).

A volcanology team analyzes samples from two recent submarine eruptions: volcano A emits fluid lavas with 49 % $\text{SiO}_2$ and less than 0,5 % $\text{H}_2\text{O}$; volcano B, located behind an oceanic trench, ejects viscous lavas and pyroclasts with 68 % $\text{SiO}_2$ and 4,2 % volatiles. a) Justify the thermal and compositional origin of the magma in volcano A, deducing why adiabatic decompression melting at mid-ocean ridges barely generates differentiation. b) Justify the two geochemical processes that have transformed the initial melt in volcano B into an explosive felsic magma, detailing the role of the overlying lithospheric mantle and the fluids released by the subducting plate.

A popular science report states the following: "The Aleutian island arc exhibits quiet and effusive volcanism because the surrounding seawater cools the magma, making it stable; in contrast, Iceland suffers continuous devastating Plinian eruptions because it is located on an open mid-ocean ridge." a) Refute both statements on a reasoned basis: classify the real tectonic setting of each area and justify what type of eruption predominates in each one according to viscosity, silica content, and magma degassing. b) Predict and compare the two main direct volcanic hazards (lahars, lava flows, pyroclastic flows, ash) that threaten the population in each of these two scenarios.

In 2021, the eruption of Tajogaite (La Palma, intraplate setting) emitted basaltic lavas that advanced down the slope, burying buildings but allowing orderly evacuations. In contrast, in 2008, the Chaitén volcano (Chile, subduction zone) woke up abruptly with a plinian column of rhyolitic ash over 20 km high and the collapse of pyroclastic domes. a) Based on mantle melting conditions and magmatic evolution in each geodynamic setting, justify why the rheology (viscosity and gases) of both magmas was so disparate. b) As an advisor to Civil Protection, justify why the evacuation and exclusion zone delimitation measures applied in La Palma would have been useless and lethal in an event like the one at Chaitén.

Observe the following chronology on the onset of surface volcanism in the Canary Islands: Fuerteventura (about 20 Ma ago), Tenerife (about 12 Ma ago), La Palma (about 2 Ma ago), and El Hierro (1,1 Ma ago). However, Fuerteventura and Tenerife have recorded volcanic activity in the Holocene, and Mount Teide is an active stratovolcano. a) If the archipelago strictly followed a stationary, thermal mantle plume of the Hawaii type beneath a moving lithospheric plate, what eruptive activity would you expect to find today in Fuerteventura and Lanzarote? Justify the discrepancy with actual data. b) To resolve this anomaly, two models have been proposed: propagating fracture tectonics from the Atlas, and a pulsating mantle plume beneath a nearly stationary plate. Design what geophysical (such as deep seismic tomography) or geochemical observations would confirm or rule out the existence of a deep thermal plume beneath the islands.

Following a major volcanic eruption, 15 million tons of $\text{SO}_2$ are released into the atmosphere. If the eruptive column does not exceed an altitude of 9 km (mid-troposphere), the resulting surface cooling is virtually zero and dissipates in weeks; in contrast, if the column reaches 24 km (stratosphere), the global mean temperature drops by 0,5 °C for three years. a) Justify at a physicochemical level what transformation $\text{SO}_2$ undergoes in the atmosphere, how it modifies the Earth's radiative balance (albedo versus greenhouse effect), and why the altitude reached determines so critically the duration of the climatic impact. b) Certain climate geoengineering plans propose injecting sulfate aerosols into the stratosphere to curb global warming. Critically evaluate this proposal by predicting two unfavorable side effects on the climatic or chemical dynamics of Earth's atmosphere.

The Volcanic Explosivity Index (VEI) is a logarithmic scale relative to the volume of tephra ejected (from VEI 2 onward, each single-step increase multiplies the ejected volume by 10). The eruption of Tambora (1815) reached a VEI 7 and expelled about $100\ \text{km}^3$ of pyroclasts, causing the "year without a summer" in 1816. a) If a modern volcano erupts with a VEI 5, calculate the approximate volume of tephra emitted and justify why two eruptions with identical VEI can produce completely disparate global climatic impacts depending on the original chemical composition of the magma and the latitude of the volcano. b) Reconstruct the cascade of impacts that the contemporary interconnected global socioeconomic system would suffer if a VEI 7 eruption were to occur today in an intertropical zone, analyzing the effect on global agricultural primary production and supply chains.

A monitoring station records the following changes in an andesitic stratovolcano over four weeks: 1) Tiltmeters show accelerated bulging of the volcanic edifice; 2) Volcano-tectonic earthquakes decrease while continuous shallow harmonic tremor is recorded; 3) The $\text{SO}_2$ flux increases tenfold, but the $\text{CO}_2/\text{SO}_2$ molar ratio drops sharply; 4) Suddenly, gas emissions fall to baseline levels within hours, without ground deformation ceasing. a) Interpret what physical and magma degassing processes signals 1, 2, and 3 reveal (taking into account the different solubilities of $\text{CO}_2$ and $\text{SO}_2$ as confining pressure changes). b) Predict what mechanical phenomenon caused the sudden change in signal 4 and justify what imminent risk of catastrophic eruption the observatory should warn Civil Protection about.

At mid-ocean ridges, 99% of magmatic rocks are tholeiitic basalts of remarkably constant composition. In contrast, in the East African Rift, alkaline basalts coexist with abundant trachytes, phonolites, and peralkaline rhyolites. a) Justify this bimodality in the continental rift petrologically: deduce how the thickness of the continental lithosphere, residence times in intermediate magma chambers, and fractional crystallization with crustal assimilation influence the diversification of these magmas compared to mid-ocean ridges. b) Predict how the composition and petrological variety of magmatism in the Horn of Africa will evolve in the long term if crustal separation culminates in the complete oceanization of the basin.